Midterm paper

PCB3702L Intermediate Human Physiology Digestive System Dr. Lisa Brinn

Gastrointestinal Lab Supplemental Resource I. Overview of the Digestive System

A. Consists of the Gastrointestinal Tract and the Accessory Digestive Organs

• Gastrointestinal tract

a. Alimentary canal

b. Everything from

mouth to anus

• Accessory digestive

organs

a. Teeth

b. Tongue

c. Salivary glands

d. Pancreas

e. Liver

f. Gallbladder

B. There are six basic digestive processes of digestion

• Ingestion – involves taking food and liquids into the mouth (eating)

• Secretion – each day, cells within the walls of the GI tract and accessory digestive

organs secrete a total of about 7 liters of water, acid, buffers, and enzymes into the

lumen (interior space) of the tract. This occurs in the mouth, esophagus, stomach,

small intestine and large intestine.

• Motility – contractions of smooth muscle in the wall of the GI tract mix food and

secretions, and propel them toward the anus. This capability to mix and move

material along its length is called motility. This occurs in esophagus, stomach, small

intestine, and the large intestine.

• Digestion - Mechanical and chemical processes break down ingested food into small

molecules. In mechanical digestion, the teeth cut and grind food before it is

swallowed, and then smooth muscles of the stomach and small intestine churn the

food. As a result, food molecules become dissolved and thoroughly mixed with

digestive enzymes. In chemical digestion, the large carbohydrate, lipid, protein, and

nucleic acid molecules in food are split into smaller molecules by hydrolysis.

Digestive enzymes produced by the salivary glands, tongue, stomach, pancreas, and

small intestine catalyze these catabolic reactions. This occurs in the mouth, stomach,

small intestine, and large intestine.

• Absorption - The movement of the products of digestion from the lumen of the GI

tract into blood or lymph is called absorption. Once absorbed, these substances

circulate to cells throughout the body. A few substances in food can be absorbed

without undergoing digestion. These include vitamins, ions, cholesterol, and water.

This occurs in the mouth, stomach, small intestine, and large intestine.

• Defecation - Wastes, indigestible substances, bacteria, cells sloughed from the lining

of the GI tract, and digested materials that were not absorbed in their journey

through the digestive tract leave the body through the anus in a process called

defecation. The eliminated material is termed feces. This occurs in the large

intestine.

C. Wall of the GI tract is comprised of four functional layers

• Mucosa - The mucosa, or inner lining of the GI tract, is a mucous membrane. It is

composed of (a) a layer of epithelium, (b) a lamina propria, and (c) a muscularis

mucosae.

a. The epithelium of the mucosa is in direct contact with the contents of the

GI tract. Several types of epithelial cells comprise the mucosal epithelium,

and the types vary from one part of the GI tract to another. Some epithelial

cells are exocrine cells that secrete fluids and other substances into the

lumen of the GI tract. Other epithelial cells are endocrine cells, collectively

known as enteroendocrine cells, that secrete hormones into the

bloodstream. Still other epithelial cells are absorptive cells that transport

nutrients from the lumen of the GI tract into the blood or lymph. Tight

junctions firmly seal neighboring epithelial cells to one another to restrict

leakage between the cells. The rate of renewal of GI tract epithelial cells is

rapid: Every 5 to 7 days they slough off and are replaced by new cells.

b. The lamina propria is a layer of connective tissue that surrounds the

epithelium of the mucosa. It contains small blood and lymphatic vessels,

which are the sites where absorbed nutrients enter blood or lymph. Also

located in the lamina propria is gut-associated lymphoid tissue (GALT).

These prominent lymphoid nodules contain immune system cells that

protect against disease. GALT is present all along the GI tract, especially in

the small intestine, appendix, and large intestine.

c. The muscularis mucosae is a thin layer of smooth muscle fibers that

surrounds the lamina propria. Contraction of the muscularis mucosae

throws the mucous membrane of the stomach and small intestine into

many small folds, which increase the surface area for digestion and

absorption.

• Submucosa - The submucosa is a thick layer of connective tissue that provides the GI

tract with distensibility and elasticity, allowing it to stretch as food passes through it

and then to return to its original shape when food is no longer present. The

submucosa contains relatively large blood and lymphatic vessels that receive

absorbed food molecules from smaller vessels in the lamina propria. Also present in

the submucosa are exocrine glands and an extensive network of neurons known as

the submucosal plexus.

• Muscularis Externa - The muscularis externa, or simply the muscularis, is the main

muscle layer of the GI tract. The muscularis externa of the mouth, pharynx, and upper

to middle parts of the esophagus contains skeletal muscle that produces voluntary

swallowing. Skeletal muscle also forms the external anal sphincter, which permits

voluntary control of defecation. Throughout the rest of the GI tract, the muscularis

externa consists of smooth muscle that is arranged in two layers: an inner layer of

circular muscle and an outer layer of longitudinal muscle. Involuntary contractions of

the circular and longitudinal smooth muscles propel food along the GI tract and mix

food with digestive secretions. Between the two smooth muscle layers is a second

plexus of neurons—the myenteric plexus.

• Serosa - The serosa, the outermost layer of the GI tract, consists of connective tissue

and epithelium. It forms part of the peritoneum, a membrane that lines the abdominal

cavity and covers the organs within that cavity. Folds of the peritoneum known as

mesenteries bind the digestive organs to one another and to the abdominal wall. The

mesenteries hold the digestive organs in place and supply the organs with blood

vessels and nerves.

D. Nervous innervation - The gastrointestinal tract is regulated by an intrinsic set of nerves

known as the enteric nervous system and by an extrinsic set of nerves that are part of the

autonomic nervous system.

• Enteric nervous system

a. Myenteric plexus

o Controls smooth muscle contraction

b. Submucosal plexus

o Controls endocrine and exocrine GI secretion

• Autonomic nervous system - Although the neurons of the ENS can function

independently, they are subject to regulation by the parasympathetic and

sympathetic divisions of the autonomic nervous system. The vagus (X) nerves supply

parasympathetic fibers to most parts of the GI tract, with the exception of the last half

of the large intestine, which is supplied with parasympathetic fibers in pelvic nerves

from the sacral region of the spinal cord. The parasympathetic nerves that supply the

GI tract form neural connections with the ENS. Parasympathetic preganglionic

neurons of the vagus (X) nerves or pelvic nerves synapse with parasympathetic

postganglionic neurons located in the myenteric and submucosal plexuses. Some of

the parasympathetic postganglionic neurons in turn synapse with neurons in the

ENS; others directly innervate smooth muscle and glands within the wall of the GI

tract. In general, stimulation of the parasympathetic nerves that innervate the GI tract

causes an increase in GI secretion and motility by increasing the activity of ENS

neurons.

• Sympathetic nerves that supply the GI tract arise from the thoracic and upper lumbar

regions of the spinal cord. Like the parasympathetic nerves, these sympathetic nerves

form neural connections with the ENS. Sympathetic postganglionic neurons synapse

with neurons located in the myenteric plexus and the submucosal plexus. In general,

the sympathetic nerves that supply the GI tract cause a decrease in GI secretion and

motility by inhibiting the neurons of the ENS. Emotions such as anger, fear, and

anxiety may slow digestion because they stimulate the sympathetic nerves that

supply the GI tract.

• Gastrointestinal reflex pathways - Many neurons of the ENS are components of

gastrointestinal (GI) reflex pathways that regulate GI secretion and motility in

response to stimuli present in the lumen of the GI tract. The initial components of a

typical GI reflex pathway are sensory receptors (such as chemoreceptors and

mechanoreceptors) that are associated with the sensory neurons of the ENS. The

axons of these sensory neurons can synapse with other neurons located in the ENS,

CNS, or ANS, informing these regions about the nature of the contents and the degree

of distension (stretching) of the GI tract. The neurons of the ENS, CNS, or ANS

subsequently activate or inhibit GI glands and smooth muscle, altering GI secretion

and motility. If the reflex pathway is confined entirely within the GI tract wall, then it

is called a short reflex. If the reflex pathway involves not only the GI tract wall but

also the CNS and autonomic nerves, then it is referred to as a long reflex.

E. GI smooth muscle displays autorhythmicity and promotes two major patterns of motility

• Autorhythmicity

a. Due to interstitial cells of Cajal (ICCs) – located in the muscularis externa

b. Cause depolarizations that don’t necessarily make it to threshold

c. Slow wave potentials or GI tract’s basic electrical rhythm (BER)

d. Patterns of motility

o Peristalsis - successive muscular contractions along the wall of a hollow

muscular tube that propel the luminal contents in a forward direction.

It occurs throughout the GI tract, from the esophagus to the anus, and

in other parts of the body, including the ureters, bile ducts, and uterine

(fallopian) tubes. In the GI tract, peristalsis involves successive

contractions of the circular and longitudinal layers of the muscularis

externa and occurs mainly in response to distension of the wall by

luminal contents. In the segment of the GI tract wall just behind a mass

of food, the circular layer contracts while the longitudinal layer relaxes;

this shortens the wall, causing the food to move forward. Meanwhile, in

the segment of the GI tract wall just in front of the food mass, the

circular muscle layer relaxes while the longitudinal layer contracts; this

causes the wall to push outward so that it can receive the food. As the

circular and longitudinal layers undergo repeated cycles of contraction

and relaxation, the food is moved along the GI tract toward the anus.

o Segmentation - refers to alternating muscular contractions that mix

luminal contents. It occurs in the small intestine in response to

distension and involves contractions of the circular muscle fibers of the

muscularis externa. Segmentation begins with the contraction of the

circular muscle fibers at various intervals along the small intestine, an

action that constricts the intestine into segments. When the circular

muscle fibers relax, the segments disappear. Then circular muscle

fibers at other points along the intestinal wall contract, causing new

segments to form. As this sequence of events repeats, the intestinal

contents slosh back and forth. Segmentation mixes food with digestive

juices and brings the molecules of food into contact with the mucosa for

absorption.

II. Mouth

A. Oral cavity

• Formed by

a. Cheeks

b. Lips

c. Soft palate

d. Tongue

B. Other structures

• Teeth

• Salivary glands

C. Cheeks and lips keep food in the mouth during chewing

D. Palate prevents food from entering nasal cavity

E. Tongue moves food toward the pharynx and produces lingual lipase

F. Salivary glands secrete saliva

o Saliva:

i. Dissolves and lubricates food for taste, initial digestion, and

swallowing

ii. Secretion of salivary amylase (carbohydrate digestion)

iii. Buffers ingested acid food

• Three large glands that produce saliva: parotid, submandibular, sublingual

G. Teeth allow for initiation of mechanical digestion

• Grind and tear apart food

H. Chemical digestion initiates in mouth

III. Pharynx and Esophagus

A. Pharynx conveys food from the mouth to the esophagus

B. Esophagus transports food into the stomach

• Requires relaxation of two sphincters

a. Upper esophageal sphincter

b. Lower esophageal sphincter

C. Deglutition

• Swallowing of food (bolus)

IV. Stomach

A. J – shaped enlargement of the GI tract

B. Connects esophagus to the small intestine

C. Serves as a mixing chamber and a temporary holding reservoir

D. Can accommodate up to 2 liters of food

E. Main regions : fundus, body and antrum

F. Stomach wall

• Four basic layers- each with specific functions

a. Mucosa

o The mucosa is thick to prevent the organ from autodigestion.

o Consists of folds called rugae: these folds increase surface area. Rugae

are visible when the stomach is empty but disappear when the stomach

is distended (full).

o Contains different cell types

i. Parietal cells (also known as oxyntic cells) = secrete HCl

ii. Chief cells (also known as zymogen cells) = secrete

pepsinogen and gastric lipase

• Pepsinogen (a zymogen = inactive form of enzyme) is

then converted into it’s active form, pepsin, in the

presence of HCl

iii. Surface Mucus cells and Mucus Neck Cells = secrete mucus

which provides a layer of protection for the stomach

o Arranged in pits and glands

b. Muscularis externa contains 3 layers

o Extra layer = oblique (allows for churning)

G. Mechanical digestion in the stomach involves propulsion and retropulsion

• Propulsion

a. Forward movement

• Retropulsion

a. Splash back of gastric contents when they hit against pyloric sphincter

H. Chemical digestion in the stomach occurs as food mixes with

gastric juice

• Gastric juice

a. Includes many different gastric secretions

o Mostly HCl

i. Produced by parietal cells

b. Also includes enzymes

o Mostly to digests proteins

I. Reflexes

• Vomiting

a. Rapidly expels contents of the GI tract

J. Parietal cell secretion - Food may remain in the fundus for about an hour without becoming

mixed with gastric juice. During this time, digestion by salivary amylase continues. Soon,

however, the churning action mixes food with acidic gastric juice, inactivating salivary

amylase and activating lingual lipase, which starts to digest triglycerides into fatty acids and

diglycerides.

K. Although parietal cells secrete hydrogen ions (H+) and chloride ions (Cl−) separately into the

stomach lumen, the net effect is secretion of hydrochloric acid (HCl). Proton pumps powered

by H+/K+ ATPases actively transport H+ into the lumen while bringing potassium ions (K+)

into the cell. At the same time, Cl− and K+ diffuse out into the lumen through Cl− and K+

channels in the apical membrane. The enzyme carbonic anhydrase, which is especially

plentiful in parietal cells, catalyzes the formation of carbonic acid (H2CO3) from water (H2O)

and carbon dioxide (CO2). As carbonic acid dissociates, it provides a ready source of H+ for

the proton pumps but also generates bicarbonate ions (HCO3−). As HCO3− builds up in the

cytosol, it exits the parietal cell in exchange for Cl− via Cl−/HCO3− antiporters in the

basolateral membrane (next to the lamina propria). HCO3− diffuses into nearby blood

capillaries. This “alkaline tide” of bicarbonate ions entering the bloodstream after a meal

may be large enough to elevate blood pH slightly.

HCl secretion by parietal cells can be stimulated by several sources: acetylcholine (ACh) released by

parasympathetic neurons; gastrin secreted by G cells; and histamine, which is a paracrine substance

released by mast cells in the nearby lamina propria. Acetylcholine and gastrin stimulate parietal cells to

secrete more HCl in the presence of histamine. In other words, histamine acts synergistically,

enhancing the effects of acetylcholine and gastrin. Receptors for all

three substances are present in the plasma membrane of parietal

cells. The histamine receptors on parietal cells are called H2

receptors; they mediate different responses than do the H1

receptors involved in allergic responses.

V. Small Intestine

A. Made up of three different regions: duodenum, jejunum and ileum

B. Functions

• Digestion (mainly in the duodenum)

• Absorption (mainly in the jejunum and ileum)

C. Secretes intestinal juice

• Contains: Enzymes, Mucus, Water, Ions

D. Contains brush border enzymes

• Attached to the villi of cells

E. Mechanical digestion

• Involves segmentation

F. Chemical digestion

• Occurs as chyme mixes with pancreatic juice, intestinal juice and bile

a. Along with digestive enzymes, pancreatic juice also contains Bicarbonate-rich

fluid that helps protect the duodenum against the corrosive action of acidic

chyme coming from the stomach.

G. Absorptive brush border - The absorptive cells of the small intestine synthesize several

digestive enzymes, called brush-border enzymes, and insert them in the plasma membrane

of the microvilli. You have already learned about one brush-border enzyme, namely

enterokinase, which converts trypsinogen (a zymogen = inactive form of enzyme) into

trypsin. Among the other brush-border enzymes are four carbohydrate-digesting enzymes

called α-dextrinase, sucrase, lactase, and maltase; a protein-digesting enzyme called

aminopeptidase; and two types of nucleotide-digesting enzymes, nucleosidase and

phosphatase. Thus, some enzymatic digestion occurs at the surface of the absorptive cells

that line the villi, rather than in the lumen exclusively, as occurs in other parts of the GI tract.

Also, as absorptive cells slough off into the lumen of the small intestine, they break apart and

release enzymes that help digest nutrients in chyme.

Even though the action of salivary amylase may continue in the stomach for a while, the

acidic pH of the stomach destroys salivary amylase and ends its activity. Thus, only a few

starches are broken down by the time chyme leaves the stomach. Those starches not already

broken down into maltose, maltotriose, and α-dextrins are cleaved by pancreatic amylase, an

enzyme in pancreatic juice that acts in the small intestine. Although pancreatic amylase acts

on both glycogen and starches, it has no effect on another polysaccharide called cellulose, an

indigestible plant fiber that is commonly referred to as “roughage” as it moves through the

digestive system. After amylase (either salivary or pancreatic) has split starch into smaller

fragments, a brush-border enzyme called α-dextrinase acts on the resulting α-dextrins,

clipping off one glucose unit at a time.

Ingested molecules of sucrose, lactose, and maltose—three disaccharides—are not acted on

until they reach the small intestine. Three brush-border enzymes digest the disaccharides

into monosaccharides. Sucrase breaks sucrose into a molecule of glucose and a molecule of

fructose; lactase digests lactose into a molecule of glucose and a molecule of galactose; and

maltase splits maltose and maltotriose into two or three molecules of glucose, respectively.

Digestion of carbohydrates ends with the production of monosaccharides, which the

digestive system is able to absorb.

VI. Large Intestine

A. Terminal part of the GI tract - The cecum is the initial portion of the large intestine. At the

junction of the ileum and cecum is the ileocecal sphincter (valve), which allows materials

from the small intestine to pass into the large intestine. The colon is the longest portion of

the large intestine. It is further subdivided into an ascending colon, transverse colon,

descending colon, and sigmoid colon. The rectum is the terminal portion of the large

intestine. The opening of the rectum to the exterior

is known as the anus. The anus is guarded by an

internal anal sphincter of smooth muscle

(involuntary) and an external anal sphincter of

skeletal muscle (voluntary). Normally these

sphincters keep the anus closed except during the

elimination of feces.

B. Involved in water and ion absorption

C. Where feces are formed

• Feces is stored within the sigmoid colon

D. Mechanical digestion

• Involves haustral churning - the haustra remain relaxed and become distended while

they fill up. When the distension reaches a certain point, the walls contract and

squeeze the contents into the next haustrum.

• Peristalsis - occurs in the large intestine but at a slower rate than in more proximal

portions of the tract.

• Mass movement - a strong wave of contraction that begins at about the middle of the

transverse colon and quickly drives the contents of the colon into the rectum. Mass

movement is similar to peristalsis, except that the contraction lasts for a longer

period of time. Mass movements usually take place three or four times a day, during

or immediately after a meal. Hence, the presence of food in the stomach triggers mass

movement in the large intestine, an event known as the gastrocolic reflex.

E. Chemical digestion

• Occurs via bacteria

F. Large intestine concentrates feces

• Defecation reflex expels feces from the body

• Assisted by the valsalva maneuver

a. Involves voluntary contractions of the abdominal muscles

G. Disorders associated with feces

• Diarrhea

• Constipation

H. Large intestinal wall - The wall of the large intestine contains the typical four layers found in

the rest of the GI tract, with certain modifications:

• The surface of the large intestinal mucosa consists of two types of epithelial cells:

absorptive cells and goblet cells. The absorptive cells absorb water and ions; the

goblet cells secrete mucus that lubricates the contents of the colon.

• As in the small intestine, the epithelium of the large intestinal mucosa extends

downward from the surface to form intestinal glands called crypts of Lieberkühn.

Recall that the crypts of Lieberkühn of the small intestine contain several types of

cells. In the large intestine, however, the crypts of Lieberkühn contain only absorptive

cells and goblet cells.

• Compared to the small intestine, the wall of the large intestine does not have as many

structural features that increase surface area. There are no circular folds or villi;

however, microvilli are present on the absorptive cells. Consequently, much more

absorption occurs in the small intestine than in the large intestine.

• Unlike other parts of the GI tract, the outer longitudinal layer of smooth muscle of the

muscularis externa is bundled into three bands called the teniae coli that run most of

the length of the large intestine. Contractions of these bands gather the colon into a

series of pouches called haustra (singular is haustrum), which give the colon a

puckered appearance.

VII. Phases of Digestion

A. Digestive activities occur in overlapping phases

• Cephalic phase

a. Prepares the mouth and stomach for food that’s about to be eaten

• Gastric phase

a. Promotes gastric juice secretion and gastric motility

• Intestinal phase

a. Promotes digestion in small intestine and slows the digestion in the stomach

B. Under significant hormonal control

a. CCK

o Produced in the duodenum

o Stimulates secretion of pancreatic juice

o Causes ejection of bile (bile is a non-enzymatic substance that aids in

mechanical digestion of fats. It does so by dispersing fat globules,

increasing the surface area available for lipases to chemically digest

these fats) from the gallbladder in the presence of fat in the duodenum

o Induces satiety - slows gastric emptying by promoting contraction of

the pyloric sphincter

b. Secretin

o Produced in duodenum when acidic chyme comes from the stomach

o Stimulates secretion of pancreatic juice due to high acidic coming from

the stomach

o Inhibits secretion of gastric juice

o Promotes normal growth and maintenance of the pancreas, and

o Enhances the effects of CCK, by acting on the liver

c. Gastrin

o Secreted in the stomach in response to:

i. distension of the stomach by chyme,

ii. partially digested proteins in chyme,

iii. the high pH of chyme due to the presence of food in the

stomach,

iv. caffeine in gastric chyme, and

v. acetylcholine released from parasympathetic neurons.

o Promotes secretion of gastric juice

o Increases gastric motility

o Strengthens the contraction of the lower esophageal sphincter to

prevent reflux of acid chyme into the esophagus

o Increases motility of the stomach; and relaxes the pyloric sphincter,

which promotes gastric emptying

o Promotes the gastroileal and gastrocolic reflexes.

o Gastrin secretion is inhibited when the pH of gastric juice drops below

2.0 and is stimulated when the pH rises. This negative feedback

mechanism helps provide an optimal low pH for the functioning of

pepsin, the killing of microbes, and the denaturing of proteins in the

stomach.

VIII. Transport of Lipids by Lipoproteins

A. Most lipids are very hydrophobic

• Makes it difficult to transport in blood

• Use lipoproteins

• Different classes of lipoproteins

a. Chylomicron

b. Very low-density lipoproteins

c. Low-density lipoproteins

d. High-density lipoproteins