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.
⏱ 45 min5 dot points5 MC · 3 Short AnswerLesson 8 of 17
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Choose how you work, type your answers below or write in your book.
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.
Know
Open vs closed circulatory systems with examples
Single vs double circulation with examples
Structure and function of arteries, veins and capillaries
Components of blood and their functions
The cardiac cycle, systole and diastole
Understand
Why double circulation is an advantage for active animals
How vessel structure relates to function
How the ECG reflects electrical events in the cardiac cycle
Can Do
Trace blood flow through the double circulatory system
Explain how structural features of vessels suit their function
Analyse ECG data to identify cardiac events and abnormalities
Core Content
The Discovery Story, Harvey and the Circulation of Blood
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.
Why this matters for the HSC: Harvey's story is a perfect example of how quantitative evidence can overturn qualitative tradition. The lesson is not just biological, it's about how science progresses through measurement and falsification.
AnalyseBand 4
Activity 01
Data Analysis, ECG Interpretation
Pattern C, Data Analysis
A physician records ECGs from three patients. Study the descriptions of each ECG trace and answer the questions.
ECG Description
Column B
Calculate the heart rate (beats per minute) for Patient A. Show your working.
Calculate the heart rate for Patient C and compare it to Patient A. What does the difference suggest about Patient C's condition?
Patient B shows no P waves and an irregular rhythm. Identify this condition and explain what has gone wrong with the cardiac conduction system.
Patient C has a prolonged QT interval. Which part of the cardiac cycle does the QT interval correspond to? Why is a prolonged QT interval clinically dangerous?
A doctor says Patient A is healthy. Identify two specific features of the ECG data that support this conclusion.
Write your responses here or in your book.
Interactive
Types of Circulatory Systems
Open Circulatory System
Blood (haemolymph) is pumped from a heart into open body cavities (sinuses), where it bathes organs directly. No capillaries. Returns slowly to the heart.
Limitation: Low pressure, suits less active animals
Single Circulation
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
Double Circulation
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
Why double circulation matters: In single circulation, blood pressure drops as blood passes through gill capillaries. In double circulation, the heart re-pressurises blood after lung gas exchange before sending it to the body, ensuring high-pressure, oxygen-rich delivery to every tissue. This is why mammals and birds can sustain high activity levels that fish cannot.
Single circulation (fish): blood through heart once. Double circulation (mammal): pulmonary + systemic circuits maintain high body pressure. Blue = deoxygenated, Red = oxygenated.
The Human Heart and Double Circulation
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
The misconception resolved: The pulmonary artery carries deoxygenated blood (from heart to lungs) and the pulmonary vein carries oxygenated blood (from lungs to heart). Arteries and veins are defined by direction of flow relative to the heart, not by oxygen content. Arteries carry blood away from the heart; veins carry blood toward the heart.
EvaluateBand 5
Activity 02
Apply to an Unfamiliar Organism
Pattern C, Apply to unfamiliar context
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.
The octopus has a closed circulatory system. Identify one advantage this gives it over an open system for an active predator.
The two branchial hearts pump blood through the gills before the systemic heart pumps it to the body. What type of circulation does this resemble, single or double? Justify your answer.
Haemocyanin is less efficient at carrying oxygen than haemoglobin at body temperature, but functions better in cold, low-oxygen deep-water environments. Explain why the type of oxygen-carrying molecule a species uses reflects its ecological niche.
If the systemic heart of the octopus fails but the branchial hearts continue, predict what would happen to oxygen delivery to body tissues. Use your knowledge of circulatory system design.
Blood clotting, aggregate at wound sites, trigger clotting cascade
Check Your Understanding
Write one sentence summarising the main idea of this section.
The Cardiac Cycle and the ECG
Beyond the syllabus. The heart's conduction system — SA and AV nodes, Bundle of His, Purkinje fibres — and reading an electrocardiogram (P wave, QRS complex, T wave) are extension: Stage 6 Biology never requires ECG interpretation. What is assessed is the structure and function of the heart, vessels and blood as a transport system. Learn the double-circulation story first and treat this card as optional depth.
The heart contracts in a coordinated sequence called the cardiac cycle. Each cycle consists of:
Atrial systole: Both atria contract, pushing blood into ventricles
Ventricular systole: Both ventricles contract, pushing blood into aorta (left) and pulmonary artery (right). Highest blood pressure point.
Diastole: All chambers relax and refill. Lowest blood pressure point.
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:
What it represents
Atrial depolarisation
Ventricular depolarisation
Ventricular repolarisation
Cardiac event
SA node fires → atria contract (atrial systole)
AV node → Bundle of His → Purkinje fibres → ventricles contract (ventricular systole)
Ventricles recover (relax) → diastole begins
Reading heart rate from an ECG: Count the number of complete R peaks (tallest point of QRS complex) in a set time period, or measure the R-R interval in seconds and divide 60 by that value to get beats per minute.
Check Your Understanding
Write one sentence summarising the main idea of this section.
Normal sinus rhythm, three complete cardiac cycles shown. P wave, QRS complex and T wave labelled on first beat.
Real World, Heart Disease & the ECG
Cardiovascular disease is Australia's leading cause of death, responsible for around 18,000 deaths annually. The ECG is the primary tool for diagnosing cardiac abnormalities because each type of dysfunction produces a characteristic change in the waveform. Atrial fibrillation (AF), irregular, chaotic electrical activity in the atria, eliminates the distinct P wave, replacing it with irregular baseline fluctuations. The QRS complexes appear at irregular intervals, producing an unmistakable irregular rhythm. AF affects around 500,000 Australians and increases stroke risk five-fold because blood can pool in the fibrillating atrium and form clots. A prolonged QT interval (abnormally long time between Q and T) indicates delayed ventricular repolarisation, a risk factor for potentially fatal ventricular fibrillation. Many over-the-counter and prescription medications can prolong the QT interval, which is why ECG monitoring is required for some drug therapies. You'll apply ECG analysis in Activity 01 and Short Answer Q3.
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.
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).
Circulatory System Types
Open, haemolymph in sinuses (insects)
Single, heart → gills → body (fish)
Double, pulmonary + systemic circuits (mammals)
Blood Vessels
Arteries, thick walls, high pressure, away from heart
Arteries = away from heart. Veins = toward heart. Pulmonary artery = deoxygenated. Pulmonary vein = oxygenated. Oxygen content does NOT define vessel type.
Activities
Assessment
Multiple Choice, 5 marks
RememberBand 1
1. Which of the following correctly describes the path of deoxygenated blood through the pulmonary circuit?
A Left ventricle → pulmonary vein → lungs → pulmonary artery → left atrium
B Right atrium → pulmonary vein → lungs → pulmonary artery → left ventricle
C Right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium
D Right atrium → aorta → lungs → vena cava → left atrium
UnderstandBand 2
2. Capillaries are only one cell thick. Which of the following best explains the advantage of this structure?
A It increases blood pressure within capillaries to drive diffusion
B It minimises the diffusion distance between blood and tissue cells, maximising exchange rate
C It allows capillaries to contract and push blood forward like arteries
D It prevents white blood cells from leaving the circulatory system
AnalyseBand 4
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?
A The heart is in atrial fibrillation
B The SA node has failed and the AV node is pacing the heart
C The ventricles are contracting before the atria
D The SA node is functioning normally and initiating each heartbeat
ApplyBand 3
4. Veins have valves; arteries do not. The best explanation for this difference is:
A Arterial blood flows too fast for valves to function
B Valves in arteries would prevent the pulse wave from propagating
C Venous blood flows at low pressure and could flow backward without valves; arterial pressure is high enough to maintain forward flow
D Veins carry oxygenated blood that requires valve protection
UnderstandBand 2
5. Double circulation in mammals is an evolutionary advantage over single circulation in fish primarily because:
A Mammals have more blood than fish
B Blood is re-pressurised by the heart after lung gas exchange, delivering high-pressure oxygenated blood to body tissues
C Mammalian blood contains more haemoglobin per red blood cell than fish blood
D Double circulation prevents deoxygenated blood from ever reaching the body
Short Answer, 9 marks
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
Q1, C: Deoxygenated blood flows: right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary vein (now oxygenated) → left atrium. Note: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, the exception to the common misconception.
Q2, B: Capillary walls are one endothelial cell thick, the shortest possible diffusion distance between blood and surrounding tissue. This maximises the rate of O₂, CO₂, nutrient, and waste exchange by minimising the distance over which diffusion must occur.
Q3, D: A P wave before every QRS complex indicates the SA node is firing normally and the atria are contracting before the ventricles, normal sinus rhythm. 72 bpm is a normal resting heart rate.
Q4, C: Arterial blood flows under high pressure from ventricular contraction, which maintains forward flow without valves. Venous blood flows at low pressure, without valves, gravity and the absence of pumping force would allow blood to pool in the lower limbs and flow backward.
Q5, B: In single (fish) circulation, blood passes through gill capillaries for gas exchange, losing pressure in the process. The body then receives low-pressure blood. In double circulation, the left ventricle re-pressurises blood after lung exchange before sending it to the body, maintaining high pressure delivery to support the high metabolic demands of endothermic mammals.
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.
Revisit Your Thinking
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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