The heart is an organ that pumps blood around the body in a double circulatory system. In the first circuit, the right ventricle pumps blood to the lungs, where gas exchange takes place. In the second circuit, the left ventricle pumps blood around the rest of the body. The blood vessels you need to know are the aorta, the vena cava, the pulmonary artery, the pulmonary vein and the coronary arteries, which supply the heart muscle itself. The resting heart rate is controlled by a group of cells in the right atrium that act as a pacemaker. An artificial pacemaker is an electrical device used to correct an irregular heart rate.
Air travels to the lungs down the trachea, which divides into two bronchi. The bronchi branch into smaller tubes that end in tiny air sacs called alveoli. Each alveolus is surrounded by a network of capillaries. The alveoli are adapted for gas exchange: there are huge numbers of them, which gives a large surface area, their walls are very thin, and the rich blood supply keeps carrying oxygen away and bringing carbon dioxide, which keeps the concentration differences high.
The body has three types of blood vessel. Arteries carry blood away from the heart at high pressure. They have thick, muscular and elastic walls and a narrow lumen. Veins carry blood back to the heart at low pressure. They have thinner walls, a wider lumen and valves that stop the blood flowing backwards. Capillaries link arteries to veins and pass close to every cell. Their walls are only one cell thick, so substances can diffuse through them easily.
Blood flow rate is a simple compound measure: blood flow rate = volume of blood divided by time. If 300 cm3 of blood passes a point in 60 seconds, the rate of blood flow is 5 cm3 per second.
