Control Of Heart Rate

Question 1

Describe the role of receptors and of the nervous system in the increasing of heart rate (4)

Answer 1

1. Chemoreceptors detect rise in CO2 / H+ / acidity / carbonic acid / fall in pH
   OR
   Baro / pressure receptors detect rise in blood pressure;
2. Send impulses to cardiac centre / medulla;
3. More impulses to SAN;
4. By sympathetic (nervous system for chemoreceptors / CO2)
   OR
   By parasympathetic (nervous system for baro / pressure receptors / blood
   pressure);

Question 2

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 2

1. Electrical activity only through Bundle of His / AVN;
2. Wave of electrical activity passes over / through both ventricles at the same time;
   For ‘electrical activity’ accept impulses / depolarisation / action potential
   Reject messages/signals/information once only

Question 3

Describe how a heartbeat is initiated and coordinated (5)

Answer 3

1. SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction;
   Accept excitation
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 wave of electrical activity / impulses down Bundle of His;Allow Purkyne fibres / tissue
5. Causing ventricles to contract from base up;

Question 4

The heart controls and coordinates the regular contraction of the atria and ventricles. Describe how (6)

Answer 4

1. SAN → AVN → bundle of His / Purkyne fibres;
2. Impulses / electrical activity (over atria);
3. Atria contract;
4. Non-conducting tissue (between atria and ventricles);
5. Delay (at AVN) ensures atria empty / ventricles fill before ventricles contract;
6. Ventricles contract from apex upwards;

Question 5

The cardiac cycle is controlled by the
sinoatrial node (SAN) and the
atrioventricular node (AVN).
Describe how (6)

Answer 5

1. SAN initiates heartbeat / acts as a pacemaker / myogenic;
   Q Must be in context
2. (SAN) sends wave of electrical activity / impulses (across atria) causing atrial contraction;
   Reject: signals / electronic / messages
   / nerve impulses once only
3. AVN delays (electrical activity / impulses);
   Neutral: reference to non-conducting tissue
   delaying impulses instead of the AVN
4. (Allowing) atria to empty before ventricles contract / ventricles to fill before they contract;
5. (AVN) sends wave of electrical activity / impulses down Bundle of His / Purkyne fibres;
6. (Causing) ventricles to contract (from base up) / ventricular systole;