Blood cells are made mainly in the bone marrow, a soft, spongy material located inside certain bones. Organs such as the spleen and thymus (an organ in the neck) also play a role in the development of some cells.
As blood moves through the body, it performs many important jobs:
It delivers oxygen and important nutrients (such as vitamins, minerals, fats, and sugars) to the body.
It carries carbon dioxide to the lungs to be exhaled.
It takes waste products to the kidneys and to the liver to be removed from the body.
It carries hormones, which are chemical messengers that help different parts of the body to communicate.
It contains cells that fight infection and help control bleeding.
Blood is made up of plasma (the liquid portion) and three main types of cells:
Red blood cells (also called erythrocytes) carry oxygen.
White blood cells (also called leukocytes) fight infection.
Platelets (also called thrombocytes) are cell fragments that start the process of blood clotting.
There are many different kinds of blood disorders. Some occur because normal responses to abnormal situations go too far. For example, the number of white blood cells normally increases during an infection or illness, but too much of an increase can cause a blood disorder.
Other blood disorders result from improper functioning of the blood cells. Still others result from a problem in the blood itself, such as when bone marrow failure causes a shortage of all blood cell types.
