9.8 - Control of heart rate in mammals

What is the equation to calculate cardiac output?

Cardiac output = stroke volume x heart rate

What is cardiac output?

The volume of blood the heart pumps through the circulatory system in one minute

What is stroke volume?

The volume of blood pumped from the left ventricle with each beat of the heart

Describe the autonomic nervous system mean

• It is self-governing & controls involuntary (subconscious) activities of internal muscles & glands

• It has two divisions: sympathetic & parasympathetic

Compare the sympathetic & parasympathetic nervous systems

Sympathetic ‘fight or flight’:

• Helps the body prepare for a perceived threat

  • inhibition of digestive & immune systems (i.e. diverting blood to muscles)

  • pupil dilation & increased heart rate

  • expansion of the lungs

Parasympathetic ‘rest & digest’:

• Effects largely mirror opposites of sympathetic system (i.e. antagonistic → bring about opposite effects)

  • stimulation of digestive & immune systems

  • decreased pupil size & heart rate

  • contraction of the lungs

The cardiac centre can send impulses to the SAN of the heart. What do impulses through sympathetic neurons release?

Noradrenaline at the SAN, causing the heart rate to increase

The cardiac centre can send impulses to the SAN of the heart. What do impulses through parasympathetic neurons release?

Acetylcholine at the SAN, causing the heart rate to decrease

Where in the brain is the cardiac control centre located & name the two types of receptors that send information to the cardiac control centre?

Medulla oblongata:

• baroreceptors

• chemoreceptors

Which type of receptor is found in the aorta & carotid arteries & is sensitive to pressure changes?

Baroreceptors

Which type of receptor is found in the walls of the carotid arteries & aorta & are sensitive to levels of blood carbon dioxide?

Chemoreceptors

Describe what the baroreceptors do in response to an increase in blood pressure above normal

1. When blood pressure rises above normal, the baroreceptors are stretched

2. They send more nerve impulses to the centre in the medulla that decreases heart rate

3. This centre sends impulses via the parasympathetic nervous system to the SAN of the heart, which decreases the rate at which the heart beats & causes vasodilation (widening of blood vessels)

4. Both of these effects lower blood pressure

Describe what the baroreceptors do in response to a decrease in blood pressure below normal

1. When blood pressure falls below normal, the baroreceptors stretch less

2. They send more nerve impulses to the centre of the medulla that increases heart rate

3. This centre sends impulses via the sympathetic nervous system to the SAN of the heart, which increases the rate at which the heart beats & causes vasoconstriction (narrowing of blood vessels)

4. Both of these effects raise blood pressure

Describe what the chemoreceptors do in response to an increase in CO₂ levels above normal

1. Increased muscular/metabolic activity (e.g. exercise)

2. More CO₂ is produced in the tissues from increased respiration

3. Carbon dioxide is an acidic gas → more CO₂ means a lower pH

4. Chemoreceptors in the carotid arteries & aorta detect this change & increase the frequency of nerve impulses that are sent to the centre of the medulla that increases heart rate

5. The centre in the medulla increases the frequency of nerve impulses to the SAN via sympathetic nervous system

6. The SAN increases heart rate

7. Increased blood flow removes CO₂ more quickly from the blood

8. CO₂ levels return to normal → chemoreceptors reduce the frequency of impulses to the medulla

How does the autonomic nervous system increase heart rate?

• In times of stress, the sympathetic nervous system stimulates the release of adrenaline from the adrenal glands

• Adrenaline stimulates the cardiac centre in the brain, increasing the impulses in the sympathetic neurons supplying the heart & SAN to increase heart rate