Unit 21 - Digestive tract


The digestive system is basically an elongate coiled tube through which food materials move. As the food moves it is broken down into small molecules which are absorbed into the body. By providing the essential nutrients that cells require, the digestive system is a major contributor to homeostasis.


A. Functions - The digestive tract functions in the following ways.


1. Ingestion of food into the mouth

2. Movement of food along the tract

3. Mechanical breakup of food

4. Secretion of digestive juices

5. Chemical digestion of food

6. Absorption of digested nutrients into the blood and lymphatics

7. Elimination of undigestible substances and waste products


B. General anatomy - The digestive tract is a long tube to which a series of accessory glands are attached.


1. Regions - The digestive tube has five major regions which are as follows.


a. Mouth

b. Esophagus

c. Stomach

d. Small intestine

e. Large intestine


2. Anatomy of the tract wall - The great bulk of the functions of the digestive tract are carried out by the wall of the tract. Beginning at the esophagus and continuing throughout the rest of the tract, the wall of the tube has the same basic structure. The wall consists of four layers with nervous plexuses connecting the layers. The layers are as follows.


a. Tunica mucosa - This is the mucous membrane that lines the tract. It consists of an epithelium and a layer of areolar tissue (lamina propria) to which the epithelium is attached. A thin layer of smooth muscle surrounds the mucosa (muscularis mucosae).


b. Tunica submucosa - This is a thick layer of either dense or areolar connective tissue. It contains the blood vessels, lymphatics, nerves, and glands.


c. Tunica muscularis - Here is found a double layer of smooth muscle. The inner layer has its fibers arranged in a circle while the outer layer is arranged longitudinally. At certain areas the circular layer is thickened to form a sphincter muscle or valve.


d. Tunica serosa - This is a connective tissue membrane which forms the outermost layer.


e. Intrinsic nerve plexuses - These are nerve complexes found between the tunics. They require no stimulation from the CNS but instead respond to receptors found in the walls of the tract. They coordinate much of the digestive tract activity. Keep in mind, that while these plexuses do not require stimulation from the CNS, the CNS still sends nerves to the tract and can alter its activity.


3. Accessory glands - Although the wall of the tract contains a large number of glands, there are three glands that lie outside of the tract and dump their secretions into the tract via ducts. These glands are the salivary glands, pancreas, and liver.


a. Salivary glands - There are three pairs of these glands which produce saliva which enters the mouth.


(1) Parotids - These are the largest of the three pairs and are located beneath the ears.


(2) Submandibular (submaxillary) - These glands lie medial to the angle of the mandible.


(3) Sublinguals - These are located on the floor of the mouth, beneath the tongue.


b. Pancreas - This large gland is located beneath the stomach. It produces pancreatic juice which contains many digestive enzymes.


c. Liver - This, the largest organ in the body, is divided into a left and right lobe.


(1) The liver produces bile which aids in the breakdown of fats and serves to rid the body of certain metabolic wastes.


(2) The liver has many more functions, over 200 at last count. Some of the major ones include the following.


(a) The production of albumin and other plasma proteins.


(b) It manufactures prothrombin and other blood clotting factors.


(c) It regulates carbohydrate, protein, and lipid metabolism.


(d) It synthesizes and stores glycogen.


(e) It produces the waste products urea and ketone bodies.


(f) It stores iron, vitamin A, and vitamin D.


(g) It detoxifies most materials which are toxic to the cells.


(h) It plays a major role in body defense by phagocytizing microorganisms.


C. Movements of materials through the tract - There are two basic types of contraction movement found in the digestive tract.


1. Segmentation - These are stationary contractions that occur long the tract. They divide the tract into constricted and unconstricted regions. Segmentation functions in the mixing of materials.


2. Peristalsis - This is a wave-like contraction that moves down the tract. It functions in the movement of material along the tract.


D. Control of flow - valves - The directional movement of materials throughout the tract is regulated by a series of valves that are often referred to as sphincters. There are five major valves.


1. Upper esophageal sphincter - This valve keeps the esophagus closed except during swallowing.


2. Lower esophageal sphincter (cardiac sphincter) - The guards the opening into the stomach. It prevents material from returning to the esophagus from the stomach.


3. Pyloric sphincter - This valve controls material flow between the stomach and the small intestine.


4. Ileo-cecal sphincter - This regulates the flow of material from the small intestine to the large intestine.


5. Anal sphincter - This is actually a dual sphincter. The inner one is composed of smooth muscle and is not under voluntary control. The outer valve is composed of skeletal muscle and is under voluntary control. The anal sphincters regulate the exit of fecal material.


E. Digestive process - In examining the process of digestion we will look at each of the five major regions of the tract, beginning with the mouth, and detail the major processes that occur in each region.


1. Mouth and pharynx - The mouth and oral cavity are the first parts of the tract. They extend from the lips to the oropharynx. The digestive activities of the mouth include the following.


a. Analysis - Receptors analyze food materials prior to swallowing. This protects against the ingestion of dangerous materials.


b. Mastication - This is chewing which functions to reduce the food particles into smaller size. Smaller particles will have a greater surface area for the digestive enzymes to act upon and mastication therefore increases the efficiency of the digestive process.


(1) The tongue plays a major role in manipulating food both during mastication and swallowing. The tongue also contains glands that secrete the enzyme lingual lipase which begins the breakdown of lipids, even before swallowing has occurred.


c. Salivation - While mastication is occurring, salivation mixes saliva with the food particles to form a moist mass known as a bolus. The salivary glands produce from 1000 to 1500 ml of saliva each day.


(1) Composition of saliva


(a) Water - 97 to 99%

(b) Mucin

(c) Salivary amylase - enzyme which hydrolyses starch

(d) Electrolytes


(2) Functions of saliva


(a) Lubrication of food bolus for swallowing

(b) Digestion of starch

(c) Taste

(d) Aids in speech

(e) Bacteriostasis


(3) Regulation of salivation - Salivation is under neural control of the ANS. Both parasympathetic and sympathetic innervation occurs. Parasympathetic stimulation increases salivation while sympathetic decreases it. The salivation reflex can be initiated in three different ways.


(a) Conscious thought - Thinking of food will initiate saliva flow.


(b) Chemical stimulation - The presence of food in the mouth initiates a neural reflex that causes saliva flow.


(c) Mechanical stimulation - The action of chewing initiates a neural reflex that increases saliva flow.


d. Swallowing - The processes involved here are as follows.


(1) The food bolus is forced into the pharynx by the tongue.


(2) A series of reflexes close off all other openings into the pharynx.


(a) The epiglottis closes off the glottis.


(b) The soft palate closes off the nasopharynx.


(c) The tongue prevents reentry into the mouth.


(3) The upper esophageal sphincter relaxes and the esophagus opens. The food bolus is forced into the esophagus by reflexive contractions of the pharynx.


(4) Peristaltic waves move the bolus down the esophagus to the stomach. When the peristaltic wave hits the lower esophageal sphincter it relaxes permitting food to move into the stomach.


2. Stomach


a. Functions


(l) It stores food and slowly releases it to the intestine.


(2) It mixes food with the stomach (gastric) secretions which results in liquification.


(3) Protein digestion begins in the stomach.


(4) Production of intrinsic factor, a glycoprotein which must be present for the absorption of vitamin B12


b.  Gastric secretions - The stomach produces several different substances which are collectively termed gastric juice. The epithelium that lines the stomach is a simple columnar one made up entirely of goblet cells that secrete an alkaline mucous that coats and protects the stomach from digestion. Gastric juice is produced by the gastric glands. These glands are made up of four types of cells.


(1) Mucous neck cells These are found at the top (neck) of the glands and produce an acidic mucous.


(2) Parietal cells These produce HCl and intrinsic factor. HCl is a strong acid that has the following functions.


(a)         speeds up the breakdown of protein

(b)         provides appropriate pH for pepsin

(c)         kills microorganisms

(d)         stimulates release of pancreatic juice

(3) Chief (zyomogenic) cells Produce pepsinogen the inactive form of the enzyme pepsin. It functions in protein hydrolysis.


(4) Enteroendocrine cells These produce at least seven different hormones that regulate the digestive process.


The combination of the secretions of these cells plus water constitutes gastric juice.


c. Control of gastric secretions - The stomach produces up to three liters of gastric juice per day. Secretion is under both neuronal and hormonal control.


(1) Stimulation of secretion - There are two distinct phases of gastric secretion.


(a) Cephalic phase - This is stimulated by vagal activity to the stomach. The trigger can be the sight, smell, or taste of food.


(b) Gastric phase - The trigger for this is distention of the stomach, increase in gastric pH, and the presence of proteins in the stomach. Actual flow is brought about by


/1/ local nervous reflexes coordinated by the nervous plexuses.


/2/ gastrin release. Gastrin is a hormone which stimulates gastric juice secretion.


/3/ histamine release by cells of the lamina propria, which stimulates acid release by the parietal cells.


(c) Intestinal phase - This occurs when acid chyme (liquified food) from the stomach begins to distend the duodenum (first section of the small intestine). There are two phases, excitatory which occurs when the chyme first enters this duodenum. This causes the release of a gastrin like hormone which further stimulates gastric juice release. The inhibitory phase begins when chemical substances found in the acid chyme interact with the duodenal cells, then gastric juice secretion is terminated. This is brought about the following mechanisms.


/1/ Neural reflex


/2/ The release of four different gastric hormones, all of which will block secretion. These four hormones are as follows.


/a/ Secretin


/b/ Cholecystokinin


/c/ Gastric inhibitory peptide, GIP

/d/ vasoactive intestinal peptide (VIP)


3. Small intestine


a. Functions


(l) Digestion - It is here that the great bulk of chemical digestion takes place.


(2) Absorption - The vast majority of absorption of nutrients into the blood occurs there. Substances absorbed here include organic nutrients, vitamins, minerals, and water.


b. Regions - The entire small intestine is about six meters long and is divided into three regions based upon its microscopic structure. The three regions are as follows.


(1) Duodenum - This is the first 25 cm of the small intestine. It receives secretions from both the liver and pancreas.


(2) Jejunum - This constitutes the next 2.5 meters.


(3) Ileum - This is the last 3.5 meters of the small intestine.


c. Digestive juices - The small intestine receives three major digestive juices. Collectively these three juices contain all of the enzymes and other substances which are necessary for the breakdown and absorption of all food categories.


(l) Pancreatic juice - Approximately 750 ml of this juice are produced per day.


(a) Composition


/1/ Digestive enzymes - proteases, lipases, and carbohydrases.


/2/ Water


/3/ Bicarbonate - This neutralizes the HCl from the stomach. This is necessary as the enzymes of the small intestine require an alkaline pH for optimal activity.


(b) Control of secretion - Both neural and hormonal mechanisms are involved.


/1/ Neural - There is a vagal reflex which increases the enzyme concentration of the pancreatic juice.


/2/ Hormonal


/a/ Secretin - This stimulates the release of bicarbonate. Secretion is in response to the presence of acid chyme in the duodenum.


/b/ Cholecystokinin - This stimulates the release of enzymes. It is released in response to certain chemical substances found in the acid chyme.


(2) Bile - About 600 to l000 ml of bile are produced per day. About 40 to 70 ml of this amount is stored in the gall bladder at any one time. Bile is released into the duodenum via the common bile duct.


(a) Composition - The principal components of liver bile are as follows.


/1/ Water

/2/ Electrolytes

/3/ Bile salts

/4/ Bile pigments (bilirubin and biliverdin)

/5/ Cholesterol


(b) Function - Liver bile carries out the following activities.


/1/ Emulsifies fat

/2/ Assists in the absorption of fat soluble vitamins

/3/ Assists in calcium absorption

/4/ Eliminates excess hemoglobin and cholesterol


(c) Control of secretion - There are three levels of control for the secretion of bile, chemical, neural, and hormonal.


/1/ Chemical - Bile salts in the plasma stimulate bile flow.

/2/ Neural - Vagal stimulation increases bile secretion.

/3/ Hormonal - Cholecystokinin stimulates the contraction of the gall bladder as well as the relaxation of the Sphincter of Odi which regulates bile flow through the common bile duct.


(3) Intestinal juice - This juice is produced by the cells of the mucosa (crypts of Lieberkuhn, intestinal glands) and Brunner's glands (submucosal glands) which are located in the submucosa of the duodenum.


(a) Control of secretion


/1/ The intestinal glands are keyed by local reflexes which respond to tactile or irritating stimuli due to the presence of acid chyme. Their secretion is largely enzymatic. There is also a hormone released by the duodenum known as vasoactive intestinal peptide (VIP), which stimulates the intestinal glands. In addition, this hormone inhibits acid production and stimulates the dilation of arterioles supplying capillary beds in the intestines.


/2/ Brunner's glands produce an alkaline mucous. Their secretion is controlled by a hormone, enterocrinin. Enterocrinin is released when acid chyme enters the duodenum. The submucosal glands also produce a hormone, urogastrone (epidermal growth factor, EGF), which stimulates division of epithelial cells that line the intestinal tract and other areas. Urogastrone also seems to inhibit gastric acid production.


d. Absorption - This occurs across the epithelial cells that line the mucosa of the small intestine. The mucosa is highly folded, with the folds being termed plicae. This effectively increases the surface area available for absorption to occur across.


(1) Intestinal villi - In addition to the plicae, there is another mechanism for increasing available surface area. These are the microscopic villi, finger-like projections that cover the mucosa. It is the villi that give the mucosa a velvety texture. Each villus consists of an epithelium with a capillary and lacteal inside.


The bulk of absorption occurs in the jejunum and this is where plicae and villi are best developed. As the ileum is approached the plicae and villi decrease, and by the end of the ileum there are no plicae at all. There are located here nodules of lymphoid tissue known as Peyer's patches, which protect the small intestine against bacteria from the large intestine.

(2) Mechanisms of absorption - Materials move into the capillary and lacteal of the villus by the usual mechanisms of transmembrane movement: diffusion, active transport, and osmosis. Most nutrients move directly into the capillaries. Larger lipid droplets are absorbed into the lacteal.


(a) Active transport - Most of the organic nutrients and electrolytes are absorbed in this manner.


(b) Osmosis - Water is absorbed by this mechanism. The active transport of nutrients and electrolytes creates an osmotic gradient which water follows. In a typical day anywhere from 5 to 10 liters of water which enter the small intestines. This volume comes from the food, drink, and digestive juices. Of this total amount, only about 0.5 liter enters the large intestine. The remainder is reabsorbed by the small intestine.


4. Large intestine


a. Anatomy - The large intestine is about 1.5 meters long. It extends from the ileocecal valve to the anus. There are six different regions.


(1) Cecum - This is a blind pouch which receives the ileum. The vermiform appendix extends downward from the cecum. The remaining regions of the large intestine are referred to as the colon.


(2) Ascending colon - This is the portion that ascends from the cecum.


(3) Transverse colon - This portion extends across the body from the ascending to the descending colon.


(4) Descending colon - This region extends downward to the sigmoid region.


(5) Sigmoid colon - This represents a curvature toward the midline of the body and into the iliac fossa.


(6) Rectum - The is the last, straight portion of the colon.


b. Functions


(1) Reabsorption of water - The final volumes of water are removed by the colon. About 1500 ml of material inters the colon each day, but only about 200 ml of feces are ejected. Much of the reduction is due to reabsorption of water (400 to 500 ml).


(2) Synthesis of vitamins - Certain symbiotic bacteria that inhabit the colon produce vitamin K and the B complex.


(3) Elimination - The colon compacts, stores, and eventually eliminates undigested food residues.


c. Movements - The colon undergoes several different types of movements.


(1) Sluggish peristalsis - This is very slow, wave-like motions. This results in the slow movement of material throughout the colon and allows time for water uptake. It frequently requires l8 to 24 hours for materials to move through the colon.


(2) Mass peristalsis - This is a rapid and forceful peristaltic wave. This usually forces material into the rectum. The resulting distention of the rectum initiates the defecation reflex.


(3) Defecation reflex - This reflex stimulates peristaltic contractions of the descending colon, sigmoid colon, and rectum. It also results in the relaxation of the internal anal sphincter. The external sphincter, being of skeletal muscle, must be relaxed voluntarily. Upon relaxation, defecation, the elimination of undigested food materials, will occur.


F. Pathologies


1. Vomiting - This is a complex reflex which is coordinated by a vomiting center in the medulla. It results in reversed peristalsis which ejects stomach contents up through the esophagus. Normally anything that irritates the stomach mucosa results in vomiting. It can also be initiated from other parts of the nervous system.


2. Diarrhea - This is where fecal material becomes very watery and defecation is frequent. It can be due to over secretion of fluids into the tract or by increased motility of the tract. Many intestinal infections induce this condition, either by stimulating over secretion (cholera) or increased intestinal motility (enteritis).


3. Constipation - This is due to reduce motility of the large intestine. Material remains in the colon for extended periods of time and as a result excessive amounts of water are absorbed leaving the feces hard and dry. It can be caused by diet, lack of exercise, and nervous upset.


4. Peptic ulcer - This is an erosion of the mucosa of the stomach or small intestine by acid and digestive enzymes. Excessive gastric juice secretion or insufficient mucous secretion is normally the immediate cause. Erosion may continue into the blood vessels causing a bleeding ulcer. Complete penetration through the tract wall is termed a perforated ulcer. Recent evidence indicates that a bacterial infection may be a contributing factor to ulcers, and most ulcer treatment now involves administration of antibiotics as well as the traditional treatments..


G. Effects of aging - There is a decrease in secretion and a loss of motility with age. As a result the digestive process is not as efficient as it was at an earlier age. Digestion takes much longer and food remains in the tract for extended periods. Various pathologies such as ulcer and cancer also increase with age although they are not necessarily a direct consequence of aging.