7 to 8% of our body weight accounts for blood. Blood is a mixture of two basic components called cells and plasma. The basic function of blood is to carry nutrients and oxygen to every cell of the body. It is pumped to every cell by the heart through the arteries, capillaries and veins. It also carries away waste products like carbon dioxide, ammonia and forms clot to prevent excess blood loss.
Red and white human blood cells as seen under a microscope using a blue slide stain
A human body contains about 5 liters of blood out of which, the liquid component plasma accounts for 3 liters. Cell component of blood consists of the red blood cells, white blood cells and the platelets. Red blood cells that float in the plasma contain a special protein called hemoglobin which gives red color to blood. It helps carry oxygen from the lungs to other parts of body and return carbon dioxide to lungs to exhale. Red blood cells can survive for 120 days only and we give birth to 100 billion red cells every day. White blood cells that help fight infection account for a mere 1% of the blood. They consist of neutrophil and lymphocytes. Platelets are not actually cells but fragments of cells. They help in blood clotting process by forming a layer of fibrin clot on the injured blood vessel and prevent excessive bleeding.
The blood cells get produced in the bone marrow by a process called hematopoiesis. All the blood cells come from hematopoietic stem cells. These stem cells could be found circulating in the blood and bone marrow. These could also be used to treat diseases like leukemia and bone marrow failure.
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