Digestion In The Small Intestine
After being churned and mixed with digestive juices in the stomach, food chyme moves slowly into the folds of the small intestine through the pyloric sphincter or valve. The small intestine (or small bowel) is the longest section of the digestive tract (approx 17 feet) and is divided into three segments: the duodenum, jejunum and ileum, each of which performs different digestive functions. Chyme from the stomach is propelled through the small intestine by a process of muscular contractions called peristalsis.
Functions Of The Small Intestine
The small intestine is where most chemical digestion takes place; peptides (complex chains of protein molecules) are broken down into amino acids; lipids (fats) are broken down into fatty acids and glycerol; and carbohydrates are broken down into simple sugars like glucose. To accomplish this, chyme is mixed with additional digestive juices including bile from the liver and pancreatic juice and amylase from the pancreas, as well as other intestinal enzymes such as maltase, lactase and sucrase to break down the chyme and assist in nutrient absorption. Absorbed nutrients flow in the bloodstream to the liver where they are further metabolised and then either stored or or sent to cells in other parts of the body. In total, food typically takes 4-5 hours to transit all three sections of the small intestine. Along the way its consistency changes from porridge (chyme) to a thin watery mixture.
Why Is It Called The Small Intestine?
Since the small intestine (about 17 feet) is much longer than the large intestine (about 5 feet) people often wonder why it is referred to as “small”. The answer lies in its diameter (3-4 cm), which is about 3 times narrower than the “large” intestine.
Absorption Of Nutrients In The Small Intestine
It is inside the small intestine that we absorb most of the nutrients in our food. Although the small intestine has a relatively small diameter, the intestinal walls are covered in wrinkles called rugae, which are themselves covered in millions of finger-like projections called villi, which are themselves studded with millions of smaller projections called microvilli. This provides a surface-area of about the size of a tennis court for nutrient absorption. Inside each villus is a series of lymph vessels (lacteals) and blood vessels (capillaries). The lacteal lymph vessel absorbs digested fat into the lymphatic system which eventually drains into the bloodstream. The blood vessels receive other nutrients and transport them via the hepatic portal vein to the liver. Here the blood is filtered, toxins are removed and the nutrients are processed. An important task performed by the liver in this context is the regulation of blood glucose levels to provide sufficient energy for the body. Excess glucose is converted in the liver to glycogen in response to the hormone insulin, and stored. Then, when blood glucose levels begin to drop, (eg. between meals), the glycogen is re-converted to glucose in response to messages conveyed by the hormone glucagon.
Digestive Function Of The Duodenum
The duodenum continues the process of food breakdown. Its name stems from the Latin “duodenum digitorum”, meaning twelve fingers or inches. It is roughly horse-shoe-shaped. Anatomically, it is is sub-divided into four segments: the superior, descending, horizontal and ascending duodenum. Inside the duodenal tube, chyme is mixed with fluids from the gallbladder (bile) and pancreas (pancreatic juice). Bile breaks down fat particles into smaller droplets, while pancreatic juice contains enzymes that convert fats into fatty acids and glycerol, plus sodium bicarbonate to neutralize stomach acid. Note: during gastric bypass to reduce severe obesity, the duodenum is bypassed to reduce the amount of nutrients and calories that can be absorbed. See Bariatric Surgery Guide.
Digestive Function Of The Jejunum
Roughly 4-7 feet in length, the jejunum is where chemical breakdown of the food chyme is completed. Pancreatic enzymes, along with enzymes produced by the jejunum wall, finalize the food digestion process. The term jejunum stems from the Latin jejunus, meaning empty. Note: in some gastric bypass operations, part of the jejunum is also “bypassed” to reduce calorie intake.
Digestive Function Of The Ileum
Roughly 7-5 feet in length, the ileum is the final section of the small intestine, linked to the large intestine by the ileocecal valve. The main function of the ileum is to absorb nutrients. Bile is also absorbed here and returns to the liver through blood vessels in the intestinal walls.
The unabsorbed watery remains of the food chyme now pass into the large intestine for water-removal and final processing, before being expelled from the body.