CM DS

The principal structure of the digestive system is an irregular tube, open at both ends, called the alimentary canal. Accessory organs of digestion comprise the rest of the system. The term gastrointestinal (or GI) tract technically refers only to the portion that includes the stomach and intestines but is often used to designate the entire digestive tract.

The digestive system uses many mechanisms:

Ingestion is the taking in of complex foods into the GI tract.

Digestion is a group of processes that break complex nutrients into simpler ones. These include mechanical digestion, the breakup of large chunks of food into smaller bits; and chemical digestion, which breaks large molecules into smaller ones.

Motility refers to gastrointestinal movements resulting from muscular contraction.

Secretion is the release of digestive juices and hormones that facilitate digestion.

Absorption is movement of digested nutrients into the internal environment of the body.

Elimination is the movement of residues of digestion out of the alimentary canal.

Regulation includes neural, hormonal, and other mechanisms that regulate digestive activity.

Now we’ll review the anatomy of the digestive system, beginning with the wall of the digestive tract.

The digestive tract is described as a tube that extends from the mouth to the anus; the inner hollow space is called the lumen.

The wall is formed by four layers: mucosa, submucosa, muscularis, and serosa.

The mucosa is the innermost layer. The type of mucosa varies depending on the GI location (tough and stratified or delicate and simple epithelium). Mucosa produces mucus.

The submucosa, a connective tissue layer, is the next layer.

The muscularis are circular and longitudinal layers of muscle important in motility. There is also an oblique layer in the stomach.

The muscularis produces peristalsis and segmentation motions.

o Peristalsis is the “wavelike” movement that pushes food down the tract.

o Segmentation is the “back-and-forth” mixing movement.

The outermost layer is the serosa, a serous membrane that covers the outside of abdominal organs; it attaches the digestive tract to the wall of the abdominopelvic cavity by forming folds called mesenteries.

To follow the sequence that food takes through the alimentary canal, we will next review the structures and functions of the mouth.

The oral cavity has a roof and a floor.

The roof is formed by the hard palate (parts of the maxillary and palatine bones) and the soft palate, an arch-shaped muscle separating the mouth from the pharynx; as well as the uvula, a downward projection of the soft palate that helps in speech and swallowing. The medical term for swallowing is deglutition.

The floor of the oral cavity is formed by the tongue and its muscles, and the lingual frenulum, the fold of mucosa that helps anchor the tongue.

Even though teeth are made of bone, they are considered part of the digestive system.

Teeth have three main parts: a crown, neck, and root.

Enamel, which covers the crown, is the hardest tissue in the body.

Be sure you know the names of the types of teeth—incisors, canines (or cuspids), premolars (or bicuspids), and molars (or tricuspids).

There are twenty teeth in the deciduous or baby set. The average age for cutting a first tooth is about 6 months; the set is complete at about 30 months of age.

There are thirty-two teeth in the permanent set; 6 years is about the average age for starting to cut the first permanent tooth; the set is complete usually between ages of 17 and 24 years.

Salivary glands are exocrine glands that secrete saliva into ducts.

They are typical of the accessory glands associated with the digestive system. They are located outside of the digestive tube itself and must convey their exocrine secretions by way of ducts into the tract.

There are three sets of salivary glands: the parotid, submandibular, and sublingual.

The parotid glands are the largest salivary glands. The parotid gland secretes sodium bicarbonate which helps neutralize bacterial acids.

The submandibular glands open into the mouth on either side of the lingual frenulum.

The sublingual glands open into the floor of the mouth.

• In addition to the three major salivary glands, there are hundreds of minor salivary glands located within the oral cavity.

Saliva contains mucus and a digestive enzyme called salivary amylase, which begins the process of carbohydrate digestion. Water and mucus moisten and lubricate the chewed food, allowing it to pass with less friction through the esophagus and on into the stomach.

The pharynx has three divisions: the nasopharynx, oropharynx, and laryngopharynx.

Tonsils form a lymphoid ring in the pharynx that prevents digestive tract infection.

The pharynx functions in deglutition.

The oropharynx is the most involved pharyngeal segment in deglutition.

Regulation of deglutition movements occurs via the motor cortex of the cerebrum (voluntary) and the “deglutition center” of the brainstem, which is involuntary.

The esophagus connects the pharynx to the stomach.

It is a dynamic passageway for food.

Sphincters at each end of the esophagus (the upper esophageal and lower esophageal sphincters) help keep ingested material moving in one direction down the tube:

The lower esophageal sphincter, also called the cardiac sphincter, normally prevents backflow of acidic stomach contents.

The stomach is divided into the fundus (an outpouched base), the body (its main part), and the pylorus (or apex, which is the lower part of the stomach).

The stomach expands after a large meal; it is about the size of a large sausage when empty.

There are many smooth muscle fibers in the three muscularis layers of the stomach; contractions produce churning movements.

The stomach’s mucosa contains many microscopic gastric glands that secrete gastric juice containing enzymes, hydrochloric acid, and intrinsic factor into the stomach.

Its mucous membrane lies in folds (called rugae) when the stomach is empty.

The pyloric sphincter muscle closes the opening between the pylorus and the beginning of the small intestine, the duodenum.

Food enters the stomach through the lower esophageal sphincter and the digestive process continues. Partial digestion of proteins occurs after chyme is held in the stomach for some time by the muscle.

A hiatal hernia occurs when the stomach is pushed through an opening in the diaphragm, which may cause gastroesophageal reflux disease.

The small intestine is about 7 m (20 feet) long but only 2 cm or so in diameter. Many coiled loops accommodate a long tube within the short abdominal cavity. Divisions of the small intestine are the duodenum, jejunum, and ileum.

The duodenum is a site of much chemical digestion. Ducts from the pancreas and liver enter the tract here, so the duodenum receives pancreatic and liver secretions. The major and minor duodenal papillae are the two bumps where the secretions enter. Most chemical digestion occurs in the duodenum.

The main functions of the small intestine are digestion and absorption; it performs most of these functions for the digestive system.

The small intestine contains many microscopic glands that secrete intestinal juice containing water, enzymes, and ions.

The small intestine has a huge absorptive surface area, which includes circular folds called plicae. These folds are themselves covered with thousands of tiny microscopic finger-shaped projections called villi, further increasing the surface area.

Blood capillaries absorb carbohydrate and protein products. Lymph capillaries called lacteals absorb fats.

Smooth muscle fibers of the small intestine contract to produce movements; peristalsis pushes chyme along, toward the large intestine, and segmentation mixes digestive juices with chyme and helps with absorption.

The liver and gallbladder are accessory organs of digestion.

The liver is the largest exocrine gland. It fills the upper right section of the abdominal cavity and extends over into the left side.

The liver secretes bile, a mixture of substances, and removes yellowish bile pigments from blood that were created from the breakdown of old red blood cells. It has many other metabolic functions.

The gallbladder is located on the undersurface of the liver; it is a sac with a folded interior.

The gall bladder concentrates and stores the bile produced in the liver.

Three ducts are associated with the liver and gall bladder.

o The hepatic duct drains bile from the liver; bile enters and leaves the gallbladder via the cystic duct; and the

o common bile duct is formed by the union of the hepatic and cystic ducts; it drains bile from the hepatic or cystic ducts into the duodenum.

Bile contains bile salts that emulsify the fats in chyme; it also contains cholesterol that can be eliminated from the body.

Cholecystokinin is a hormone triggered by the fat in chyme; it causes the gallbladder to contract and push stored bile into ducts leading to the duodenum. Gallstones can block ducts and possibly cause discolored stool and accumulation of yellow bile pigments in the blood and throughout the body—a condition called jaundice.

The pancreas is both an exocrine and an endocrine gland. It lies behind the stomach.

Pancreatic cells secrete pancreatic juice, the most important digestive juice. Pancreatic juice contains enzymes to digest carbohydrates, proteins, and lipids; and sodium bicarbonate that neutralizes stomach acid.

Pancreatic juice is secreted into the pancreatic ducts; the main duct empties into the duodenum.

Pancreatic islets of Langerhans are the endocrine cells not connected with pancreatic ducts; they secrete the hormones glucagon and insulin into the blood.

The large intestine is divided into subdivisions: the cecum, colon, rectum, and anal canal.

The cecum is a blind-end pouch at the beginning of the large intestine; chyme enters the cecum through the ileocecal valve.

The colon has four segments: the ascending, transverse, descending, and sigmoid colon. The rectum empties feces through the anal canal and the external opening called the anus.

Microbiome (or flora) of the large intestine helps digest nutrients, produce vitamins, and support immune protection; it also produce gases, called flatulence or flatus.

The large intestine absorbs water, salts, and vitamins.

Increased motility of the large intestine may produce diarrhea and decreased motility may result in constipation.

Defecation is the elimination of feces; it is regulated by voluntary and involuntary anal sphincters.

The appendix is a blind, worm-shaped tube off the cecum.

It functions as an incubator for bacteria of the intestinal microbiome.

Appendicitis is inflammation of the appendix.

The peritoneum is a continuous serous membrane lining the abdominal cavity and covering abdominal organs.

The parietal layer of the peritoneum lines the abdominal cavity while the visceral layer of the peritoneum covers abdominal organs.

The peritoneal space lies between the parietal and visceral layers; it produces lubricating peritoneal serous fluid.

The term “retroperitoneal” describes structures outside the parietal peritoneum.

The largest extensions of the peritoneum are the mesentery and greater omentum. The mesentery attaches most of the small intestine to the posterior abdominal wall. The greater omentum, or “lace apron,” hangs down from the lower edge of the stomach and transverse colon over the intestines.

The last section in this chapter reviews digestion. Digestion transforms foods into nutrient substances that can be absorbed and used by cells.

In mechanical digestion mastication, deglutition, and peristalsis break food into tiny particles, mix them well with digestive juices, and move them along the digestive tract.

Chemical digestion breaks up large nutrient molecules into compounds that have smaller molecules; it is brought about by digestive enzymes.

Enzymes are protein molecules that act as catalysts. The breakdown process they promote is called hydrolysis. Note that enzyme names often end in -ase.

Carbohydrate digestion occurs mainly in the small intestine. Pancreatic amylase breaks polysaccharides down to disaccharides. Three intestinal juice enzymes break sugars into glucose:

Maltase changes maltose to glucose;

Sucrase changes sucrose to glucose and fructose; and

Lactase changes lactose to glucose and galactose.

Protein digestion starts in the stomach and is completed in the small intestine,

Hydrochloric acid in gastric juice unfolds large proteins and converts pepsinogen to active pepsin. The gastric juice enzyme pepsin partially digests proteins.

The pancreatic enzyme, trypsin, continues digestion of proteins.

Intestinal enzymes, peptidases, complete digestion of partially digested proteins and convert them to amino acids.

Fat digestion occurs mainly in the small intestine.

Bile contains no enzymes, but emulsifies fats, meaning that it breaks fat droplets into very small droplets.

Pancreatic lipase changes emulsified fats to fatty acids and glycerol in the small intestine.

The end product of digestion of carbohydrates is the sugar glucose. The end products of digestion of protein are amino acids. Fatty acid and glycerol are the end products of fat digestion.

Absorption is the process by which digested nutrients move from the intestine into the blood or lymph.

Mechanisms of absorption include diffusion, osmosis, and active transport.

Nutrients and most water, minerals, and vitamins are absorbed from the small intestine; some water and vitamin K are also absorbed from the large intestine.

Structural adaptations in the intestines increase the absorptive surface area.

Fractal geometry is the study of irregular “fragmented” geometric shapes, such as those in the lining of intestine that have an almost unlimited surface area.

This concludes the audio review of Chapter 16.