The heart, ventilation and exercise: exam questions

• SAN (in right atrium wall) initiates / generates a wave of excitation / depolarisation
• wave spreads across both atria, causing them to contract
• non-conducting tissue between atria and ventricles prevents the wave passing straight to the ventricles
• AVN delays the wave, so atria empty before ventricles contract
• wave passes down the bundle of His (in the septum) to the apex
• Purkyne fibres carry the wave up the ventricle walls, so ventricles contract from the apex upwards
• Max 5

Answer: B – cardiac muscle contracts and relaxes rhythmically without nervous stimulation.

• heart rate = 60 ÷ 0.48
• 125 beats per minute (accept 125 bpm)
• patient 2 has tachycardia because their heart rate is above 100 beats per minute (patient 1 = 71 bpm)

• atrial fibrillation / irregular atrial activity
• ECG records the electrical activity of the heart
• changes in the timing / spacing / shape of waves (e.g. R–R interval, P wave) can be compared with a normal trace to identify a rhythm problem such as tachycardia, bradycardia or ectopic beats

• resting cardiac output = 80 × 65 = 5200 cm³ min⁻¹
• stroke volume = cardiac output ÷ heart rate = 19 200 ÷ 160
• 120 cm³

• respiration in muscles produces more CO₂
• CO₂ lowers blood pH / chemoreceptors detect the change
• impulses sent to the ventilation centre / cardiovascular control centre in the medulla oblongata
• ventilation centre sends impulses to the diaphragm and intercostal muscles, so breathing is faster and deeper
• impulses along the sympathetic nerve to the SAN increase heart rate
• so oxygen delivered faster to muscles / CO₂ removed faster
• Max 5

• adrenaline binds to receptors on target cells
• increases heart rate / stroke volume, so more oxygen and glucose delivered to muscles
• liver converts glycogen to glucose, raising blood glucose for respiration
• bronchioles dilate / ventilation increases, so more oxygen enters the blood
• blood redirected to muscles by vasodilation to them and vasoconstriction elsewhere
• Max 4

• rest: 0.50 × 14 = 7.0 dm³ min⁻¹
• exercise: 1.80 × 26 = 46.8 dm³ min⁻¹
• increase in oxygen consumption = 1.80 − 0.30 = 1.50
• (1.50 ÷ 0.30) × 100
• 500 (%)

• soda lime absorbs the CO₂ breathed out
• so the fall in volume of the trace is due to oxygen being used / so the volunteer does not rebreathe CO₂
• precautions, one mark each, max 2:
• sterilise / use a clean mouthpiece
• check the volunteer has no health problem (e.g. asthma, heart condition) before exercise
• volunteer can stop at any time / stop if they feel dizzy or unwell
• keep flames away from the oxygen
• Max 4