Small intestine is the part of gastrointestinal (GI) tract, extending between the pyloric sphincter of stomach and ileocecal valve, which opens into large intestine. It is called small intestine because of its small diameter, compared to that of the large intestine. But it is longer than large intestine. Its length is about 6 meters.
Small intestine consists of three portions-
- Proximal part known as duodenum
- Middle part known as jejunum
- Distal part known as ileum.
Mucous membrane of small intestine is covered by minute projections called villi. The height of villi is about 1 mm and the diameter are less than 1 mm.
Villi are lined by columnar cells, which are called enterocytes. Each enterocyte gives rise to hair-like projections called microvilli. Villi and microvilli increase the surface area of mucous membrane by many folds.
Intestinal Glands (Crypts of Lieberkuhn)-
Crypts of Lieberkühn or intestinal glands are simple tubular glands of intestine. Intestinal glands do not penetrate the muscularis mucosa of the intestinal wall, but open into the lumen of intestine between the villi.
These cells which move to villi are called enterocytes. Enterocytes secrete the enzymes. Old enterocytes are continuously shed into lumen along with enzymes.
Types of cells interposed between columnar cells of intestinal glands-
- Argentaffin cells or enterochromaffin cells, which secrete intrinsic factor of Castle
- Goblet cells, which secrete mucus
- Paneth cells, which secrete the cytokines called defensins.
In addition to intestinal glands, the first part of duodenum contains some mucus glands, which are called Brunner glands. These glands penetrate muscularis mucosa and extend up to the submucus coat of the intestinal wall. Brunner glands open into the lumen of intestine directly. Brunner gland secretes mucus and traces of enzymes.
The superior mesenteric artery supplies the whole of the small intestine, and venous drainage is by the superior mesenteric vein which joins other veins to form the portal vein.
Innervation of the small intestine is both sympathetic and parasympathetic.
About 1500 ml of intestinal juice are secreted daily by the glands of the small intestine. It consists of:
- Mineral salts
- Enzyme- enterokinase (enteropeptidases).
The pH of intestinal juice is usually between 7.8 and 8.0.
Functions of the small intestine-
The functions are-
- Onward movement of its contents which is produced by peristalsis
- Secretion of intestinal juice
- Completion of chemical digestion of carbohydrates, protein and fats in the enterocytes of the villi
- Protection against infection by microbes that have survived the antimicrobial action of the hydrochloric acid in the stomach, by the solitary lymph follicles and aggregated lymph follicles
- Secretion of the hormones cholecystokinin (CCK) and secretin
- Absorption of nutrients.
|Sucrase||Sucrose||Fructose and glucose|
|Maltase||Maltose and maltriose||Glucose|
|Lactase||Lactose||Galactose and glucose|
|Dextrinase||Dextrin, maltose and maltriose||Glucose|
|Intestinal lipase||Triglycerides||Fatty acids|
Chemical digestion in the small intestine-
When acid chyme passes into the small intestine it is mixed with pancreatic juice, bile and intestinal juice, and is in contact with the enterocytes of the villi. In the small intestine the digestion of all the nutrients is completed:
- Carbohydrates are broken down to monosaccharides
- Proteins are broken down to amino acids
- Fats are broken down to fatty acids and glycerol.
Pancreatic juice enters the duodenum at the hepatopancreatic ampulla and consists of:
- Mineral salts
- Inactive enzyme precursors –
Pancreatic juice is alkaline (pH 8) because it contains significant quantities of bicarbonate ions, which are alkaline in solution.
- Digestion of proteins-
Trypsinogen and Chymotrypsinogen are inactive enzyme precursors activated by enterokinase (Enteropeptidase), an enzyme in the microvilli, which converts them into the active proteolytic enzyme’s trypsin and chymotrypsin. These enzymes convert polypeptides to tripeptides, dipeptides and amino acids. It is important that they are produced as inactive precursors and are activated only upon arrival in the duodenum, otherwise they would digest the pancreas.
- Digestion of Carbohydrates–
Pancreatic amylase converts all digestible polysaccharides (starches) not acted upon by salivary amylase to disaccharides.
- Digestion of Fats-
Lipase converts fats to fatty acids and glycerol. To aid the action of lipase, bile salts emulsify fats, i.e. reduce the size of the globules, increasing their surface area.
- Control of Secretion-
The secretion of pancreatic juice is stimulated by secretin and CCK, produced by endocrine cells in the walls of the duodenum. The presence in the duodenum of acid material from the stomach stimulates the production of these hormones.
Bile, secreted by the liver, is unable to enter the duodenum when the hepatopancreatic sphincter is closed therefore it passes from the hepatic duct along the cystic duct to the gall bladder where it is stored.
Bile has a pH of 8 and between 500 and 1000 ml are secreted daily. It consists of
- Mineral salts
- Bile salts
- Bile pigments, Mainly bilirubin
- The bile salts, sodium taurocholate and sodium glycocholate, emulsify fats in the small intestine.
- The bile pigment, bilirubin, is a waste product of the breakdown of erythrocytes and is excreted in the bile rather than in the urine because of its low solubility in water. Bilirubin is altered by microbes in the large intestine. Some of the resultant urobilinogen, which is highly water soluble, is reabsorbed and then excreted in the urine, but most is converted to stercobilin and excreted in the faeces.
- Fatty acids are insoluble in water, which makes them very difficult to absorb through the intestinal wall. Bile salts make fatty acids soluble, enabling both these and fat-soluble vitamins (e.g. vitamin K) to be readily absorbed.
- Stercobilin colours and deodorises the faeces.
The principal constituents of intestinal secretions are:
- Mineral salts
- Enzyme- enterokinase (enteropeptidase).
The enzymes involved in completing the chemical digestion of food in the enterocytes of the villi are-
- Sucrase, maltase and lactase.
Chemical digestion associated with enterocytes-
- Alkaline intestinal juice (pH 7.8 to 8.0) assists in raising the pH of the intestinal contents to between 6.5 and 7.5.
- Enterokinase activates pancreatic peptidases such as trypsin which convert some polypeptides to amino acids and some to smaller peptides. The final stage of breakdown to amino acids of all peptides occurs inside the enterocytes.
- Lipase completes the digestion of emulsified fats to fatty acids and glycerol partly in the intestine and partly in the enterocytes.
- Sucrase, maltase and lactase complete the digestion of carbohydrates by converting disaccharides such as sucrose, maltose and lactose to monosaccharides inside the enterocytes.
Absorption of Nutrients-
Absorption of nutrients occurs by two possible processes-
Monosaccharides, amino acids, fatty acids and glycerol diffuse slowly down their concentration gradients into the enterocytes from the intestinal lumen.
- Active transport-
Monosaccharides, amino acids, fatty acids and glycerol may be actively transported into the villi; this is faster than diffusion. Disaccharides, dipeptides and tripeptides are also actively transported into the enterocytes where their digestion is completed before transfer into the capillaries of the villi.