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KS5 Biology Lesson Plan 8 – Heart and Circulation Science at Work in Healthcare Post – 16 Science Education Pack Resource Sheet 8.1 – The Human Heart Your heart is a pump. Its job is to pump blood around your body so that your cells receive vital materials such as glucose and oxygen and can get rid of waste products such as carbon dioxide. Blood is pumped into a system of arteries which then split into smaller capillaries inside the tissues. It is through these capillaries that exchange of materials between the blood and cells take place. Your heart is made up of four chambers and connects to a number of different blood vessels. The reason that the heart is structured in this way is because it has to pump blood around two circulatory systems. Double Circulation Firstly, blood is pumped through the pulmonary circuit. This circuit takes blood from the heart to the lungs to pick up oxygen and drop off carbon dioxide. It then returns to the heart. Blood is then pumped through the systemic circuit which takes it around the body so that it can distribute oxygen which it picked up in the lungs. This means that any given blood cell actually passes through the heart twice for every time it passes around the whole body. Diagram 1 shows how this happens. How Does Your Heart Work? Possibly the most remarkable feature of the heart is its ability to work continuously without fatigue. During the course of a typical human lifespan a heart will beat something like 2.5 billion times and will pump more than 150 million litres of blood through each ventricle. Diagram 1. The systemic and pulmonary circuits of the human circulatory system KS5 Biology Lesson Plan 8 – Heart and Circulation Science at Work in Healthcare Post – 16 Science Education Pack The reason that it can maintain this rate of work is down to the nature of cardiac muscle. This consists of a network of interconnected muscle fibres divided up into uninucleate cells containing fine contractile myofibrils. The nature of the connections between the muscle fibres ensure that there is a rapid, synchronised spread of excitation through the heart wall, creating a uniform contraction. Your heart works by continually contracting and relaxing. These periods are called systole and diastole respectively. When it contracts blood is pushed out and around the pulmonary and systemic circuits. When it relaxes, blood re-enters the heart ready to be pushed out again on the next contraction. Take a look at diagram 2 and go through the steps laid out below to see how it all works in a bit more detail: 1. Blood from the systemic circuit (body) fills the right atrium and blood from the pulmonary circuit (lungs) fills the left atrium. The walls of both atria are now stretched outwards. 2. When the pressure in the atria is high enough the cuspid valves open and the blood flows from the atria into the ventricles. The cuspid valves are one-way valves letting blood into the ventricles, but not out. Tendons fixed to the ventricle walls stop the cuspid valves from being forced backwards and allowing blood from the ventricles to backflow into the atria. 3. The walls of the ventricles then contract. The left ventricle walls are thicker than the right and create a higher pressure. This is because the left ventricle pushes the blood around the systemic circuit, which is much bigger than the pulmonary circuit. Diagram 2. The structure of the heart KS5 Biology Lesson Plan 8 – Heart and Circulation Science at Work in Healthcare Post – 16 Science Education Pack 4. As pressure in the ventricles increases, the pocket valves at the base of the pulmonary artery and aorta are forced open, allowing blood to be forced out of the heart. The pulmonary artery takes blood to the lungs (pulmonary circuit) and the aorta directs blood to the body (systemic circuit). 5. Blood from the pulmonary circuit arrives in the left atrium through the pulmonary vein and blood from the systemic circuit arrives in the right atrium and the whole process repeats... Keeping the Pace But why does your heart beat? Most muscles contract as a result of impulses reaching them through nerves. The heart, however, is different. Cardiac muscle is myogenic, meaning that the contractions arise from within the heart muscle itself. It will continue to beat even when removed from the body. This is because it contains its own pacemaker, called the sino-atrial node (SAN), which is located in the wall of the right atrium. The SAN is actually connected to two nerves, the vagus nerve and the sympathetic nerve but these nerves only change the rate at which the SAN causes the heart to contract. The vagus nerve slows the heart rate and the sympathetic nerve speeds it up.