The result of the heart beating faster is that more blood can be sent to the muscles. The arteries serving the muscles widen, whereas those serving less needful organs get narrower. The result is that extra blood is diverted to the structures that need it most.
Nerves and Hormones
Alterations in heart rate are brought about by nerves or hormones. These include; nerves that are connected to the heart, which carry impulses from an area of the brain called the medulla, and adrenaline, which is a hormone that is released from the adrenal glands, which are small organs above the kidneys.
There are two nerves which bring messages to the heart from the brain. The first is the accelerator nerve, and an impulse along this nerve causes the heart to beat faster. The second is the vagus nerve, and this nerve causes the heart rate to decrease. Events such as performing a vigorous activity, such as running a race can cause the brain to send these messages. When your brain sends the signals to your muscles to begin exercise, it simultaneously sends signals along the accelerator nerve to the heart, to increase the heart rate. Therefore, your heart starts beating faster straight away, providing your muscles with extra oxygen, Another factor which can trigger signals in these nerves is the blood pressure in the aorta and carotid arteries. These vessels have stretch receptors in their walls. If blood pressure in the arteries is high, the stretch receptors are stimulated, and impulses are sent along the vagus nerve, which slows the rate of heart beat. On the other hand, if blood pressure is low, impulses are sent along the accelerator nerve speeding the rate of heart beat.
The main hormone that helps to change the heart rate is adrenaline. Adrenaline is secreted from the adrenal glands in times of stress or need for action, and is carried in the blood to all areas of the body. It stimulates the heart to increase, and so increasing the heart rate.
Importance of maintaining a constant blood pressure
It is vital for the blood pressure to be as constant as possible. If a person’s blood pressure is too high, they may suffer from hypertension, which is a condition commonly associated with narrowing of the . This causes blood to be pumped with excessive force against the artery walls, pushing them out, often bursting them. The risk of this happening is greatest in old people whose arteries have become fragile. It is a sign that the heart and blood vessels are being overworked. Untreated, hypertension will cause the heart to eventually overwork itself to the point where serious damage can occur. Hypertensive patients are also at increased risk of and , which is when an artery bursts inside the brain, and the spillage of blood kills the cells in that part of the brain. The stroke may leave a person partly paralysed and unable to speak properly, and a severe stoke may be fatal. Hypertensive patients are also at risk of hardened (), which is caused by the fatty substance, cholesterol being laid down in the walls of the vessels, making them narrower and slowing the flow of blood through them. Where this happens, a blood clot may occur inside the artery, blocking it completely and this can often cause a heart attack. Other symptoms of hypertension are aneurysm, a weakness in the artery wall where it balloons out to more than 1.5 times its normal size and is in danger of rupturing, often resulting in sudden cardiac death, and retinopathy (loss of vision) and kidney failure which means that the kidney loses its ability to filter the blood.
Having a low blood pressure is often seen as a good thing, but people with too low a blood pressure may suffer from hypotension. This is a sign that the force of blood within the artery walls is weak and this can lead to slight or even severe oxygen deprivation to the brain.
Changes which occur in blood pressure and pulse during and after exercise
The contracting of the heart muscles forces blood into your arteries. This is quite a force so the walls of the arteries are elastic so they can take this pressure. They expand and contract as the blood is pushed through and this is what is felt as your pulse. And as the blood pressure is higher, the pulse rate is also quicker as more blood is pumped through the arteries.
When you take exercise, your heart beats faster. This is due to the extra carbon dioxide being produced by the active muscles, and it builds up in the bloodstream. The brain senses this and sends a nerve impulse to the heart making it beat faster. It is an automatic reflex and so you are not aware of it. The heart beating faster means that more blood is sent to the muscles. The arteries serving the muscles widen, and those serving less needful organs get narrower. The result is extra blood is sent to the muscles. This would mean that whilst doing exercise, the blood pressure would increase, as the heart works harder to supply the muscles with more blood and so the pressure on the artery walls is larger. Therefore the blood pressure would rise and also the pulse rate. The blood carries the glucose and sugar which the muscles need to carry out their work and this is why more blood is needed to be pumped there. Immediately after exercise, the pulse will still be higher than normal, because the muscle may have used more energy than the blood could supply in the form of oxygen and glucose. The muscle will have to repay this "oxygen debt" by taking in more blood than normal for a time. A short while after exercise, the pulse rate would be closer to the resting pulse rate than immediately after exercise, as the muscles will have had some time to recover. The time it takes to regain the resting pulse rate after exercise is a useful way of telling a person’s fitness.
