THE SMALL INTESTINE

STRUCTURE OF THE SMALL INTESTINE 

Exocrine secretions from the salivary glands, the stomach, and the pancreas and liver, plus secretions of the small intestine itself, are mixed with food for digestion and absorption. Several anatomic features of the small intestine amplify the surface area for absorption, including transverse folds in the mucosa, the arrangement of the mucosa into villi, and the presence of microvilli on the enterocytes that line the small intestine . 

The area of the intestine absorption surface is 4-5 m2 , i.е. 2-3 times more than the surface of the human body.

There are two major types of motility that occur in the small intestine during the fed state:

 

1. Segmentation contractions produce a string of segments that constantly form and reform. The main function of segmentation contractions are mixing and distribution of the luminal contents. Mixing of food stuffs with digestive secretions and enzymes. Distribution of the luminal contents around the mucosa for absorption. 

2. Peristalsis consists of a wave of contractions that moves a bolus aborally. The function of peristalsis is propulsion of luminal material. Peristalsis is a reflex, and the main stimulus is moderate distension of the gut wall.

MOTILITY of small intestine 

Fluid absorption 

The intestine receives a large daily fluid load that must be absorbed. A typical daily fluid load to the jejunum is 7–10 L per day, consisting of about 1–2 L each of dietary water, saliva, gastric juice, pancreatic juice, and intestinal secretion, and about 0.5 L of bile. The small intestine reabsorbs about 6–8 L per day of fluid by isosmotic transport, with a maximum possible absorption rate of approximately 12 L per day.

Fluid secretion Both the small intestine and the large intestine secrete fluid from the crypt cells. Secretion is necessary for lubrication. Fluid secretion also provides a source of Na+ for coupling to nutrient absorption (sugar and amino acid). Antibodies secreted in the area of the intestinal crypts also require fluid secretions to reach the lumen in the gut. 

The mechanism of intestinal fluid secretion is clinically important because it is activated by a number of bacterial enterotoxins that cause secretory diarrhea (Enterotoxic Escherichia coli and Vibrio cholera).

Nutrient digestion and absorption Most nutrients are large molecules that must be broken down into smaller molecules that can be absorbed. Digestion is the chemical breakdown of food by enzymes and can occur at three sites :

1. Luminal digestion occurs within the lumen of the gastrointestinal tract and is mediated by enzymes from the salivary glands, the stomach, and the pancreas. 

2. Membrane digestion is the action of enzymes fixed to the brush-border membrane of enterocytes. Such enzymes are synthesized by enterocytes and inserted into the membrane. 

3. Intracellular digestion is mediated by cytoplasmic enzymes within enterocytes.

Nutrient absorption occurs in the small intestine. The proximal small intestine absorbs almost all of the iron and Ca2+ that is assimilated from food. Carbohydrates can be absorbed along all parts of the small intestine, but their absorption is usually completed in the proximal small intestine. Absorption of other nutrients (including protein, fat, salts, and water) is spread more uniformly along the small intestine. Bile acids and vitamin B12 are absorbed specifically in the distal ileum.