3.6.1.3 - CONTROL OF HEART RATE

Question 1

explain how the left atrioventricular valve maintains a unidirectional flow of blood (2)

Answer 1

1. pressure in left atrium is higher than in left ventricle, causing the valve to open OR when pressure above valve is higher than below it the valve opens
2. pressure in left ventricle is higher than in atrium causing the valve to close OR when pressure below valve is higher than pressure above valve, it closes

Question 2

exercise can cause an increase in heart rate.

describe the role of receptors and of the nervous system in this process (4)

Answer 2

1. chemoreceptors detect fall in pH/rise in CO2 OR baroreceptors detect rise in blood pressure
2. send impulses to the medulla oblongata
3. results in more impulses to SAN (increase in frequency)
4. by sympathetic nervous system for chemoreceptors OR by parasympathetic nervous system for baroreceptors

Question 3

when the heart beats, both ventricles contract at the same time.

explain how this is coordinated in the heart after initiation of the heartbeat by the SAN (2)

Answer 3

1. electrical activity only through Bundle of His/AVN
2. wave of electrical activity passes through Purkyne fibres to both ventricles at the same time

Question 4

describe how a heart beat is initiated and coordinated (5)

Answer 4

1. SAN sends wave of electrical activity/impulses across atria, causing atrial contraction
2. non-conducting tissue prevents immediate contraction of ventricles/prevents impulses reaching the ventricles
3. AVN delays impulse whilst blood leaves atria/ventricles fill
4. AVN sends impulses down Bundle of His/Purkyne fibres
5. causing ventricles to contract from base up

Question 5

explain how the heart muscle and the heart valves maintain a one-way flow of blood from the left atrium to the aorta (5)

Answer 5

1. atrium has higher pressure than ventricle (due to filling/contraction) causing atrioventricular valves open
2. ventricle has higher pressure than atrium (due to filling/contraction) causing atrioventricular valves to close
3. ventricle has higher pressure than aorta, causing semilunar valve to open
4. higher pressure in aorta than ventricle (as heart relaxes) causing semilunar valves to close
5. muscle/atrial/ventricular contraction causes increase in pressure

Question 6

explain, in terms of pressure, why the semilunar valves open (1)

Answer 6

pressure is greater below the valve/in ventricle than artery

Question 7

when a wave of electrical activity reaches the AVN, there is a short delay before a new wave leaves the AVN.

explain the importance of this short delay (2)

Answer 7

ANY PAIR:

1. allows atria to empty
2. before ventricles contract

OR

1. delays contraction of ventricles
2. until after atria have contracted/ventricles have filled

Question 8

a woman takes moderate exercise.

explain what causes her heart rate to increase while she exercises (6)

Answer 8

1. rate of respiration increases in muscle cells
2. CO2 concentration increases/pH falls
3. chemoreceptors in aorta and carotid arteries
4. send impulses to medulla oblongata
5. increased frequency of impulses
6. along sympathetic nervous system to SAN

Question 9

describe the route taken when electrical impulses are transmitted from the sinoatrial node to the muscles of the ventricles in a healthy heart. (2)

Answer 9

1. through cardiac muscle
2. to atrioventricular node
3. along Bundle of His/Purkyne fibres