Blood is the life fluid of the human body and the liquid that transports nutrients and removes waste. It makes up about 1/13 of the total weight of the human body. CompositionBlood cells, which come from the bone marrow, make up about half the volume of blood. Blood cells begin as stem cells, which can then develop into any of the other kinds of cells: red cells, white cells, or platelets. Red cellsRed cells are the most numerous kind of blood cells. Each drop of blood contains about 300 million of them. The main job of red cells is to transport oxygen to all parts of the body. Red cells have an iron-containing substance called hemoglobin that picks up oxygen as it passes through the lungs. A red-colored compound called oxyhemoglobin is formed as a result of this process. As this blood, now bright red, flows through the arteries, it releases some of its oxygen to the surrounding tissues. In return, the tissues send a waste gas, carbon dioxide, into the blood. White cellsTechnically known as leukocytes, white cells are outnumbered by red cells at a ratio of about 1 to 700. There are three different types of white blood cell, but they all help protect the body from disease. Some do this by rushing to places that have been invaded by bacteria or other foreign substances and swallowing up the invaders. Others actually change or fight against the invading substance. PlateletsThe smallest of the blood cells are called platelets. Under certain circumstances they are able to stick to one another and form blood clots. Such clots help to plug holes that may develop in the blood-vessel walls. When injured tissue triggers the clotting process, the tissue is repaired and the clot is dissolved. The blood system of an average adult contains about 2 trillion platelets. PlasmaThe watery part of the blood is the plasma. Most of the thousands of chemicals carried by the blood are found there. These chemicals include minerals, such as sodium and potassium, as well as complex vitamins, hormones, and enzymes. Some of these substances are manufactured in the body. Others gain access to the body through the process of eating or by means of the lungs. CirculationThe circulatory system carries blood to all parts of the body. It is a complex system composed of a powerful pump, the heart, and a vast network of different kinds of blood vessels such as arteries, veins, and capillaries. The walls of these vessels are made up of living cells through which substances, oxygen and nutrients as well as waste products, can pass in to and out of the blood. FunctionsOne of the most important substances to enter the blood is oxygen. As blood passes through the lungs it picks up oxygen, which it then transports to all parts of the body. As the oxygen-laden blood proceeds on its journey, oxygen passes out through the cell walls of the circulatory system. It is picked up by the cells of tissues, which are the working parts of the body and must have oxygen for fuel. Blood also carries nutrients to the muscle cells. As the muscles work they generate waste products that must be disposed of. These wastes pass through the walls of the circulatory system and into the blood. The wastes are then transported to the kidneys and removed from the body. Lastly, the work of the muscles and other tissues creates heat. Once again the blood provides the means of transportation as that heat is carried throughout the system and warms the body. |