The Heart
Created | Updated Jan 3, 2003
The heart is a muscle that forces blood to circulate throughout the body. Cardiac output is the total amount of blood pumped out from the heart each minute. It is proportional to the volume of blood leaving the ventricles per beat (stroke volume), and the number of heart beats per minute (heart rate or pulse).
The stroke volume ranges between 70-80ml per beat when the person is stood upright and is at rest. However, in trained male endurance athletes, this can increase to 100-110ml per beat. The heart rate is in the range of 60-80 beats per minute for untrained men and women. However, in athletes, it can fall to 40-55bpm.
Cardiac output and stroke volume are difficult to measure. However, the pulse can be measured easily, either by the use of a pulse meter, or can be measured manually. This is done by placing the first two fingers over the radial artery of the wrist, or over the carotid artery in the neck, and the number of beats per minute are counted.
The cardiovascular centre for the body is situated in the medulla. During exercise, this receives information from various areas of the body, and varies the cardiac output accordingly. However, as it is difficult to measure cardiac output, it is possible to use the heart rate to determine how it alters.
The following factors transmit information to the medulla, causing it to alter the heart rate:
1. When athletes take part in a competitive event, there may be an increase in the heart rate prior to exercise. This is due to emotion or anxiety connected with the event. This results in stimulation of the limbic system in the brain stem, and the adrenal glands. This produces increased stimulation of the cardiac nerves and an increase in the amount of adrenaline and noradrenaline released into the blood.
2. Prioreceptors measure the position of joints and muscles. When any form of exercise begins, these detect changes in positioning, and send an ‘increased movement’ message to the cardiovascular centre. This results in a rapid rise in the heart rate.
3. The heart contains nerve endings from both the sympathetic and parasympathetic nerve systems. The sympathetic nerve secretes adrenaline and noradrenaline. Together, these increase heart rate. The parasympathetic nerve (vagus nerve) secretes acetylcholine. This results in a decrease in heart rate. During exercise, it is believed that inhibition of the vagus nerve causes the largest increase of heart rate during exercise.
4. During exercise, the contracting muscles release chemicals including potassium and lactic acid. These stimulate nerve receptors, which transmit messages to the cardiovascular centre, increasing the heart rate.
5. There are nerve receptors in the aorta and carotid arteries. These detect changes in carbon dioxide content in the blood. However, it has not been definitively proven whether these have an effect in increasing the heart rate or not.
6. Within the heart itself, there are 3 further factors which cause a change in pulse rate:
a.) Temperature of heart. As the temperature of the heart increases, the heart rate increases. Conversely, as the temperature decreases, the heart rate decreases.
b.) If an excess amount of blood enters the heart, it stretches the cardiac muscle and the sino-atrial node. This causes an increase in the heart rate.
c.) Any change in the concentration of sodium or potassium ions causes a change in the heart rate.
The stroke volume ranges between 70-80ml per beat when the person is stood upright and is at rest. However, in trained male endurance athletes, this can increase to 100-110ml per beat. The heart rate is in the range of 60-80 beats per minute for untrained men and women. However, in athletes, it can fall to 40-55bpm.
Cardiac output and stroke volume are difficult to measure. However, the pulse can be measured easily, either by the use of a pulse meter, or can be measured manually. This is done by placing the first two fingers over the radial artery of the wrist, or over the carotid artery in the neck, and the number of beats per minute are counted.
The cardiovascular centre for the body is situated in the medulla. During exercise, this receives information from various areas of the body, and varies the cardiac output accordingly. However, as it is difficult to measure cardiac output, it is possible to use the heart rate to determine how it alters.
The following factors transmit information to the medulla, causing it to alter the heart rate:
1. When athletes take part in a competitive event, there may be an increase in the heart rate prior to exercise. This is due to emotion or anxiety connected with the event. This results in stimulation of the limbic system in the brain stem, and the adrenal glands. This produces increased stimulation of the cardiac nerves and an increase in the amount of adrenaline and noradrenaline released into the blood.
2. Prioreceptors measure the position of joints and muscles. When any form of exercise begins, these detect changes in positioning, and send an ‘increased movement’ message to the cardiovascular centre. This results in a rapid rise in the heart rate.
3. The heart contains nerve endings from both the sympathetic and parasympathetic nerve systems. The sympathetic nerve secretes adrenaline and noradrenaline. Together, these increase heart rate. The parasympathetic nerve (vagus nerve) secretes acetylcholine. This results in a decrease in heart rate. During exercise, it is believed that inhibition of the vagus nerve causes the largest increase of heart rate during exercise.
4. During exercise, the contracting muscles release chemicals including potassium and lactic acid. These stimulate nerve receptors, which transmit messages to the cardiovascular centre, increasing the heart rate.
5. There are nerve receptors in the aorta and carotid arteries. These detect changes in carbon dioxide content in the blood. However, it has not been definitively proven whether these have an effect in increasing the heart rate or not.
6. Within the heart itself, there are 3 further factors which cause a change in pulse rate:
a.) Temperature of heart. As the temperature of the heart increases, the heart rate increases. Conversely, as the temperature decreases, the heart rate decreases.
b.) If an excess amount of blood enters the heart, it stretches the cardiac muscle and the sino-atrial node. This causes an increase in the heart rate.
c.) Any change in the concentration of sodium or potassium ions causes a change in the heart rate.
