The Circulatory System
The Circulatory System
The heart is a muscular pump that continuously and rhythmically beats to pump blood and its contents around the body and is made of cardiac muscle. It is divided into four chambers. The two upper chambers are called atria and receive blood into the heart. The two lower chambers are known as ventricles, again one on the left and one on the right and pump blood from the heart. Both ventricle walls are thick and muscular especially the left ventricle wall as their contraction pushes the blood to the lungs and heart. All four chambers are connected to their own blood vessel:
- The right atrium - the vena cava
- The left atrium - the pulmonary vein
- The right ventricle - the pulmonary artery
- The left ventricle - the aorta
- Oxygen from lungs to body cells
- Carbon dioxide from body to lungs
- Waste products and water from cells to kidneys
- White blood cells to sites of infection
Blood flows into the heart from the body and head into the right atrium through the vena cava. Blood from the lungs flows into the left atrium through the pulmonary vein. Blood from the right ventricle goes out through the pulmonary artery to the lungs. Blood from the left ventricle goes out through the aorta to the head and body.
When the body is at rest the heart can beat at between 50 to 80 times a minute, pumping the 4.7 litres or so of blood around the body.
The heart can respond immediately to any extra demands, for example, during exercise. Heart rate can increase to over 200 beats per minute, pumping 45 litres around the body in a minute.
Heart rate can be used to measure if the intensity of exercise is sufficient to improve fitness levels. This is dependent on the age of the person exercising and can be calculated in the following way:
75% of maximum heart rate
maximum heart rate = 220 - age
For example, a person who is 30 years old has a maximum heart rate of 220 - 30 = 190. 75% of 190 = 142.
Arteries carry freshly oxygenated blood from the heart to the capillary system.
One exception to this rule is the pulmonary artery that carries deoxygenated blood.
Most arteries lie deep in the body, but some are nearer the surface and it is these places that a pulse can be felt.
Artery walls are thicker than the walls of veins because they have a thicker layer of smooth muscle and elastic fibres between the protective covering and connective tissue.
Veins are often near the surface. Being near the surface allows any excess heat that is generated during exercise to be lost to the atmosphere. Veins have valves to prevent blood flowing backwards.
Capillaries are minute vessels with walls of only one cell thick. This allows food and oxygen to pass out to the body cells and carbon dioxide and other waste products to pass into the bloodstream.