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3.5.2Exchange and Transport

The Human Heart — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 35 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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Lesson summary

3.5.2 The Human Heart — AQA A Level Biology 7402 notes: Describe the structure of the heart and explain how the cardiac cycle, with its valves and pressure changes, moves blood in one direction.The heart is a muscular pump made of cardiac muscle, which contracts rhythmically with…

  • ventricles
  • Atrioventricular (AV) valves
  • Semilunar valves
  • coronary arteries
  • Atrial systole
  • Ventricular systole

Section 1 of 9Structure of the heart

The heart is a muscular pump made of cardiac muscle, which contracts rhythmically without fatigue.

Structure of the heart
Atria
It has four chambers: two thin-walled atria that receive blood from the veins, and two thick-walled ventricles that pump blood into the arteries.
The right side pumps blood to the lungs, and the left to the whole body, so the wall of the left ventricle is thicker, because it generates higher pressure.
Septum
A septum separates the two sides, preventing the oxygenated and deoxygenated blood from mixing.
Structure of the heart
Atrioventricular (AV) valves
(tricuspid on the right, bicuspid on the left) prevent backflow into the atria; they are held by tendinous cords (tendons) attached to papillary muscles.
Semilunar valves
Semilunar valves in the aorta and pulmonary artery prevent backflow into the ventricles.
Coronary arteries
The coronary arteries supply the cardiac muscle with oxygen and glucose.

Section 2 of 9The cardiac cycle

1
Atrial systole
Atrial systole the atria contract, increasing pressure and pushing the remaining blood into the ventricles through the open AV valves.
2
Ventricular systole
Ventricular systole the ventricles contract; pressure rises above that in the atria so the AV valves shut (the first heart sound, "lub"); pressure then exceeds that in the aorta and pulmonary artery, so the semilunar valves open and blood is ejected.
3
Diastole
Diastole both atria and ventricles relax; the pressure in the ventricles falls below that in the arteries, so the semilunar valves shut (second sound, "dub"); blood from the veins flows into the atria and, once the ventricular pressure falls lower than that of the atria, passes through the AV valves into the ventricles.

Valves open and close passively as a result of the pressure differences, so blood always flows from high pressure to low pressure and in one direction.

Section 3 of 9Control of the heartbeat

Heart muscle is myogenic: it initiates its own contraction. The sequence is:

1Sinoatrial node (SAN)
The sinoatrial node (SAN) in the wall of the right atrium sends out a wave of electrical excitation across both atria, making them contract.
2
A layer of non-conducting fibrous tissue between atria and ventricles prevents the wave from passing directly, so there is a short delay.
3Atrioventricular node (AVN)
The wave reaches the atrioventricular node (AVN) which, after a delay of about 0.1 s, sends impulses down the bundle of His in the septum.
4Purkyne fibres
The impulse spreads along the Purkyne fibres to the walls of the ventricles, causing them to contract from the apex upwards, which pushes blood up and out through the arteries.

Section 4 of 9Pressure and volume data

StageAtrial pressure vs ventricleAV valveSemilunar valve
Atrial systoleAtrium higherOpenClosed
Early ventricular systoleVentricle higherClosedClosed
Late ventricular systoleVentricle higher than arteryClosedOpen
DiastoleAtrium higher after pressure dropOpensClosed

Cardiac output = stroke volume × heart rate. At rest, stroke volume is about 70 cm3 and the heart rate is about 70 beats per minute, so cardiac output is about 4.9 dm3 min−1. The cycle at 70 beats per minute takes 60 ÷ 70 = 0.86 s.

Section 5 of 9Reading pressure graphs and ECGs

A pressure graph for the left side has the left ventricle trace rising from about 0 to about 16 kPa in systole, while the aortic pressure follows it above about 12 kPa.

The point where the ventricular line crosses the aortic line is where the semilunar valve opens, and where it falls below shows where it shuts.
Electrocardiogram (ECG)
An electrocardiogram (ECG) records electrical activity: the P wave is atrial depolarisation, the QRS complex is ventricular depolarisation and the T wave is ventricular repolarisation.
A fast, regular heart rate is tachycardia, a slow one bradycardia, and an irregular one an arrhythmia, for example fibrillation.

Section 6 of 9Worked analysis of a pressure trace

A typical exam item shows pressures in the left atrium, left ventricle and aorta against time over 0.8 s.

1
Work through it in order.
2
When the ventricular line rises above the atrial line, the AV valve closes: this is at the start of ventricular systole.
3
When the ventricular line crosses above the aortic line, the semilunar valve opens, and the ventricular pressure peaks at about 16 kPa.
4
When the ventricular line falls below the aortic line, the semilunar valve shuts, producing a small dip (the dicrotic notch) in the aortic trace.
5
When the ventricular pressure falls below the atrial, the AV valve opens and the ventricle fills.

To calculate the time for each phase, read off the times on the x axis and subtract. If the heart rate is 75 beats per minute, one cycle lasts 60 ÷ 75 = 0.8 s. Systole lasts about 0.3 s and diastole about 0.5 s at rest; at higher heart rates, it is mainly diastole that becomes shorter.

The heart rate is regulated by the medulla through the autonomic nervous system: the sympathetic nerve increases the rate of SAN firing (noradrenaline), and the parasympathetic vagus nerve decreases it (acetylcholine). Chemoreceptors in the carotid arteries and aorta detect CO2 and pH, and baroreceptors detect pressure; adrenaline from the adrenal glands also raises heart rate.

Section 7 of 9Exam-style questions

  1. Explain how the valves in the heart ensure one-way flow of blood during ventricular systole. [4]4 marks

  2. Describe how the heart's contraction is coordinated by the SAN and AVN. [4]4 marks

  3. A person has a stroke volume of 75 cm3 and a cardiac output of 5.25 dm3 min−1. Calculate the heart rate. [2]2 marks

Section 8 of 9Key terms

TermMeaning
SystoleContraction phase of the cardiac cycle.
DiastoleRelaxation phase of the cardiac cycle.
Sinoatrial nodeGroup of cells in the right atrium that initiates the heartbeat.
Atrioventricular nodeTissue that relays the impulse to the ventricles after a delay.
MyogenicContraction that starts in the muscle itself.
Cardiac outputVolume of blood pumped by a ventricle per minute.
Stroke volumeVolume of blood pumped per beat.

Section 9 of 9Exam technique

1
Writing that valves are opened by nerves or muscle; they respond to pressure differences.
2
Saying the left ventricle is thicker because it pumps more blood; it pumps against a higher pressure.
3
Stating that the SAN controls the ventricles directly.
4
Forgetting the delay at the AVN.
5
Confusing systole (contraction) with diastole (relaxation).

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