The Cardiovascular System: The Heart
The heart is a four chambered muscular pump, about the size of a clenched fist, and is responsible for pumping blood around the human body. When the heart contracts it generates pressure that forces blood to pass along the blood vessels. A normal healthy adults heart will pump over 7000Litres of blood each day. This works out at about 5Litres of blood every minute. The heart consists of two collecting chambers (the right and left atrium) and two pumping chambers (the right and left ventricles). The atria are separated from the ventricles by the atrioventricular valves. Theses allow blood to pass from the atria to the ventricles but prevent blood passing back from the ventricles to atria when the ventricles contract.
The right atrium receives de-oxygenated blood from two large veins called the superior vena cava and inferior vena cava. When inside the right atrium this de-oxygenated blood drops through the atrioventricular valve into the right ventricle. The right ventricle is responsible for pumping de-oxygenated blood to the lungs. The right ventricle contracts rhythmically forcing blood through the pulmonary artery which carries the de-oxygenated blood to the lungs. At the base of the large arteries, that carry blood from the ventricles away from the heart, are semilunar valves. The semilunar valves are non-returnable, meaning they allow blood to pass through them from the ventricles but will not allow the blood to pass back from the arteries to the ventricles. This prevents blood from passing back into the ventricles, from the arteries, when the ventricles relax.
As the blood in the pulmonary artery reaches the lungs it is oxygenated and carried back to the heart along the pulmonary vein. The pulmonary artery and vein are the only exceptions to the rule that arteries carry oxygenated blood and veins carry de-oxygenated blood.
The oxygenated blood returns to the heart and is received by the left atrium and drops through the atrioventricular valve into the left ventricle. The left ventricle is the main pumping chamber of the heart. It is larger in size than the right ventricle and is responsible for pumping blood around the rest of the body. When the left ventricle contracts, oxygenated blood is pumped through the aorta. The aorta subdivides into arteries and then into arterioles and finally into capillaries which is where oxygen is delivered to tissues such as the muscle or skin. The blood then becomes de-oxygenated and travels back along a series of veins until it reaches the heart where it is received by the right atrium and the whole process starts again.
The heart muscle contracts through the action of cardiac muscle cells. The contraction of cardiac muscle cells is controlled by the sinoatrial node. The sinoatrial node contains specialized cardiac muscle cells called pace maker cells. These specialist cardiac muscle cells generate action potentials that spread throughout the heart muscles through a specialised conducting system, causing cardiac muscle cells to contract rhythmically. The action potentials generated by the sionatrial node passes to the atrioventricular node and then to the purkinje fibres which carry the action potentials to the ventricle walls where it causes the cardiac muscle cells to contract. The action potentials generated by the pacemaker cells occur at regular intervals and cause the heart of a healthy adult to contract around 60-80 times per minute. Following a period of aerobic training an individuals resting heart rate may decrease to as low as 30beats per minute.
The term cardiac cycle refers to the series of events that occur with each heartbeat. The cardiac cycle begins with the contraction of cardiac muscle cells (i.e. when the ventricle contracts to squeeze blood out of the heart), and ends with the relaxation of the cardiac muscle cells (i.e. when the ventricle relaxes and refills with blood from the atria). The contraction phase of the cardiac cycle is called the systole and the relaxation phase is called the diastole.
Blood pressure is the term used to describe the force generated by blood as it passes through our arteries. When taking blood pressure exercise professionals will measure both the systolic and diastolic blood pressure. Therefore when looking at an individuals blood pressure we take into account the pressure generated during the contraction and relaxation phase of the cardiac cycle. An average healthy adult would have a systolic blood pressure of 120 and a diastolic blood pressure of 80. Our blood pressure is in a constant state of change and is affected by a number of factors:
- Physical activity – as we increase our activity levels our heart rate also increases. As our heart rate increases so does blood pressure. This is due primarily to the increased amount of blood that is being pumped around the body.
- Mental stress – As we our put under mental stress our bodies respond by increasing the levels of stress hormones in our blood. Stress hormones have varying effects on the body but two primary affects include an increase in heart rate and constriction of the blood vessels. Both of these factors lead to an increase in blood pressure. This is why exercises such as yoga, which lower stress hormone levels, are good for lowering blood pressure.
Blood pressure is important for two main reasons:
- 1) It helps to push blood uphill against gravity and therefore it is important that blood pressure is high enough to maintain an adequate supply of blood to the brain and heart when we are standing. This is why people with low blood pressure are more prone to fainting. Fainting is actually a naural response by the body to preserve blood supply to the brain – this sounds strange but when you faint and you fall to the ground your heart no longer has to pump against gravity to maintain a blood supply to the brain.
- 2) An adequate blood pressure is needed to maintain blood flow through the capillaries so that vital oxygen is delivered to the important body tissues.
However, when blood pressure is too high – a blood pressure above 140 (systolic) and 90 (diastolic) is considered above normal and is cause for concern – we are at increased risk of heart disease and stroke as well as hardening of the arteries.
The stroke volume refers to the volume of blood that is pumped by the heart with each contraction. With aerobic training the heart is able to pump greater blood with each contraction. As stroke volume increases there will normally be a concomitant decrease in the resting heart rate and since the resting heart rate is lower so will blood pressure at rest as well.
The main effects of exercise training on the heart:
- The heart becomes thicker and stronger (particularly the left ventricle). In fact in highly trained endurance athletes the heart may be twice the size of normal.
- The heart becomes more efficient – it contracts more powerfully, empties its chamber more quickly, and is able to pump more blood with each contraction – and is able to transport a greater amount of oxygen around the body with each contraction.
- The resting heart rate decreases