Blood is the red fluid in your arteries and veins. It provides the oxygen, water, and nutrients that your tissues and organs need to survive. You have about 5 liters of blood in your body. Your heart constantly pumps blood throughout your body.
Blood is your body's delivery service. Your blood:
All the cells in your body need oxygen and water to live. Your cells also need nutrients such as sugar, proteins, and fats. Your blood picks up oxygen in your lungs, and water and nutrients from your stomach and intestines.
Your cells create waste products when they process nutrients. Oxygen and nutrients turn into carbon dioxide and the chemical waste products that become part of urine. Your blood carries carbon dioxide to the lungs, where it is breathed out. Waste products are carried to your kidneys, where they are filtered out of blood and into urine. Your blood carries other waste products to your liver for further processing and removal.
Your immune system is your body's defense system against invaders such as germs and cancer cells. Your blood carries the special cells and proteins of your immune system to where they're needed.
Blood is made up of:
Red and white blood cells and platelets are constantly being made in your bone marrow, which is inside your bones.
White blood cells are part of your immune system. White blood cells travel through your blood to find and fight foreign substances such as germs and cancer cells. Once they fight a certain substance, white blood cells usually remember it so they can fight it more quickly the next time it appears.
Common blood problems usually involve:
Having too few red blood cells (anemia) causes weakness and tiredness because the tissues of your body can't get enough oxygen. Having too many red blood cells can cause a health problem called polycythemia vera.
Too few platelets increases your risk of bleeding after injuries, surgery, or even for no reason at all. Having too many platelets makes it hard for your blood to clot normally. This can cause too much clotting or not enough clotting.