DSE Biology | Human Circulation Lesson Prep (for tutors) | From heart rate to capillary velocity

2026-11-04

Tutor lesson-prep guides · Secondary Biology

Students can recite that arteries carry blood away from the heart, yet freeze when asked why capillary flow is slowest or why standing all day swells the ankles. This lesson obeys two numbers: cardiac output and total cross-sectional area. Vessel function, tissue fluid and oedema all follow from them.

Objectives — Students can: (1) calculate cardiac output from heart rate and stroke volume, then use blood volume to state circulation time; (2) compare the three vessel types by wall thickness, lumen, valves and total cross-section, and explain why capillary velocity is lowest; (3) explain tissue fluid formation with hydrostatic and osmotic pressure, and why failing lymphatic return causes oedema.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Four chambers	Where does the right ventricle pump blood to?	The lungs, pulmonary circulation
Vessel names	What do we call a vessel leaving the heart?	An artery
Diffusion	What is movement from high to low concentration called?	Diffusion, passive

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three-question check	Chamber destinations, artery definition, diffusion direction	Answer orally
8-24 min	Double circulation and valves	Right to lungs, left to body, one-way valves	Draw four outlets
24-38 min	Three vessel types	Wall, lumen, valves, total cross-section	Fill the table and compute
38-50 min	Tissue fluid and lymph	Hydrostatic push, osmotic pull	Explain ankle swelling
50-57 min	Timed practice	Cardiac output and a pathway question	Work alone
57-60 min	Exit check	Three checkpoints, one question each	Self-mark and circle slips

03 Teaching script (read it out)

Scene 1: Double circulation: one pump, two circuits

Anchor the four outlets first: right atrium receives the venae cavae, right ventricle sends the pulmonary artery, left atrium receives pulmonary veins, left ventricle sends the aorta. The right side serves the lungs only, the left side serves the body only, and the septum keeps the two streams apart. Then compute: heart rate 70 beats per minute times stroke volume 70 mL gives 4900 mL/min, about 4.9 litres per minute. With roughly 5 litres of blood, one circuit takes about 1.02 minutes. Valves enforce one direction: atrioventricular valves stop backflow into the atria, semilunar valves stop backflow into the ventricles.

Have students repeat the pulmonary circuit in the same pattern: right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium.

Scene 2: Three vessel types: wall and total cross-section decide the job

Arteries have thick muscular and elastic walls to survive systolic pressure; veins are thin-walled, wide, and valved, relying on skeletal muscle to push blood back; capillaries are one endothelial cell thick, about 1 micrometre, so diffusion is short-range. Velocity follows continuity: the same 82 mL/s through 3 square centimetres of aorta gives about 272 mm/s, while 4500 square centimetres of capillary bed gives about 0.18 mm/s, a factor of 1500. Slow flow is the point, because exchange needs time.

Close with one line: larger total cross-section means slower flow; capillaries are designed to be slow.

Scene 3: Tissue fluid: pushed out at one end, pulled back at the other

At the arterial end of a capillary, hydrostatic pressure exceeds osmotic pressure, so water and small solutes leave to form tissue fluid. At the venous end hydrostatic pressure falls and plasma proteins pull most water back. The remainder, with a little protein, returns through lymph vessels and drains into the subclavian vein. Standing for hours raises venous pressure, so filtration outruns reabsorption and ankles swell; low plasma protein from malnutrition or liver disease lowers the osmotic pull and causes the same sign.

Separate the two causes: one is pushing too hard, the other is pulling too weakly. Answers must say which.

04 Question types and worked steps

Type	Example	Steps	Answer
Cardiac output	Heart rate 70, stroke volume 70 mL	Multiply, then convert to litres	4.9 L/min
Vessel comparison	Artery versus vein versus capillary	Compare wall, lumen, valves, velocity	Capillaries slowest
Oedema	Ankle swelling after standing	Hydrostatic rise, filtration exceeds return	Lymphatic drainage overwhelmed
Pathway	Right big toe to the brain	Veins to right heart, lungs, left heart, aorta	One pulmonary pass

Sanity check (teach this): The numbers must agree: 70 times 70 gives 4900 mL/min, the same order as total blood volume, so one circuit takes about a minute. Aortic flow of 272 mm/s against capillary 0.18 mm/s differs by 1500, exactly the cross-section ratio, and students should use that as a consistency check. A pressure of 120 over 80 mmHg fits systolic ejection followed by elastic recoil of the aorta.

05 Independent practice (3 questions)

Question	Answer	What to watch
Cardiac output with heart rate 75 and stroke volume 68 mL	5.1 L/min	Correct product and units
Explain why capillary velocity stays low during exercise	Total cross-section also rises	Names cross-section, not just faster heart
Write the full route from right ventricle to right big toe and back	Pulmonary artery, lungs, left atrium, left ventricle, aorta, arteries, capillaries, veins, vena cava	Four chambers and both vessel sets present

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating the pulmonary artery as oxygen-rich	Says pulmonary artery blood is oxygenated	The pulmonary artery is the only artery carrying deoxygenated blood, because it goes to the lungs, not the body.
Assuming every artery carries oxygenated blood	Says all arterial blood is bright red	Artery describes direction only: away from the heart. Oxygen content is a separate question.
Explaining slow capillary flow as a weak pump	Says blood is tired by the time it reaches the foot	Same cardiac output through 1500 times the cross-section means 1500 times slower flow.
Answering oedema with too much water	No mention of pressure or protein	State whether hydrostatic push rose or plasma protein pull fell.

07 Homework (paper, section and question numbers)

Biology chapter index — All chapters
Biology practice papers — Circulation: cardiac output and pathway questions

08 Exit check (last 5 minutes)

Question	Answer	Decision
Cardiac output	Heart rate 70, stroke volume 70 mL in L/min	Cannot compute: reteach act one
Vessel comparison	Why capillaries are slowest	Answers because they are far: return to cross-section
Oedema	Which imbalance explains standing oedema	Only says extra water: add hydrostatic and osmotic terms

09 Tutor FAQ

Why is the left ventricular wall thicker?

It drives blood round the systemic circuit against far higher resistance than the pulmonary circuit.

How does lymph move without a pump?

Skeletal muscle squeeze, negative intrathoracic pressure during breathing, and one-way valves in lymph vessels.

Is cardiac output examined numerically?

The arithmetic is easy; the marks come from using it to explain circulation time and velocity with units.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.