Control of the heartbeat and cardiac outputEdexcel A-Level Biology A: Revision notes
Section 1
The heart is myogenic
Cardiac muscle is myogenic: it contracts and relaxes rhythmically without needing a nerve impulse. A heart removed from the body and kept in oxygenated saline keeps beating for hours.
The beat is initiated by the sinoatrial node (SAN), a patch of specialised muscle in the wall of the right atrium. It is the heart's pacemaker. Nerves only change the rate at which the SAN fires.
Do not say the heart 'has its own nerves'. Myogenic means the muscle itself generates the excitation.
Section 2
Coordinating the beat
- The SAN sets off a wave of depolarisation across both atria, so they contract (atrial systole).
- A layer of non-conducting fibrous tissue between atria and ventricles stops the wave crossing directly.
- The wave reaches the atrioventricular node (AVN), which delays it by about 0.1 s so the atria finish emptying before the ventricles contract.
- The wave passes down the bundle of His in the septum to the apex.
- Purkyne fibres carry it up through the ventricle walls, so the ventricles contract from the apex upwards and push blood out through the arteries.
The AVN delay is the idea examiners most often want: it lets the atria empty into the ventricles before ventricular systole.
Section 3
Electrocardiograms (ECGs)
An electrocardiogram (ECG) records the electrical activity of the heart from electrodes on the skin.
- P wave: depolarisation of the atria.
- QRS complex: depolarisation of the ventricles.
- T wave: repolarisation of the ventricles.
Heart rate = 60 ÷ time between successive R peaks (s). An R–R interval of 0.50 s gives 120 beats per minute.
ECGs help diagnose problems: tachycardia (resting rate above 100 beats per minute), bradycardia (below 60), atrial fibrillation (irregular timing, no clear P waves), ventricular fibrillation (chaotic activity, no QRS pattern) and ectopic beats (an extra early beat followed by a pause). Damage after a heart attack can change the shape of the trace.
Section 4
Cardiac output
Cardiac output (CO) is the volume of blood pumped by one ventricle per minute.
CO = stroke volume × heart rate
Stroke volume is the volume pumped per beat. Worked example: stroke volume 70 cm³ and heart rate 72 beats per minute gives CO = 70 × 72 = 5040 cm³ min⁻¹ (5.04 dm³ min⁻¹). Rearranged, stroke volume = CO ÷ heart rate.
During exercise both stroke volume and heart rate rise, so cardiac output can increase several times.
Check units: 1 dm³ = 1000 cm³. Convert before comparing values.
Section 5
Controlling heart rate
The cardiovascular control centre in the medulla oblongata adjusts heart rate. Chemoreceptors in the carotid arteries and aorta detect a fall in pH (more CO₂ from respiring muscle). They send impulses to the centre.
- Impulses along the sympathetic (accelerator) nerve to the SAN increase heart rate.
- Impulses along the vagus (parasympathetic) nerve to the SAN decrease it.
Once CO₂ falls the heart rate returns to normal: negative feedback.
Section 6
Ventilation and Core practical 17
The ventilation centre in the medulla sends impulses to the diaphragm and intercostal muscles. When blood CO₂ rises (pH falls), chemoreceptors raise the rate of impulses, so breathing becomes faster and deeper. Increased ventilation and cardiac output deliver oxygen to tissues quickly and remove CO₂.
In the spirometer investigation, a trace of gas volume is recorded while the subject breathes through a chamber with soda lime absorbing CO₂.
- Tidal volume: volume of each breath.
- Minute ventilation = tidal volume × breathing rate (dm³ min⁻¹).
- Oxygen consumption is the gradient of the trace (the soda lime means the fall is only due to oxygen being used).
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Control of the heartbeat and cardiac output
- A researcher removes the heart from an anaesthetised frog and places it in oxygenated saline solution at room temperature. All nerves to the heart have been cut. The isolated heart continues to beat rhythmically for several hours.Explain why the isolated heart continues to beat after its nerves have been cut, naming the structure that sets its rhythm.2 marks
- A 55-year-old man attends a cardiology clinic complaining of a fluttering sensation in his chest. A technician attaches electrodes to his chest and records an electrocardiogram (ECG) while he rests. The trace shows a normal sequence of P wave, QRS complex and T wave, with a constant interval of 0.50 s between the R peaks of successive heartbeats.Another patient's ECG shows no regular P waves, QRS complexes or T waves, only rapid, irregular, disorganised electrical activity. Deduce what this indicates about the ventricles and explain why this is a medical emergency.2 marks
- A student volunteers for a spirometer investigation. She breathes from a closed spirometer chamber containing soda lime, which absorbs carbon dioxide, and a pen recorder traces the volume of gas in the chamber. At rest her tidal volume is 0.50 dm³ and her breathing rate is 14 breaths per minute. After five minutes of cycling these values are 2.0 dm³ and 28 breaths per minute. The trace shows her oxygen consumption rising from 0.30 dm³ min⁻¹ at rest to 2.4 dm³ min⁻¹ during cycling.Calculate the student's minute ventilation at rest and during cycling, and state how many times greater it was during cycling.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).