- SALIVARY GLANDS
In the last lesson, we looked at the major organs in the digestive tract, and now we are going to look at the accessory organs of digestion. The first of these organs are the salivary glands, of which there are three pairs. These are the parotid, the mandibular and the sublingual salivary glands and they are found in the mouth of the animal.
Figure 1: Shows the Position of the Three Salivary Glands
The function of the salivary glands is to produce fluid which pours into the mouth while the animal is eating, and which softens and lubricates the food and assists in the chewing process which breaks down the food, and makes the bolus slippery so that it can pass easily down the oesophagus. Salivary glands produce two types of fluid, a thin, clear, watery fluid and a thick, sticky fluid called mucus. These two fluids are mixed to produce saliva which is the liquid in the mouth. The other important function of the salivary glands is to produce an enzyme called ptyalin (the P is not pronounced). This enzyme starts the process of digestion in the mouth by turning some of the starch in the food into sugar. It is important because it starts the chemical process of digestion, as opposed to the mechanical process which is carried out by the grinding action of the teeth.
- 2. THE PANCREAS
If you refer to the diagram in Lecture 3, the digestive tract, you will see that the pancreas is an organ situated below the stomach which joins the first part of the small intestine; the part called the duodenum. This can be seen in more detail in the diagram on next page.
Figure 2: The Pancreas
The pancreas is made up of two types of glands, exocrine and endocrine glands. You can see from the diagram above that the largest part of the pancreas is made up of endocrine glands, but the part which consists of exocrine glands leads to the pancreatic duct which flows directly into the duodenum.
An endocrine gland has no duct system and pours its secretions directly into blood vessels which run through the pancreas.
Figure 3: The Two Types of Glands Found in the Pancreas
An exocrine gland pours its secretions into ducts, and these join up and lead into the pancreatic duct and into the duodenum.
The function of the pancreas is to produce two chemicals, enzymes and hormones. The enzymes produced are trypsin, chymotrypsin, carboxypeptidase, lipase and amylase. These appear as rather complicated names but you do not have to learn them off by heart; just remember that they are enzymes, and their function is to assist in the breakdown of complex nutrients, starches, sugars, oils, fats, etc., into the simple sugar glucose which is the end product of digestion, and which is the fuel for
the body. The endocrine part of the pancreas produces two hormones, insulin and glucagon, and their task is to regulate the breakdown of the complex nutrients into simpler compounds; they decide how much is broken down, and how quickly.
If you look at the diagram on page 23, you will see that the pancreas pours its secretions into the duodenum very close to the point where the bile duct leading from the liver also joins the duodenum. This is important because it means that the secretions from both organs can work together in the process of digestion.
The two hormones, insulin and glucagon, control the levels of glucose in the blood. Glucagon increases the glucose levels and insulin lowers the levels in the blood. Although insulin performs several functions, its main task is to pass glucose across the cell membranes or walls into the cells, where it can be further broken down and converted into energy.
| Hormones: a regulatory substance produced in an organism and transported in tissue fluids such as blood or sap to stimulate specific cells or tissues into action. |
If there is a shortage of insulin in the body, glucose remains in the blood and builds up; it cannot be utilised by the body, and so is wasted. This condition in humans is known as diabetes, and if a person is a severe diabetic he has to inject himself with insulin every day and avoid eating sugary foods which are easily converted into glucose. Glucagon controls the release of sugar stored in the liver into the bloodstream, and so increases the glucose levels in the blood. Insulin and glucagon work together to maintain the ideal level of glucose in the blood.
- 3. THE LIVER
The liver is a large, irregular shaped organ made up of lobes. Refer to the diagram of the digestive tract and you will see that it lies below the stomach and next to the duodenum, just below the diaphragm.
The liver produces liquid called bile. This collects in the hepatic ducts which are channels running through the liver tissue and which lead into an organ called the gall bladder; this acts as a storage tank for bile. All farm animals, except the horse, have a gall bladder. Bile is collected in the gall bladder and released from the gall bladder through the cystic duct.
Figure 4: The Flow of Fluids
Figure 5: The Liver, Gall Bladder and the Pancreas
The cystic duct leads into the common bile duct which empties into the duodenum near the pancreatic duct. The level of bile in the bile duct is controlled by a hormone called cholecystokinin (CCK).
| Diaphragm: a dome-shaped muscular partition separating the thorax from the abdomen in mammals. It plays a major role in breathing, as its contraction increases the volume of the thorax and so inflates the lungs. |
Bile is a greenish-yellow liquid consisting of water, bile salts, bile pigments (pigments are colouring materials), and a substance called cholesterol. Bile assists the digestion and absorption especially of fats and the fat soluble vitamins.
The liver performs functions other than the production of bile, but this will be covered later on in the course when we come to digestion and absorption.
