When food reaches the small intestine and has been reduced by the action of chewing and digestion, it becomes "chyme". Chyme entering into the duodenum from the stomach is very acidic and contains a concentration of hydrochloric acid and enzymes which are required to further reduce the larger molecules so that absorption becomes easier and faster.
The small intestine secretions contain bicarbonate, an alkaline substance which makes for a neutralization of the stomach acid. Special cells in the intestinal wall secrete these substances and are combined with juices flowing from the ball bladder (bile) and pancreatic juices which flow by way of the pancreatic duct into the duodenum.
Bile salts which are produced in the liver and stored in the gall bladder act like a detergent to emulsify the fatty acids and glycerides, making very small particles to be absorbed into the intestinal walls. By way of hormonal secretions, the small intestine is able to control the digestive processes.
How Absorption is Facilitated
The marvelous construction of the small intestine allows for efficient nutrient absorption. A large inner surface area is provided by the accordion-like folds of the intestinal wall. Lining the wall are finger-like structures called villi.
Villi project into the interior from all directions and the average adult has a small intestinal area of approximately 200 square feet. The small molecular particles of the broken down food are able to pass into the cells lining the villi and are taken up by the tiny blood capillaries and eventually find their way into the hepatic portal vein where they are carried to the liver and reduced even further. From the liver, digested food substances are delivered to cells in the body that support life-sustaining body systems.