control of heart beat

What does it mean that cardiac muscle is myogenic? (1 mark)

It can contract and relax without receiving electrical impulses from nerves.

Describe the pathway of electrical activity through the heart. (5 marks)

1. SAN sends out electrical impulses.

2. Impulses spread across atria — atria contract.

3. Impulses delayed at AVN.

4. Pass to Bundle of His → Purkyne fibres.

5. Ventricles contract from base upwards.

Why is there a delay at the AVN? (1 mark)

To allow the atria to contract and empty fully before the ventricles contract.

What is the role of the SAN? (1 mark)

Acts as the pacemaker — initiates regular heartbeat.

Where are chemoreceptors located? (1 mark)

In the aorta and carotid arteries.

What do chemoreceptors detect? (1 mark)

Changes in blood CO₂ concentration and pH.

Where are baroreceptors located? (1 mark)

Changes in blood pressure.

How does the body respond to increased CO₂ in the blood? (3 marks)

• Chemoreceptors detect high CO₂/low pH.

• Impulses sent to medulla → SAN via sympathetic neurones.

• Heart rate increases.

How does the body respond to decreased CO₂ in the blood? (3 marks)

1. Chemoreceptors detect low CO₂/high pH.

2. Impulses sent to medulla → SAN via parasympathetic neurones.

3. Heart rate decreases.

How does the body respond to high blood pressure? (3 marks)

1. Baroreceptors detect increase in pressure.

2. Impulses sent to medulla → SAN via parasympathetic neurones.

3. Heart rate decreases.

How does the body respond to low blood pressure? (3 marks)

• Baroreceptors detect low pressure.

• Impulses sent to medulla → SAN via sympathetic neurones.

• Heart rate increases.

What part of the brain controls heart rate? (1 mark)

The medulla oblongata.

Which part of the nervous system increases heart rate? (1 mark)

The sympathetic nervous system.

Which part of the nervous system decreases heart rate? (1 mark)

The parasympathetic nervous system.

Which neurone increases heart rate? (1 mark)

Sympathetic neurone to the SAN.

Which neurone decreases heart rate? (1 mark)

Parasympathetic neurone to the SAN.

What effect does exercise have on blood CO₂ levels? (1 mark)

Increases CO₂ due to higher respiration rate.

Why does heart rate increase during exercise? (2 marks)

• Chemoreceptors detect increased CO₂.

• More impulses to SAN via sympathetic neurones.

What is the benefit of increased heart rate during exercise? (2 marks)

• More oxygen and glucose delivered to muscles.

• More efficient removal of CO₂ and lactate.

Why does heart rate stay high after exercise? (2 marks)

• To repay oxygen debt.

• To remove excess CO₂ and lactic acid.

What happens to pH when CO₂ increases? (1 mark)

pH decreases (more acidic).

What happens to pH when CO₂ decreases? (1 mark)

pH increases (less acidic).

What does the AVN do? (1 mark)

delays the impulse to allow atria to contract before ventricles.

What does the Bundle of His do? (1 mark)

Conducts impulses to the base of the ventricles.

What do the Purkyne fibres do? (1 mark)

Distribute impulses through the ventricles causing contraction from base up.

How does the heart beat in a coordinated way? (2 marks)

1. Electrical impulses follow a defined pathway.

2. AVN delay ensures atria empty before ventricles contract.

What happens if the SAN fails? (2 marks)

1. No pacemaker activity.

2. Heart may beat irregularly or stop.

What happens if the AVN fails? (2 marks)

• Atria and ventricles beat out of sync.

• Ventricular contraction is inefficient.

What is the sequence of electrical activity in the heart? (1 mark)

SAN → Atria → AVN → Bundle of His → Purkyne fibres → Ventricles.

Why is heart rate lower at rest? (1 mark)

Body tissues need less oxygen and glucose.

Why doesn’t heart rate drop immediately after exercise? (1 mark)

Body is still removing CO₂ and restoring normal levels.

Why is the control of heart rate described as involuntary? (1 mark)

It is controlled by the autonomic nervous system, not conscious thought.

What is the advantage of reflex control of heart rate? (1 mark)

It provides a fast, automatic response to changes in internal conditions.

Define and explain the medulla Oblogata:

• Located in the brain, and controlls the heart rate via the autonomic response.

Autonomic = Automatic response.

Explain the Sympathetic nervous system:

Any impulses sent down the sympathetic nerve will increase the heart rate

Explain the Para-Sympathetic nervous system:

Opposite effect to the sympathetic nerve - where any impulses sent down the parasympathetic nervous system; the SAN releases a wave of depolarisation slowly; decreasing the heart rate.

Where is the bundle of HIS located?

In between the septum

Where is the Purkyn Fibres located?

In the walls of the ventricles.

Explain the response of an increased amount of pressure (stress etc...

• Increased in pressure: Baroreceptors are present in the walls of the aorta and the carotid artery.

• If blood pressure is too high, the blood vessels become stretched.

• This stretching activates the pressure receptors, triggering an action potential along the sensory neuron.

• More impulses are sent to the medulla oblongata in the brain, leading to increased impulses along the parasympathetic nervous system to the sinoatrial node (SAN).

• This decreases the frequency of electrical impulses, resulting in reduced cardiac muscle depolarization waves.

• The response is a decrease in heart rate.

Explain the effect of a decrease in pH:

• Decrease in PH

• Noticed by chemoreceptors in the aorta wall

• Increase in the rate of impulses sent to the medulla oblegata

• Increased impulses sent to the SAN via the sympathetic nervous system

• Increase in heart rate leads to an increase in delivery of oxygen to respiring cells

• Increases the Ph

What can an increased blood pressure do to you?

Damages the cell lining of the walls of the atreiols 
Lead to blood clotts
heart attacks.

what happens if blood pressure is too low?

not enough oxygenated blood being delivered to the respiring cells, and less removal of waste products from respiration.

When does PH Decrease (become more acidic):

• Increased respriation during high intense excercise due to the production of carbon dioixde and lactic acid

• If waste products not removed, Acid may denature important enzymes.