Control of Heart Rate

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Control of the Cardiac Cycle

1) The sinoatrial node (SAN) is a group of cells found in right atrium of the heart, that acts as a pacemaker.

2) A wave of electrical activity spreads out from the SAN, across the atria causing them to contract simultaneously.

3) The atriventricular septum prevents the wave crossing to the ventricles (this allows the atria to complete their contraction and completely empy of blood).

4) The wave passes to the atrioventricular node (AVN), which lies between the atria.

5) After a short delay, the AVN passes on the wave along specialised fibres called the Bundle of His.

6) When the wave of electrical activity is released from the fibres, the ventricles can contract.

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The Autonomic Nervous System

- two divisions:

1) Sympathetic Nervous System

- stimulates effectors so speeds up activity

- helps us to cope with stressful situations (i.e. by heightening awareness and   preparation for activity).

2) Parasympathetic Nervous System

- inhibits effectors so slows down activity

- controls activity under normal resting conditions

- focussed on conserving energy and replenishing the body's reserves

* the two systems are antagonistic because they oppose one another

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The Medulla Oblongata

- the region of the brain that contols heart rate

- it again has two centres:

1) the cetre that increases heart rate - it is linked to the sinoatrial node by the sympathetic nervous system

2) the centre that decreases heart rate - it is linked to the sinoatrial node by the parasympathetic nervous system

- which centre is stimulated depends on the information they recieve from two types of receptor which respond to:

- chemical changes in the blood (chemoreceptors are found in the aorta and carotid arteries)

- pressure changes in the blood (pressure receptors are found in the aorta and vena cava)

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Control by Chemoreceptors (1)

- High pH level (low O2, high CO2)

1) Chemoreceptors detect a drop in CO2 levels and increase the frequency of     nerve impulses to the medulla oblongata.

2) The frequency of impulses via the parasympathetic nervous system increase      to the SAN, which decreases heart rate.

3) The reduced blood flow keeps CO2 in the blooed stream as it takes longer for     the CO2 to reach the lungs and be expelled from the body.

4) Blood pH therefore returns to normal.

5) The heart rate then increases back to normal speed as the chemoreceptors    detect normal CO2 levels.

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Control by Chemoreceptors (2)

- Low pH level (low O2, high CO2)

1) Chemoreceptors detect a rise in CO2 concetration and increase the frequency of impulses to the medulla oblongata.

2) Frequency of impulses via the sympathetic nervous system increase to the SAN which increases heart rate.

3) Increased blood flow means that CO2 reaches the lungs faster and is therefore expelled faster.

4) Blood pH returns to normal.

5) Heart rate decreases as chemoreceptors detect normal blood pH.

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Control by Pressure Receptors

- blood pressure higher than normal

1) Pressure receptors transmit nervous impulse to centre of medulla oblongata.

2) The centre sends impulses via the parasympathetic nervous system to the SAN of the heart.

3) Heart rate decreases.

- blood pressure lower than normal

1) Receptors transmit a nervous impulse to the medulla oblongata.

2) The centre sends impulses via the sympathetic nervous system to the SAN of the heart.

3) Heart rate increases.

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The affect of exercise on cardiac output

1) Increased muscular/metabolic activity.

2) This means more CO2 is produced by tissues from increased respiration.

3) Therefore the blood pH is lowered as CO2 makes blood more acidic.

4) Chemical receptors in the carotic arteries increase the frequency of impulses to the medulla oblongata.

5) This increases the frequency of impulses to the SAN via the sympathetic nervous system.

6) The SAN increases heart rate.

7) Increased blood flow removes CO2 quickly and blood pH returns to normal.

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