What Does the Small Intestine Do?

by admin on January 7, 2010

Your small intestine is a bundled up, continuous tube located in your abdomen that receives food from the stomach at the duodenum, digests and absorbs food in the jejunum, and deposits food into the large intestine at the ileocecal valve. Stretched out, the small intestine of the average adult measures 5 meters in length and about 3 centimeters in diameter. Although, it is much longer than the large intestine, it has a much narrower diameter, thus accounting for its name. The majority of food digestion and nutrient absorption occurs in the small intestine.

The small intestine receives partially digested food from your stomach. At the point when food reaches the duodenum of the small intestine, it has been mixed with stomach acid and manually broken up by the contraction of the stomach walls. The smaller the pieces of food, the better able your body is to absorb the nutrients because many small pieces of food have much greater surface area than fewer large pieces of food. This surface area allows for greater exposure to the microvilli of the small intestine to absorb the maximum possible number of nutrients. Let’s examine how the body deals with carbohydrates, fats and proteins in the small intestine. Microvilli are small, finger like appendages attached to the larger villi that line the walls of your small intestine. Combined, the microvilli and villi increase the surface area inside your small intestine to 500 meters squared, whereas, if the intestinal walls were smooth with no ridges, the surface area would only be 1/2 a meter squared. Think of your microvilli as little nutrient vacuums inside your small intestine.


Lipid molecules (lipid means fat) must be broken down into very small globules in order to be taken up by the microvilli in your small intestine. Pancreatic lipase (“lip” refers to fat and “ase” means the “breaking down of”) is made in the pancreas and secreted through the pancreatic duct into the duodenum, the first portion of the small intestine after the stomach. Lipase is aided by bile from the gallbladder. Bile orients the fatty acids so that their hydrophobic (“water fearing”) heads point towards each other in the center, away from the watery intestinal walls, to allow the pancreatic lipase to break down triglycerides (fancy name for fat molecule) into glycerol and free fatty acids which can be absorbed by the microvilli.


Starchy and sugary foods consist mostly of carbohydrates. Carbohydrates are long strings of sugar molecules. Pancreatic amylase breaks down starch molecules into oligosaccharides. Then, sucrase, lactase and maltase (remember “ase” means the “breaking down of” and sucrose, lactose and maltose are different types of sugar molecules), break down the smaller components into molecules that can be absorbed and used in the body. It is interesting to note that lactase is absent in many adults; thus rendering them “lactose intolerant”. Lactose is the sugar in milk. Without lactase, our intestines are unable to digest milk products, resulting in a gaseous, upset, sick feeling, with diarrhea, and flatulence.


Protein digestion begins in the stomach with chemical and mechanical breakdown into smaller protein pieces and polypeptide chains. A protein is a long peptide chain (poly means “many”). Polypeptides are long chains of amino acids. Amino acids are absorbed by the microvilli of the small intestine. The pancreas secretes trypsin and chymotrypsin into the duodenum of the small intestine to break down proteins into amino acids.

How Does Absorption Happen?

Within the microvilli of the small intestine are networks of capillaries and lymph vessels called lacteals. Capillaries are the smallest components of your blood vessels, the site at which your blood exchanges nutrients, oxygen and waste products with your body’s tissues. Simply put, nutrients diffuse into your blood vessels through chemical and electrical gradients: in a very short, very oversimplified summary, substances will move from an area of high concentration to an area of low concentration and the cells in our tissues facilitate this by changing their chemical concentrations and electrical charges. Fat molecules are taken up by lacteals; little balloons of lymph tissue that merge with other lacteals to form lymphatic vessels responsible for circulating lymph fluid. Lacteals make it into our bloodstream for distribution to our tissues via the subclavian vein (sub = under and clavian = clavicle). Fats in the bloodstream are called chylomicrons.

Not all digested matter from our food makes it to the bloodstream. Whatever is left after our small intestine sucks out all of the nutrients passes onto our large intestine. Here, water and salt are reclaimed for the body, the waste is concentrated and comes out our anus as stool. What I find really interesting is that between the mouth and the anus is one, long continuous, open ended tube. What goes in, must come out. At risk of being too gross, I will mention that the color of your stool can be indicative of a number of illnesses of the pancreas, the liver, the gallbladder and more; since the color of stool is dependent both upon what we eat and what we use to digest food. for example, grey or whitish stool can indicate a problem with your liver or gallbladder. What can your poo tell you? :)

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