Small intestine functional anatomy

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 meter. Important function of small intestine is absorption. Maximum absorption of digested food products takes place in small intestine.

Small intestine consists of three portions:

1. Proximal part known as duodenum

2. Middle part known as jejunum

3. Distal part known as ileum.

Wall of the small intestine has all the four layers as in stomach.

INTESTINAL VILLI AND GLANDS OF SMALL INTESTINE

INTESTINAL VILLI

Mucous membrane of small intestine is covered by minute projections called villi. The height of villi is about 1 mm and the diameter is 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. Within each villus, there is a central channel called lacteal, which opens into lymphatic vessels. It contains blood vessels also.

CRYPTS OF LIEBERKÜHN OR INTESTINAL GLANDS

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. Intestinal glands are lined by columnar cells. Lining of each gland is continuous with epithelial lining of the villi . Epithelial cells lining the intestinal glands undergo division by mitosis at a faster rate. Newly formed cells push the older cells upward over the lining of 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:

1. Argentaffin cells or enterochromaffin cells, which secrete intrinsic factor of Castle

2. Goblet cells, which secrete mucus

3. Paneth cells, which secrete the cytokines called defensins.

FUNCTIONS OF SUCCUS ENTERICUS

1. DIGESTIVE FUNCTION

Enzymes of succus entericus act on the partially digested food and convert them into final digestive

products. Enzymes are produced and released into succus entericus by enterocytes of the villi.

Proteolytic Enzymes

Proteolytic enzymes present in succus entericus are the peptidases. These

peptidases convert peptides into amino acids.

Amylolytic Enzymes

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

i. Mucus present in the succus entericus protects the intestinal wall from the acid chyme, which

enters the intestine from stomach; thereby it prevents the intestinal ulcer.

ii. Defensins secreted by paneth cells of intestinal glands are the antimicrobial peptides.

These peptides are called natural peptide antibiotics because of their role in killing the phagocytosed

bacteria.

3. ACTIVATOR FUNCTION

Enterokinase present in intestinal juice activates trypsinogen into trypsin. Trypsin, in turn activates other

enzymes.

4. HEMOPOIETIC FUNCTION

Intrinsic factor of Castle present in the intestine plays an important role in erythropoiesis. It is

necessary for the absorption of vitamin B12.

BRUNNER GLANDS

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.

5. HYDROLYTIC PROCESS

Intestinal juice helps in all the enzymatic reactions of digestion.

 

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