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By the end of this lesson you should be able to explain and apply each part of this topic — External Anatomy, Internal Anatomy and Blood Flow, The Cardiac Cycle and Cardiac Output — and use these ideas accurately in exam-style questions.
Spec Mapping — OCR H420 Module 3.1.2 — Transport in animals, content statements covering the external and internal anatomy of the mammalian heart, the cardiac cycle (atrial systole, ventricular systole, diastole), the pressure changes that open and close the AV and semilunar valves, and the calculation of cardiac output (refer to the official OCR H420 specification document for exact wording). This lesson supplies the central pump that powers the circulation described in the previous lesson.
The mammalian heart is a remarkable pump: around the size of a clenched fist, it beats approximately 100,000 times per day without rest, shifting about 7,000 dm³ of blood. It consists of four chambers that work in synchrony to maintain two parallel circulations — pulmonary and systemic — at different pressures. This lesson examines the external and internal anatomy of the heart, the roles of its valves and blood vessels, and explores the cardiac cycle: the sequence of events in a single heartbeat.
William Harvey's De Motu Cordis (1628) is the founding document of cardiac physiology. By dissecting living and dead animals and calculating the volume ejected by the heart per minute (paraphrasing his school of thought, far in excess of the body's total blood volume), Harvey established that the same blood circulates repeatedly — overturning Galenic dogma and laying the groundwork for two centuries of further work. Starling then formalised the relationship between end-diastolic ventricular volume and stroke volume: more filling produces greater contraction (Starling's law of the heart, 1918), an intrinsic regulatory mechanism that matches output to venous return without nervous input.
Key Definitions:
- Cardiac cycle — the sequence of events making up one complete heartbeat, lasting about 0.8 seconds at rest in humans.
- Systole — phase during which a chamber of the heart contracts.
- Diastole — phase during which a chamber relaxes.
- Stroke volume — the volume of blood ejected from the left ventricle during one contraction (~70 cm³ at rest).
- Cardiac output — stroke volume × heart rate; typically ~5 dm³ min⁻¹ at rest.
The heart lies between the lungs in the mediastinum, slightly tilted to the left. It is enclosed in a tough, fibrous sac — the pericardium — which is separated from the outer surface of the heart (the epicardium) by a thin film of pericardial fluid that lubricates movement. The heart's own muscular wall is the myocardium, composed of cardiac muscle (myocytes). Its inner lining is the endocardium, a smooth squamous layer continuous with the endothelium of the blood vessels.
The myocardium itself requires its own blood supply; this is provided by the coronary arteries, which branch off the aorta immediately above the aortic valve. Blockage of a coronary artery causes myocardial infarction — heart attack — because the cardiac muscle downstream is starved of oxygen.
The mammalian heart has four chambers:
The left ventricle has a much thicker muscular wall than the right ventricle, because it must generate enough pressure to push blood through the entire systemic circulation. The right ventricle only needs to push blood a short distance through the pulmonary circulation and at much lower pressure — high pressure would damage the delicate pulmonary capillaries.
The left and right sides are completely separated by the septum, preventing mixing of oxygenated and deoxygenated blood (this is essential for an efficient double circulation).
The wall-thickness ratio (typically left:right ≈ 3:1 in healthy adults) is a direct consequence of the pressure each ventricle generates. The left ventricle is essentially a high-pressure muscular pump, while the right ventricle is a low-pressure volume pump matched to the pulmonary circuit. The two ventricles eject the same stroke volume each beat — in a healthy person, mismatches would rapidly cause pulmonary or systemic congestion — but the work done is very different.
flowchart TB
VC[Venae cavae] --> RA[Right atrium]
RA -->|Tricuspid valve| RV[Right ventricle]
RV -->|Pulmonary valve| PA[Pulmonary artery]
PA --> LUNGS[Lungs]
LUNGS --> PV[Pulmonary veins]
PV --> LA[Left atrium]
LA -->|Bicuspid / mitral valve| LV[Left ventricle]
LV -->|Aortic valve| AO[Aorta]
AO --> BODY[Body tissues]
BODY --> VC
Valves prevent backflow, ensuring one-way movement of blood:
At a resting heart rate of 75 beats per minute, each cardiac cycle lasts 60/75 ≈ 0.8 s. It is conventionally divided into three phases:
flowchart TB
A[Atrial systole: atria contract, ventricles fill final 20 percent]
B[Ventricular systole: AV valves close, pressure rises, semilunar valves open, blood ejected]
C[Diastole: ventricles relax, semilunar valves close, AV valves open, passive filling]
A --> B --> C --> A
| Stage | Left atrial pressure | Left ventricular pressure | Aortic pressure | AV valve | Semilunar valve |
|---|---|---|---|---|---|
| Atrial systole | Rising | Low | Falling | Open | Closed |
| Early ventricular systole | Falling | Rising rapidly | Falling | Closes | Closed |
| Late ventricular systole | Low | Above aortic | Rising | Closed | Open |
| Early diastole | Rising slowly | Falling rapidly | Falling slowly | Closed | Closes |
| Late diastole | Above ventricular | Low | Slowly falling | Opens | Closed |
Exam Tip: OCR frequently asks students to interpret a pressure–time graph showing left atrial, left ventricular and aortic pressures during one cardiac cycle. Practise spotting where each valve opens and closes — this is where the lines cross.
Cardiac output (CO) is the volume of blood ejected by the heart per unit time:
CO=Stroke volume×Heart rate
For a typical adult: 70 cm³ × 75 min⁻¹ ≈ 5250 cm³ min⁻¹ = 5.25 dm³ min⁻¹ at rest. During strenuous exercise, both stroke volume and heart rate rise, and cardiac output can exceed 25 dm³ min⁻¹ in a fit adult.
The cardiac output equation is one of the most frequently examined calculations in this module, and it rewards fluency in rearranging it as well as simply plugging in. Start with a resting adult whose stroke volume is 70 cubic centimetres and whose heart rate is 75 beats per minute. The cardiac output is the product of the two, which is 70 multiplied by 75, giving 5250 cubic centimetres per minute. Converting to the more usual units, that is 5.25 cubic decimetres per minute, or a little over five litres of blood passing through the heart every minute at rest. A memorable way to appreciate the scale of this is that the entire blood volume of the body, about five litres, is pumped around roughly once every minute even while sitting still.
Now let the same person begin vigorous exercise. Suppose their heart rate rises to 180 beats per minute and, because of increased venous return stretching the ventricle in accordance with Starling's law, their stroke volume rises to 120 cubic centimetres. The new cardiac output is 120 multiplied by 180, which is 21600 cubic centimetres per minute, or 21.6 cubic decimetres per minute. Comparing this with the resting value, cardiac output has risen from 5.25 to 21.6 cubic decimetres per minute, an increase of more than fourfold. Once again the crucial teaching point is that the body recruits both variables at once: heart rate more than doubles and stroke volume rises by over half, and because the two multiply, the combined effect is dramatic. This extra output is what delivers the additional oxygen that exercising muscle demands.
The equation is equally useful rearranged. If a physiologist measures a patient's cardiac output as 4.8 cubic decimetres per minute using a clinical technique, and independently records a heart rate of 60 beats per minute, the stroke volume can be found by dividing cardiac output by heart rate. That is 4800 cubic centimetres per minute divided by 60 beats per minute, which gives a stroke volume of 80 cubic centimetres per beat. Being confident to move between the three quantities in any direction, rather than only calculating cardiac output from the other two, is exactly the flexibility that examiners test with unfamiliar data.
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