William Harvey proved in 1628 that blood circulates — against 1400 years of accepted wisdom. Understanding how that circulation works, and what happens when it fails, is the foundation of modern cardiology.
Here is a statement many students believe:
"Arteries always carry oxygenated blood and veins always carry deoxygenated blood."
Do you agree or disagree? Write your reasoning. If you agree, explain why. If you disagree, identify a specific exception.
Come back to this at the end of the lesson.
Core Content
For 1400 years after Galen (2nd century AD), the accepted view was that blood was produced in the liver, consumed by the body as fuel, and never returned — a one-way flow. Physicians believed arteries carried air (pneuma), not blood.
In 1628, William Harvey published Exercitatio Anatomica de Motu Cordis — a mathematical demolition of Galen. Harvey calculated that the heart pumps approximately 245 mL of blood per beat. At 72 beats per minute, that is 17.6 litres per minute — more than three times the total blood volume of the body. The liver could not possibly produce blood this fast. Therefore, Harvey argued, the same blood must be circulating continuously.
Harvey demonstrated valves in veins that allowed flow in only one direction, proved the heart pumped blood outward through arteries, and showed blood returned through veins. He could not see capillaries — the microscope had not yet been applied to anatomy — but predicted they must exist. Malpighi confirmed them four years after Harvey's death.
Blood (haemolymph) is pumped from a heart into open body cavities (sinuses), where it bathes organs directly. No capillaries. Returns slowly to the heart.
Examples: Insects, crustaceans, molluscs (except cephalopods)
Advantage: Simple, low energy cost
Limitation: Low pressure — suits less active animals
Blood passes through the heart once per circuit. Heart → gills (gas exchange) → body → heart. Blood pressure drops after gills, limiting delivery speed.
Examples: Fish
Advantage: Sufficient for aquatic animals with lower metabolic demands
Limitation: Low pressure to body after gill capillaries
Blood passes through the heart twice per complete circuit — once through the pulmonary circuit (heart → lungs → heart) and once through the systemic circuit (heart → body → heart). Oxygenated and deoxygenated blood are fully separated.
Examples: Mammals, birds (4-chambered); reptiles and amphibians (partial separation)
Advantage: High, maintained pressure to body tissues — supports high metabolic rate
Single circulation (fish): blood through heart once. Double circulation (mammal): pulmonary + systemic circuits maintain high body pressure. Blue = deoxygenated, Red = oxygenated.
The human heart has four chambers: right atrium, right ventricle, left atrium, left ventricle. The two circuits operate simultaneously:
| Circuit | Path | Function | Blood type |
|---|---|---|---|
| Pulmonary circuit | Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium | Gas exchange — CO₂ released, O₂ absorbed | Deoxygenated → oxygenated |
| Systemic circuit | Left ventricle → aorta → body tissues → vena cava → right atrium | Delivers O₂ and nutrients; collects CO₂ and waste | Oxygenated → deoxygenated |
Wall: Thick, elastic, muscular — three distinct layers (tunica intima, media, externa)
Lumen: Relatively narrow
Blood flow: High pressure, pulsatile — away from heart
Why thick walls? Must withstand and smooth out high-pressure pulses from ventricular contraction
Wall: Thinner, less muscle and elastic tissue
Lumen: Wider than arteries
Blood flow: Low pressure — toward heart. Valves prevent backflow. Skeletal muscle contraction assists flow.
Why valves? Low pressure means blood could pool or flow backward without valves
Wall: Single layer of endothelium — one cell thick
Lumen: Extremely narrow — red blood cells squeeze through single file
Blood flow: Very slow — maximises time for exchange
Why one cell thick? Minimises diffusion distance for O₂, CO₂, nutrients, waste — exchange happens here
The heart contracts in a coordinated sequence called the cardiac cycle. Each cycle consists of:
This cycle is initiated by the sinoatrial (SA) node — the heart's natural pacemaker — located in the right atrium wall. The SA node generates an electrical impulse that spreads across both atria (causing atrial systole), then passes through the atrioventricular (AV) node, which delays the signal briefly before it travels down the Bundle of His and Purkinje fibres to trigger ventricular systole from the apex upward.
An electrocardiogram (ECG) records the electrical activity of the heart. A normal ECG shows a characteristic waveform per heartbeat:
Normal sinus rhythm — three complete cardiac cycles shown. P wave, QRS complex and T wave labelled on first beat.
Human heart cross-section. Right side (blue) handles deoxygenated blood via pulmonary circuit. Left side (red) handles oxygenated blood via systemic circuit. Note: pulmonary artery carries deoxygenated blood; pulmonary veins carry oxygenated blood.
Misconception: Arteries always carry oxygenated blood and veins always carry deoxygenated blood.
Arteries carry blood away from the heart; veins carry blood toward the heart. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. The pulmonary veins carry oxygenated blood from the lungs to the left atrium. Vessels are named for their direction of flow, not their oxygen content.
Misconception: The heart pumps blood by squeezing from the top down.
The ventricles contract from the apex (bottom) upward — triggered by Purkinje fibres spreading from the bottom of the ventricle. This "wringing" motion efficiently ejects blood upward into the aorta and pulmonary artery.
Misconception: The ECG measures the heart's mechanical pumping activity.
The ECG records electrical activity — the depolarisation and repolarisation of cardiac muscle cells. The mechanical pumping (contraction and relaxation) follows each electrical event with a brief delay. An ECG can look normal even if the heart is not pumping effectively (pulseless electrical activity — a medical emergency).
Arteries = away from heart. Veins = toward heart. Pulmonary artery = deoxygenated. Pulmonary vein = oxygenated. Oxygen content does NOT define vessel type.
Activities
A physician records ECGs from three patients. Study the descriptions of each ECG trace and answer the questions.
Write your responses here or in your book.
The blue-ringed octopus has a closed circulatory system with three hearts: one systemic heart and two branchial (gill) hearts. The blood contains haemocyanin (copper-based) rather than haemoglobin (iron-based), giving it a blue colour when oxygenated.
Write your responses here or in your book.
Assessment
1. Which of the following correctly describes the path of deoxygenated blood through the pulmonary circuit?
2. Capillaries are only one cell thick. Which of the following best explains the advantage of this structure?
3. An ECG shows a regular rhythm with a clear P wave before every QRS complex, and a heart rate of 72 bpm. What can be concluded?
4. Veins have valves; arteries do not. The best explanation for this difference is:
5. Double circulation in mammals is an evolutionary advantage over single circulation in fish primarily because:
1. Compare the structure of arteries, veins, and capillaries. For each vessel type, explain how its structure is suited to its function. (3 marks)
1 mark per vessel type: structural feature correctly described and linked to function
2. Trace the complete path of a red blood cell from the right ventricle, through the pulmonary circuit, back to the heart, and then through the systemic circuit to a muscle cell in the leg and back to the heart. Name every structure the cell passes through in order. (3 marks)
1 mark per circuit correctly sequenced with key structures named
3. Explain how an ECG can be used to diagnose atrial fibrillation. In your answer, describe what a normal ECG looks like, what changes are seen in AF, and why AF is clinically dangerous. (3 marks)
1 mark normal ECG description; 1 mark AF changes with mechanism; 1 mark clinical danger
Answers
SA1: Arteries have thick, elastic, muscular walls with three distinct layers. This structure allows them to withstand and buffer the high-pressure pulses generated by ventricular contraction, maintaining smooth blood flow. Veins have thinner walls with less muscle and elastic tissue, and contain valves. The thin walls reflect the lower pressure in veins; the valves prevent backflow of blood as it returns to the heart against gravity at low pressure, assisted by skeletal muscle contractions. Capillaries have walls only one cell (endothelium) thick and a very narrow lumen. This minimises the diffusion distance between blood and surrounding cells, maximising the rate of gas, nutrient, and waste exchange — the primary function of capillaries.
SA2: Right ventricle → pulmonary artery → pulmonary capillaries (in lungs — O₂ absorbed, CO₂ released) → pulmonary vein → left atrium → left ventricle → aorta → systemic arteries → arterioles → leg muscle capillaries (O₂ delivered, CO₂ collected) → venules → femoral vein → inferior vena cava → right atrium.
SA3: A normal ECG shows a distinct P wave (representing atrial depolarisation — SA node firing and atria contracting), followed by a QRS complex (ventricular depolarisation — ventricles contracting), then a T wave (ventricular repolarisation — ventricles relaxing). The rhythm is regular with consistent R-R intervals. In atrial fibrillation, the SA node loses coordinated control of atrial activity. Instead, multiple disorganised electrical impulses fire chaotically throughout the atria. This eliminates the distinct P wave — replaced by irregular low-amplitude fluctuations — and results in QRS complexes appearing at irregular intervals. AF is clinically dangerous because the fibrillating atria fail to contract effectively, allowing blood to pool and stagnate. Pooled blood can clot, and these clots can travel to the brain and cause a stroke — AF increases stroke risk approximately five-fold.
You were asked whether arteries always carry oxygenated blood. The verdict: this is a misconception.
Arteries are defined by direction — they carry blood away from the heart. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. The pulmonary veins carry oxygenated blood from the lungs to the left atrium. Vessels are named for their direction of flow, not their oxygen content.
If you disagreed and identified the pulmonary artery as the exception — excellent. If you agreed, you now have the correct framework: artery = away, vein = toward. Oxygen content depends on which circuit you're in.
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