Chapter 24

The Digestive System

The organ of digestion is called alimentary canal or gastrointestinal tract (GI). The tract consists of the mouth, pharynx, esophagus, stomach, small intestine, large intestine and ends in anus.

The digestive system includes other accessory organs like the teeth, tongue, gall bladder, and the glands (salivary, liver and pancreas). The process of digestion involves ingestion, propulsion, digestion (mechanical and chemical), absorption and defecation.

Structure of the GI tract

Wall

The wall consists of 4 layers:

Peritoneum

This is the largest serous membrane of the body, the lining is simple squamous tissues.

There are two membranes, parietal (outer membrane serous) peritoneum and visceral peritoneum (inner membrane serous). The parietal peritoneum lines the inside of the abdominal cavity while the visceral peritoneum covers the organs inside the abdominal cavity. Organs that lie in the posterior part of the abdominal cavity are described as retroperitoneal, examples, are the pancreas, kidneys and some part of the large intestine. These organs lose their peritoneal mesentery, while those that maintain it are described as intraperitoneal or peritoneal organs, an example is the small intestine. In between the parietal and visceral peritonea is a cavity (peritoneal cavity) which contains serous fluid. An extension of the parietal peritoneum forms the mesentery.

Mesentery is a double membrane that provides routes for blood vessels. Mesentery forms the omentum (greater and lesser omenta).

Mouth (oral cavity)

Within the oral cavity is the tongue, attached to the floor by the lingual frenulum. The lingual frenulum allows the movement of the tongue and if injured or deformed speech is distorted. The oral cavity is surrounded by the cheeks; on the roof of the cavity are the hard and soft palates. A projection from the soft palate is the uvula that regulates the entrance to the pharynx. The uvula closes off the nasopharynx during swallowing. There are salivary glands that secrete saliva. There are two groups of salivary glands: extrinsic salivary glands which are parotid, sublingual, and submandibular glands; the intrinsic salivary glands are the buccal glands that secrete small amount of saliva. These glands secrete mucus saliva, and salivary enzymes (example, salivary amylase). Inflammation of the glands causes mump.

Teeth are located in the gum (or gingiva) within the oral cavity. A typical tooth consists of three principal regions: the crown, the neck, and the root.

Enamel is the visible outer covering of the crown. It consists of calcium phosphate and calcium carbonate. It is regarded as the hardest substance in the body. It is avascular, and does not heal if broken, rather enamel filling has to be performed by a dentist.

Underneath the enamel is the Dentin, it forms the bulk of the tooth. It is also avascular and it surrounds a cavity which is highly vascular (the pulp cavity). Dentin is a bone-like substance.

This is another bone-like substance that lines the outer root section of the dentin.

The pulp is made up of loose connective tissues, blood vessels, and nerves. The pulp extends to form the root canal.

Dentition

There are two sets of teeth, deciduous (milk, falling, temporary or baby) teeth and permanent teeth. The deciduous starts appearing at about 6 months of age and continues up to 24 months. There are 20 deciduous teeth altogether: 2 incisors, 1 canine, and 2 molars respectively, in both the upper and lower jaws of both sides of the oral cavity. These deciduous teeth start falling out at the age of 6 years and are gradually replaced by the permanent teeth. Between the ages of 6 to 12 years of age the permanent teeth start to replace them. The permanent teeth are 32 in number; they are 2 incisors, 1 canine, 2 premolars, 2 molars, and 1 wisdom teeth on each side of the upper and lower oral cavity. Wisdom teeth usually appear after the age of 17 years.

Dental Disorders:

Physiology of Digestion

Mouth

With the help of the lips, the food enters the oral cavity and the teeth bite and chew it. The food is ground and mixed with the saliva. The grinding reduces the food into a bolus. At this point the food is swallowed by a voluntary action. In the mouth, the salivary amylase starts the digestion of carbohydrate food. There is also lingual amylase that starts the digestion of fats in the mouth.

Esophagus

The bolus now moves through the laryngopharynx and passes through the sphincter at the entrance of the esophagus. The bolus moves down by a wave-like motion (peristalsis) produced by the contraction of the esophageal wall. The esophagus secretes only mucus that helps to propel the food into the stomach, no digestion occurs here. It takes about 8 seconds for the bolus to reach the stomach but liquid may reach much faster (1-2 seconds).

Structurally the mucosa lining of the esophagus consists of the non-keratinized stratified squamous tissues. Other tissues that make up the wall of the mucosa include the lamina propria, and muscularis mucosa.

*The non-keratinized stratified squamous tissue epithelial lining is also seen in the lips, mouth, tongue, and esophagus.

Stomach

At the junction of the esophagus / stomach, there is another sphincter, the esophageal sphincter or cardia sphincter. The stomach is 'J' shaped. In the stomach the food is mixed properly. The food is held up in the stomach and released at intervals into the small intestine. In the stomach wall are also special cells known as pace maker cells found in the longitudinal smooth muscle layer. These muscle cells set the pace for the contraction of the stomach during the peristalsis producing cyclic slow waves of peristalsis.

Structurally, the stomach's wall has 3 layers of muscles: longitudinal, circular, and oblique. The stomach is divided into four regions: cardia (neck or the region closest to the esophageal junction); fundus (elevated and curved region at the superior part of the stomach); body (the main part of the stomach) and the pylorus (lower part that contains the pyloric sphincter). The mucosa of the stomach is modified to form special folds called RUGAE.

Histology of the stomach

The mucosa epithelial lining of the stomach has simple columnar cells with goblet cells. This epithelial lining extends into the lamina propria forming channels called gastric pits. Some of the tissues form secretory cells that are collectively called gastric glands. These secretory cells are of 4 kinds:

The tissues that form the gastric glands vary in composition depending on the part of the stomach lining. The gastric glands secrete 2 to 3 liters of gastric juice a day. The stomach secretes thick mucus that helps to protect the stomach wall against the acid of the gastric juice. In addition to the mucus protection, the epithelial lining of the stomach has tight junctions, which does not allow the gastric juice to leak into the stomach tissues. There is also a complete renewal of the epithelial lining of the stomach every 3-6 days. Damage to the underlying tissues causes gastric ulcers.

*Hyposecretion of mucus or hypersecretion of pepsin or HC1 causes peptic ulcers. Other factors that can cause ulcers include smoking, alcohol, coffee, and stress.

Chemistry of Digestion & Absorption in the Stomach

The food in the stomach is mixed gently by slow mixing waves or slow peristalsis. The food is now in semi-liquid form called chyme. The mixing waves starts slow and become strong as the food proceeds from the fundus through the body to the pylorus region of the stomach. As the chyme reaches the pylorus region, the pyloric sphincter opens and the chyme is forced into the duodenum a little at a time.

In the body of the stomach protein digestion starts with the pepsin secreted in inactive form by the chief cells. Protein is digested into short peptides by the pepsin. Gastric lipase is also secreted but the enzyme remains inactive at the stomach pH of 2.0 to 3.0. In children the stomach gland also secretes rennin, an enzyme that acts on milk protein (casein). Certain drugs like aspirin can pass through the stomach lining, irritates the stomach and possibly causes bleeding resulting in gastric ulcers.

Regulation of Gastric Secretion and Motility

Gastric juice secretion is regulated by both nervous and endocrine system. The regulation of gastric juice release is a negative feedback process that occurs in 3 phases.

The olfactory receptors detect the smell of food and sends impulses to the hypothalamus. These impulses are relayed through the vagus nerves of the medulla oblongata. These parasympathetic impulses sent through the vagus nerves cause peristalsis in the stomach followed by the release of gastric juice.

As the bolus enters the stomach, its presence causes stomach distension, and the stretch receptors become stimulated. Impulses from parasympathetic nerves stimulate the stomach peristaltic waves and more gastric secretions are produced. The presence of much gastric secretions in the stomach stops further stimulation of gastric secretions. These parasympathetic fibers also release acetylcholine that stimulates gastrin release by the enteroendocrine cells called G-cells. Gastrin stimulates the release of enzymes and more HCL, however low pH inhibits gastrin release (a type of negative feedback process).

The chyme moves into the small intestine and triggers reflexes that produce secretions that inhibit gastric juice secretions in the stomach. The enteroendocrine cells in the intestine secrete a group of hormones collectively called enterogastrones; these include the following:

Gastric inhibitory peptides (GIP), that inhibit gastric secretions and gastric motility.   

Secretin that inhibits gastric secretions only, Cholecystokinin (CCK) that inhibits gastric juice secretions and stomach motility and vasoactive intestinal peptide that also inhibit gastric juice secretions only.

A neural reflex or enterogastric reflex, which occurs due to the presence of food in the duodenum, signals the medulla to send impulses to inhibit the gastric secretion and motility, inhibit parasympathetic responses and stimulate sympathetic responses.

The accessory Glands of the Digestive System:

Pancreas: This is located in between the curved upper region of the small intestine (duodenum). The pancreas has one major duct called Pancreatic Duct (or duct of Wirsung), as it joins the common bile duct, it is called hepatopancreatic ampulla (Ampulla of Vater). There is a small duct that leads directly from the pancreas and empties into the duodenum, it is called duct of santorino. Histologically, the pancreas is made up of small clusters of cells - acini cells which secrete pancreatic juice. Among the acini cells are few clusters of cells known as pancreatic islets (or Islets of Langerhans) that secrete insulin & glucagon hormones. The pancreatic juice secretion is about 1.2 to 1.5 L a day. The juice contains digestive enzymes, water, and salts. The juice is alkaline with a pH of 7.1 - 8.2. This helps bring up the pH of the food from acidic to neutral or alkaline thereby, stopping the action of the pepsin. Pancreatic juice enzymes include:

·         Trypsinogen which is inactive and is converted into active form trypsin by enterokinase, trypsin digests protein and convert them into amino acids.

·         Chymotrypsinogen is inactive and is activated by trypsin to form chymotrypsin, the enzyme also assists trypsin in protein digestion.

·         Procaboxypeptidase is inactive, it is converted into active carboxypeptidase by trypsin; it also assists trypsin in protein digestion.

·         Pancreatic lipase digests lipids into fatty acids and glycerol.

·         Ribonuclease and deoxyribonuclease digest nucleic acids.

Pancreatic juice secretions are regulated just like the gastric juice secretions by both neural and endocrine negative feedback mechanisms. The presence of fatty acids, and amino acids makes the enteroendocrine cells secrete cholecystokinin (CCK), and secretin that stimulates the flow of pancreatic juice.

Clinical Significance:

Elevated values of pancreatic lipase and pancreatic amylase are symptoms of acute pancreatitis or chronic pancreatitis. Measurements of these enzyme levels in the serum help in the diagnosis of the conditions of the pancreas.

In order to evaluate gastric secretion activities, gastric function tests are usually performed. Patients are administered gastrin or pentagastrin, after 12 hours of fasting, the gastric residue is obtained, the HC1 content of the stomach is usually measured.

Liver:

Structurally, the liver has two major lobes and two small minor lobes. Each lobe consists of many lobules. Each lobule is made up of specialized cells called hepatocytes. Within this liver tissue are large spaces (or liver capillaries) called sinusoids through which the blood flows. Kupffer's cells (lymphoid or immune cells) or stellate or reticuloendothelial cells line the sinusoids. In the liver are found small canals (bile canaliculi) through which bile flows. These canaliculi merge to form the common hepatic duct that later joins the cystic duct (from the gallbladder) and forms the common bile duct.

The gall bladder temporary stores the bile. The liver secretes about 0.5 to 1.0 liter of bile a day. It is a yellow brownish or green liquid, and it is alkaline (pH 7.6 - 8.6). The bile is mainly water, bile acids / salts (cholic and chenodeoxycholic acids), cholesterol, phospholipids (lecithin), bile pigment (bilirubin) and ions (Na and K). The bile salts function in the emulsification of fats or lipids. The bile pigment, bilirubin comes from hemoglobin (or RBC) breakdown, the bilirubin breaks further into stercobilin which gives feces its brown color. Excess of bilirubin causes jaundice.

Disorders associated with bile secretion

Regulation of Bile secretion

Bile secretion is regulated by all the factors that regulate the pancreatic juice secretion.

The function of the liver is not just bile production but the liver participates in many metabolic processes, which include carbohydrate, lipid, protein, drugs, and various other substances including vitamin D metabolism, storage of vitamins, and formation of ferritin.

Anatomy of the small intestine:

The small intestine is divided into the duodenum (upper), jejunum (middle), and the ileum (lower) region, that joins the large intestine. The junction between the stomach and the small intestine carries the pyloric sphincter, while at the junction between the small intestine and the large intestine is the ileocecal sphincter.

The small intestine has the 4 layers of tissues (mucosa, submucosa, muscularis, and serosa). In the mucosa, are circular folds (plicae circulares), which help increase the surface area of the small intestine for absorption. Also are small fingerlike processes called villi (that make this region feel velvety) which also help increase the surface area of absorption. Within each villus are blood capillaries and lymphatic capillaries (known as lacteal).

The epithelial lining consists of simple columnar cells, goblet cells, and enteroendocrine cells (Paneth cells). These cells bear microvilli that appear as brush (brush border). The microvilli carry special digestive enzymes called brush border enzymes. Examples of brush border enzymes include lactase, nucleosidase, sucrase, and maltase, which continue further the digestion of carbohydrates and nucleic acids.

The mucosa has pits or openings lined with glandular cells (crypts of Lieberkuhn) which secrete intestinal juice. Within these pits or openings, (apart from the crypts of Lieberkuhn) are other specialized cells, for example Paneth cells that secrete lyzozyme, Enteroendocrine cells secrete SECRETIN,and CCK and Lymphatic nodules (Peyer's-Patches).

The submucosa of the duodenum (upper region of the small intestine) has special mucus secreting gland (Brunner's gland)

The intestinal juice is a clear yellow alkaline liquid and about l.to 2 liters are secreted daily into the lumen of the small intestine. It contains mainly water and mucus.

Physiology of Digestion in the Small intestine        

The food moves by two muscle contracting processes of segmentation and peristalsis. The presence of food and nerve impulses from the parasympathetic fibers (vagus-X) promote these movements, while the sympathetic fibers inhibit them. All carbohydrates are chemically converted into monosaccharides, all proteins to into amino acids, and all lipids into fatty acids, glycerol and monoglycerides. These molecules are absorbed from the intestinal lumen into the blood stream, for absorption. Absorption takes place mainly in the small intestine, very little absorption occurs in the stomach and in the large intestine.

Anatomy of the large intestine:

The large intestine is divided into cecum, colon, rectum, and the anal canal. The cecum is a blind pouch, and the beginning part of the large intestine that joins the ileum through the ileocecal valve. Attached to the cecum is the vermiform appendix. The colon is divided into ascending, transverse, descending, and sigmoid.

The epithelial lining of the mucosa is made up of simple columnar cells, in between them are found goblet cells. The longitudinal muscles of the muscularis tissue layer thicken to form long flat bands, taenia coli, on the surface of the large intestine. Contraction of the bands produces a series of pouches in the colon called haustra.

Bacterial flora can be found in the lumen of the large intestine; they help to ferment any undigested food, (example cellulose). These bacteria can synthesize Vit.K and Vit.B.

Peristalsis occurs but slowly. Another type of movement is haustra churning, which moves food from one haustra to the next. A strong peristaltic movement, mass peristalsis moves the colon content forcefully into the rectum.

The remaining water in the chyme is absorbed in the large intestine, the chyme now becomes feces (semi solid). A defecation reflex, a parasympathetic response allows the anal sphincter to relax. The rectum empties into the anus. The anal canal has two sphincters (external and internal). The internal sphincter is involuntarily controlled, while the external sphincter is voluntarily controlled, the later allows the food to be expelled as needed.

Disorders of the Digestive System.

Gastritis

This is the inflammation of the gastric mucosa. It could be as the result of drugs like aspirin or emotional or physical stress, bacterial infection, or ingestion of strong alkaline or acid.

Peptic Ulcers

This disorder is caused by the presence of acid in the stomach that penetrates the mucosa of the stomach or by the excessive production of acid in the stomach. Inadequate secretion of alkaline mucus in the stomach can cause this disorder also.

Gastroenteritis

This is caused by an irritation of the small intestine. Symptoms include diarrhea, and vomiting.

Diarrhea and Constipation

Diarrhea's symptoms include watery feces. Constipation's symptoms include hard feces. Both disorders are caused by the inability of the intestine to absorb just the adequate amount of water from the digested food. Bacterial or viral infection can cause inadequate water absorption by the colon, therefore producing watery feces.           

Cholera is a special type of diarrhea, in which the bacteria releases toxin that stimulates a massive fluid secretion by the intestine. This causes a dramatic loss in fluid and ions from the body. Following this fluid and ions loss, the blood volume is dropped, the kidneys are damaged, and dehydration occurs, this is followed by death.

In the case of constipation, excessive absorption of water occurs, this makes the feces hard.

Mumps

This is caused by the mump's virus. The salivary glands in children are usually attacked, other organs that the virus may attack include the central nervous system, the testes and the pancreas.

 

 

 

KEY POINTS

  1. The digestive system also known as the gastrointestinal tract consists of the mouth, pharynx, esophagus, stomach, small intestine, large intestine and the anus. Other organs of digestion are considered accessory, these include teeth, tongue, gall bladder, pancreas, liver and salivary glands.
  2. The wall of the digestive tract structurally has 4 layers of tissues: mucosa (inner lining), submucosa, muscularis, and serosa.
  3. The mouth contains the tongue that contains receptors for taste, teeth for masticating the food, salivary glands that produces saliva to mix with the food and salivary enzymes for carbohydrate digestion.
  4. When food is masticated and mixes with saliva, it is called a bolus. This bolus is swallowed and through a wave-like peristalsis movement, the bolus travels from the pharynx through esophagus into the stomach.
  5. The stomach has two sphincters: the cardia sphincter that regulates the entrance of the bolus into the stomach and the pyloric sphincter that regulates the exit of the chyme (watery food) from the stomach.
  6. Mucosa of the stomach is modified to form folds called rugae. Special tissues in the mucosa (chief cells, parietal cells, G-cells and goblet cells) together form gastric glands. The gastric gland produces secretions known as gastric juice, a mixture of HCL, pepsin, gastric lipase, mucus and gastrin.
  7. Gastric secretions are usually acidic and therefore the pH of the stomach is usually 3.0 or less. Digestion of protein by pepsin requires this low pH, therefore protein is usually digested into short peptides in the stomach.
  8. The gastric juice secretion is regulated in three phases: cephalic, gastric and intestinal phases. In the cephalic phase, the smell of food is detected by the olfactory receptors that sends impulses to the hypothalamus and through vagus nerves, impulses are relayed to the stomach and gastric juice is secreted. In the gastric phase, the presence of food in the stomach causes the stretch receptors to stimulate the secretion of gastric juice. In the intestinal phase, the exit of chyme into the intestine stimulates the enteroendocrine cells to produce secretions of secretin, GIP, VIP and CCK to inhibit gastric juice secretions.
  9. The addition of gastric juice into the bolus makes the bolus very watery, now called chyme. The chyme moves by slow peristalsis into the small intestine.
  10. In the small intestine, the pancreatice juice containing enzymes (enterokinase, trypsin, chymotrypsin, pancreatic lipase, pancreatic amylase and nucleases) is secreted by the pancreas to mix with the chyme; also the bile containing bile salts is released by the gall bladder into the intestine to mix with the chyme. At this time, the food is completely liquefied and all the food molecules are digested completely into single units (amino acids, glucose, fatty acids, and glycerol).
  11. Absorption of food occurs. Any molecule not absorbed or digested moves by segmentation and peristalsis into the large intestine. In the large intestine, further absorption of water and fermentation of undigested food occurs. The fermented food proceeds to the rectum as feces by mass peristalsis.

APPENDIX

Sample Multiple Questions.

  1. In the large intestine, the food moves by this process------

a. peristalsis b. mass peristalsis c. segmentation d. a and b

 

  1. Regulation of gastric juice secretion is in three phases, this is one of the phases---------

a. intestinal phase b. duodenal phase c. peptic phase d. none of the above.

  1. Special folds like these ----- are found in the stomach.

a. rugae b. villi c. microvilli d. none of the above.

  1. This vitamin can be synthesized in the large intestine.

a. Vitamin B b. Vitamin A c. Vitamin K d. a and c

  1. Pouches in the form haustra are found in this part of the gastrointestinal tract.

a. esophagus b. mouth c. small intestine d. large intestine.

 

Clinical cases

  1. A patient's blood analysis showed elevated serum lipase and amylase. This patient complains of severe pain in the abdominal region of the body. Trypsin and chymotrypsin activities were highly elevated in this patient's serum. What is the probable disease that this patient may be suffering from?
  2. Mrs. A experiences acute pain in the right hypochondriac region of the abdomen. Her stool appeared gray and her eyes and skin appear pale with yellow pigmentation. It is diagnosed that Mrs. A suffers from Jaundice. What could have been the cause for this jaundice?