Lec.1
Medical Physiology
Z.H.Kamil
PHYSIOLOGY OF THE GASTROINTESTINAL TRACT (GIT)
The alimentary tract provides the body with a continual supply of water, electrolytes, and
nutrients. To achieve this requires (1) movement of food through the alimentary tract; (2)
secretion of digestive juices and digestion of the food; (3) absorption of water, various
electrolytes, and digestive products; (4) circulation of blood through the gastrointestinal
organs to carry away the absorbed substances; and (5) control of all these functions by
local, nervous, and hormonal systems.
Functions of GIT
Overall, the digestive system performs six basic processes:
1. Ingestion. This process involves taking foods and liquids into the mouth (eating).
2. 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.
3. Mixing and propulsion. Alternating contractions and relaxations of smooth muscle in
the walls of the GI tract mix food and secretions and propel them toward the anus. This
capability of the GI tract to mix and move material along its length is called motility.
4. 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. A few
substances in food can be absorbed without chemical digestion. These include vitamins,
ions, cholesterol, and water.
5. Absorption. The entrance of ingested and secreted fluids, ions, and the products of
digestion into the epithelial cells lining the lumen of the GI tract is called absorption. The
absorbed substances pass into blood or lymph and circulate to cells throughout the body.
6. 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.
Divisions of The Digestive System
The gastrointestinal system consists of the gastrointestinal tract and the accessory exocrine
glands. The gastrointestinal tract includes the mouth, the esophagus, the stomach, the
small intestine, and the large intestine. The major accessory glands are the salivary glands,
the liver, the gallbladder, and the pancreas.
Figure 1 shows the entire alimentary tract. Each part is adapted to its specific functions:
some to simple passage of food, such as the esophagus; others to temporary storage of
food, such as the stomach; and others to digestion and absorption, such as the small
intestine.
Types of Digestion
The food we eat is broken down in two complementary processes: mechanical digestion
and chemical digestion.
Mechanical digestion is the physical breaking up of food into smaller pieces. Chewing is
an example of this. As food is broken up, more of its surface area is exposed for the action
of digestive enzymes.
The work of the digestive enzymes is the chemical digestion of broken-up food particles,
in which complex chemical molecules are changed into much simpler chemicals that the
body can utilize. Such enzymes are specific with respect to the fat, protein, or
carbohydrate food molecules each can digest. For example, protein-digesting enzymes
work only on proteins, not on carbohydrates or fats. Each enzyme is produced by a
particular digestive organ and functions at a specific site. However, the enzyme’s site of
action may or may not be its site of production.
Figure 1: Divisions of GIT
Neural Control of Gastrointestinal Function—
Enteric Nervous System
The gastrointestinal tract has a nervous system all its own called the enteric nervous
system. It lies entirely in the wall of the gut, beginning in the esophagus and extending all
the way to the anus. The number of neurons in this enteric system is about 100 million,
almost exactly equal to the number in the entire spinal cord. This highly developed enteric
nervous system is especially important in controlling gastrointestinal movements and
secretion.
In general, stimulation of the sympathetic nervous system inhibits activity of the
gastrointestinal tract, causing many effects opposite to those of the parasympathetic
system. It exerts its effects in two ways: (1) to a slight extent by direct effect of secreted
norepinephrine to inhibit intestinal tract smooth muscle (except the mucosal muscle, which
it excites) and (2) to a major extent by an inhibitory effect of norepinephrine on the
neurons of the entire enteric nervous system.
Strong stimulation of the sympathetic system can inhibit motor movements of the gut so
greatly that this literally can block movement of food through the gastrointestinal tract.
-Hormonal Control of Gastrointestinal Motility
Although the motility effects are usually less important than the secretory effects of the
hormones, some of the more important of them are the following:
Gastrin is secreted by the “G” cells of the antrum of the stomach in response to stimuli
associated with ingestion of a meal, such as distention of the stomach, the products of
proteins, and gastrin releasing peptide. The primary actions of gastrin are
(1) stimulation of gastric acid secretion.
(2) stimulation of growth of the gastric mucosa.
Cholecystokinin is secreted by “I” cells in the mucosa of the duodenum and jejunum
mainly in response to digestive products of fat, fatty acids, and monoglycerides in the
intestinal contents. This hormone strongly contracts the gallbladder, expelling bile into the
small intestine where the bile in turn plays important roles in emulsifying fatty substances,
allowing them to be digested and absorbed. Cholecystokinin also inhibits stomach
contraction moderately. Therefore, at the same time that this hormone causes emptying of
the gallbladder, it also slows the emptying of food from the stomach to give adequate time
for digestion of the fats in the upper intestinal tract.
Secretin was the first gastrointestinal hormone discovered and is secreted by the “S” cells
in the mucosa of the duodenum in response to acidic gastric juice emptying into the
duodenum from the pylorus of the stomach. Secretin has a mild effect on motility of the
gastrointestinal tract and acts to promote pancreatic secretion of bicarbonate which in turn
helps to neutralize the acid in the small intestine.
Gastric inhibitory peptide is secreted by the mucosa of the upper small intestine, mainly
in response to fatty acids and amino acids but to a lesser extent in response to
carbohydrate. It has a mild effect in decreasing motor activity of the stomach and therefore
slows emptying of gastric contents into the duodenum when the upper small intestine is
already overloaded with food products.
Motilin is secreted by the upper duodenum during fasting, and the only known function of
this hormone is to increase gastrointestinal motility. Motilin secretion is inhibited after
ingestion by mechanisms that are not fully understood.
Movements in the Gastrointestinal Tract
Two types of movements occur in the gastrointestinal tract:
(1) propulsive movements, which cause food to move forward along the tract at an
appropriate rate to accommodate digestion and absorption.
The basic propulsive movement of the gastrointestinal tract is peristalsis, which is a reflex
response that is initiated when the gut wall is stretched by the contents of the lumen, and it
occurs in all parts of the gastrointestinal tract from the esophagus to the rectum. The
stretch initiates a circular contraction behind the stimulus and an area of relaxation in front
of it (Figure 2). The wave of contraction then moves in an oral-to-caudal direction,
propelling the contents of the lumen forward at rates that vary from 2 to 25 cm/s.
Peristaltic activity can be increased or decreased by the autonomic input to the gut, but its
occurrence is independent of the extrinsic innervations. Indeed, progression of the contents
is not blocked by removal and resuture of a segment of intestine in its original position and
is blocked only if the segment is reversed before it is sewn back into place. Peristalsis is an
excellent example of the integrated activity of the enteric nervous system.
(2) mixing movements, which keep the intestinal contents thoroughly mixed at all times.
When the meal is present, the enteric nervous system promotes a motility pattern that is
related to peristalsis, but is designed to retard the movement of the intestinal contents
along the length of the intestinal tract to provide time for digestion and absorption (Figure
2). This motility pattern is known as segmentation, and it provides for ample mixing of the
intestinal contents (known as chyme) with the digestive juices. A segment of bowel
contracts at both ends, and then a second contraction occurs in the center of the segment to
force the chyme both backward and forward. Unlike peristalsis, therefore, retrograde
movement of the chyme occurs routinely in the setting of segmentation. This mixing
pattern persists for as long as nutrients remain in the lumen to be absorbed. It presumably
reflects programmed activity of the bowel dictated by the enteric nervous system, and can
occur independent of central input, although the latter can modulate it.
Figure 2: Peristalsis and Segmentation