Human Body Systems: Digestion, Circulation, Lymphatic
The Human Digestive System
The human digestive system consists of a sophisticated series of organs and glands that processes food. Food digestion starts in the mouth, where a transparent liquid called saliva begins the process of breaking up chemicals and forms a mass easy to swallow. This mass, bolus, goes through the throat into the esophagus, which is a 25-cm long tube that squeezes and moves the bolus into the stomach. This organ is a sack made up of strong muscle walls that squeeze food and, with the help of stomach juices, break down the food into a liquid mixture that is slowly emptied into the small intestine. The gastric juice also kills bacteria that may be present in food.
The small intestine receives the food mixture and, with the juices produced by the liver and pancreas, continues to digest food. The pancreas secretes enzymes useful to digest fats and proteins, while the liver secretes bile, a juice useful to absorb fat into the bloodstream. After a few hours in the small intestine, most nutrients, such as vitamins, minerals, proteins, carbohydrates, and fats, have passed to the blood. This blood goes to the liver for filtration of harmful substances and then goes to the rest of the body. On the other hand, the food remnants go on to the large intestine; in the section named colon, some minerals and water are absorbed into the blood, while the rest of the waste material goes on to the rectum and then out of the body.
The Human Circulatory System
The circulatory system is the main transportation and cooling system for the body. The heart pumps blood, a fluid made up of red cells, which carry oxygen and carbon dioxide; white cells, or leukocytes, which attack and destroy germs; platelets, which control bleeding when some blood vessels are broken; and plasma, a transporting fluid for the formed elements of blood, i.e., red cells, white cells, and platelets. The heart pumps blood to the lungs where cells are oxygenated and get rid of wastes such as carbon dioxide. The now oxygen-rich blood cells flow through arteries and capillaries to near and distant organs, such as the brain, carrying not only oxygen but also nutrients and taking away wastes such as the carbon dioxide that is generated as cells work. Eventually, the oxygen-depleted blood flows back through veins to the heart to be pumped out again to the lungs to continue with the pulmonary circulation. As blood flows throughout our body, white cells detect, attack, and destroy infecting microorganisms, and in case of bleeding, platelets clump together assisting in blood clotting.
The Human Lymphatic System
A secondary circulatory system is the circulation of the lymphatic system. This system is composed of organs, such as the spleen and the thymus; then, of nodes, which are distributed throughout the body and are a source of lymph and lymphocytes; and finally, of vessels, which are structures that carry the lymph. The lymphatic system has three major functions:
- It transports the lymph, which is a watery fluid that carries immune cells (lymphocytes) throughout the body.
- It absorbs the percentage of blood fluid that usually escapes from arteries or veins, dangerously filling interstitial spaces, and then returns the fluid to the bloodstream.
- It absorbs lipids from the intestine and transports them to the blood.
Disorders, malfunctioning, or diseases of the lymphatic system may significantly reduce its normal protective role and may result in the development of cancer, AIDS, and many other dangerous illnesses.