As it courses through the circulatory system, propelled by the contractions of the heart, the blood performs any number of vital functions. It picks up oxygen from the lungs and nutrients from the digestive tract and delivers them to the tissues. It carries away carbon dioxide and other waste products, so that they can be eliminated. Blood also provides a means of transport for the crucial, disease-fighting cells and chemical substances that form immune system.
About five liters of blood flow in the arteries and veins, making up about 7 percent of total body weight. Human blood is roughly 55 percent fluid; the rest is solid matter, mostly cells. The most numerous are the red blood cells, or erythrocytes, some 5 million of them or erythrocytes, some 5 million of them in ever cubic millimeter of blood, more than 25 million in all. Red cells are minute discs, less than one hundredth of a millimeter in diameter and concave on both sides; their job is to carry oxygen. They look red because they contain a red molecule called haemoglobin; this protein is the crucial substance that combines with oxygen molecules in the lungs and transports the oxygen to the tissues.
The next most numerous elements in the blood are platelets, or thrombocytes, which are actually fragments of a type of cell called megakaryocytes. Platelets are about half the size of red cells, and there are 250,000 to 400,000 of them in a cubic millimeter of blood. Platelets initiate the clotting process that keeps you from losing too much blood whenever you cut yourself.
The third type of blood cells is the white cells, or leucocytes, of which there are 5,000 to 10,000 per cubic millimeter of blood. White cells travel in the bloodstream but do most of their work elsewhere in the tissues. Of the several different kinds of white cells, the two most important are neutrophils, which ingest and kill bacteria, and lymphocytes, which recognize foreign cells and substances and produce antibodies to fight them. Monocytes are large scavenger cells that clear tissue spaces of dead or foreign material. Eosinophils and basophils are involved in allergic reactions; their role is not fully understood.
Plasma is the liquid part of the blood. It is a hazy, yellowish fluid, 90 percent water, a broth containing thousands of essential substances, including salts and minerals (sodium, potassium, calcium); nutrients (sugars, amino acids, vitamins); fats (cholesterol and others); hormones; antibodies; the proteins (prothrombin, fibrinogen and others) involved in blood clotting; and waste products (carbon dioxide, urea, lactic acid). Why do we need blood?