The small intestine is the grand central of the digestive system. Most of the real digestive process happens here.
Most of the protein, fat and carbohydrate breakdown happens within the small intestine. Most of the nutrient absorption also happens here. It is roughly 22 feet long, muscular tube that sits between the stomach and the large intestine.
The small intestine connects to stomach through a muscle called the pylorus. This connection is the pyloric valve. Whenever small intestine is not busy with digestion, stomach releases chyme through the pyloric valve.
Small intestine works with liver, pancreas and gallbladder for digestion. There are digestive enzymes that secrete within its wall. But the digestive enzymes from liver, gallbladder and pancreas are also delivered here to help with digestion.
The liver sends bile to the gallbladder through the hepatic ducts for storage and concentration. Pancreas delivers pancreatic enzymes into the small intestine through the pancreatic duct. The gallbladder is connected to the pancreatic duct through the cystic duct and delivers concentrated bile.
The small intestine has three major parts, duodenum, jejunum and ileum. Duodenum is a Latin word and it means 12 fingers. It is about 10 inches long. Jejunum is also a Latin word and it mean empty at death. It is about 6 to 8 feet long. And the ileum is the last part, which could be up to 11 and a half feet long.
Chyme is partially digested stomach content. Stomach squirts chyme into the duodenum - the first part of the small intestine. The liver and gallbladder deliver bile through ducts to the duodenum. Pancreas delivers a very complex mix of enzymes through the pancreatic duct into the duodenum.
Pancreatic enzymes include alkalis such as bicarbonates, which neutralize the stomach acid. Besides alkalis, pancreas secretes 15 different enzymes that work on three major food components carbohydrates, fats and proteins.
The enzymes in the small intestine carry out two stage enzymatic breakdown of the nutrients. Complex nutrient molecules are first decomposed into less complex molecules and later less complex molecules are broken into most basic forms.
Bile salts emulsify large fat droplets and create an emulsion of tiny fat droplets. Thus increasing the surface area for enzyme action.
Pancreatic amylase converts long chain carbohydrates like starch into disaccharides (two molecule sugars) - mainly maltose sugar. Pancreatic lipase works on small fat droplets and converts triglycerides into monoglycerides and fatty acids. The pancreatic protease enzyme breaks down protein into short chain peptide and amino acids.
Remaining of the small intestine - jejunum and ileum - is the site for the last breakdown of the food and its absorption into the blood and lymphatic fluids. The bile and the pancreatic juices continue to work within jejunum and ileum, although intestinal wall also releases few enzymes.
The enzymes work within the lining cells and on their surface. These enzymes include lactase and maltase, which work on disaccharide carbohydrates and convert them into simple sugars like glucose and galactose. Intestinal peptidases convert short peptide chains into their sub-units amino acids.
This way, finally carbohydrates turn into simple sugars, the proteins turn into amino acids and fats turn to monoglycerides and fatty acids. These are the simplest form of molecules, easily absorbed into the blood stream.
Finger-like villi of the small intestine lining give a large area for absorption of nutrients resulting from digestion. Through the surface of the villi, nutrients enter into the blood stream.