How does blood circulate around the body?

Blood is always red: brighter in arteries and darker in veins. Veins look blue because the skin absorbs and scatters light in a way that makes the dark red under it look bluish. In the 3D view, blue is only used to tell veins from arteries.

An adult has about 5 liters, roughly 7% of body weight. A child has less, depending on size. The body replaces donated blood within a few weeks.

Arteries carry blood away from the heart under high pressure and have thick walls. Veins bring blood back under low pressure, have thin walls and many have valves. Between them are capillaries, very thin tubes where blood swaps oxygen and nutrients with the body.

A bit more than half is plasma, a yellowish liquid with water, salts and proteins. The rest is cells: red blood cells, which carry oxygen; white blood cells, which defend the body; and platelets, which seal cuts. These cells are made in the marrow inside bones.

It is the pressure wave that travels through the arteries with each heartbeat. You can feel it at the wrist, on the thumb side, or on the neck. At rest, adults usually have 60 to 100 beats per minute; children have more.

Leg veins have valves that only let blood go upward. When you walk, the calf muscles squeeze the veins and push the blood along. That is why standing or sitting still for a long time can make your feet swell.