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The Human Body:
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Small Intestine
The small intestine has 3 parts: duodenum, jejunum & ileum and the total length is approximately 5 meters. Its main functions are Digestion, Absorption, Mixing and moving the chyme forward. Its major function Motility ,Secretion (secretions that enter the lumen from liver (bile) and pancreas (pancreatic juice) and secretions of the intestinal cells), Digestion and Absorption.
Motility is such that part of peristaltic movement can cause mixing of content. There is local intermittent constrictive contraction every few centimeters and is either regularly or irregularly spaced.
It occurs mainly in small intestines, at about 12/min in duodenum & proximal jejunum; 8-9/min in terminal ileum and lasts for 5-30 seconds
The main digestion occurs in duodenum whereby the Pancreatic amylase breaks carbohydrate to disaccharides and the enzymes on enterocyte breaks it further to monosaccharides. Glucose is the major product of carbohydrate digestion. However,
Plant cell wall (cellulose) is indigestible.
Absorption is facilitated with the structure of the small intestine. There is a need to increase the surface area to increase absorption. Therefore, on the wall there are folds on the luminal surface (also called ‘folds of Kerckring’, plicae circulares) diagram 2.5. On them are villi. Furthermore, on the villi, there are microvilli forming a brush border. This creates a total surface area for absorption in small intestine to be about 250-300 m2.
Diagram 2.5 Lateral view of a section of the small intestine and an enlarged view of a villi
The major mechanisms of absorption of nutrients are Diffusion-Simple diffusion where the monoglycerides and fatty acids can easily diffuse across into the blood stream from the lumen. Another is
Facilitated diffusion used by fructose and amino acids. Another mechanism is Active transport where the Primary transport is of sodium and the Secondary are glucose, amino acids and peptides.
The absorption of peptides and amino acids are absorbed in the form of free amino acids, di-peptides, tri-peptides. The energy of absorption from sodium co-transport system allows the secondary active transport to take place. The movement of Na down the electrochemical gradient provides energy to bring in amino acids and peptides. Some amino acids brought in by facilitated diffusion.
For NaCl, Na enters in 3 ways. About 30% in co-transport systemand another 30% by a neutral Na+-Cl- co-transport system. The remainder goes passively down an electrochemical gradient. Cl- flows passively down electrochemical gradient established by transport of Na.
The absorption of Vitamins also occurs in a few ways. Fat soluble vitamins (A,D,E,K) become part of micelles and absorbed with other lipids. While water soluble vitamins (B complex [B1-3,5,6,9,12 , choline, inositol, biotin], C) are absorbed by facilitated transport or Na dependent transport system. Vitamin B12 on the other hand, is bound to intrinsic factor (from stomach) and its absorption can occur only after the complex binds to receptors on enterocytes in terminal ileum. In the absence of intrinsic factor, minimum amounts of B12 can be absorbed by diffusion.
For fat digestion & absorption. It occurs in a different manner. Triglycerides (TG) are the most abundant lipids which are digested by lipases mostly in the small intestine. Bile salts and lecithin (in bile) are important to emulsify TG first then hydrolyzed into monoglycerides (MG) and fatty acids (FA). Then,
lipids can be absorbed by simple diffusion. The Short-chain fatty acids directly diffuse across the villi into the capillaries. Whereas the Long-chain fatty acids and monoglycerides (MG) requires bile to form micelles.
FA and MG diffuse through the cell, leaving the bile salts behind, to be reused. Thus, bile salts serves a ferrying function.
The absorption of water is passive, iso-osmotic absorption and the active reabsorption of electrolytes and nutrients creates osmotic gradient. In the duodenum , water flows into lumen due to hyper-osmotic chyme. In the jejunum & ileum, water flows out due to reabsorption of NaCl. The source of the water are mainly from ingested water and water in saliva, gastric, biliary, pancreatic secretions.
Calcium is regulated to maintain the Ca+ balance in the body. About 25-80% of daily intake (~1000 mg) is absorbed and absorption occurs via a membrane-bound carrier that is activated by vitamin D. Iron absorption too is necessary to maintain normal iron balance. About 10% of daily ingested iron (15-25 mg) absorbed. They can be absorbed either as heme (haem) or free iron. Ferrous iron better absorbed than ferric iron. The stomach acid breaks down insoluble iron complexes and facilitates iron absorption. Moreover, Ascorbic acid promotes absorption by reducing ferric iron to ferrous iron.
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