Heart Rate

Question 1

Describe how a heartbeat is initiated and coordinated.

Answer 1

SAN initiates heartbeat / acts as a pacemaker / myogenic;

(SAN) sends wave of electrical activity across the atria causing them to contract

Non-conducting tissue prevents the impulses from reaching the ventricles

AVN delays electrical activity;

Allowing atria to empty and ventricles to fill, before the ventricles contract

AVN sends wave of electrical activity down the Bundle of His and up the Purkyne fibres;

Causing the ventricles to contract from the base upwards

Question 2

Increased intensity of exercise leads to an increased heart rate. Explain how

Answer 2

1. (Oxygen / carbon dioxide) detected by chemoreceptors / 
(pressure) detected by baroreceptors;
2. Medulla / cardiac centre involved;
3. More impulses to SAN / along sympathetic nerve;

Question 3

Explain how a rise in blood pressure results in a decrease in the rate of heartbeat

Answer 3

1 pressure receptors / baroreceptors / stretch receptors;
2 in aorta / carotid arteries / carotid sinus; (reject carotid body)
3 send impulses;
 (reject signals / messages / electronic)
4 to cardiovascular centre / medulla / cardio-inhibitory centre;

5 send impulses;(once only)
6 parasympathetic nerves / vagus; (accept inhibitory nerve)
7 to SAN;

8 release of ACh / inhibits SAN / decreases impulses from SAN;

9 decreases impulses to AVN / decreased stimulation of AVN /decreases impulses from AVN;

Question 4

Some drugs inhibit the transmission of nerve impulses to the heart. Explain how these drugs reduce high blood pressure.

Answer 4

1 inhibit impulses in sympathetic nerves / from cardio-acceleratory centre;

2 SAN not stimulated / noradrenaline not released
3 heart rate lowers / does not increase;

Question 5

Explain how nervous control in a human can cause increased cardiac output during exercise.

Answer 5

1. Coordination via medulla (of brain) / cardiac centre;

2. (Increased) impulses along sympathetic (/ cardiac accelerator)
 nerve;

3. To S.A. node / pacemaker;

4. Release of noradrenalin;
5. More impulses sent from / increased rate of discharge of
 S.A. node / pacemaker;
6. Increased heart rate / increased stroke volume;

Question 6

Explain why increased cardiac output is an advantage during exercise.

Answer 6

In exercise – More energy release / more respiration / actively respiring muscles / for aerobic respiration;
Higher cardiac output – Increases O2 supply (to muscles);
Increases glucose supply (to muscles);

Increases CO2 removal (from muscles)/lactate removal;
Increases heat removal (from muscles) / for cooling;