The Accessory Organs
The accessory organs (Fig. 15-10) release secretions through ducts into the digestive tract. The salivary glands deliver their secretions into the mouth. All of the others accessory organs release secretions into the duodenum.
The Salivary Glands
While food is in the mouth, it is mixed with saliva, which moistens the food and facilitates mastication (chewing) and deglutition (swallowing). Saliva helps to keep the teeth and mouth clean. It also contains some antibodies and an enzyme (lysozyme) that help reduce bacterial growth. This watery mixture contains mucus and an enzyme called salivary amylase, which begins the digestive process by converting starch to sugar. Saliva is manufactured by three pairs of glands (see Fig. 15-4):
* The parotid glands, the largest of the group, are located inferior and anterior to the ear.
* The submandibular, or submaxillary, glands are located near the body of the lower jaw.
* The sublingual glands are under the tongue.
All these glands empty through ducts into the oral cavity.
Figure 15-4 The digestive system.
Figure 15-10 Accessory organs of digestion.
The liver, often referred to by the word root hepat, is the body’s largest glandular organ (see Fig. 15-10). It is located in the upper right portion of the abdominal cavity under the dome of the diaphragm. The lower edge of a normal-sized liver is level with the lower margin of the ribs. The human liver is the same reddish brown color as animal liver seen in the supermarket. It has a large right lobe and a smaller left lobe; the right lobe includes two inferior smaller lobes. The liver is supplied with blood through two vessels: the portal vein and the hepatic artery. These vessels deliver about 1.5 quarts (1.6 L) of blood to the liver every minute. The hepatic artery carries oxygenated blood, whereas the venous portal system carries blood that is rich in digestive end products.
This most remarkable organ has many functions that affect digestion, metabolism, blood composition, and elimination of waste. Some of its major activities are:
* The manufacture of bile, a substance needed for the digestion of fats.
* The storage of glucose (simple sugar) in the form of glycogen, the animal equivalent of the starch found in plants. When the blood sugar level falls below normal, liver cells convert glycogen to glucose, which is released into the blood restoring the normal blood sugar concentration.
* The modification of fats so that they can be used more efficiently by cells all over the body.
* The storage of some vitamins and iron.
* The formation of blood plasma proteins, such as albumin, globulins, and clotting factors.
* The destruction of old red blood cells and the recycling or elimination of their breakdown products. One byproduct, a pigment called bilirubin, is eliminated in bile and gives the stool its characteristic dark color.
* The synthesis of urea, a waste product of protein metabolism. Urea is released into the blood and transported to the kidneys for elimination.
* The detoxification (removal of the poisonous properties) of harmful substances, such as alcohol and certain drugs.
The main digestive function of the liver is the production of bile, a substance needed for the processing of fats. The salts contained in bile act like a detergent to emulsify fat, that is, to break up fat into small droplets that can be acted on more effectively by digestive enzymes.
Bile also aids in fat absorption from the small intestine. Bile leaves the lobes of the liver by two ducts that merge to form the common hepatic duct. After collecting bile from the gallbladder, this duct, now called the common bile duct, delivers bile into the duodenum. These and the other accessory ducts are shown in Figure 15-10.
The gallbladder is a muscular sac on the inferior surface of the liver that stores bile. Although the liver may manufacture bile continuously, the body needs it only a few times a day. Consequently, bile from the liver flows into the hepatic ducts and then up through the cystic duct, connected with the gallbladder (see Fig. 15-10). When chyme enters the duodenum, the gallbladder contracts, squeezing bile through the cystic duct and into the common bile duct, leading to the duodenum.
The pancreas is a long gland that extends from the duodenum to the spleen (see Fig. 15-10). The pancreas produces enzymes that digest fats, proteins, carbohydrates, and nucleic acids. The protein-digesting enzymes are produced in inactive forms which must be converted to active forms in the small intestine by other enzymes. The pancreas also produces large amounts of alkaline (basic) fluid, that neutralizes the acidic chyme in the small intestine, thus protecting the lining of the digestive tract. These juices collect in a main duct that joins the common bile duct or empties into the duodenum near the common bile duct. Most people have an additional smaller duct that opens into the duodenum. The pancreas also functions as an endocrine gland, producing the hormones insulin and glucagon that regulate sugar metabolism. These islet cell secretions are released directly into the blood.