Small Intestine Functional Anatomy
- The small intestine is part of Gl tract extending between the pyloric sphincter of the stomach and the ileocecal valve, which opens into the large intestine. It is called the small intestine because of its small diameter compared to that of the large intestine. But it is longer than the large intestine. Its length is about 6 meters.
- The functional importance of the small intestine is absorption. Maximum absorption of digested food products takes place in the small intestine.
Read And Learn More: Medical Physiology Notes
Table of Contents
The small intestine consists of three portions:
- The proximal part known as the duodenum
- The middle part known as the jejunum
- The distal part is known as the ileum.
The wall of the small intestine has all four layers as in the stomach.
Intestinal Villi And Glands Of Small Intestine
1. Intestinal Villi:
- The mucous membrane of the small intestine is covered by minute projections called villi. The height of the villi is about 1 mm, and the diameter is less than 1 mm.
- The villi are lined by columnar cells, which are called enterocytes. Each enterocyte gives rise to hair-like projections called microvilli. The villi and microvilli increase the surface area of the mucous membrane by many folds.
- Within each villus, there is a central channel called lacteal, The lacteal opens into lymphatic vessels. It contains blood vessels also.
2. Crypts Of Lieberkuhn Or Glands:
- The crypts of Ueberkuhn or intestinal glands are simple tubular glands of the intestine. The intestinal glands do not penetrate the muscularis mucosa of the intestinal wall but open into the lumen of the intestine between the villi. The intestinal glands are lined by columnar cells. The lining of each gland is continuous with the epithelial lining of the villi.
- Epithelial cells lining the intestinal glands undergo division by mitosis at a faster rate. The newly formed cells push the older cells upward over the lining of the villi. The cells which move to villi are called enterocytes. The enterocytes secrete the enzymes. The old enterocytes are continuously shed into the lumen along with enzymes.
Three types of cells are interposed between columnar cells of the glands:
- Argentaffin cells which are otherwise known as enterochromaffin cells. These cells secrete an intrinsic factor that is essential for the absorption of vitamin B12
- Goblet cells that secrete mucus
- Paneth cells which secrete the cytokines called defensins.
3. Brunner’S Glands: In addition to intestinal glands, the first part of the duodenum contains some mucus glands, which are called Brunner’s glands. These glands penetrate the muscularis mucosa and extend up to the submucus coat of the intestinal wall. The Brunner’s glands open into the lumen of the intestine directly. Brunner’s gland secretes mucus and traces of enzymes.
Properties And Composition Of Succus Entericus
Secretion from the small intestine is called succus entericus.
Properties Of Succus Entericus
Volume : 1800 mL/day
Reaction : Alkaline
pH : 8.3
Composition Of Succus Entericus: The succus entericus contains water (99.5%) and solids (0.5%). Solids include organic and inorganic substances. The bicarbonate concentration is slightly high in succus entericus.
Functions Of Succus Entericus
1. Digestive Function: The enzymes of succus entericus act on the partially digested food and convert them into final digestive products. The enzymes are produced and released into succus entericus by enterocytes of the villi.
- Proteolytic Enzymes: The proteolytic enzymes present in succus entericus are the peptidases which are given in Fig. 41-2. The pepti¬dases convert peptides into amino acids.
- Amylolytic Enzymes:
- The carbohydrate splitting enzymes of succus entericus are listed in. Lactase, sucrase, and maltase convert the disaccharides (lactose, sucrose, and maltose) into two molecules of monosaccharides.
- Dextrinase converts dextrin, maltose, and maltriose into glucose. Trehalase or trehalose glucohydrolase causes hydrolysis of trehalose (carbohydrate present in mushrooms and yeast) and converts it into glucose.
- Lipolytic Enzyme: Intestinal lipase acts on triglycerides and converts them into fatty acids.
2. Protective Function:
- The mucus present in the succus entericus protects the intestinal wall from the acid chyme, which enters the intestine from the stomach; thereby it prevents the intestinal ulcer
- Paneth cells of intestinal glands secrete defensins which are antimicrobial peptides.
- These peptides are considered natural peptide anti’ biotics because of their role in killing the phagocytosed bacteria.
3. Activator Function: The enterokinase present in intestinal juice activates trypsinogen into trypsin. Trypsin, in turn activates other enzymes.
4. Hemopoietic Function: The intrinsic factor of Castle, which is present in the intestine, plays an important role in erythropoiesis.
5. Hydrolytic Process: Intestinal juice helps in all the enzymatic reactions of digestion.
Functions Of Small Intestine
- Mechanical Function: The mixing movements of the small intestine help in the thorough mixing of chyme with digestive juices like succus entericus, pancreatic juice, and bile.
- Secretory Function: Small intestine secretes succus entericus, enterokinase, and the Gi hormones.
- Hormonal Function: Small intestine secretes many GI hormones such as secretin, cholecystokinin, etc. These hormones regulate the movement of the GI tract and secretory activities of the small intestine and pancreas.
- Absorptive Functions: The presence of villi and microvilli in small intestinal mucosa increases the surface area of the mucosa. This facilitates the absorptive function of the intestine.
- The digested products of foodstuffs, proteins, carbohydrates, fats, and other nutritive substances such as vitamins, minerals, and water are absorbed mostly in the small intestine. From the lumen of the intestine, these substances pass through the lacteal of villi, cross the mucosa and enter the blood directly or through lymphatics.
Absorption Of Water And Minerals
- In the small intestine, sodium is absorbed actively. It is responsible for the absorption of glucose, amino acids, and other substances by means of sodium co-transport.
- Water moves in or out of the intestinal lumen until the osmotic pressure of intestinal contents becomes equal to that of plasma.
- In the ileum, chloride ion is actively absorbed in exchange for bicarbonate. The significance of this exchange is not known.
- Calcium is actively absorbed mostly in the upper part of the small intestine.
Absorption Of Vitamins: Most of the vitamins are absorbed in the upper part of the small intestine and vitamin B12 is absorbed in the ileum. The absorption of water-soluble vitamins is faster than fat-soluble vitamins.
Regulation Of Secretion Of Succus Entericus
The secretion of succus entericus is regulated by both nervous and hormonal mechanisms.
1. Nervous Regulation:
- Stimulation of parasympathetic nerves causes vasodilatation and increases the secretion of succus entericus. Stimulation of sympathetic nerves causes vasoconstriction and decreases the secretion of succus entericus. But, the role of these nerves in the regulation of intestinal secretion in physiological conditions is uncertain.
- However, the local nervous reflexes play an important role in increasing the secretion of intestinal juice. When chyme enters the small intestine, the mucosa is stimulated by tactile stimuli or irritation. It causes the development of local nervous reflexes, which stimulate the glands of the intestine.
2. Hormonal Regulation: When the chyme enters the small intestine, the intestinal mucosa secretes enterocrinin, secretin, and cholecystokinin which promote the secretion of succus entericus by stimulating the intestinal glands.
Methods Of Collection Of Succus Entericus
1. In Man:
- In men, intestinal juice is collected by using the multi-lumen tube. The multi-lumen tube is inserted through the nose or mouth until the tip of this tube reaches the intestine. A line is marked on the tube.
- The entrance of the tip of the tube into the small intestine is indicated when this line comes near the mouth. The tube has three lumens. To the outer two lumens, small balloons are attached.
- When these balloons are inflated, the intestine is enlarged. Now, the intestinal juice is collected through the middle lumen by means of aspiration.
2. In Animals
- Thiry Loop: A portion of the intestine is separated from the gut by incising at both ends. The cut ends of the main gut are connected and the continuity is re-established. One end of the isolated segment is closed and the other end is brought out through the abdominal wall. It is called the Thiry loop or Thiry fistula.
- Thiry-Vella Loop: Thiry-Vella loop is the modified Thiry loop. In this, a long segment of the intestine is cut and separated from the main gut. Both the ends of this segment are brought out through the abdominal wall, The cut ends of the main gut are joined.
Applied Physiology
1. Malabsorption: Malabsorption is difficulty in the digestion or absorption of nutrients from the small intestine. It may be the failure to absorb either the specific substances such as proteins, carbohydrates, fats and vitamins or some general non-specific substances of food. Malabsorption affects the growth and development of the body. It also causes specific diseases (see below).
2. Malabsorption Syndrome: Malabsorption syndrome is a condition characterized by failure.of digestion and absorption in the small intestine. Malabsorption syndrome is generally caused by Crohn’s disease, tropical sprue, steatorrhea, and celiac disease.
3. Crohn’s Disease Or Enteritis: It is an inflammatory bowel disease (IBD) characterized by inflammation of the small intestine. Usually, it affects the lower part of the small intestine, the ileum. The inflammation causes malabsorption and diarrhea.
- Crohn’s Disease Causes: Crohn’s disease develops because of abnormalities of the immune system. The immune system reacts to a virus or a bacterium resulting in inflammation of the intestine.
- Crohn’s Disease Features:
- Malabsorption of vitamin
- Weight loss
- Abdominal pain
- Diarrhea
- Rectal bleeding, anemia, and fever
- Delayed or stunted growth in children.
4. Tropical Sprue: Tropical sprue is a malabsorption syndrome affecting the residents of or the visitors to tropical areas where the disease is epidemic.
- Tropical Sprue Cause: The cause of this disease is not known and it may be related to infectious organisms.
- Tropical Sprue Features:
- Indigestion
- Diarrhea
- Anorexia and weight loss
- Abdominal and muscle cramps.
5. Steatorrhea: Malabsorption of fat occurs in steatorrhea, a condition caused by a deficiency of pancreatic lipase.
6. Celiac Disease: Celiac disease is an autoimmune disorder characterized by the damage of mucosa and atrophy of villi in the small intestine resulting in impaired digestion and absorption. It is also known as gluten-sensitive enteropathy, celiac sprue, and nontropical sprue.
- Celiac Disease Cause: This disease is caused by gluten. It is a protein present in wheat, oats, rye, barley, and other grains. Gluten is like a poTcn – viduals with celiac disease, because it damages the intestine severely.
- Celiac Disease Features:
- Ularmea
- Steatorrhea
- Abdominal pain
- Weight loss
- Irritability
- Depression.
Leave a Reply