The heart, the pump
The human heart is a muscular pump with four chambers. The two upper chambers are the atria (singular: atrium) and the two lower chambers are the ventricles. The atria receive blood coming back to the heart, and the ventricles pump blood out. Valves between the chambers and in the veins act like one-way doors that prevent the backflow of blood.
Key idea
Four chambers: two atria (receive) on top, two ventricles (pump) below. Valves keep blood flowing one way.
Blood vessels
There are three kinds of blood vessels, each suited to its job.
- Arteries carry blood away from the heart under high pressure and have thick, elastic walls.
- Veins carry blood back to the heart at lower pressure and contain valves.
- Capillaries are tiny and thin-walled; here gases and nutrients are exchanged with the body cells.
Double circulation
Humans have a double circulation: blood passes through the heart twice for each complete trip around the body.
- Pulmonary circulation carries blood between the heart and the lungs to pick up oxygen.
- Systemic circulation carries oxygen-rich blood from the heart to the rest of the body through the aorta, the largest artery.
Remember
- Most arteries carry oxygenated blood, but the pulmonary artery carries deoxygenated blood to the lungs.