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In
Biology the is the part of the
Gastrointestinal Tract (gut) between the
Stomach and the
Large Intestine and includes the
Duodenum ,
Jejunum , and
Ileum . It is where the vast majority of digestion takes place.
In
Human s over 5 years old it is approximately 7 m (21 ft) long and can vary from 4-7 m (13.12-22.97 ft).
It is divided into three structural parts:
- 26 cm (9.84 in) in length
- 2.5 m (8.2 ft)
- 3.5 m (11.5 ft)
Although the small intestine is much longer than the
Large Intestine (typically 4-5 times longer), it is referred to as such due to its comparatively smaller diameter. On average, the diameter of the small intestine of an adult human measures approximately 2.5-3cm, and the large intestine measures about 7.6 cm in diameter.
Food from the stomach is allowed into the duodenum by a muscle called the pylorus, or
Pyloric Sphincter , and is then pushed through the small intestine by a process of muscular-wavelike contractions called
Peristalsis .
The small intestine is the site where most of the nutrients from ingested food are absorbed and is covered in wrinkles or folds called
Plicae Circulares . These are considered permanent features in the wall of the organ. They are distinct from
Rugae which are considered non-permanent or temporary allowing for distention and contraction. From the plicae circulares project microscopic finger-like pieces of tissue called
Villi . The small intestine is lined with simple columnar epithelial tissue. The epithelial cells also have finger-like projections known as microvilli. The function of the plicae circulares, the villi and the microvilli is to increase the amount of surface area available for secretion of enzymes and absorption of nutrients.
The purpose of these
Wrinkles and projections is to increase surface area for absorption of nutrients. Each villus is covered in
Microvilli , which increase the surface area manyfold. Each villus contains a
Lacteal and capillaries. The lacteal absorbs the digested
Fat into the
Lymphatic System which will eventually drain into the
Circulatory System . The capillaries absorb all other digested
Nutrient s.
The surface of the cells on the microvilli are covered with a
Brush Border of proteins which helps to catch a molecule-thin layer of water within itself. This layer, called the "unstirred water layer," has a number of functions in absorption of nutrients.
Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:
The digestion of proteins into peptides and amino acids principally occurs in the stomach but some also occurs in the small intestine. The small intestine is where the most chemical digestion takes place:
- peptides are degraded into Amino Acid s. Chemical break down begins in the stomach and is further broken down in the small intestine. Proteolytic enzymes, Trypsin and Chymotrypsin , which are secreted by the pancreas cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the end amino acid products.
- Lipid s (fats) are degraded into Fatty Acid s and Glycerol . Lipid digestion is the sole responsibility of the small intestine. Pancreatic lipase is secreted here. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase preforms its job with the help of bile salts. Bile salts attach to triglycerides which aids in making them easier for pancreatic lipase to.
- Carbohydrate s are degraded into simple sugars (e.g., Glucose ). In the small intestine pancreatic amylase breaks down carbohydrates into oligosaccharides. Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase which further break down oligosaccharides. Other brush border enzymes are maltase, sucrase and lactase.
The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences.
The parts of the intestine are as follows:
-
- Solomon et al (2002) Biology Sixth Edition, Brooks-Cole/Thomson Learning ISBN 0-03-033503-5
- Townsend et al (2004) Sabiston Textbook of Surgery, Elsevier ISBN 0-7216-0409-9
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