Haemoglobin and Oxygen Transport — AQA A-level Biology

SPEC 3.3.4.1 - MASS TRANSPORT IN ANIMALS

The Heart & Blood Vessels

Double circulation, the cardiac cycle, structure of arteries, veins and capillaries, and the formation of tissue fluid.

Why a Double Circulation?

Mammals have a closed, double circulatory system: blood passes through the heart twice in one complete circuit.

Pulmonary Circuit

Right side of heart → lungs → left side of heart. Picks up O₂, loses CO₂. Low pressure — protects delicate alveolar capillaries.

Systemic Circuit

Left side of heart → body → right side of heart. Delivers O₂/nutrients, collects CO₂/waste. High pressure — needed to reach every cell.

Why Two Loops?

A single circulatory system (like in fish) would lose pressure at the gas-exchange organ. A double circuit lets the heart re-pressurise blood between lungs and body, delivering blood rapidly to respiring tissues at high pressure.

Named Blood Vessels

AQA requires names only of: coronary arteries and vessels entering/leaving the heart, lungs, and kidneys. Learn these exactly.

VesselFromToBlood

AortaLeft ventricleBodyOxygenated

Vena cavaBodyRight atriumDeoxygenated

Pulmonary arteryRight ventricleLungsDeoxygenated ⚠

Pulmonary veinLungsLeft atriumOxygenated ⚠

Coronary arteriesAortaHeart muscle itselfOxygenated

Renal arteryAortaKidneyOxygenated

Renal veinKidneyVena cavaDeoxygenated

The Exception

The pulmonary artery carries DEOXYGENATED blood and the pulmonary vein carries OXYGENATED blood — the only artery/vein pair where this is the case. The 2024 examiner report noted students still mix these up.

Gross Structure of the Heart

Four Chambers

Right atrium — receives deoxygenated blood from vena cava

Right ventricle — pumps deoxygenated blood to lungs via pulmonary artery

Left atrium — receives oxygenated blood from pulmonary vein

Left ventricle — pumps oxygenated blood to body via aorta

Why the Left Ventricle Has a Thicker Wall

The left ventricle pumps blood around the entire systemic circulation at high pressure. The right ventricle only pumps to the lungs (lower pressure, shorter distance). Thicker muscular wall → greater force of contraction → higher pressure generated.

Valves

Atrioventricular (AV) valves: between atria and ventricles. Left side = bicuspid (2 flaps); right side = tricuspid (3 flaps). Prevent backflow into atria during ventricular contraction.

Semilunar (SL) valves: in the aorta and pulmonary artery. Prevent backflow into ventricles when ventricles relax.

Heart — Labelled Diagram

The Cardiac Cycle

CARDIAC CYCLE

The sequence of events making up one complete heartbeat. Consists of diastole (relaxation, chambers fill) and systole (contraction, blood ejected).

The cycle has three main phases, each controlled by pressure differences that open and close the valves:

1

Atrial systole — atria contract, pushing the last of the blood into the ventricles.

2

Ventricular systole — ventricles contract, ejecting blood into aorta and pulmonary artery.

3

Diastole — whole heart relaxes, blood flows passively into atria and through to ventricles.

Golden Rule

Valves open and close passively — driven by pressure differences. Always state which pressure exceeds which when explaining a valve movement.

Phase 1: Atrial Systole

1

Both atria contract simultaneously.

2

Atrial pressure exceeds ventricular pressure.

3

AV valves are open → blood is pushed from atria into ventricles, topping them up.

4

Semilunar valves remain closed — aortic/pulmonary pressure still exceeds ventricular pressure.

Valve Status

AV valves: OPEN · Semilunar valves: CLOSED

Phase 2: Ventricular Systole

1

Ventricles contract (atria now relaxed).

2

Ventricular pressure rises rapidly and exceeds atrial pressure → AV valves close (the "lub" sound) → prevents backflow into atria.

3

Ventricular pressure continues to rise and exceeds aortic / pulmonary artery pressure → semilunar valves open.

4

Blood is ejected into the aorta (from left ventricle) and pulmonary artery (from right ventricle).

Valve Status

AV valves: CLOSED · Semilunar valves: OPEN

Phase 3: Diastole

1

Whole heart relaxes. Ventricular pressure falls rapidly.

2

Aortic/pulmonary artery pressure exceeds ventricular pressure → semilunar valves close (the "dub" sound) → prevents backflow into ventricles.

3

Blood returns to the atria via the venae cavae and pulmonary veins. Atrial pressure rises.

4

Atrial pressure exceeds ventricular pressure → AV valves open → blood flows passively into ventricles, filling them before the next atrial systole.

Valve Status

AV valves: OPEN (once ventricular pressure drops below atrial) · Semilunar valves: CLOSED

Reading the Pressure Graph

How to Read It

Whenever two lines cross, a valve opens or closes — because the pressure difference reverses.

Ventricular > atrial → AV valves shut.

Ventricular > aortic → semilunar valves open.

Ventricular < aortic → semilunar valves shut.

Ventricular < atrial → AV valves open.

Cardiac Output

CARDIAC OUTPUT (CO)

The volume of blood pumped by the left ventricle per minute.

CO = Stroke Volume × Heart Rate

Stroke Volume (SV)

Volume of blood ejected by the left ventricle per beat. Typical resting value: ~70 cm³.

Heart Rate (HR)

Number of beats per minute. Typical resting value: ~70 bpm.

Worked Example

Resting CO = 70 cm³ × 70 bpm = 4900 cm³ min⁻¹ (~5 dm³ min⁻¹).

During vigorous exercise, both SV and HR rise → CO can reach 25–30 dm³ min⁻¹.

Exam Skill

AQA regularly asks you to rearrange this equation — e.g. "given CO and HR, calculate SV." Practise SV = CO ÷ HR and HR = CO ÷ SV.

Blood Vessel Structure

The structure of each vessel type matches its function. Always link feature → function.

VesselWall StructureFunction

ArteryThick wall; much elastic tissue AND smooth muscle; narrow lumen; smooth endotheliumCarry blood from heart at high pressure; elastic tissue stretches in systole and recoils in diastole to smooth/maintain pressure

ArterioleMostly smooth muscle , less elastic tissueSmooth muscle contracts ( vasoconstriction ) or relaxes ( vasodilation ) to control blood flow to tissues

CapillaryOne-cell-thick endothelium only; no muscle/elastic tissue; very narrow lumen (~RBC diameter)Exchange of O₂, CO₂, nutrients and waste between blood and tissues — short diffusion distance, slow flow

VenuleThin wall; little muscle or elastic tissueCollects blood from capillary beds

VeinThin wall; wide lumen; pocket valvesReturn blood to heart at low pressure; valves prevent backflow; skeletal muscle contraction squeezes veins to push blood along

Arteries: Structure → Function

1

Thick muscular wall → withstands the high pressure generated by ventricular systole without tearing.

2

Elastic tissue → stretches when blood is pumped in (storing pressure), then recoils during diastole. This maintains a continuous flow of blood and smooths out pressure fluctuations.

3

Smooth endothelium → reduces friction, allowing smooth blood flow.

4

Narrow lumen (compared to veins) → helps maintain high pressure.

2024 Examiner Report

Students attributed "pumping" or "vasoconstriction" to the aorta — muscle action (vasoconstriction) occurs in arterioles, not arteries/aorta.

Only mentioning recoil missed marks — the examiner wanted both stretch AND recoil.

Many said recoil increases pressure — it doesn't. Recoil maintains/smooths pressure.

Few mentioned the smooth endothelium reducing friction — an easy mark often missed.

Capillaries: Exchange Surfaces

Capillaries form dense networks (capillary beds) throughout every tissue. Their structure is adapted for efficient exchange.

Structural Adaptations

Wall only one endothelial cell thick → very short diffusion distance

Lumen very narrow (~7 μm — barely wider than a red blood cell) → RBCs pass in single file, travel slowly, and are pressed against the capillary wall → short diffusion distance from Hb to tissue cells

Extensive branching → very large total surface area across all capillary beds

Many capillaries in a bed → reduces blood velocity → more time for exchange

Capillary walls are leaky — small gaps between endothelial cells allow some plasma (but not plasma proteins) to pass into tissue fluid

Synoptic Link — Fick's Law

Capillaries show all three factors from Fick's Law (Topic 3.3.1): large surface area, short diffusion distance, steep concentration gradient (maintained by constant blood flow). Great Paper 3 essay example.

Formation & Return of Tissue Fluid

TISSUE FLUID

The fluid that bathes cells. Formed by ultrafiltration from blood capillaries. Similar composition to plasma but contains no large plasma proteins and no red blood cells.

1

Arterial end of capillary: Hydrostatic pressure (blood pressure) is HIGH. It forces water and small dissolved substances out through the capillary wall by ultrafiltration. Large plasma proteins are too big to pass through the wall and remain in the blood.

2

In the tissues: fluid bathes cells → exchange of O₂, glucose, amino acids, CO₂, urea between fluid and cells.

3

Venous end of capillary: Hydrostatic pressure has DROPPED. Retained plasma proteins lower the water potential of the blood → water re-enters capillary by osmosis down the water potential gradient.

4

Excess tissue fluid (~10%): drained by lymphatic capillaries → becomes lymph → returned to the blood via the thoracic duct near the subclavian veins.

Examiner Alert

Water is returned to the capillary by osmosis driven by the low water potential of plasma (due to retained proteins) — NOT simply "because blood has more water." The plasma proteins are the key — they stay in the capillary throughout.

Cardiovascular Disease: Risk Factors

AQA requires you to analyse and evaluate data on risk factors and CVD incidence — and to distinguish correlation from causation.

Risk FactorEffect on CVD Risk

Diet (high saturated fat, high salt)Raises blood cholesterol and blood pressure

High blood pressure (hypertension)Damages artery walls → initiates atheroma formation

SmokingNicotine raises blood pressure; CO reduces O₂ carriage; damages endothelium

High blood cholesterol (LDLs)Promotes atheroma deposition in artery walls

Lack of exerciseAssociated with obesity, raised BP, reduced cardiac efficiency

Genetic factorsFamily history of CVD increases individual risk

Correlation ≠ Causation

AQA always wants you to distinguish these. A correlation in data doesn't prove one thing causes another — there may be confounding variables or a common cause. Causation requires a plausible biological mechanism AND controlled experiments.

Structuring Extended Answers

A typical AQA question: "Explain how the structure of arteries is related to their function. (5 marks)"

DEEP: Arteries Structure-Function

DDefine: Arteries carry blood away from the heart at high pressure in the systemic and pulmonary circulations.

EExplain mechanism: Thick wall with smooth muscle withstands high pressure. Elastic tissue stretches during ventricular systole and recoils during diastole, smoothing/maintaining pressure between beats. Smooth endothelium reduces friction.

EEvidence/Example: Aorta has especially thick walls because it receives blood ejected directly from the left ventricle at highest pressure. The pulse you feel at your wrist is the elastic recoil of arterial walls after each ventricular systole.

PPractical/Quantitative: If elastic tissue is damaged (e.g. aneurysm), the vessel cannot stretch/recoil properly → pressure surges become dangerous and can cause rupture.

Practice Question

A person's resting cardiac output is 4800 cm³ min⁻¹ and their stroke volume is 60 cm³.

(a) Calculate their resting heart rate. [2 marks]

(b) During exercise, cardiac output rises to 20 dm³ min⁻¹ and heart rate to 160 bpm. Calculate the new stroke volume. [2 marks]

(c) Explain how the heart can increase cardiac output during exercise. [3 marks]

(a)

HR = CO ÷ SV = 4800 ÷ 60 = 80 bpm (2 marks: 1 for rearrangement, 1 for answer with units)

(b)

SV = CO ÷ HR. Convert units: 20 dm³ = 20 000 cm³. SV = 20 000 ÷ 160 = 125 cm³ (2 marks: 1 for unit conversion, 1 for answer)

(c)

Heart rate increases (MP1) AND stroke volume increases (MP2); both raise CO because CO = SV × HR (MP3). Either: ventricles contract more forcefully / more blood returns to the heart during exercise so ventricles fill more.

Summary

The Essentials

Double circulation: pulmonary (low P) + systemic (high P). Heart has 4 chambers; left ventricle wall thicker (pumps to body).

Named vessels: aorta, vena cava, pulmonary artery/vein, coronary arteries, renal artery/vein. Pulmonary vessels break the usual "artery = oxygenated" rule.

Cardiac cycle: atrial systole → ventricular systole → diastole. Valves open/close due to pressure differences .

CO = SV × HR — know how to rearrange.

Arteries: thick wall, elastic tissue (stretch AND recoil), smooth muscle, narrow lumen, smooth endothelium.

Arterioles: smooth muscle → vasoconstriction/dilation controls local flow.

Capillaries: one-cell-thick endothelium → short diffusion distance.

Veins: wide lumen, pocket valves, thin wall.

Tissue fluid: formed by ultrafiltration (arterial end, high hydrostatic P); returned by osmosis (venous end, low water potential from retained plasma proteins) + lymphatic drainage.

Top Exam Tips

For valves: always state which pressure exceeds which.

For arteries: mention both stretch AND recoil, and say recoil maintains/smooths pressure (doesn't raise it).

Don't confuse arteries (thick wall, elastic) with arterioles (smooth muscle for vasoconstriction).

For tissue fluid return: the plasma proteins are the reason — not just "blood has more water."