Every section of the colon and anus secretes mucous. This serves to lubricate the mucosal layer of the intestine to allow for the easy passage of stool. Although bacterial degradation of waste material occurs in the colon, this is not a secretory process.
Absorption is the process by which the nutrients are transported across the mucosal layer into the blood stream. Each part of the small intestine is anatomically different to absorb specific nutrients. The brush border membrane in each portion of the small intestine has been adapted to facilitate easy passage of the products of digestion that are preferentially absorbed there. The methods by which the molecules are transported vary according to the type of nutrient. Some nutrients are actively transported across the membrane. This requires the build- up of an electrochemical gradient whose energy can be used to transport larger nutrient molecules.
Passive transport is the most common mechanism of transport. This requires the presence of an electrolyte on the other side of the membrane to trade places with the nutrient that desires entry. As the electrolyte enters the lumen of the intestine, the nutrient crosses the barrier and moves into the blood stream. Some passive transport requires the help of a carrier molecule. In the case of fat, micelles carry the products of fat digestion across the border in a passive fashion. Micelles are complex carrier molecules that are made in the liver.
Diffusion is the other method by which nutrients can cross over the barrier. Because of different gradients on either side of the membrane, some molecules can move freely between the lumen and blood stream in a direction from highest to lowest concentration. Water is moved in and out of the lumen in this fashion. The first place that absorption occurs is in the mouth. Because of the enormous amount of blood vessels in the mouth, small molecules and water can diffuse directly into the blood stream at this level. There is minimal absorption in the stomach. The small intestine and colon are responsible for the remainder of absorption. In the small intestine, protein, carbohydrates, fats, vitamins, and minerals are absorbed. In the colon, large amounts of water are reabsorbed so that very little water is eliminated through the stool.
Assimilation is the process whereby the nutrients are delivered to the rest of the cells and organs in the body. Once the nutrients are transported into the blood stream, they are released from their transport molecules. Through efficient circulatory methods, the nutrients are delivered to the rest of the body. The awaiting cells respond by recognizing the nutrients they need and transporting them into the cell. The nutrients are then used appropriately. Waste products are transported into the blood stream and eliminated via the kidneys or the digestive tract.
Elimination is the removal of undigested food and waste products accumulated during digestion. They remain in the lumen during the process of digestion. Peristalsis serves a housekeeping function to keep the waste products moving through the gut toward the anus. The stool enters the colon in a liquid form. Once in the colon, bacteria assist in the final stages of digestion. The gaseous by-products of bacterial degradation are eliminated via the rectum and anus. As the stool moves towards the rectum, water is aggressively reabsorbed. The mucosa of the colon has an incredible absorptive capacity that enables almost complete reabsorption of all the water from the stool. The stool becomes firm and bulky and moves into the rectum, which acts as a reservoir until proper evacuation can be performed.