The Heart The circulatory system consists of the heart, complete circuit of the blood vessels, and the blood. The job of the circulatory system is to move oxygen, nutrients and other substances to the body cells and carry carbon dioxide and other waste away from the cells. Blood consists of the fluid plasma, which contains dissolved substances such as glucose and proteins, making 55 per cent of blood volume. Suspended in the plasma are red blood cells, white blood cells and platelets making 45 per cent of blood volume. The blood transports substances and heat around the body and is a defence against infection. Blood composition Blood vessels help the pumping heart circulate blood around the body. Blood is pumped at high pressure into the arteries causing them to swell. When the heart relaxes, the arteries contract, helping to pump the blood around the body with an even flow. The rhythmic beat in the arteries is called the pulse and can be felt where the arteries comes near the surface of the body, e.g. on the wrist, side of the forehead (temple), sides of the neck, etc. A network of very thin-walled blood vessels called capillaries take the blood from the arteries to the tissues of the body, where various exchanges occurs between the blood and the cells of tissues. For example, oxygen, water, nutrients, antibodies and hormones are diffused from the blood to the tissues and carbon dioxide is diffused away from the tissues back to the blood. After having passed through the tissues in the capillaries, the blood returns under low pressure in the veins to the right atrium of the heart. Veins have thinner walls than arteries and have pocket valves along their length, which prevent blood flowing back under gravity. If people stand still too long, gravity can prevent proper flow of blood to the heart. This leads to less oxygenated blood reaching the brain, causing a person to faint. During exercise, blood is moved from other places such as the stomach liver and kidneys to the working muscle, where it is needed most. This is called vascular shunt and is achieved by shutting down capillary beds in those areas and opening extra capillary beds in working muscles. Blood vessels and blood The heart has four chambers. Two upper chambers called atria (singular; atrium) and two lower chambers called ventricles. The heart is divided down the middle into two parts by a wall called the septum. There are two valves between the atria and ventricles: the bicuspid and tricuspid valves that prevent blood from flowing back to the atria from the ventricles. There are also two semi-lunar valves between the heart and the arteries that prevent the back flow of blood to the heart from the arteries. The heart is a double pump for double circulation. The right side pumps blood to the lungs and then it returns to the heart. This is called pulmonary circulation. The left side pumps blood to the body and then it returns to the heart. This is called systemic circulation. The heart pumps blood by contracting. When the heart relaxes, both sides are filled with blood from the veins. The atria contracts and the veins, where they join the atria, also contract and blood is forced into the ventricles. The ventricles contract, the valves from the atria close and blood is forced out of the heart into the arteries. This repeated cycle is called cardiac cycle. – A heartbeat is one complete cycle. – The heart rate is the number of beats per minute. – The stroke volume is the amount of blood pumped from the left ventricle in one heart beat. – The cardiac output is the volume of blood pumped from the left ventricle in one minute. Therefore, cardiac output = stroke volume X heart rate. The cardiac output to the body increases during exercise as a result of increase in heart rate and stroke volume. The working muscles, therefore, receive much more blood and much more oxygen and nutrients. Also heat, carbon dioxide and lactic acid are carried away from the tissues more quickly. The heart gets stronger as a result of all the extra work it has to do during exercise.