Blood circulates endlessly through the human body to keep it alive. This fluid contains trillions of cells and countless chemicals, all floating in watery plasma. Blood is pumped by the heart through a network of blood vessels to deliver nutrients, oxygen, and other essential substances to cells. Blood also transports waste; helps keep the body temperature steady, and fights germs.
Blood flows into the kidneys through the renal arteries and out through the renal veins. The kidneys filter substances such as urea, uric acid, and creatinine out of the blood plasma and into the ureters. The liver also removes toxins from blood. During digestion, it cleans blood that has been enriched with vitamins before sending it back out to the rest of the body.
Blood vessels expand and contract when they react to outside organisms, such as bacteria, and to internal hormone and chemical changes. These actions move blood and heat closer to or farther from the skin surface, where heat is lost.
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