As a vital part of the circulatory system, human blood cells are responsible for carrying out several important biological tasks within the body. Blood cells are divided into three main types: white blood cells, red blood cells and platelets. White blood cells primarily are responsible for fighting off infections and other harmful substances. Red blood cells transfer oxygen and carbon dioxide throughout the bloodstream. Platelets form blood clots when the body is injured and help to prevent excessive bleeding from minor wounds.
White blood cells, also called leukocytes, work to destroy bacteria, viruses, fungi and parasites that can invade and damage the human body. Like most human blood cells, leukocytes are produced by a special stem cell inside bone marrow. The lifespan of white blood cells varies considerably — from hours to years — depending on the cells' role within the immune system. The body may produce larger amounts of leukocytes when the immune system is actively fighting disease, and elevated levels of these cells are often a sign of infection. Some diseases and disorders can affect production of white blood cells and leave a person with a compromised immune system.
Red blood cells, or erythrocytes, are the second type of human blood cells. They perform an integral function of the circulatory system, transferring oxygen from the lungs out to the muscles. These cells also are responsible for moving carbon dioxide to the lungs, where it is exhaled. It is erythrocytes that give blood its distinctive red coloring. A shortage of red blood cells is known as anemia and may lead to weakness and fatigue.
Platelets are the third main type of human blood cells. These cells also are called thrombocytes and are responsible for forming clots when the body has been injured. Without platelets, blood could continue to flow freely from wounds, as it does in people suffering from hemophilia. Thrombocytes can cause problems within the body by forming clots that can break off and damage internal organs. Many strokes and heart attacks are caused by unwanted clot formations.
All of these types of human blood cells are essential for normal body function. Physicians will often draw blood from patients to make sure their bodies are producing the right concentration of these cells. Disorders of the blood cells are often treated by a special doctor known as a hematologist. Patients with minor blood disorders may need to take iron supplements or make diet and lifestyle changes to treat the conditions.