controlling heart rate Flashcards
controlling heart rate
Heart rate is involuntary and controlled by the autonomic nervous system.
The medulla oblongata in the brain is responsible for controlling heart rate and making any necessary changes. There are two centres within the medulla oblongata, linked to the sinoatrial node (SAN) in the heart by motor neurones:
• one centre increases heart rate by sending impulses through the sympathetic nervous system, these impulses are transmitted by the accelerator nerve
• one centre decreases heart rate by sending impulses through the parasympathetic nervous system, these impulses are transmitted by the vagus nerve.
Which centre is stimulated depends on the information received by receptors in the blood vessels. There are two types of receptors which provide information that affects heart rate:
• baroreceptors (pressure receptors) - these receptors detect changes in blood pressure. For example, if a person’s blood pressure is low, the heart rate needs to increase to prevent fainting. Baroreceptors are present in the aorta, vena cava, and carotid arteries.
-chemoreceptors (chemical receptors) - these receptors detect changes in the level of particular chemicals in the blood such as carbon dioxide. Chemoreceptors are located in the aorta, the carotid artery (a major artery in the neck that supplies the brain with blood), and the medulla.
chemoreceptors
Chemoreceptors are sensitive to changes in the pH level of the blood.
If the carbon dioxide level in the blood increases, the pH of the blood decreases because carbonic acid is formed when the carbon dioxide interacts with water in the blood. If the chemoreceptors detect a decrease in blood pH, a response is triggered to increase heart rate - blood therefore flows more quickly to the lungs so the carbon dioxide can be exhaled.
When the carbon dioxide level in the blood decreases, the pH of the blood rises. This is detected by the chemoreceptors in the wall of the carotid arteries and the aorta. This results in a reduction in the frequency of the nerve impulses being sent to the medulla oblongata. In turn, this reduces the frequency of impulses being sent to the SAN via the sympathetic nervous system, and thus heart rate decreases back to its normal level.
Baroreceptors
Baroreceptors present in the aorta and carotid artery wall detect changes in pressure. If blood pressure is too high, impulses are sent to the medulla oblongata centre which decreases heart rate. The medulla oblongata sends impulses along parasympathetic neurones to the SAN which decreases the rate at which the heart beats. This reduces blood pressure back to normal.
If blood pressure is too low, impulses are sent to the medulla oblongata centre which increases heart rate. The medulla oblongata sends impulses along sympathetic neurones to the SAN which increases the rate at which the heart beats. This increases blood pressure back to normal.
hormonal control
Heart rate is also influenced by the presence of hormones. For example, in times of stress adrenaline and noradrenaline are released.
These hormones affect the pacemaker region of the heart itself - they speed up your heart rate by increasing the frequency of impulses produced by the SAN. Look back at Topic 14.1, Hormonal control and Topic 14.5, Coordinated responses to remind yourself how adrenaline is released and its importance in the ‘fight or flight’ response.
