Blood Supply

Blood cells come from bone marrow. They begin as stem cells and then develop into red or white blood cells, or platelets. They don't live forever and must be replaced by new ones. Red blood cells live an average of 120 days before wearing out. Then they are captured and destroyed in the liver and spleen. Platelets live only for about 10 days.

The amount of blood in your body depends on your size, weight and the altitude at which you live. An adult who weighs 80 kg has about 5 litres of blood, a 40 kg child about half the amount. People who live in high areas where the air is thinner need more blood to deliver more oxygen to the body.

High-altitude training in the Swiss Alps
Professional athletes often carry out training in mountain regions. Such training produces more red blood cells, which can then carry more oxygen to other parts of the body.
Image: Christof Sonderegger von Photoplus.ch, CC BY-SA 3.0,
via Wikimedia Commons