Unit 6 - 3.6.1.3 Control of heart rate

Question 1

Give two safety precautions that should be followed when dissecting a heart (2 marks).

Answer 1

1. Use a sharp scalpel/scissors
2. Wash hands/wear gloves
3. Disinfect bench/equipment
4. Cover any cuts
5. Cut away from self/others/on a hard surface
6. Safe disposal

Question 2

Caffeine affects the autonomic nervous system.
Suggest how caffeine could increase heart rate. (2 marks)

Answer 2

1.   More impulses/action potentials along sympathetic;
Ignore signals/information/ messages
2.   To SAN increasing the heart rate

Question 3

Exercise causes an increase in heart rate.
Describe the role of receptors and of the nervous system in this process. (4 marks)

Answer 3

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 4

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 marks).

Answer 4

1. Electrical activity only through Bundle of His / AVN;
2. Wave of electrical activity / impulse passes over / through both ventricles at the same time;

Question 5

Describe how a heartbeat is initiated and coordinated (5 marks).

Answer 5

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 wave of electrical activity / impulses down Bundle of His / Purkyne fibres;
5. Causing ventricles to contract from base up;

Question 6

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

Answer 6

1. Atrium has higher pressure than ventricle (due to filling / contraction) causing atrioventricular valves to open (accept bicuspid, reject tricuspid);
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 valve (allow aortic valve) to close;
5. (Muscle / atrial / ventricular) contraction causes increase in pressure; Start anywhere in sequence, but events must be in the correct order.

Question 7

Explain, in terms of pressure, why the semilunar valves open (1 mark).

Answer 7

1. Pressure is greater below valve / in ventricle than (artery);
   Must be comparative
   Reject: pressure is greater in ventricle than atrium
   Neutral: pressure in ventricle increases

Question 8

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 marks).

Answer 8

1. Allow atria to empty / contract / ventricles to fill;
2. Before ventricles contract;
   OR
3. Delays contraction of ventricles;
4. Until after atria have contracted / ventricles have filled;
   Neutral: ‘to pump blood’

Question 9

The sinoatrial node (SAN) is in the right atrium of the heart. Describe the role of the sinoatrial node (2marks).

Answer 9

1. Sends out electrical activity / impulses;
2. Initiates the heartbeat / acts as a pacemaker / (stimulates) contraction of atria;

Question 10

Explain why increased cardiac output is an advantage during exercise (3 marks).

Answer 10

1. During exercise – More energy release / more respiration / actively respiring muscles / for aerobic respiration;
2. 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;
   If no “increase” – max 2 marks

Question 11

Explain how a rise in blood pressure results in a decrease in the rate of heartbeat (6 marks).

Answer 11

1. pressure receptors / baroreceptors / stretch receptors in aorta / carotid arteries / carotid sinus; (reject carotid body)
2. send impulses to cardiovascular centre / medulla / cardio-inhibitory centre;
   (reject signals / messages / electronic)
3. impulses via parasympathetic nerves
4. to SAN;
5. release of ACh / inhibits SAN / decreases impulses from SAN;
6. decreases impulses to AVN / decreased stimulation of AVN / decreases impulses from AVN;