Lec.3 Medical Physiology Z.H.Kamil
PHYSIOLOGY OF THE GASTRO-
INTESTINAL TRACT (GIT)
Small Intestine
Thesmall intestine is the body's major digestive organ. Within its twisted passageways, usable food is finally prepared for its journey into the cells of the body. It is the longest section of the GIT, with an average length of 2.5 to 7 m in a living person.
Thesmall intestine has three subdivisions: the duodenum, the jejunum, and the ileum, which contribute 5%, nearly 40%, and almost 60% of the length of thesmall intestine respectively.
Activities of Small Intestine
- Movements of the Small Intestine
The movements of the small intestine, like those elsewherein the gastrointestinal tract, can be divided intomixing contractions and propulsive contractions.
-Mixing Contractions (SegmentationContractions)
When a portion of the small intestine becomes distendedwith chyme, stretching of the intestinal wallelicits localized concentric contractions spaced atintervals along the intestine and lasting a fraction of aminute. The contractions cause “segmentation” of the small intestine.
As one set of segmentationcontractions relaxes, a new set often begins,but the contractions this time occur mainly at newpoints between the previous contractions. Therefore,the segmentation contractions “chop” the chyme twoto three times per minute, in this way promoting progressivemixing of the food with secretions of the smallintestine.
-Propulsive Movements
Chyme is propelledthrough the small intestine by peristaltic waves. Thesecan occur in any part of the small intestine, and theymove toward the anus at a velocity of 0.5 to 2.0 cm/sec,faster in the proximal intestine and slower in the terminalintestine. They normally are very weak andusually die out after traveling only 3 to 5 centimeters,very rarely farther than 10 centimeters, so that forwardmovement of the chyme is very slow, so slow in factthat net movement along the small intestine normally averages only 1 cm/min. This means that 3 to 5 hoursare required for passage of chyme from the pylorus tothe ileocecal valve.
In addition to the nervous signals that may affectsmall intestinal peristalsis, several hormonal factorsalso affect peristalsis. They include gastrin,insulin, motilin, and serotonin, all of which enhanceintestinal motility and are secreted during variousphases of food processing. Conversely, secretin andglucagon inhibit small intestinal motility.
- Food Breakdown
Chemical digestion of foods begins in earnest in the small intestine. The small intestine is able to process only a small amount of food at one time. The pyloric sphincter controls food movements into the small intestine from the stomach and prevents the small intestine from being overwhelmed. Though the C-shaped duodenum is the shortest subdivision of the small intestine, it has the most interesting features.
Food reaching the small intestine is only partially digested. Carbohydrates and protein digestion have been started, but virtually no fats have been digested up to this point. The process of chemical food digestion is accelerated as the food takes about 3-6 hours journey through the small intestine. By the time the food reaches the end of the small intestine, digestion is complete and nearly all food absorption has occurred.
The microvilli of small intestine cells bears a few important enzymes, the so called brush border enzymes that break down double sugars into simple sugars and complete protein digestion. Intestinal juice itself is relatively enzyme-poor, and protective mucus. However, foods entering the small intestine are literally deluged with enzyme-rich pancreatic juice.
Pancreas
The pancreasis a large, soft, pink, triangular gland that extends across the abdomen from the spleen to the duodenum. The pancreas produces enzymes that breakdown all categories of digestible foods. The pancreatic enzymes are secreted into the duodenum in an alkaline fluid, which neutralizes the acidic chyme coming in from the stomach. The pancreas also has an endocrine function; it produces the hormones glucagon andinsulin (but this is not secreted by the samepancreatic tissue that secretes intestinal pancreatic juice. Instead, insulin is secreted directly into theblood—not into the intestine—by the islets of Langerhansthat occur in islet patches throughout the pancreas.
Pancreatic juice is secreted most abundantly inresponse to the presence of chyme in the upper portionsof the small intestine, and the characteristics ofthe pancreatic juice are determined to some extent bythe types of food in the chyme.
Pancreatic secretion contains multiple enzymes fordigesting all of the three major types of food: proteins, carbohydrates, and fats.
Pancreatic juice contains enzymes that:
(1)Along with brush border enzymes, complete the digestion of starch (pancreatic amylase).
(2)Carry out about half of protein digestion (via the action of trypsin, chymotrypsin, carboxypeptidase, and others)
(3)Are totally responsible for fat digestion since the pancreas is essentially the only source of lipases.
(4)Digest nucleic acids (nucleuses).
In addition to enzymes, pancreatic juice also contains large quantitiesof bicarbonate ions(make it very basic with pH about 8), which play an important rolein neutralizing the acidity of the chyme emptied from the stomach into the duodenum and provides the proper environment for intestinal activity and pancreatic digestive enzymes.
The release of pancreatic juice into the duodenum is stimulated by both nerves and local hormones.
Liver and Gallbladder
The liver is the largest gland in the body. It is located under the diaphragm, more to the right side of the body. It has many metabolic and regulatory roles; however, its digestive function if to produce bile that enters the duodenum through the bile duct.
Bile is a yellow-to-green, watery solution containing bile salts, bile pigments (chiefly bilirubin, a breakdown product of hemoglobin), cholesterol, phospholipids and a variety of electrolytes.
The gallbladder is a small, thin-walled green sac in the inferior surface of the liver. When food digestion is not occurring, bile backs up the cystic duct and enters the gallbladder to be stored. While being stored in the gallbladder, bile is concentrated by the removal of water. Later, when fatty food enters the duodenum, a hormonal stimulus prompts the gallbladder to contract and spurt out stored bile, making it available to the duodenum.
Secretion of Bile by the Liver
One of the many functions of the liver is to secrete bile,normally between 600 and 1000 ml/day. Bile serves twoimportant functions:
First, bile plays an important role in fat digestionand absorption, not because of any enzymes in the bilethat cause fat digestion, but because bile acids in thebile do two things:
(1) they help to emulsify the largefat particles of the food into many minute particles,
the surface of which can then be attacked by lipaseenzymes secreted in pancreatic juice, (2) they aidin absorption of the digested fat end products throughthe intestinal mucosal membrane.
Second, bile serves as a means for excretion ofseveral important waste products from the blood.These include especially bilirubin, an end product ofhemoglobin destruction, and excesses of cholesterol.
Bile is not an enzyme, act like a detergent to emulsify, or mechanically break down. Its bile salts emulsify fats by physically breaking large fat globules into smaller ones, thus providing more surface area for the fat-digesting enzymes to work on. Bile is also necessary for absorption of fats and other fat-soluble vitamins [K,D and A] that are absorbed along with them.
- Food Absorption
Thesmall intestine wall has three structures that increase the absorptive surface tremendously; microvilli, villi and circular folds,
- Microvilli are tiny projections of the plasma membrane of the mucosa cells that give the cell surface a fuzzy appearance, sometimes referred to as the brush border.
- Villi are fingerlike projections of the mucosa. Within each villus is arich capillary bed and a modified lymphatic capillary called a lacteal. The digested foodstuffs are absorbed through the mucosal cells into both the capillaries and the lacteal.
- Circular folds are deep folds of both mucosa and submucosa layers.
Absorption of water and of the end products of digestion occurs all along the length of the small intestine. Most substances are absorbed through the intestinal cell plasma membranes by the process of active transport. Then they enters the capillary beds in the villi to be transported in the blood to the liver via the hepatic portal vein. The exception seems to be lipids, or fats, which are absorbed passively by the process of diffusion. Lipid breakdown products enter the capillary bed in the villi and are carried to the liver by both blood and lymphatic fluids.
At the end of the ileum, all that remains is some water, indigestible food materials (plant fibers such as cellulose) and large amount of bacteria. This debris enters the large intestine through the ileocecal valve.
Function of the Ileocecal Valve
A principal function of the ileocecal valve is to preventbackflow of fecal contents from the colon into the small intestine. Theileocecalvalve itself protrudes into the lumen of the cecum andtherefore is forcefully closed when excess pressurebuilds up in the cecum and tries to push cecal contentsbackward against the valve lips. The valve usually canresist reverse pressure of at least 50 to 60 centimetersof water.
In addition, the wall of the ileum for several centimetersimmediately upstream from the ileocecalvalve has a thickened circular muscle called the ileocecalphincter.This sphincter normally remains mildlyconstricted and slows emptying of ileal contents intothe cecum. Resistance to emptying at the ileocecal valve prolongsthe stay of chyme in the ileum and therebyfacilitates absorption. Normally, only 1500 to 2000 millilitersof chyme empty into the cecum each day.
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