6A - Control of heart rate Flashcards
What controls the regular beating of the heart?
Cardiac muscle.
What does it mean that the heart is myogenic?
It can contract and relax without receiving signals from nerves - this pattern of contractions controls the regular heartbeat.
How is heart rate controlled?
1) SAN in wall of RA sends out regular waves of electrical activity (excitation) to the atrial walls.
2) This causes the atria to contract at the same time.
3) Waves transferred from SAN to AVN (atrioventricular node).
4) Slight delay at AVN before electrical impulse is passed onto the bundle of His.
5) Bundle of His is a group of muscle fibres responsible for conducting the waves of excitation between the ventricles to the apex (bottom) of the heart. The bundle splits into finer muscle fibres in the right and left ventricle walls called the Purkinye tissue.
6) The Purkinye tissue carries the wave of excitation into the muscular walls of the right and left ventricles, causing them to contract simultaneously, from the bottom up.
What does the sinoatrial node do?
Sends out regular waves of electrical activity (excitation) to the atrial walls.
It is like a pacemaker setting the rhythm of the heartbeat.
Why can’t the wave of excitation be passed straight from the atria to the ventricles?
Because a band of non-conducting collagen tissue (atrioventricular septum) prevents this.
Why does the AVN delay the transmission of the impulse to the bundle of His?
To make sure the atria have emptied before the ventricles contract.
How do the ventricles contract?
From the bottom (apex) up.
What branch of the nervous system does control of heart rate involve?
Autonomic.
What does the SAN do?
Generates electrical impulses that cause the cardiac muscles to contract.
The rate at which the SAN fires is controlled by what?
Unconsciously by a part of the brain called the medulla oblongata.
Why do animals need to alter their heart rate?
To respond to internal stimuli, e.g. to prevent fainting due to low blood pressure or to make sure the heart rate is high enough to supply the body with enough oxygen.
What are stimuli in the blood detected by?
Pressure receptors and chemical receptors.
What are the pressure receptors called?
Baroreceptors.
What are baroreceptors?
Pressure receptors.
Where are baroreceptors?
Aorta and carotid arteries.
Where are the carotid arteries?
(Major arteries) In the neck.
What are baroreceptors stimulated by?
High and low blood pressure.
What are the chemical receptors called?
Chemoreceptors.
What are chemoreceptors?
Chemical receptors.
Where are chemoreceptors?
Aorta, carotid arteries and medulla.
What do chemoreceptors do?
Monitor the oxygen level in the blood and also carbon dioxide and pH (which are indicators of O2 level).
How are electrical impulses from receptors sent to the medulla?
Along sensory neurones.
What does the medulla do?
Processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones (which are part of the autonomic nervous system).
Where are the cardioacceleratory and cardioinhibitory centres?
Medulla oblongata.
What does the cardioacceleratory centre do to HR?
Increases it.
What does the cardioinhibitory centre do to HR?
Decreases it.
How is the cardioacceleratory centre linked to the SAN?
By the sympathetic NS.
How the the cardioinhibitory centre linked to the SAN?
By the parasympathetic NS.
How is heart rate controlled when there are high blood O2 levels/low CO2/high pH levels?
Chemoreceptors (in carotid arteries) detect chemical changes in blood.
Increases frequency of impulses sent to the medulla/cardioinhibitory centre.
Increases frequency of impulses sent along parasympathetic neurones.
These secrete acetylcholine, which binds to receptors on the SAN.
Effector = cardiac muscles.
HR decreases to return O2, CO2 and pH levels back to normal.
How is heart rate controlled when there are low blood O2 levels/high CO2/low pH levels?
Chemoreceptors (in carotid arteries) detect chemical changes in blood.
Increases frequency of impulses sent to the medulla/cardioacceleratory centre.
Increases frequency of impulses sent along sympathetic neurones.
These secrete noradrenaline, which binds to receptors on the SAN.
Effector = cardiac muscles.
HR increases to return O2, CO2 and pH levels back to normal.
Explain how heart rate is controlled DURING exercise
Increase in respiration increases CO2 levels and decreases pH levels in blood.
Chemoreceptors (in carotid arteries) detect chemical changes in blood.
Increases frequency of impulses sent to the medulla/cardioacceleratory centre.
Increases frequency of impulses sent along sympathetic neurones.
These secrete noradrenaline, which binds to receptors on the SAN.
Effector = cardiac muscles.
HR increases to return O2, CO2 and pH levels back to normal.
Explain how heart rate is controlled AFTER exercise
Decrease in respiration decreases CO2 levels and increases pH levels in blood.
Chemoreceptors (in carotid arteries) detect chemical changes in blood.
Increases frequency of impulses sent to the medulla/cardioinhibitory centre.
Increases frequency of impulses sent along parasympathetic neurones.
These secrete acetylcholine, which binds to receptors on the SAN.
Effector = cardiac muscles.
HR decreases to return O2, CO2 and pH levels back to normal.
What is the equation for blood pressure?
BP = CO x Total peripheral resistance
Total peripheral resistance = diameter of blood vessels (Increase in diameter = decrease in resistance).
What is the equation for cardiac output?
CO = SV x HR
What happens to BP when CO decreases?
BP decreases.
What happens to BP when HR decreases?
BP decreases.
How is heart rate controlled when there is high blood pressure?
Baroreceptors (in walls of carotid arteries and aorta) detect high BP.
Increase in impulses sent to the medulla/cardioinhibitory centre.
Increase in impulses sent along parasympathetic neurones.
These secrete acetylcholine, which binds to receptors on the SAN.
Effector = cardiac muscles.
Heart rate slows down to reduce blood pressure back to normal.
How is heart rate controlled when there is low blood pressure?
Baroreceptors (in walls of carotid arteries and aorta) detect low BP.
Increase in impulses sent to the medulla/cardioacceleratory centre.
Increase in impulses sent along sympathetic neurones.
These secrete noradrenaline, which binds to receptors on the SAN.
Effector = cardiac muscles.
Heart rate speeds up to increase blood pressure back to normal.
