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This lesson covers the structure and function of the mammalian heart and the circulatory system as required by the Edexcel A-Level Biology specification (9BI0). You need to understand the double circulatory system, the detailed anatomy of the heart, and how blood flows through the heart chambers, valves, and associated vessels. You should also be prepared for the Edexcel required practical involving dissection of the mammalian heart.
Mammals are large, metabolically active organisms. As discussed in Lesson 1, their small surface area to volume ratio means that diffusion alone is far too slow to transport substances between exchange surfaces and cells deep within the body. A circulatory system is therefore essential to:
Mammals have a double circulatory system, meaning that blood passes through the heart twice for each complete circuit of the body.
| Circuit | Description |
|---|---|
| Pulmonary circulation | Right side of the heart → lungs → left side of the heart. Deoxygenated blood is pumped to the lungs for gas exchange and returns as oxygenated blood. |
| Systemic circulation | Left side of the heart → body tissues → right side of the heart. Oxygenated blood is pumped to all body tissues and returns as deoxygenated blood. |
Key Definition: Double circulatory system — a circulatory arrangement in which blood passes through the heart twice during one complete circuit of the body, with separate pulmonary and systemic circuits.
The heart is a muscular organ located in the thoracic cavity, between the lungs, slightly to the left of the midline. It is enclosed in a tough, protective membrane called the pericardium.
The heart has four chambers:
| Chamber | Position | Function |
|---|---|---|
| Right atrium | Upper right | Receives deoxygenated blood from the venae cavae |
| Right ventricle | Lower right | Pumps deoxygenated blood to the lungs via the pulmonary artery |
| Left atrium | Upper left | Receives oxygenated blood from the pulmonary veins |
| Left ventricle | Lower left | Pumps oxygenated blood to the body via the aorta |
The septum is a thick wall of muscle that separates the left and right sides of the heart. It prevents mixing of oxygenated and deoxygenated blood.
Exam Tip: If asked to identify the left and right ventricles on a photograph or dissection, remember that the left ventricle has a much thicker wall. Also remember that diagrams of the heart are drawn as if the person is facing you — so the left side of the heart appears on the right of the diagram.
Valves ensure that blood flows in one direction only through the heart. They open and close in response to pressure differences.
| Valve | Location | Function |
|---|---|---|
| Tricuspid valve | Between right atrium and right ventricle | Prevents backflow from right ventricle to right atrium |
| Bicuspid (mitral) valve | Between left atrium and left ventricle | Prevents backflow from left ventricle to left atrium |
| Pulmonary semilunar valve | At the base of the pulmonary artery | Prevents backflow from pulmonary artery to right ventricle |
| Aortic semilunar valve | At the base of the aorta | Prevents backflow from aorta to left ventricle |
Key Definition: Heart sounds — the "lub-dub" sounds of the heart are caused by the closing of valves. The first sound ("lub") is the AV valves closing; the second sound ("dub") is the semilunar valves closing.
The pathway of blood through the heart follows this sequence:
The following diagram summarises the pathway of blood flow through the heart and lungs:
graph TD
A["Vena Cava"] --> B["Right Atrium"]
B --> C["Right Ventricle"]
C -->|"Pulmonary Artery"| D["Lungs<br/>(gas exchange)"]
D -->|"Pulmonary Vein"| E["Left Atrium"]
E --> F["Left Ventricle"]
F -->|"Aorta"| G["Body"]
G --> A
To examine the internal and external structure of a mammalian heart (typically a pig or lamb heart).
| Feature | Observation |
|---|---|
| Left ventricular wall | Much thicker than right — generates higher pressure for systemic circulation |
| Chordae tendineae | String-like structures attaching valve flaps to papillary muscles |
| Coronary arteries | Visible on the external surface; supply the myocardium |
| Semilunar valves | Pocket-shaped; found at the base of the aorta and pulmonary artery |
Exam Tip: You may be shown a photograph from a heart dissection and asked to identify structures. Practise labelling diagrams and linking each structure to its function. Remember that the left ventricle is thicker, the aorta is the largest artery, and the AV valves have chordae tendineae.
Coronary heart disease (CHD) occurs when the coronary arteries become narrowed or blocked by a build-up of atheroma (fatty deposits including cholesterol) in the artery walls — a process called atherosclerosis.
| Risk factor | Explanation |
|---|---|
| Smoking | Damages the endothelium; increases blood clotting tendency |
| High blood cholesterol (LDL) | Increases atheroma formation |
| High blood pressure | Damages the endothelium; increases the workload of the heart |
| Diet high in saturated fat | Increases LDL cholesterol levels |
| Lack of exercise | Associated with higher blood pressure and cholesterol |
| Genetics | Family history of CHD increases risk |
| Obesity | Associated with high blood pressure, high cholesterol, and diabetes |
| Feature | Detail |
|---|---|
| Circulatory type | Double (pulmonary + systemic) |
| Chambers | 4 — RA, RV, LA, LV |
| AV valves | Tricuspid (right), bicuspid/mitral (left) |
| Semilunar valves | Pulmonary, aortic |
| Thickest wall | Left ventricle |
| Blood supply to heart | Coronary arteries |
| CHD | Atherosclerosis of coronary arteries → myocardial infarction |
Detailed knowledge of the heart's structure, the double circulatory system, and coronary heart disease is essential — these are among the most commonly examined topics in Edexcel A-Level Biology.
This material sits centrally within Edexcel 9BI0 Topic 7 (Run for your life — Exchange and Transport), where statements on the mammalian circulatory system require candidates to describe the gross anatomy of the heart, justify the chamber-wall thickness pattern in pressure terms, account for the existence of a double circulation, and explain valve operation through pressure-gradient logic. It is the structural foundation for the next three lessons of the topic: the cardiac cycle (atrial systole, ventricular systole, diastole) and its control by the SAN-AVN-Purkyne pathway (lesson 5), the matched structure-function of arteries, veins and capillaries (lesson 6), and the loading/unloading behaviour of haemoglobin along the systemic and pulmonary circuits (lesson 7). It is heavily synoptic with Topic 5 (Energy for Biological Processes) because cardiac output exists to deliver oxygen at the rate cellular respiration consumes it, and with Topic 8 (Grey Matter) because renal blood flow — set by perfusion of the renal arteries from the systemic circulation — determines glomerular filtration rate. Examiners pair anatomical recall (AO1) with quantitative cardiac-output work (AO2) and stretch synthesis on the evolutionary logic of double circulation (AO3). Refer to the official Pearson Edexcel 9BI0 specification document for the exact wording of the relevant statements.
Question (8 marks):
A resting adult has a stroke volume of 70 cm3 and a heart rate of 72 beats per minute.
(a) Calculate the cardiac output in dm3 min−1. (2)
(b) During moderate exercise, stroke volume rises to 110 cm3 and heart rate to 140 beats per minute. Calculate the new cardiac output and the fold-increase from rest. (3)
(c) Explain, in terms of the four chambers of the heart and the double circulatory system, why this fold-increase has to be matched simultaneously by the right and left sides of the heart. (3)
Solution with mark scheme:
(a) Step 1 — apply the cardiac-output equation. Cardiac output (CO) = stroke volume (SV) × heart rate (HR) =70 cm3×72 min−1=5040 cm3 min−1=5.0 dm3 min−1 (to 2 sf).
M1 — correct multiplication. A1 — value with correct units. A common pitfall is leaving the answer in cm3 min−1 when the question asks for dm3 min−1; that costs the A1.
(b) M1 (AO2) — exercise CO =110×140=15400 cm3 min−1=15.4 dm3 min−1.
M1 (AO2) — fold-increase =15400/5040≈3.1×.
A1 — both values with correct units, fold-increase to a sensible number of significant figures.
(c) M1 (AO1) — recall that the two ventricles contract simultaneously during ventricular systole, so each beat ejects approximately equal volumes from left and right ventricles.
M1 (AO2) — apply the double-circulation logic: blood ejected by the left ventricle into the systemic circuit must be matched by blood ejected by the right ventricle into the pulmonary circuit, otherwise blood accumulates in one circuit and is depleted in the other.
A1 (AO3) — conclude that any sustained mismatch between right and left output rapidly causes pulmonary or systemic congestion (the physiological basis of left- and right-sided heart failure); the double circulation is therefore self-balancing only because the two pumps share a single contraction cycle. Many candidates lose marks here by treating the two sides as independent pumps rather than two halves of one synchronised double-pump unit.
Total: 8 marks (M5 A3).
Question (6 marks): Explain how the structure of the mammalian heart is adapted to support a double circulatory system in which the systemic circuit operates at a much higher pressure than the pulmonary circuit.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that the heart has four chambers, with a complete septum separating left and right sides |
| 2 | AO1.2 | Identifying that the left ventricle has a thicker myocardial wall than the right ventricle |
| 3 | AO2.1 | Linking the thicker left-ventricular wall to the generation of higher systolic pressure for the systemic circuit |
| 4 | AO2.1 | Linking the thinner right-ventricular wall to the lower pressure required for the short pulmonary circuit and the protection of delicate alveolar capillaries from hydrostatic damage |
| 5 | AO2.7 | Explaining that the complete septum prevents mixing of oxygenated and deoxygenated blood and prevents pressure equilibration between the two circuits |
| 6 | AO3.1 | Synthesis: the double circulation is mechanically possible only because the heart is a single double-pump unit in which two pressure regimes are isolated by the septum yet synchronised by a shared contraction cycle |
Total: 6 marks (AO1 = 2, AO2 = 3, AO3 = 1). Edexcel structure–function questions of this type reliably split AO marks roughly 30/50/20 across AO1/AO2/AO3.
Lesson 5 (Cardiac cycle and control of heart rate). The chamber and valve anatomy described here is the static stage on which the dynamic pressure-volume cycle of atrial systole, ventricular systole and diastole plays out. Valve opening and closure in lesson 5 are entirely driven by the chamber-pressure inequalities generated by the wall structure described here.
Lesson 6 (Blood vessels and blood). The pressure regimes set by the left and right ventricles are precisely what the structural differences between elastic arteries, muscular arteries, capillaries and veins are evolved to handle. The thick elastic wall of the aorta absorbs the systolic pressure pulse from the thick left ventricle; the thin-walled pulmonary artery handles the lower pulmonary pressure generated by the thinner right ventricle.
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