The food that we eat is complex in nature and is hence broken down into simpler substances by our digestive system so that it can be utilized for various purposes. Thus, we derive energy for various internal and external body works such as repairing of cells, synthesis of new cells, playing, exercising and walking.
Nutrition in Humans
The nutrition in humans takes place through a specialized system, known as the digestive system. The digestive system starts from the mouth and buccal cavity and ends in an anus. The digestive system is responsible for the breakdown of food and supply of energy from the food to cells of the body.
The food that we eat through mouth undergoes mechanical/physical treatment via chewing process of our teeth. The teeth are responsible for breaking down the food and the mouth mixes the food with saliva, secreted from the sebaceous glands. The saliva contains specialized digestive enzymes like salivary amylase, which breaks down the complex starch present in the food into simple sugar molecules. The food is passed down through the food pipe or esophagus.
The food then enters stomach which constantly expands and contracts to aid the movement of digestion. This movement of a stomach is known as peristalsis. The stomach contains muscular walls which aid in the mixing of food with digestive juices. The stomach contains gastric glands which secrete hydrochloric acid. Hydrochloric acid creates an acidic pH, which helps in activation of pepsin enzyme. Hydrochloric acid also assists in the breakdown of proteins to peptones.
The food is churned well in the stomach and only when food is partially digested, the stomach opens up to the small intestine through a small opening called sphincter muscle. The sphincter muscle releases the food to the small intestine in small amounts.
The food passed down to the small intestine where it is completely digested. The small intestines are the largest organ of the digestive system and are coiled in a small space. The small intestines receive digestive juices, bile from liver and glucagon and insulin from the pancreas. These digestive juices help to complete digestion of the food.
The food received from the stomach is in acidic conditions is converted to an alkaline condition by bile salts secreted from the liver. The bile salts are primarily responsible for breaking down of fats. The pancreas produces insulin which regulates blood glucose levels. The pancreas also produces pancreatic juice which consists of enzymes like pepsin, trypsin which are responsible for breaking down the peptones to peptides and lipase produced is responsible for breaking down fats to fatty acids.
In addition to this, the walls of intestine also secrete intestinal juice, which finally converts the remaining proteins to amino acids, carbohydrates to simple sugars and emulsified fats to fatty acids. The wall of the intestine has an inner lining which consists of numerous finger-like projections called villus (plural – villi). These villi are surrounded with blood vessels.
The villi are responsible for expanding the surface area of the intestine so that the food is completely digested. The simpler substances of the food in the intestine are then absorbed by the blood vessels are transported to various cells of the body, where the molecules are assembled to generate specialized proteins for varied repair mechanisms of the body, carbohydrates, and fats for energy utilization purpose.
The unabsorbed food is then passed on to the large intestine. In the large intestine, the excess water and nutrients are absorbed and then the remaining wastes in passed to the anus via rectum and colon.