6.1.3 - Control of heart rate

Question 1

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

Answer 1

• Chemoreceptors detect rise in CO2 / H+ / acidity / carbonic acid / fall in pH
  /Baro / pressure receptors detect rise in blood pressure
• Send impulses to cardiac centre / medulla
• More impulses to SAN
• By sympathetic (nervous system for chemoreceptors / CO2)
  /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

• Electrical activity only through Bundle of His / AVN
• Wave of electrical activity passes over / through both ventricles at the same time

Question 3

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

Answer 3

• SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction
  ‘accept excitation’
• Non-conducting tissue prevents immediate contraction of ventricles / prevents impulses reaching the ventricles
• AVN delays (impulse) whilst blood leaves atria / ventricles fill
• (AVN) sends wave of electrical activity / impulses down Bundle of His
  ‘accept Purkyne fibres / tissue’
• Causing ventricles to contract from base up

Question 4

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 4

• Atrium has higher pressure than ventricle (due to filling / contraction) causing atrioventricular valves to open
  ‘allow: blood passes through the valve = valve open / blood stopped from passing through the valve = valve closed’
• Ventricle has higher pressure than atrium (due to filling / contraction) causing atrioventricular valves to close
• Ventricle has higher pressure than aorta causing semilunar valve to open
• Higher pressure in aorta than ventricle (as heart relaxes) causing semilunar valve to close
• (Muscle / atrial / ventricular) contraction causes increase in pressure

Question 5

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

Answer 5

• Pressure is greater below valve / in ventricle than (artery)

Question 6

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 6

• Allow atria to empty / contract / ventricles to fill
• Before ventricles contract
  or
• Delays contraction of ventricles
• Until after atria have contracted / ventricles have filled

Question 7

A woman takes moderate exercise. Explain what causes her heart rate to increase while she exercises. (6)

Answer 7

• Rate of respiration increases (in muscle cells)
• Carbon dioxide concentration increases / pH falls / H+ increases / acidity increases
• Chemoreceptors in aortic / carotid bodies / medulla (accept reference to aorta / carotid arteries not sinus)
• (Impulses to) medulla / cardioaccelerator centre
• Increased frequency of impulses
• Along sympathetic pathway to sinoatrial node / SAN (not pacemaker)

Question 8

Describe the route taken when electrical impulses are transmitted from the sinoatrial node to the muscles of the ventricles in a healthy heart. (3)

Answer 8

• Through cardiac muscle
• To atrioventricular node
• Along bundle of His / Purkyne fibres