Small Intestine

Submitted by Thiruvelan on 19th Nov 2011 | Last updated 4 August 2014

What is Small Intestine? Small intestine is a tubular organ, which absorbs most of the nutrients from the food that we are in need.

Small intestine – Digestive System

The small intestine is the gastrointestinal tract followed by stomach and then large intestine. The small intestine in a well developed human measure 6 meters (19 feet) long and 2.5 to 3 cm diameter. When chyme (food from stomach) reaches the cavity of the small intestine, two types of contraction starts; segmental contraction that chops, mix and roll the chyme and peristaltic contraction that slowly push food towards the large intestine.

Small Intestine

When there is no chyme in the cavity of the small intestine, housekeeping contraction starts from the stomach to the entire small intestine to sweep debris and make it clean. These contractions are signal by nervous system and moderate by gastrointestinal hormones.

Villi

Small intestinal tract contains numerous fingers like protrusions of approximately 0.5 to 1mm length called Villi (singular: Villus). The surface of villus has numerous hairs like protrusions called as Microvilli.

  • Villi and microvilli are performing the duty of nutrient absorption.
  • Intestinal tracts with villi and microvilli increases the surface area of the intestinal wall by 30 to 60 folds, this increases the nutrient absorption area of the tract.
  • Digestive nutrients such as sugars and amino acids enter the villi by diffusion (transfer from a region of higher concentration to lower concentration).

It absorbs nutrients are then carried by the circulating blood.

Intestine Villi

The small intestine is dividing into three parts; they are Duodenum, Jejunum, and Ileum.

Duodenum

Duodenum is the first part of the small intestine; it is a hollow ‘c-shaped’ tube of about 10 to 12 inches long where iron absorption takes place. Bile and pancreatic juice are entering in the duodenum around its midway. The duodenum absorbs glucose (from carbohydrates) and amino acids (from proteins). New studies showed that around 80% of obese type 2 diabetic undergoes gastric bypass surgery (means bypassing the duodenum) has cured from diabetes.

  • Duodenum has Brunner's gland which secret’s mucus helps for smooth travel of chyme.
  • Duodenum regulates the rate of stomach emptying through hormones Secretin and Cholecystokinin, which are releasing with respect to acidity and fatty stimuli in the chyme.
  • Once gastric chyme reach duodenum, the liver, & gall bladder release bile. In addition, the pancreas releases sodium bicarbonate (neutralize acid chyme from the stomach) and digestive enzymes (trypsin, lipase and amylase) that break down fats, proteins and carbohydrates.

Jejunum

Jejunum is the middle part of the small intestine of length 2.5 meters where absorption of amino acids (from proteins), glucose (from carbohydrates), vitamins, and minerals takes place. The process of peristalsis pushes the food in the jejunum. The pH in the jejunum is maintaining at neutral or slightly alkaline.

Ileum

Ileum is the final part of the small intestine of length about two to four meters. The function of the ileum is to absorb vitamin B12, bile salts, and those digestion products that are not absorbed by the jejunum.