The small intestine is involved in the digestion, absorption and transport of food. Food breaks down further in the small intestine after passing through the stomach, and the absorption of vitamins, minerals, carbohydrates, protein, and fat takes place. The small intestine is the largest part of the gastrointestinal tract and is composed of the duodenum which is about one foot long, jejunum (5-8 feet long), and ileum (16-20 feet long).
The Digestive Process
The duodenum is the major portion of the small intestine where enzyme secretion takes place. The small intestine secretes sucrase (breaks sucrose into glucose and fructose); maltase (breaks maltose into glucose); and lactase (breaks lactose into glucose and galactose – lactase is missing in many people). It also secretes petidase to split peptides (from protein) into amino acids, and lipase to break down fat into glycerol and fatty acids.
The duodenum receives bile from the liver and gallbladder, to decrease the surface tension between the large fat globules and water, and break them into smaller globules that can be acted upon by lipase. Lipase, amylase, trypsin, chymotrypsin and sodium bicarbonate are received from the pancreas upon hormonal signals from pancreozymin and other hormones (produced in the small intestine) and neural signals from the vagus nerve.
Epithelial cells in the small intestine secrete over half a gallon of a neutral fluid daily to supply a watery substance to mix with the chyme and provide a substance to aid in electrolyte and vitamin absorption through the villi.
Brunner's glands in the duodenum secrete mucus in response to secretin, vagal stimulation, and direct stimulation of food in the small intestine. This mucus protects the duodenal wall from the digestive juices. Goblet cells in the mucosa also produce mucus.
Approximately 50% of the carbohydrate digestion is performed by amylase from the pancreas, 40% by amylase from saliva, and 10% by intestinal amylase. About 95% of the fat digestion is performed by pancreatic lipase and 5% by intestinal and stomach lipase.
Almost 90% of our daily fluid intake is absorbed in the small intestine. The small intestine is covered with villi and microvilli, which increase by a factor of 600 the surface area of the intestinal wall exposed to chyme. The increased surface area makes the small intestine very efficient in absorption. Capillaries in the villi absorb amino acids, glucose, fructose, and galactose while lacteals absorb fatty acid and glycerol to travel through the lymphatic vessels.
As chyme enters the small intestine, its acidity causes secretion of the hormone secretin, which signals the pancreas to secrete alkaline juices to neutralize the chyme. The chyme also initiates a type of small intestine contraction known as segmentation which helps to mix and chop the chyme and propel it along. These contractions occur about once every 5 seconds in the duodenum but only half as fast in the ileum. Peristaltic waves also occur and aid in chyme propulsion. It takes about 2-3 minutes for the chyme to advance 1 inch; food will normally remain in the small intestine for between 3 and 10 hours.
Harmful irritants reaching the small intestine can initiate what is called a peristaltic rush which can empty the entire small intestine into the colon within a few minutes. Food that you are allergic to can also do this and lead to diarrhea due to the lack of time for fluid absorption.
Acidophilus is essential in the absorption of nutrients from our food. It colonizes the walls of the small intestine, secretes substances that help maintain a balance of intestinal microflora (by creating a favorable environment for the growth of beneficial flora), and helps keep the intestinal wall functioning properly by keeping clear the spaces between the microvilli so that they can function efficiently. Acidophilus also promotes proper peristalsis – the normal movement of food through the small intestine. The beneficial flora dramatically influence metabolism and overall health.
Garlic will kill harmful intestinal bacteria and promote the growth of beneficial acid-producing bacteria.
If you have eaten too many irritating foods and suspect your villi to be "plugged" and your absorption decreased, two things may help: Take a mixture of comfrey leaves and papain (the milky white sap from the stem of the papaya, also plentiful in green papayas). The papain will dissolve the mucus while the comfrey will heal the membranes. Do this for one week, once daily with one-quarter of a cup of the mixture.
Charcoal tablets and anise tea both are effective in relieving gas.