Hormonal Control of Digestion in Human
Hormonal control of the secretion of digestive juices is brought about by the hormones secreted by the digestive tract. These hormones are all proteins chemically. The major hormones that control the secretion of digestive juices are:
Certain glandular cells present in the lining of the stomach produce the hormone gastrin. It influences the secretion of digestive juices in the stomach.
Within a few minutes after the chyme has entered the small intestine, the cells lining it are stimulated to secrete the hormone secretin. The hormone is absorbed into the blood and carried by the blood to the pancreas. There it stimulates pancreas and more pancreatic juice is secreted.
It induces the flow of pancreatic enzymes.
It influences the flow of bile from the gall bladder.
This hormone is also released from the duodenum. It helps in the stoppage of gastric secretion. It has been shown that the pancreozymin and cholecystokinin is, in fact, the same hormone.
Metabolism of the Major Food Stuffs and their Nutritive Value
Metabolism involves both constructive and destructive processes. The body synthesizes various tissue components from the absorbed food materials utilising energy. This process is referred to as anabolism and is a constructive process. At the same time the body needs energy for its various activities and also for the anabolic reactions. The energy is liberated by the complete breakdown of the major food materials like carbohydrates, proteins and fats through a series of chemical reactions. This process is called catabolism. Through this process the energy is released in small and usable amounts. The energy liberated thus can be stored in the body in the form of special energy rich phosphate compounds. The most important of such compounds is Adenosine triphosphate (ATP). The energy is also stored in the form of proteins, carbohydrates and fats, synthesised from simpler molecules by the anabolic process. Catabolism and anabolism together constitute the metabolism. The amount of energy liberated by catabolism appears as external work, heat or energy storage in the body.
Role of the Liver in Metabolism
The liver is a very important organ of the body in which certain important changes in proteins, carbohydrates and fats take place. The role of the liver in general metabolic reactions can be summarised as follows:
- It helps in the carbohydrate metabolism by converting excess of glucose into glycogen (glycogenesis) which is stored in the liver. When the level of glucose in the blood falls, glucose is mobilised from the liver by reconverting glycogen into glucose (glycogenolysis). The glucose thus formed enters the blood.
- A very high protein diet results in absorption of an excess amount of amino acid in the blood. In the liver these amino acids are converted into carbohydrates. This process is called gluconeogenesis.
- Individuals on a high carbohydrate diet tend to be obese. This is because the excess of carbohydrates is converted into fat in the liver. This fat is stored in the body.
- Another function of liver in the metabolic process is the detoxification of toxic substances. The metabolic process yields substances which are harmful to the body. These harmful substances are rendered harmless (detoxification) in the liver.
Calorific Value of Foods
A calorie is the standard unit of heat energy. One gram of carbohydrate yields 4.2 kilocalories (kcal). One kilocalorie is equal to 1000 calories. Similarly one gram of fat yields 9.5 kcal and one gram of protein 4.3 kcal. Thus, it can be seen that fat has an energy value about twice that of protein or carbohydrates. The daily requirement of calories by a person depends largely on the kind of work done, the state of health, the bulk of body, etc. If a person is resting, he expends the minimum of energy, which is 1 cal/h/kg of body weight. The energy expended by an organism at rest is called the basal metabolic rate (BMR). A person indulging in manual labour may require about 3000-3500 calories.