Every adult has approximately 5 liters of blood circulating in his body. Blood is made up of both liquid and solid components: the liquid portion is called plasma, while the solid portion consists of red blood cells, white blood cells, and platelets.

Plasma consists mainly of water (92%) but it also contains a complex mixture of proteins, vitamins, and hormones.

Red blood cells make up over 99% of your blood cells. These cells are very important because contain the protein hemoglobin that enables red blood cells to transport oxygen throughout the body. When hemoglobin combines with oxygen, it forms a bright red compound called oxyhemoglobin; when oxygen is released, the compound darkens and becomes deoxyhemoglobin.

White blood cells, or leukocytes, protect your body from infections. They can either eat bacteria (phagocytosis) or produce substances that destroy external particles. Though the majority of white blood cell activity takes place outside of the circulatory system, these cells use the blood to reach sites of infection.

Platelets help repair blood vessels by adhering to damaged walls and help to trigger blood coagulation (or clotting), which prevents bleeding and blood escaping from the blood vessel. Their size is less than half the size of red blood cells.

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