Small intestine
In
biology the small
intestine is the part of the
gastrointestinal tract between the
stomach and the large intestine. In
humans over 5 years old it is 17 to 21 feet long. It is divided into three structural parts: duodenum,
jejunum and
ileum. 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 contractions called
peristalsis.
The small intestine is the site where most of the nutrients from ingested food are absorbed.
Encyclopedia
In
biology the
small intestine is the part of the
gastrointestinal tract between the
stomach and the large intestine. In
humans over 5 years old it is 17 to 21 feet long. It is divided into three structural parts:
duodenum, jejunum and ileum. 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 contractions called
peristalsis.
The small intestine is the site where most of the nutrients from ingested food are absorbed. It is covered in wrinkles called rugae, from which project microscopic finger-like pieces of tissue called
villi. The purpose of these wrinkles and projections is to increase surface area for absorption of nutrients. Furthermore, each villus is covered in
microvilli, which again 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
nutrients. As well, the surface of the cells on the microvilli are covered wih 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.
Although the small intestine is much longer than the large intestine , it is referred to as such due to its comparatively smaller diameter. On average, the diameter of the large intestine of an adult human measures approximately 3 times the diameter of the small intestine. Upon viewing accurate diagrams of the digestive system, non-medically trained personnel often initially express bemusement as to why the obviously longer portion of the digestive system is referred to as the small intestine.
Functions
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 acids;
lipids are degraded into
fatty acids and
glycerol; and
carbohydrates are degraded into simple sugars .
Small Intestine Disorders
References
Solomon et. al Biology Sixth Edition, Brooks-Cole/Thomson Learning ISBN 0-03-033503-5
See also
External links
