5.5.12 Factors Affecting Heart Rate Flashcards
How is heart beat regulated by both the nervous and endocrine system?
The cardiac muscle present in the heart is myogenic
It does not require any external stimuli to initiate contraction, contraction is initiated by the muscle itself
This allows the heart to beat at its own regular intervals
The length of the intervals between the beats (the rate of beating) can be regulated by both the nervous system and endocrine system
How is heart rate controlled by the autonomic nervous system?
There are several circumstances that can cause an individual’s heart rate to increase, such as during exercise
The brain is involved in this response of the heart however it does not require any thinking
There is a specific region of the brain that plays a vital role in controlling the heart rate
This cardioregulatory centre in the brain is called the medulla
The medulla is found at the base of the brain near the top of the spinal cord
The medulla is made up of two distinct parts:
The acceleratory centre, which causes the heart to speed up
The inhibitory centre, which causes the heart to slow down
Both centres are connected to the sinoatrial node (SAN) by nerves
These specific nerves are different from the nerves that control conscious activities. They make up the autonomic nervous system
The autonomic nervous system is self-controlling
What is the acceleratory centre?
Once the acceleratory centre has been activated impulses are sent along the sympathetic neurones to the SAN
Noradrenaline is secreted at the synapse with the SAN
Noradrenaline causes the SAN to increase the frequency of the electrical waves that it produces
This results in an increased heart rate
What is the inhibitory centre?
Once the inhibitory centre has been activated impulses are sent along the parasympathetic neurones to the SAN
Acetylcholine is secreted at the synapse with the SAN
This neurotransmitter causes the SAN to reduce the frequency of the electrical waves that it produces
This reduces the elevated heart rate towards the resting rate
How are either the acceleratory or inhibitory centres activated?
Exercise causes several internal conditions to change, creating internal stimuli:
Carbon dioxide concentration in the blood increases
There is an initial fall in blood pressure caused by the dilation of muscle arterioles
These internal stimuli can be detected by chemoreceptors and pressure receptors located in the aorta (close to the heart) and in the carotid arteries (they supply the head with oxygenated blood)
These receptors release nerve impulses that are sent to the acceleratory and inhibitory centres (coordinators)
The frequency of the nerve impulses increases or decreases depending on how stimulated the receptors are:
Lower frequency impulses activate the inhibitory centre to slow down the heart rate
Higher frequency impulses activate the acceleratory centre to speed up the heart rate
How can heart rate be controlled by the endocrine system?
The endocrine glands that produce hormones in animals are known collectively as the endocrine system
A gland is a group of cells that produces and releases one or more substances (a process known as secretion)
The hormones noradrenaline and adrenaline are both secreted by the adrenal glands and they both cause an increase in heart rate
Another hormone called thyroxine, which is produced by the thyroid gland, also causes an increase in heart rate
What is the importance of adrenaline as a ‘fight-or-flight’ hormone?
Adrenaline is a hormone released during a ‘fight-or-flight’ response
For example, when a zebra sees a hunting lion
It causes the heart rate to increase. The increase in heart rate is beneficial as it allows for a rapid increase in blood supply to respiring muscles
This means the muscles will have more oxygen and glucose for respiration
It enables high-intensity activities like running away from a predator to be an immediate response
