Controlling Heart Rate

What does “myogenic” mean?

Cardiac muscle can contract and relax on its own without nervous stimulation. The nervous system only changes the rate.

Describe how the heartbeat is initiated and coordinated.

1. The sinoatrial node (SAN) in the right atrium wall sends out a wave of depolarisation across both atria, causing them to contract (atrial systole).

2. A layer of non-conducting tissue prevents the wave from passing directly to the ventricles.

3. The wave reaches the atrioventricular node (AVN), which delays it briefly so the atria can finish emptying.

4. The AVN sends the wave down the bundle of His and along the Purkyne fibres in the septum to the base of the ventricles.

5. The ventricles contract from the base upwards (ventricular systole).

Why is there a delay at the AVN?

It allows the atria to contract and empty fully into the ventricles before the ventricles contract.

Why must the ventricles contract from the base upwards?

It pushes blood efficiently towards the arteries at the top of the heart and ensures that as much blood as possible is expelled.

What is the role of the medulla oblongata in controlling heart rate?

It contains the cardiac control centres. One centre increases heart rate via the sympathetic (accelerator) nerve, which releases noradrenaline to the SAN.

The other decreases heart rate via the parasympathetic (vagus) nerve, which releases acetylcholine to the SAN.

What are chemoreceptors and where are they found?

They detect changes in blood pH or CO₂ concentration. They are found in the carotid arteries and the aorta.

How does the body respond to increased CO₂ levels (e.g. during exercise)?

CO₂ lowers blood pH. Chemoreceptors detect this and send more impulses to the medulla, which increases impulses along the sympathetic nerve to the SAN. Heart rate rises, so blood flows faster, more CO₂ is carried to the lungs and removed, and blood pH returns to normal.

What are baroreceptors and where are they found?

They are pressure receptors in the walls of the carotid arteries and the aorta.

How does the body respond to high blood pressure?

Baroreceptors detect the rise and send more impulses to the medulla. The medulla sends impulses via the parasympathetic nerve to the SAN, so heart rate decreases and blood pressure falls.

How does the body respond to low blood pressure?

Baroreceptors send fewer impulses to the medulla. The medulla sends more impulses via the sympathetic nerve to the SAN, so heart rate increases and blood pressure rises.

Why is the control of Heart Rate beneficial for survival?

For the fight or flight response, adrenaline is released which increases the heart rate , this increases blood supply to respiring muscles, giving them oxygen and glucose for respiration, enabling high-intensity activities like running away from a predator to be an immediate response

Why is the control of heart rate an example of negative feedback?

A change in CO₂ level or blood pressure is detected, and the response reverses the change and brings the level back to normal.

How do hormones affect heart rate?

Noradrenaline, released from the adrenal glands during stress, binds to receptors on the SAN and increases heart rate.Whereas Ach does the opposite effect