Module 6
Digestive and Urinary Systems
a. Overview of the Digestive System
The digestive system represents a marvel of biological engineering, intricately
designed to process ingested food and extract vital nutrients essential for sustaining life.
Comprising a complex network of organs and tissues, the digestive tract serves as the
primary site for the mechanical and chemical breakdown of food, as well as the
absorption of nutrients into the bloodstream for distribution to cells throughout the body.
Beginning at the mouth, where the process of digestion initiates with the
mechanical grinding and chemical breakdown of food by teeth and salivary enzymes, the
digestive tract extends along a convoluted path through various anatomical structures,
culminating at the anus. This continuous tube, known as the gastrointestinal (GI) tract,
traverses multiple regions of the body, including the oral cavity, pharynx, esophagus,
stomach, small intestine, and large intestine, each with distinct functions and specialized
adaptations for digestion and absorption.
As food progresses along the GI tract, it undergoes a series of coordinated
processes orchestrated by specialized organs and tissues. In the stomach, gastric glands
secrete hydrochloric acid and digestive enzymes to further break down food particles and
initiate protein digestion. Concurrently, rhythmic contractions of the stomach wall churn
and mix the contents, forming a semi-liquid mixture known as chyme.
The small intestine represents the primary site for nutrient absorption, where the
majority of digestion and absorption processes occur. The small intestine consists of three
segments: the duodenum, jejunum, and ileum, each optimized for specific functions.
Within the small intestine, pancreatic enzymes and bile secreted from the liver aid in the
digestion and emulsification of fats, while intestinal brush border enzymes facilitate the
breakdown of carbohydrates and proteins into absorbable molecules.
Nutrient absorption in the small intestine occurs across the mucosal surface,
where specialized structures such as villi and microvilli increase the absorptive area and
facilitate the transport of nutrients into the bloodstream. Absorbed nutrients, including
glucose, amino acids, fatty acids, vitamins, and minerals, are transported via blood
vessels to various tissues and organs, where they serve as building blocks for cellular
metabolism and energy production.
After nutrient absorption is completed, undigested food residues and waste
products pass into the large intestine, where water reabsorption and formation of feces
occur. The large intestine also harbors a diverse microbial ecosystem, known as the gut
microbiota, which plays a crucial role in nutrient metabolism, immune function, and host-
microbe interactions.
Finally, waste materials are expelled from the body through the anus during
defecation, completing the digestive process and maintaining gastrointestinal
homeostasis. The intricate coordination of digestive processes, regulated by neural,
hormonal, and local factors, ensures efficient nutrient assimilation, waste elimination, and
overall digestive function.
In summary, the gastrointestinal tract represents a dynamic and highly organized
system responsible for the digestion, absorption, and metabolism of ingested food. From
the mouth to the anus, this remarkable anatomical structure facilitates the conversion of
complex nutrients into usable forms, supporting cellular function, growth, and overall
health. Understanding the anatomy and physiology of the digestive system provides
insights into the complexities of human nutrition and metabolism, as well as the
pathophysiology of digestive disorders and diseases.
An interesting fact about the GI tract is that while food or leftover residues are in
it, technically the material is still outside the body. Nutrients don’t “officially” enter the
body until they move from the lumen the space inside the digestive tube into the
bloodstream. Blood delivers nutrients to cells throughout the body. From beginning to
end, epithelium lines the surfaces facing the lumen. The lining is coated with thick, moist
mucus that protects the wall of the tube and enhances the diffusion of substances across
it.
From the esophagus onward, the digestive tube wall has four layers. The
innermost layer is a mucosa of epithelial cells. It lines the lumen, the space through
which food passes. The mucosa is surrounded by the submucosa, a layer of connective
tissue with blood and lymph vessels and nerve cells. The next layer is smooth muscle—
usually two sublayers, one circling the tube and the other oriented lengthwise. An outer
layer, the serosa, is a very thin serous membrane. Circular arrays of smooth muscle called
sphincters are located at the junctions between sections of the GI tract. As sphincters
contract and relax, they control the movement of material in the tube. For example, a
gastroesophageal sphincter controls the passage of food from the esophagus into the
stomach.
b. Chewing and Swallowing: Food Processing Begins
In the oral cavity, or mouth, the food you eat begins to be broken apart by
chewing. Most adults have thirty-two teeth. Young children have just twenty so-called
primary teeth. A tooth’s crown is coated with tooth enamel. It consists of hardened
calcium deposits and is the hardest substance in the body. The enamel covers a living,
bonelike layer called dentin. Dentin and an inner pulp extend into the root. The pulp
cavity contains blood vessels and nerves. The shape of a tooth fits its function. Chisel-
shaped incisors bite off chunks of food, and cone-shaped canines (cuspids) tear it.
Premolars and molars, with broad crowns and rounded cusps, grind it.
Chewing mixes food with saliva from several salivary glands. A large parotid
gland nestles just in front of each ear. Submandibular glands lie just below the lower jaw
in the floor of the mouth, and sublingual glands are under your tongue. The tongue itself
is skeletal muscle covered by a membrane.
Saliva is mostly water, but it includes other substances. An important one is the
enzyme salivary amylase, which breaks down starch; chew on a soda cracker and you can
feel it becoming mushy as salivary amylase goes to work. A buffer, bicarbonate (HCO3
2), keeps the pH of your mouth between 6.5 and 11.5, a range within which salivary
amylase can function. Saliva also contains mucins, proteins that help bind food bits into a
lubricated ball. Once it is swallowed, this ball of chewed food is called a bolus (bow-lus).
Starch digestion continues in the stomach until acids there inactivate salivary amylase.
Behind the upper teeth is a bone-reinforced section of the palate—the roof of the
mouth. It provides a hard surface against which the tongue can press food it is mixing
with saliva. Tongue muscle contractions force the bolus into the pharynx (fare-inks), the
throat. This passageway connects with the windpipe, or trachea , which leads to the lungs.
It also connects with the esophagus, which leads to the stomach. Mucus secreted by the
membrane lining the pharynx and esophagus lubricates the bolus, helping move food on
its way.
Swallowing food might seem simple, but it involves a sequence of events.
Swallowing begins when voluntary skeletal muscle contractions push a bolus into the
pharynx, stimulating sensory receptors in the pharynx wall. The receptors trigger a reflex
in which involuntary muscle contractions keep food from moving up into your nose and
down into the trachea. As this reflex occurs, the vocal cords are stretched tight across the
entrance to the larynx (your “voice box”). Then, the flaplike epiglottis is pressed down
over the vocal cords as a secondary seal. For a moment, breathing stops as food moves
into the esophagus, so you normally don’t choke when you swallow. When swallowed
food reaches the lower esophagus, it passes through a sphincter into the stomach. Waves
of muscle contractions called peristalsis (pare-ih-stal-sis) help push the food bolus along.
c. The Stomach: Food Storage, Digestion, and More
The surface of the stomach wall facing the lumen is lined with glandular
epithelium. Each day, gland cells in the lining release about 2 liters (roughly 2 quarts) of
hydrochloric acid (HCl), mucus, and other substances. These include pepsinogens,
precursors of digestive enzymes called pepsins. Other gland cells secrete intrinsic factor,
a protein required for vitamin B12 to be absorbed later on, in the small intestine. Along
with water, these substances make up the stomach’s strongly acidic gastric juice.
Combined with mixing due to stomach contractions, the acidity converts swallowed
boluses into thick, pasty chyme (kime). The acidity also kills most microbes in food.
The digestion of proteins starts when the high acidity denatures proteins and
exposes their peptide bonds. The acid also converts pepsinogens to active pepsins, which
break the bonds, “chopping” the protein into fragments. Meanwhile, gland cells secrete
the hormone gastrin, which stimulates cells that secrete HCl and pepsinogen. mulates
cells that secrete HCl and pepsinogen. Usually, mucus and bicarbonate prevent gastric
juice from harming the stomach lining. These protections form the “gastric mucosal
barrier.” When the barrier breaks down, an ulcer can develop.
Waves of peristalsis move food out of the stomach. These waves mix chyme and
build force as they approach the pyloric sphincter at the stomach’s base. When a strong
contraction arrives, the sphincter closes, squeezing most of the chyme back. Only a small
amount moves into the small intestine at a given time. In this way the stomach regulates
the rate at which food moves onward, so that food is not passed along faster than it can be
processed. Depending mainly on the fat content and acidity of chyme, it can take from 2
to 6 hours for a full stomach to empty. When the stomach is empty, its walls crumple into
folds called rugae.
Water and alcohol are two of a few substances that begin to be absorbed across
the stomach wall. Liquids imbibed on an empty stomach pass rapidly to the small
intestine, where absorption continues. Putting food into your stomach slows its emptying.
This is why the effects of alcohol are more gradual when drinking accompanies a meal,
especially one that contains fat.
d. The Small Intestine: A Huge Surface for Digestion and Absorption
Your small intestine is about an inch and a half in diameter and 6 meters (20 feet)
long. It absorbs most nutrients in the food you eat. How densely folded the mucosa is,
and how the folds all stick out like ruffles into the lumen. Each fold has even smaller,
hairlike projections. Each “finger” is a villus (plural: villi). Small blood vessels (an
arteriole and a vein) and a lymph vessel in each villus move substances to and from the
bloodstream. Gland cells in the mucosal lining release digestive enzymes.
Most cells in the epithelium covering a villus have a threadlike projection of their
plasma membrane. This projection is called a microvillus (plural: microvilli). Each
epithelial cell has about 1,700 microvilli—a dense array that gives the epithelium of villi
its common name, the brush border.
The intricate folds and projections present in the intestinal mucosa, including villi
and microvilli, serve as essential anatomical adaptations that vastly increase the surface
area available for nutrient absorption within the small intestine. This extensive surface
area enhancement is crucial for optimizing the efficiency of nutrient absorption from the
chyme, the semi-liquid mixture of partially digested food and gastric secretions, as it
traverses through the intestinal lumen.
The villi, which are finger-like projections that protrude from the mucosal lining
of the small intestine, significantly augment the absorptive surface area by increasing the
mucosal surface area exposed to the chyme. These villi are densely packed with
specialized epithelial cells, known as enterocytes, which are responsible for nutrient
absorption. Furthermore, the surface of each villus is adorned with even smaller
structures called microvilli, forming a brush border that further amplifies the absorptive
surface area.
The microvilli, often referred to as the "brush border membrane," are tiny, finger-
like extensions of the plasma membrane of enterocytes. These microvilli dramatically
increase the surface area available for nutrient absorption at the cellular level, allowing
for more efficient exchange of nutrients and waste products across the epithelial barrier.
The microvilli contain numerous transport proteins and enzymes involved in the uptake
and processing of nutrients, facilitating their efficient absorption into the bloodstream.
Collectively, the presence of villi and microvilli within the small intestine
significantly magnifies the absorptive surface area, enabling the efficient absorption of
nutrients, including carbohydrates, proteins, lipids, vitamins, and minerals, from the
chyme into the bloodstream. This expansive surface area enhancement ensures rapid and
effective nutrient uptake, allowing for the efficient extraction of essential nutrients
required for cellular metabolism, growth, and overall physiological function.
Without the extensive surface area provided by the folds and projections of the
intestinal mucosa, nutrient absorption would be severely compromised, leading to
inadequate nutrient uptake and potential malnutrition. The increased surface area
provided by villi and microvilli is essential for maintaining optimal nutrient absorption
rates, ensuring the timely delivery of nutrients to cells throughout the body to support
vital physiological processes.
Furthermore, the enhanced absorptive surface area of the intestinal mucosa
facilitates the absorption of water and electrolytes, contributing to fluid balance and
electrolyte homeostasis within the body. This regulatory function is essential for
maintaining proper hydration, blood volume, and electrolyte concentrations, which are
critical for cardiovascular function, nerve transmission, and cellular hydration.
In summary, the extensive folds and projections present in the intestinal mucosa
serve as essential anatomical adaptations that greatly enhance the absorptive surface area
of the small intestine. This surface area amplification is crucial for optimizing the
efficiency of nutrient absorption from the chyme, ensuring the timely delivery of
essential nutrients required for cellular metabolism, growth, and overall physiological
function. Understanding the functional significance of these mucosal adaptations
provides insights into the complexities of nutrient absorption and gastrointestinal
physiology, highlighting the remarkable efficiency and adaptability of the digestive
system.
e. Accessory Organs: The Pancreas, Liver, and Gallbladder
The pancreas nestles behind and below the stomach. It contains exocrine cells
that release digestive enzymes into the duodenum, the first section of the small intestine.
The pancreas also contains endocrine cells that release hormones into the blood.
There are four types of pancreatic enzymes, which can chemically dismantle the
four major categories of food—complex carbohydrates, proteins, lipids, and nucleic
acids. These enzymes work best when the pH is neutral or slightly alkaline, so the
“pancreatic juice” also contains bicarbonate (HCO3 2), which neutralizes the acid in
chyme moving into the duodenum from the stomach. Depending on how often and what
type of food you eat, your pancreas may make 2 quarts of this fluid each day.
When the digestive system is processing food, a yellowish fluid called bile is
released into the upper small intestine. Making bile is a digestive role of the liver. This
large organ secretes as much as 1,500 mL, or almost 1.6 quarts, of bile every day. Bile is
a blend of substances including water and bile salts synthesized from cholesterol. Bile
salts aid in the digestion and absorption of fats. Bile is stored in the gallbladder, a small
sac tucked behind the liver. As needed, the gallbladder contracts and empties bile into the
small intestine where it aids in the digestion and absorption of fats. When no food is in
the small intestine, a sphincter closes off the main bile duct, and bile backs up into the
gallbladder.
Besides its digestive functions, the liver processes incoming nutrients into
substances the body requires. A system of blood vessels called the hepatic portal system
diverts blood from the small intestine to the liver. Blood entering the liver in this system
arrives in the hepatic portal vein and returns to the general circulation via the hepatic
vein. After a meal, when blood from the small intestine enters the system loaded with
nutrients, liver cells manage this bonanza in various ways. For example, if the blood
contains more of the sugar glucose than body cells can take up at the time, the liver
removes some of the excess and stores it as glycogen. The liver also stores several
vitamins and minerals and forms the active form of vitamin D, which is essential for the
uptake of calcium from digested food. Liver cells use arriving amino acids to synthesize
proteins such as the albumin in blood plasma, or process and reship them in a form cells
throughout the body can use to make ATP.
The chart lists some other major liver functions. For instance, the liver removes
alcohol and other potential toxins, such as ammonia produced by the breakdown of amino
acids. The ammonia is converted to urea, a much less toxic waste product that is excreted
in urine. Liver cells also take up bilirubin, a pigment that forms as aging or damaged red
blood cells are broken down and the hemoglobin in them is recycled. Bilirubin is added
to bile and eventually is excreted in feces. In addition, the liver also inactivates many
hormones, which move via the blood to the kidneys and are excreted in urine.
Bile often contains cholesterol apart from that used to synthesize bile salts. This
excess may form a gallstone in the gallbladder. If the gallbladder is surgically removed—
usually due to the painful presence of gallstones— the duct that connects it to the small
intestine enlarges and takes on the role of bile storage. This is why millions of people are
walking around minus their gallbladder, with few or no ill effects.
f. Digestion and Absorption in the Small Intestine
Each day about 9 liters (10 quarts) of fluid enters the first section of the small
intestine, the duodenum (doo-oh-deenum). This fluid includes chyme along with enzymes
and other substances from the pancreas, liver, and gallbladder. Most digestion and
nutrient absorption occurs in the next section, the 3-foot-long jejunum. Some nutrients
are absorbed while the remaining material is moving through the ileum, the last section of
the small intestine, on its way to the large intestine.
Chyme entering the duodenum triggers hormone signals that stimulate a brief
flood of digestive enzymes from the pancreas. As part of pancreatic juice, these enzymes
act on carbohydrates, fats, proteins, and nucleic acids. For example, like pepsin in the
stomach, the pancreatic enzymes trypsin and chymotrypsin digest the polypeptide chains
of proteins into peptide fragments. The fragments are then broken down to amino acids
by different peptidases (which are on the surface of the intestinal mucosa). Recall from
that the pancreas also secretes bicarbonate that buffers stomach acid, maintaining a
chemical environment in which pancreatic enzymes can function.
Fat digestion requires enzymes called lipases. Bile salts in bile secreted by the
liver (and delivered via the gallbladder) make fat digestion more efficient. Bile salts are
like a detergent—they emulsify, or break up, large units of fat into smaller ones. How
does this process work? Most fats in the average diet are triglycerides, which tend to
clump into big fat globules in chyme. When peristalsis mixes chyme, the globules break
up into droplets that become coated with bile salts. These droplets, called micelles
(mycells), give fat-digesting enzymes a much greater surface area to act on. So, because
triglycerides are emulsified, they can be broken down much faster to monoglycerides and
fatty acids, molecules that are small enough to be absorbed. Micelles also may contain
fat-soluble vitamins.
When a substance is absorbed, it crosses the intestine lining into the bloodstream.
Due partly to the vast absorptive surface area of the small intestine, this process is very
efficient. Segmentation helps, too. In this process, rings of smooth muscle in the wall
repeatedly contract and relax. The result is a back-and-forth movement that mixes
digested material and forces it against the wall:
By the time food is halfway through the small intestine, most of it has been
broken apart and digested. Water crosses the intestine lining by osmosis, and cells in the
lining also selectively absorb minerals. Transport proteins in the plasma membrane of
brush border cells actively move some nutrients, such as the monosaccharide glucose and
amino acids, across the lining. After glucose and amino acids are absorbed, they move
into tissue fluid and then directly into blood vessels.
Additional steps occur before digested lipids move into the bloodstream. After
lipases digest micelles, the fatty acids and monoglycerides enter brush border cells, just
as glucose and amino acids do. (The bile salts that formed the droplets are recycled.)
There, fatty acids and monoglycerides quickly reunite into triglycerides. Then
triglycerides combine with proteins into particles that leave the cells by exocytosis and
enter tissue fluid. They don’t directly enter blood vessels, however. Instead they cross
into lymph vessels known as lacteals, which drain into the general circulation.
g. Controls over Digestion
The regulation of digestion is a finely tuned process orchestrated by a complex
interplay of signals from the nervous system and hormones secreted by endocrine cells
distributed throughout the gastrointestinal (GI) tract. These regulatory mechanisms
operate in concert to modulate digestive processes in response to various stimuli,
including the presence of food, its chemical composition, and the physiological needs of
the body.
The nervous system exerts both intrinsic and extrinsic control over digestion
through a network of neurons embedded within the walls of the GI tract, collectively
known as the enteric nervous system (ENS), as well as through extrinsic nerve fibers
originating from the central nervous system (CNS). The ENS coordinates local reflexes
that regulate gastrointestinal motility, secretion, and absorption, enabling the efficient
processing and propulsion of food along the digestive tract. Sensory neurons within the
ENS detect mechanical and chemical stimuli in the GI lumen, signaling the release of
neurotransmitters that modulate smooth muscle contraction, glandular secretion, and
blood flow in response to changing conditions.
Extrinsic neural signals from the CNS, conveyed via sympathetic and
parasympathetic nerve fibers, further regulate digestive function by modulating the
activity of the ENS and influencing GI motility and secretion. Parasympathetic
stimulation, mediated by the vagus nerve, promotes digestion by increasing salivary
gland secretion, gastric motility, and intestinal peristalsis, while sympathetic activation
inhibits digestive processes and redirects blood flow away from the GI tract during
periods of stress or physical exertion.
In addition to neural regulation, digestion is modulated by a diverse array of
hormones secreted by specialized endocrine cells located throughout the GI mucosa,
collectively referred to as the enteroendocrine system. These hormones act as chemical
messengers, transmitting signals to target cells and tissues to coordinate various aspects
of digestion and nutrient absorption. Examples of GI hormones include gastrin, secretin,
cholecystokinin (CCK), ghrelin, and glucagon-like peptide-1 (GLP-1), each with specific
roles in regulating gastric acid secretion, pancreatic enzyme release, bile production,
appetite regulation, and glucose homeostasis.
The release of GI hormones is tightly regulated in response to luminal stimuli,
such as the presence of food components (e.g., proteins, fats, carbohydrates), changes in
pH, and mechanical distension of the GI tract. For example, the ingestion of protein-rich
foods triggers the release of gastrin and CCK, stimulating gastric acid secretion and
pancreatic enzyme release, respectively, to facilitate protein digestion and nutrient
absorption. Similarly, the presence of dietary fats in the duodenum stimulates the
secretion of CCK and GLP-1, promoting bile release from the gallbladder and insulin
secretion from pancreatic beta cells to facilitate lipid digestion and glucose uptake.
Furthermore, the coordinated actions of neural and hormonal signals allow for the
precise regulation of digestive processes in accordance with the nutritional status and
metabolic demands of the body. These regulatory mechanisms ensure the efficient
extraction and utilization of nutrients from ingested food, while also maintaining GI
function and integrity. Dysregulation of neural and hormonal control mechanisms can
lead to digestive disorders, such as dyspepsia, gastroesophageal reflux disease (GERD),
irritable bowel syndrome (IBS), and malabsorption syndromes, underscoring the
importance of integrated signaling pathways in maintaining digestive health and
homeostasis.
In summary, digestion is regulated by a sophisticated network of neural and
hormonal signals that coordinate GI motility, secretion, and absorption in response to
luminal stimuli and physiological cues. The integration of nervous and endocrine
signaling pathways ensures the efficient processing and utilization of ingested nutrients,
while also maintaining GI function and homeostasis. Understanding the intricate
mechanisms underlying digestive regulation provides insights into the complexities of
gastrointestinal physiology and the pathophysiology of digestive disorders, paving the
way for novel therapeutic approaches aimed at restoring digestive health and function.
When you take food in your mouth—and sometimes when you merely think about
eating— sensory receptors in your mouth stimulate the salivary glands to release saliva.
Food entering the stomach stretches the stomach walls, and then those of the small
intestine. This stretching also triggers signals from sensory receptors. Some of the signals
give you (by way of processing in your brain) that “full” feeling after you eat. Others can
lead to the muscle contractions of peristalsis or the release of digestive enzymes and other
substances. Centers in the brain coordinate these activities with factors such as how much
blood is flowing to the small intestine, where nutrients are being absorbed.
There are several types of endocrine cells in the GI tract. For example, one type
secretes the hormone gastrin into the bloodstream when the stomach contains protein.
Gastrin mainly stimulates the release of hydrochloric acid (HCl), which you may recall is
a key ingredient in gastric juice. After the stomach has emptied out, the increased acidity
there causes another type of endocrine cell to release somatostatin, which shuts down
HCl secretion so that conditions in the stomach are less acidic. Notice that this is an
example of negative feedback.
Hormones also come from endocrine cells in the small intestine. One of them,
secretin, signals the pancreas to release bicarbonate when acid enters the duodenum.
When fat enters the small intestine, a hormone called CCK (for cholecystokinin) is
released. CCK spurs the pancreas to release enzymes and triggers gallbladder
contractions that deliver bile into the small intestine. Secretin and CCK also slow the rate
at which the stomach empties—the mechanism mentioned that prevents food from
entering the small intestine faster than it can be processed there. Yet another hormone,
GIP (for glucose-dependent insulinotropic peptide) is released when fat and glucose are
in the small intestine. GIP stimulates the release of insulin from the pancreas, which is
required for cells to take up glucose.
h. Digestive System Disorders
The main symptom of gastroesophageal reflux disease, or GERD, is often called
“heartburn,” but it has nothing to do with the heart. With this common disorder acidic
chyme backs up into the esophagus when the lower esophageal sphincter doesn’t close
properly. The irritation causes burning in the upper chest and throat. Mild cases often can
be controlled by over-the-counter drugs that reduce stomach acid and by limiting intake
of acidic foods such as tomatoes, orange juice, coffee, and alcoholic beverages.
Hepatitis is inflammation of the liver. Obesity, certain drugs, and environmental
toxins may trigger it. Some types are caused by viruses that are transmitted in body fluids
such as blood and semen. The inflammation may subside with treatment, although some
patients suffer major, irreversible damage for which the only option is a liver transplant.
Long-term inflammation due to heavy alcohol consumption causes alcoholic cirrhosis
(sir-oh-sis), in which damaged liver cells are replaced by connective tissue “scars”.
It’s normal to “move the bowels,” or defecate, from three times a day to once a
week. In constipation, food residues remain in the colon for too long, too much water is
reabsorbed, and the feces become dry, hard, and difficult to eliminate. Constipation is
uncomfortable, and it is a common cause of the enlarged rectal blood vessels known as
hemorrhoids.
Constipation is often caused by a lack of bulk in the diet. “Bulk” is the volume of
fiber (mainly cellulose from plant foods) and other undigested food material that is not
decreased by absorption in the colon. Much of it is insoluble fiber such as cellulose and
other plant compounds that humans cannot digest (we lack the required enzymes) and
that does not easily dissolve in water. Wheat bran and the edible skins of fruits are just
two examples. (Plant carbohydrates such as fruit pectins that swell or dissolve in water
are soluble fiber.)
If you eat too little fiber, you are much more likely to be in the 50 percent of the
U.S. population in whom the colon has formed diverticula—knoblike sacs where the
inner colon lining protrudes through the wall of the large intestine. Inflammation of a
diverticulum is called diverticulitis, and it can have quite serious complications, including
peritonitis, if an inflamed diverticulum ruptures. Much more common is diverticulosis, in
which diverticula are there but have not (yet) become inflamed.
Have you ever heard of someone having a “spastic colon”? This problematical
condition also is known as IBS, or irritable bowel syndrome. IBS is the most common
intestinal disorder. It often begins in early to mid adulthood, and it affects twice as many
women as men. The direct trigger of IBS symptoms—abdominal pain and alternating
diarrhea and constipation—is a disturbance in the smooth muscle contractions that move
material through the colon. New research implicates a prior bacterial infection as the root
cause in some cases. Reports that consuming probiotics (“gut-friendly” bacteria such as
Bifidobacterium infantis) helps tame post-infectious IBS support this hypothesis.
Crohn’s disease is an inflammatory disorder that affects various organs including
the eyes, liver, skin, and intestines. In some patients the intestinal lining is so severely
damaged that much of the intestine must be removed. Although Crohn’s isn’t curable,
new treatment options are helping patients live with the disease more comfortably than
ever before.
Cancer of the colon or rectum—or colorectal cancer— is one of the most
frequently diagnosed cancers in the United States. It accounts for about 20 percent of all
cancer deaths. The first internal sign of colorectal cancer may be a round, depressed area
of abnormal cells. Another common early warning sign is a growth called a polyp that
develops on the colon wall and becomes malignant. Fortunately, many precancerous
growths and early cases of colon cancer can be detected by colonoscopy. After the patient
is mildly sedated, a physician inserts a viewing tube into the colon and can examine it for
polyps and other signs of disease.
Outward signs of colorectal cancer include a change in bowel habits, blood in
feces, or rectal bleeding. People over age 50 have the highest risk. The tendency to
develop polyps, and colorectal cancer, can run in families, but usually there is no obvious
genetic link. Because colorectal cancer is much more common in Western societies, some
experts have proposed that the typical high-fat, low-fiber Western diet may be a factor,
and there is a lot of active research on the issue. Studies suggest that low doses of aspirin
or NSAIDs (non-steroidal anti-inflammatory drugs such as ibuprofen) may reduce the
risk of developing precancerous polyps.
Anything that interferes with the small intestine’s ability to take up nutrients can
lead to a malabsorption disorder. As many as 50 million adults in the United States
develop lactose intolerance, a disorder that results from a deficiency of the enzyme
lactase. It prevents normal digestion and absorption of lactose, the sugar found in milk
and many milk products. Nausea, cramps, bloating, and diarrhea are common symptoms.
People who have celiac disease, or gluten intolerance, are hypersensitive to gluten, a form
of protein in wheat, rye, and barley. The disorder involves an autoimmune response in
which lymphocytes attack the villi of brush border cells. Symptoms can range from
lethargy and rashes to joint pain, mouth sores, and osteoporosis. People with gluten
intolerance can control their symptoms by eating a gluten-free diet.
Other malabsorption disorders are associated with diseases that affect the
pancreas, including the genetic condition cystic fibrosis (CF). Patients with CF don’t
make the necessary pancreatic enzymes for normal digestion and absorption of fats and
other nutrients.
i. The Challenge: Ever-Changing Extracellular Fluid
If you are an adult female in good health, by weight your body is about 50 percent
fluid. If you are an adult male, the ratio is about 60 percent. This fluid is extremely
important both in the composition of body structures and in nearly all body introduced
the concept of two “fluid compartments” in the body—one that is inside cells, and a
second that is outside cells.
Explained that tissue fluid fills the spaces between cells and other components of
tissues. Blood, which is mostly watery plasma, circulates in blood vessels. As you may
remember, tissue fluid, blood plasma, and the relatively small amounts of other fluids
(such as in lymph) outside cells together make up the body’s extracellular fluid, or ECF.
The fluid inside cells is intracellular fluid. A variety of gases and other substances
move constantly between intracellular and extracellular fluid. Those exchanges are
crucial for keeping cells functioning smoothly. They can’t occur properly unless the
volume and composition of the ECF are stable.
Yet the ECF is always changing, because gases, cell products, ions, and other
materials enter or leave it. To maintain stable conditions in the ECF, especially the
concentrations of water and vital ions such as sodium (Na1) and potassium (K1), there
must be mechanisms that remove substances as they enter the extracellular fluid or add
needed ones as they leave it. The urinary system performs this task. Before examining
how it operates, we’ll now take a general look at the traffic of substances into and out of
extracellular fluid.
Ordinarily, each day you take in about as much water as your body loses. Some of
the water is absorbed from foods and liquids you consume. The rest is produced during
metabolic reactions, including cellular respiration and condensation reactions. Thirst
influences how much water we take in. When there is a water deficit in body tissues, the
brain “urges” us to seek out water—for example, from a water fountain or a cold drink
from the refrigerator.
Water leaves the body in four ways: excretion in urine, evaporation from the
lungs and skin, sweating, and in feces. Of these four routes, urinary excretion is the form
of water loss over which the body has the most control. Urinary excretion eliminates
excess water, as well as excess or harmful solutes, in the form of urine. Some water also
evaporates from our skin and from the respiratory surfaces of the lungs. These are
sometimes called “insensible” water losses, because a person is not always aware they are
taking place.
Three main sources add solutes to the body’s extracellular fluid. Food supplies
nutrients (including glucose) and mineral ions (such as potassium and sodium ions) that
are absorbed from the GI tract. Many of us also consume many drugs and food additives.
The respiratory system brings oxygen into the blood. Last but not least, living cells
continually secrete substances, including carbon dioxide, into tissue fluid and circulating
blood.
Metabolic wastes, mineral ions, and other solutes leave extracellular fluid in
several ways. Metabolism produces more than 200 waste substances. Carbon dioxide is
the most abundant one, and we get rid of it by exhaling it from our lungs. All other major
wastes leave in urine.
Important metabolic wastes include by-products of processes that break down
nucleic acids and proteins. Dismantling nucleic acids produces one of these wastes, uric
acid. Another one, ammonia, forms in “deamination” reactions, which remove the
nitrogen-containing amino groups from amino acids. Ammonia is highly toxic if it
accumulates in the body. Reactions in the liver combine ammonia with carbon dioxide,
producing the much less toxic urea. Accordingly, urea is the main waste product when
cells break down proteins. About half of the urea filtered from blood in the kidneys is
reabsorbed. The rest is excreted. Protein breakdown also produces creatine, phosphoric
acid, sulfuric acid, and small amounts of other nitrogen-containing compounds, some of
which are toxic. These also are excreted.
Sweat carries away a small percentage of urea, but most nitrogen-containing
wastes are removed by the kidneys while they filter other wastes and excess water from
the blood. The kidneys also help maintain the balance of important ions such as sodium,
potassium, and calcium. These ions are sometimes called electrolytes because a solution
in which they are dissolved will carry an electric current.
Normally only a little of the water and solutes that enter the kidneys leaves as
urine. In fact, except when you drink lots of fluid (without exercise), all but about 1
percent of the water is returned to the blood. However, the chemical composition of the
fluid that is returned has been adjusted in vital ways. Just how this happens will be our
focus in the next few sections.
j. The Urinary System: Built for Filtering and Waste Disposal
Each kidney is a bean-shaped organ about the size of a rolled-up pair of socks. It
has several roughly triangular internal lobes. In each lobe, an outer cortex wraps around a
central region, the medulla, as you can see sketched. The whole kidney is wrapped in a
tough coat of connective tissue, the renal capsule (from the Latin renes, meaning
kidneys). A kidney’s central cavity is called the renal pelvis.
Our kidneys have several functions. They produce the hormone erythropoietin,
which stimulates the production of red blood cells. They also convert vitamin D to a form
that stimulates the small intestine to absorb calcium in food. In addition, kidneys make
the enzyme renin, which helps regulate blood pressure, as you will read later. The main
function of kidneys, however, is to remove metabolic wastes from the blood and adjust
fluid balance in the body.
In addition to the two kidneys, the urinary system includes “plumbing” that
transports or stores urine. Once urine has formed in a kidney, it flows into a tubelike
ureter, then on into the urinary bladder, where it is stored until you urinate. Urine leaves
the bladder through the urethra, a tube that opens at the body surface.
Each kidney lobe is a complex microcosm of physiological activity, housing a
vast network of blood vessels and over a million intricately structured nephrons, which
are the functional units responsible for kidney filtration and urine formation. Nephrons
represent an exquisitely engineered system designed to regulate fluid balance, electrolyte
concentrations, and waste excretion within the body, playing a critical role in maintaining
overall homeostasis and metabolic equilibrium.
Within the renal cortex and medulla, each kidney lobe contains numerous
nephrons arranged in parallel, interconnected arrays that collectively form the renal
parenchyma, the functional tissue of the kidney. Nephrons consist of a series of
specialized tubular segments, each with distinct morphological and physiological
characteristics tailored to specific functions in the process of urine formation.
The nephron begins with the renal corpuscle, comprising the glomerulus, a tuft of
fenestrated capillaries, and Bowman's capsule, a double-walled epithelial structure that
encloses the glomerular capillaries. The glomerulus serves as the primary site of
filtration, where blood plasma is sieved through fenestrations in the capillary endothelium
and the podocyte-lined filtration barrier, allowing water, electrolytes, and solutes to pass
into Bowman's space while retaining larger molecules such as proteins and blood cells in
the circulation.
From Bowman's capsule, the filtrate enters the renal tubule, a convoluted tubular
system consisting of proximal convoluted tubules (PCTs), loop of Henle, distal
convoluted tubules (DCTs), and collecting ducts. Each segment of the renal tubule is
characterized by specific transport mechanisms and selective permeability properties that
regulate the reabsorption and secretion of water and solutes, thereby modulating the
composition and volume of the urine.
In the proximal convoluted tubule, reabsorption of filtered substances, including
glucose, amino acids, sodium ions, and bicarbonate ions, occurs via active transport and
facilitated diffusion processes, allowing for the recovery of valuable nutrients and
electrolytes from the filtrate. The loop of Henle establishes a hypertonic medullary
interstitium through countercurrent multiplication, enabling the concentration of urine
and conservation of water in the collecting ducts.
In the distal convoluted tubule and collecting ducts, additional reabsorption and
secretion processes fine-tune the composition of the urine under the influence of
hormonal signals, such as aldosterone, antidiuretic hormone (ADH), and atrial natriuretic
peptide (ANP). These hormones regulate renal sodium reabsorption, water permeability,
and blood pressure control, ensuring proper fluid balance and electrolyte homeostasis in
response to physiological demands.
Ultimately, the coordinated activities of nephrons within the kidney lobes
culminate in the formation of urine, a concentrated solution of metabolic waste products,
electrolytes, and water that is excreted from the body via the ureters, bladder, and urethra.
The efficient functioning of nephrons is essential for maintaining renal function, fluid-
electrolyte balance, and acid-base equilibrium, safeguarding systemic health and
metabolic stability.
In summary, nephrons represent the fundamental structural and functional units of
the kidney, comprising a complex network of tubular segments and specialized vascular
components dedicated to the regulation of fluid and solute balance in the body. The
remarkable sophistication of nephron physiology underscores the kidneys' pivotal role in
maintaining internal homeostasis and preserving overall physiological equilibrium.
Understanding the intricate mechanisms of nephron function provides insights into renal
physiology and the pathogenesis of kidney diseases, guiding the development of
therapeutic interventions aimed at preserving renal function and mitigating renal
disorders.
Each hour, about 75 gallons of blood course through your kidneys, delivered by
the renal arteries. An afferent arteriole brings blood to each nephron (afferent means
“carrying toward”). The blood flows into the glomerulus inside Bowman’s capsule.
These capillaries are not like capillaries in other parts of the body. Specialized pores
between the cells of their walls make them much more permeable than other capillaries.
Thus it is much easier for water and solutes to move across the wall.
Glomerular capillaries don’t channel blood to venules, as other capillaries do.
Instead, they merge to form an efferent (“carrying away from”) arteriole. This arteriole
branches into peritubular (“around the tubule”) capillaries. The peritubular capillaries
weave around a nephron’s tubules. They merge into venules, which carry filtered blood
out of the kidneys.
k. How Urine Forms: Filtration, Reabsorption, and Secretion
Blood pressure is the driving force for filtration, the first step in forming urine.
Afferent arterioles are narrow, so they deliver blood to the glomerulus under high
pressure. This pressure forces about 20 percent of the blood plasma into Bowman’s
capsule. Blood cells, platelets, proteins, and other large solutes stay in the blood.
Everything else—water and small solutes such as glucose, amino acids, sodium, urea, and
vitamins—can filter out of the glomerular capillaries and into Bowman’s capsule. From
there the filtrate flows into the proximal tubule, where the next phase of urine formation
can begin.
The body cannot afford to lose the huge amounts of water and valuable solutes
such as glucose, amino acids, and electrolytes that are filtered from the blood by the
kidneys. Fortunately, most of the filtrate is recovered by tubular reabsorption. In this
process, substances leak or are pumped out of the nephron tubule and then enter
peritubular capillaries and so return to the bloodstream.
Most reabsorption takes place across the walls of proximal tubules. As in all parts
of the tubule, the walls in this area are only one cell thick. What happens with water,
glucose, and salt (ions of sodium, Na1, and chloride, Cl2). All these substances can
diffuse from the filtrate in a tubule into and through the cells of the tubule wall. On the
outer side of the cells, active transport (through proteins in the cells’ plasma membranes)
moves glucose and Na1 into the tissue fluid. Sodium ions (Na1) are positively charged,
and negatively charged ions, including chloride (Cl2), follow the sodium.
As the concentration of solutes rises in the fluid, water moves out of the tubule
cells by osmosis. In a final step, solutes are actively transported into peritubular
capillaries and water again follows by osmosis. These substances now have been
reabsorbed. The solutes and water that remain in the tubule become part of urine.
Reabsorption usually returns almost 99 percent of the filtrate’s water, all of the glucose
and most amino acids, all but about 0.5 percent of the salt (sodium and chloride ions),
and 50 percent of the urea to the blood.
Tubular secretion is a crucial physiological process within the renal tubules that
plays a pivotal role in the fine-tuning of urine composition and the elimination of
unwanted substances from the body. As urine forms within the nephron tubules, tubular
secretion selectively transports specific solutes from the peritubular capillaries into the
tubular lumen, where they are ultimately excreted in the urine. This highly regulated
process serves multiple functions, including the removal of metabolic waste products, the
maintenance of electrolyte balance, and the regulation of acid-base equilibrium.
One of the primary functions of tubular secretion is the elimination of urea, a
nitrogenous waste product resulting from the breakdown of proteins and amino acids.
Urea is actively transported from the peritubular capillaries into the tubular lumen, where
it contributes to the osmolality of urine and helps to maintain nitrogen balance in the
body. By facilitating the excretion of urea, tubular secretion helps prevent the
accumulation of toxic levels of nitrogenous waste in the bloodstream, promoting renal
health and metabolic homeostasis.
In addition to urea, tubular secretion also plays a crucial role in the elimination of
excess hydrogen ions (H+) and potassium ions (K+) from the body. Hydrogen ions,
generated through metabolic processes and acid-base regulation, are actively transported
from the peritubular capillaries into the tubular lumen, where they combine with various
substances, such as phosphate ions, ammonia, or bicarbonate, to form weak acids that are
excreted in the urine. This process helps regulate systemic pH balance and prevent the
development of metabolic acidosis.
Similarly, potassium ions, which are essential for maintaining cellular membrane
potential and neuromuscular function, are actively secreted from the peritubular
capillaries into the tubular lumen, where they contribute to the electrolyte composition of
urine. By regulating potassium excretion, tubular secretion helps maintain potassium
homeostasis and prevent hyperkalemia, a condition characterized by elevated serum
potassium levels that can lead to cardiac arrhythmias and other serious complications.
Moreover, tubular secretion also facilitates the excretion of other waste products,
such as creatinine, uric acid, and certain drugs and toxins, helping to detoxify the body
and maintain overall health. This process involves the selective transport of these
substances from the bloodstream into the tubular lumen, where they are eliminated in the
urine, thereby preventing their accumulation and potential toxicity.
Overall, tubular secretion is a highly controlled and dynamic process that
contributes to the elimination of unwanted substances from the body, including urea,
excess hydrogen ions, potassium ions, and other metabolic waste products. By selectively
transporting these solutes from the peritubular capillaries into the tubular lumen, tubular
secretion helps maintain renal function, electrolyte balance, and acid-base equilibrium,
ensuring optimal physiological function and metabolic stability. Understanding the
mechanisms of tubular secretion provides insights into the pathophysiology of renal
disorders and informs therapeutic interventions aimed at preserving renal health and
preventing renal dysfunction.
Secretion is crucial to maintaining the body’s acid–base balance, which you will
read. It also helps ensure that some wastes (such as uric acid and some breakdown
products of hemoglobin) and foreign substances (such as antibiotics and some pesticides)
do not build up in the blood. The drug testing noted in the introduction relies on the use
of urinalysis to detect drug residues that have been secreted into urine.
Homeostasis requires that the total volume of fluid in the blood and tissues stays
fairly stable. Blood and tissue fluid are mostly water, and while your kidneys are
removing impurities from your blood they are also adjusting the amount of water that is
excreted in urine or returned to the bloodstream.
You probably don’t need to be told that urination is urine flow from the body.
Urination is a reflex response. As the bladder fills, tension increases in the smooth muscle
of its strong walls. Where the bladder joins the urethra, an internal urethral sphincter built
of smooth muscle helps prevent urine from flowing into the urethra. As tension in the
bladder wall increases, though, the sphincter relaxes; at the same time, the bladder walls
contract and force urine through the urethra.
Skeletal muscle plays a crucial role in the voluntary control of urination through
the formation of the external urethral sphincter, a specialized muscular structure located
closer to the urethral opening. This sphincter functions as a mechanical barrier that
regulates the flow of urine from the bladder into the urethra, allowing for precise control
over the initiation and cessation of urination. Learning to voluntarily contract and relax
the external urethral sphincter is a fundamental aspect of urinary "toilet training" in
young children, enabling them to develop bladder control and achieve continence.
The external urethral sphincter is composed primarily of skeletal muscle fibers,
which are under conscious control and subject to voluntary modulation by the central
nervous system. During the process of urination, the external urethral sphincter remains
tonically contracted to maintain urinary continence, effectively sealing the urethral orifice
and preventing involuntary urine leakage. When it is time to void, voluntary relaxation of
the external urethral sphincter allows for the expulsion of urine from the bladder through
the urethra.
Urinary "toilet training" in young children involves the gradual acquisition of
voluntary control over the external urethral sphincter and coordination of bladder and
sphincter function to achieve effective voiding habits. Through repeated practice and
reinforcement, children learn to recognize the sensations associated with bladder filling
and develop the ability to consciously inhibit or facilitate urination as appropriate.
Caregivers play a crucial role in guiding and supporting children through the toilet
training process, providing encouragement, reinforcement, and positive reinforcement to
facilitate skill acquisition and promote successful bladder control.
The mastery of urinary control and continence relies on the maturation and
coordination of neural pathways involved in the regulation of bladder and sphincter
function. As children grow and develop, the central nervous system undergoes structural
and functional changes that enhance the ability to voluntarily modulate bladder and
sphincter activity, leading to improved urinary control and continence over time.
Successful toilet training represents a significant milestone in a child's development,
reflecting the acquisition of essential self-care skills and fostering independence and
autonomy in daily life.
In addition to its role in urinary continence, the external urethral sphincter also
serves as a protective mechanism against urinary incontinence and involuntary urine
leakage in response to increased intra-abdominal pressure, such as during coughing,
sneezing, or physical exertion. By voluntarily contracting the external urethral sphincter,
individuals can temporarily increase urethral resistance and prevent urine leakage,
providing an additional level of control over bladder function and urinary continence.
Overall, the external urethral sphincter plays a pivotal role in urinary control and
continence, allowing individuals to regulate the timing and frequency of urination and
maintain dryness and comfort throughout the day. Through urinary "toilet training" and
ongoing practice, individuals learn to harness the power of skeletal muscle control to
achieve optimal bladder function and urinary continence, supporting overall health and
well-being.
l. How Kidneys Help Manage Fluid Balance and Blood Pressure
Overall, the total volume of your body fluids, including blood plasma, doesn’t
very much. This is because during reabsorption, the kidneys adjust how much water and
salt (sodium 1 chloride ions) the body conserves or excretes in urine. As you know, blood
and tissue fluid are mostly water. In general, when the volume of blood increases or
decreases, so does blood pressure. The kidneys help ensure that the volume of
extracellular fluid, and blood in particular, stays within a normal range.
Although about two-thirds of filtered salt and water is reabsorbed in the proximal
tubule, the filtrate usually still contains more of both than the body can afford to lose in
urine. This situation is addressed as the filtrate enters the loop of Henle, which descends
into the kidney medulla. There the loop is surrounded by extremely salty tissue fluid.
Water can pass through the thin wall of the loop’s descending limb, so more water moves
out by osmosis and is reabsorbed. As the water leaves, the salt concentration in the fluid
still inside the descending limb increases until it matches that in the fluid outside.
Now the filtrate “rounds the turn” of the loop and enters the ascending limb. The
wall of this part of the nephron tubule doesn’t allow water to pass through. This is an
important variation in the tubule’s structure, because here sodium is actively transported
out of the ascending limb— but water can’t move with it. The filtrate now moves into the
distal tubule. Its cells continue to remove salt but don’t also let water escape. Hence, a
dilute urine moves on into the collecting duct.
Naturally, as salt leaves the filtrate moving through a nephron tubule, the
concentration of solutes rises outside the tubule and falls inside it. This steep gradient
helps drive the reabsorption of valuable solutes, which move into peritubular capillaries.
It also draws water out of the descending limb by osmosis.
Urea boosts the gradient. As water is reabsorbed, urea left in the filtrate becomes
concentrated. Some of it will be excreted in urine, but when filtrate enters the final
portion of the collecting duct, some urea also will diffuse out— so the concentration of
solutes in the inner medulla rises even more.
Drink a large glass of water and the next time you “go,” your urine may be pale
and dilute. If you sleep 8 hours without a break, your urine will be concentrated and
darker yellow. As described next, hormones control how much water the kidneys add to
urine. These controls also adjust blood pressure.
When you don’t take in as much water as your body loses, the salt concentration
in your blood rises. In the brain, receptors sense this change and trigger the release of
antidiuretic hormone, or ADH. It acts on cells in distal tubules and collecting ducts so
that more water moves out of them and is reabsorbed into the blood. As a result, the urine
becomes more concentrated. Gradually the additional water in blood reduces the salt
concentration there. It also increases the blood volume and blood pressure. Then a
negative feedback loop inhibits the release of ADH.
Reduced blood volume also affects cells in the afferent arterioles that bring blood
to nephrons. These cells release the enzyme renin. They are part of the juxtaglomerular
apparatus. Juxta- means “next to,” and this “apparatus” is an area where arterioles of the
glomerulus come into contact with a nephron’s distal tubule.
Renin triggers reactions that produce a protein called angiotensin I and then
convert it to angiotensin II. Among other effects, angiotensin II stimulates cells of the
adrenal cortex, the outer portion of a gland perched on top of each kidney, to secrete the
hormone aldosterone. Aldosterone causes cells of the distal tubules and collecting ducts
to reabsorb sodium faster, so less of it and less water are excreted. By limiting the loss of
water, this process also influences blood pressure.
What must the kidneys do to make dilute urine? Not much. Urine is automatically
dilute as long as ADH levels are low, so little of the hormone acts on the distal tubules
and collecting ducts. A diuretic is a substance that promotes the loss of water in urine.
For example, caffeine reduces the reabsorption of sodium along nephron tubules, so more
water is excreted.
What makes you thirsty when you don’t drink enough? The concentration of salt
in your blood has risen, and this change reduces the amount of saliva your salivary glands
produce. A drier mouth stimulates nerve endings that signal a thirst center in the brain.
The center also receives signals from the same sensors that stimulate the release of ADH.
In this case the signals are relayed to a part of the brain that “tells” you to find and drink
fluid.
m. Removing Excess Acids and Other Substances in Urine
You may recall from that normal pH in the blood and other body fluids is between
7.3 and 7.5. Because acids lower pH and bases raise it, pH reflects the body’s acid– base
balance—the relative amounts of acidic and basic substances in extracellular fluid.
Remember also that a buffer system involves substances that reversibly bind and release
H1 and OH2 ions. Buffers minimize pH changes as acidic or basic molecules enter or
leave body fluids.
Described how bicarbonate (HCO3 2) serves as a buffer in the lungs. It forms
when carbon dioxide combines with water. The bicarbonate then reacts with H1 to form
carbonic acid, and enzyme action converts carbonic acid into water and carbon dioxide.
The CO2 is exhaled, while the hydrogen ions are now a part of water molecules. H1 is
not eliminated permanently, however. Only the kidneys can do that. They also restore the
buffer bicarbonate.
Depending on changes in the acid–base balance of the blood that enters nephrons,
the kidneys can either excrete bicarbonate or form new bicarbonate and add it to the
blood. The necessary chemical reactions go on in the cells of nephron tubule walls. For
example, when the blood is too acidic (a too high concentration of H1), water and carbon
dioxide combine with the help of an enzyme. They form carbonic acid that then can be
broken into bicarbonate and H1.
As you can see, bicarbonate produced in the reactions moves into peritubular
capillaries. It ends up circulating in the blood, where it buffers excess H1. When the
blood is too basic (alkaline), chemical adjustments in the kidneys normally ensure that
less bicarbonate is reabsorbed into the bloodstream.
The process of acid-base regulation within the renal tubules involves the secretion
of excess hydrogen ions (H+) into the filtrate, facilitating the elimination of acidic waste
products from the body and maintaining systemic pH balance. Within the tubule cells of
the nephrons, hydrogen ions are actively transported across the epithelial membrane and
into the tubular lumen, where they are subsequently excreted into the urine.
Once in the tubular filtrate, excess hydrogen ions can interact with various
substances to form conjugate bases, facilitating their excretion from the body. One
common mechanism involves the combination of hydrogen ions with phosphate ions
(HPO42-) to form dihydrogen phosphate (H2PO4-), a weak acid that can be readily
excreted in the urine. Additionally, excess hydrogen ions may react with ammonia (NH3)
to form ammonium ions (NH4+), which are then excreted in the urine, contributing to
urinary acidification and acid-base balance.
Furthermore, hydrogen ions can also combine with bicarbonate ions (HCO3-)
within the tubular lumen to form carbonic acid (H2CO3), which can then be converted
into water and carbon dioxide by the enzyme carbonic anhydrase. This reaction generates
bicarbonate ions that can be reabsorbed into the bloodstream to help buffer systemic
acidosis and maintain plasma pH within normal physiological ranges.
The excretion of excess hydrogen ions in the urine is essential for preventing
metabolic acidosis and maintaining acid-base homeostasis in the body. By eliminating
acidic waste products generated through cellular metabolism, renal tubular secretion
plays a crucial role in regulating systemic pH balance and preventing the accumulation of
toxic levels of hydrogen ions in the bloodstream.
In addition to hydrogen ion secretion, the renal tubules also contribute to acid-
base regulation through processes such as bicarbonate reabsorption and ammonia
synthesis. Bicarbonate ions reabsorbed from the tubular lumen are returned to the
bloodstream via the renal veins, helping to replenish bicarbonate reserves and buffer
metabolic acids in the body. Meanwhile, ammonia synthesized within the renal tubules
acts as a potent buffer, trapping hydrogen ions and facilitating their excretion as
ammonium ions in the urine.
Overall, the renal tubules play a central role in the excretion of excess hydrogen
ions and the maintenance of acid-base balance in the body. Through the secretion of
hydrogen ions into the tubular filtrate and their subsequent interaction with various
substances, the kidneys help regulate systemic pH and prevent the development of
metabolic acidosis, ensuring optimal physiological function and metabolic stability.
Understanding the mechanisms of renal tubular acid-base regulation provides insights
into the pathophysiology of acid-base disorders and informs therapeutic interventions
aimed at restoring acid-base homeostasis in clinical settings.
Maintaining proper blood pH is crucial to homeostasis. If the pH of blood falls
outside the normal range for long, the most serious impact occurs in the central nervous
system (brain and spinal cord). When severe diarrhea, kidney disease, or some other
problem prevents kidneys from excreting enough acid, the result is metabolic acidosis.
Then nerve cells cannot communicate properly and an affected person may fall into a
fatal coma.
Severe vomiting or dehydration, hormonal disorders, and overuse of antacids are
common causes of metabolic alkalosis, or blood that is too basic. Then nerve cells are
overstimulated, so a person may suffer muscle spasms, nervousness, or convulsions. In
the next two sections you will find information about other major disorders that prevent
the urinary system from functioning normally.
