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Small Intestine

The majority of digestion occurs in the small intestine. This compartment has three distinct portions, each of which is highly specialized for different digestive functions. The first third is the duodenum. This part is responsible for signalling the distal digestive tract that food is arriving and that the stomach should stop sending food. Chemical digestion is very active at this stage, and food is broken down into basic proteins, carbohydrates, and fats. Digestive enzymes are released from the pancreas to enable the degradation process. Bile is released from the gall bladder and mixes with the chyme to aid in fat digestion and absorption.

The second portion of the small intestine is the jejunum. It is composed of many folds that increase the surface area for absorption, known as the brush border membrane.

Figure %: Brush Border Membrane
Chemical degradation continues at this point and absorption begins. Once the chyme is broken down into its basic constituents, the process of absorption occurs. Nutrients are moved across the membrane via active and passive diffusion. In some instances, carrier molecules made by the liver, known as micelles, help carry fat across the border into the blood stream.

As the digestive products move through the small intestine, different parts of the brush border membrane selectively allow the absorption of different nutrients. The third portion of the small intestine is the ileum. It is responsible for the very selective absorption of some significant nutrients, including vitamin B-12 and vitamin C. By the end of the small intestine, the majority of the nutrients has been absorbed. The material is now mostly waste products and water, and is referred to as liquid stool.

Large Intestine

The large intestine (the colon) has several major components. The cecum is the large boggy entrance to the right side of the colon. There is a circular muscle, or sphincter, that separates the small intestine from the large intestine. The sphincter protects the small intestine from the concentrated bacteria of the large intestine. Following the cecum, the ascending colon rises up and leads into the transverse colon. From there, the descending colon dives back down on the left side of the abdomen into the sigmoid colon and rectum. Waste is finally eliminated through the anus.

Accessory Organs

Salivary Glands

There are three pairs oF salivary glands: the parotid, submandibular, and sublingual glands. Each gland releases salivary amylase that begins digestion by degrading starch in the mouth. The glands are composed of specialized cells that make enzymes. They are connected to the mouth by a duct. These ducts are lined with epithelium, which serves as a protective barrier from autodigestion. They also have some muscle in them that allows the ducts to contract. This enables the one-way flow of enzymes from the glands into the mouth.

Pancreas

The pancreas has both an endocrine and an exocrine function. The endocrine function allows the production of insulin, which aids in the regulation of sugar but is not involved in digestion. The exocrine function, on the other hand, is responsible for digestion. It is in this portion of the pancreas where enzymes of digestion are made. A neutral buffer, bicarbonate , is also produced here. Bicarbonate keeps the intestinal environment at a neutral pH. The enzymes and the bicarbonate are then released into the lumen of the intestine through the pancreatic duct.

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