What is in blood

More than half of your blood is plasma, a liquid made of water, salts and protein.

A liquid and three kinds of cells

Blood looks like a single red liquid, but it is a mixture. The liquid part is called plasma, and it is made of water, salts and protein. Over half of your blood is plasma. Floating in it are the solid parts: red blood cells, white blood cells and platelets. If blood is placed in a tube and spun fast in a lab machine, the heavier cells sink and the pale yellow plasma rises to the top, which is how the layers in the figure come about.

Red blood cells

Red blood cells deliver oxygen from your lungs to your tissues and organs, and they are the reason blood is red. They are the most common cells in blood. A red blood cell lives about 120 days, then it is removed and replaced. In the heart model the red and blue dots stand for blood carrying more or less oxygen, which is mainly the oxygen carried by red blood cells.

White blood cells and platelets

White blood cells fight infection and are part of the immune system. There are several kinds with different jobs. Some white blood cells live less than a day, but others live much longer. Platelets are small cell fragments that help blood clot when a vessel is cut, so that bleeding stops. Platelets live about 6 days. Because blood cells are always wearing out, your body keeps making new ones. They are made in bone marrow, the spongy material inside your bones.

What blood carries

Blood is the transport system of the body. It carries oxygen from the lungs to the tissues and carbon dioxide from the tissues back to the lungs. It carries nourishment from digestion, and hormones from glands to the organs they act on. It carries the cells and substances that fight disease to where they are needed, and it carries waste to the kidneys to be removed. Because it contains living cells, blood is itself a living tissue.

Blood types

People do not all have the same blood. There are four main blood types: A, B, AB and O. The type depends on markers on the surface of the red blood cells. Blood type matters most when someone receives blood from a donor, because the blood has to be matched. Testing and matching blood is done by medical staff in a lab.

Where blood cells are made

Making blood cells is called hematopoiesis, and it mostly takes place in the red marrow of the bones. In babies, red marrow fills the cavities of the bones. With age much of it is replaced by yellow marrow, which stores fat. In adults, red marrow is found mainly in the spongy bone of the skull, ribs, sternum, collarbones, vertebrae and pelvis. Red marrow forms red blood cells, white blood cells and platelets, so the skeleton and the blood are closely linked. The skeleton course on this site looks at those bones in 3D.

Think about what the lifespans mean. If a red blood cell lasts about 120 days, then roughly every four months the red cells you have now will all have been replaced. Platelets turn over even faster, in about 6 days. The marrow has to keep up this production every day of your life, quietly, without you noticing.