A grasshopper and a blue whale both need to move oxygen, nutrients, and waste products around their bodies. They have evolved radically different solutions to the same problem — and understanding why illuminates one of biology's most fundamental design principles.
Content from this lesson that appears directly in HSC Biology exams
Comparing structure, pressure, efficiency, and examples of open vs closed systems. Tested as a 3–4 mark comparison question in most HSC papers. Must name specific animals and link structural differences to functional consequences.
Identifying the four blood components, what each carries, and how composition changes along the circuit. Appears in microscopy interpretation questions (Section I, 1–2 marks) and mechanism questions (Section II, 2–3 marks).
Applying SA:V ratio to justify the need for circulatory systems. Tested as a 2–3 mark justification question: "Explain why large organisms cannot rely on diffusion alone."
Identifying RBCs, WBCs, and platelets from microscope images. Tested in Section I working scientifically — 1–2 marks. Must describe distinguishing features: shape, size, nucleus presence or absence.
Core Content
Diffusion works for single cells. It fails spectacularly for whales.
In Lesson 10 you saw that as organisms grow larger, their SA:V ratio drops — the outer surface can no longer supply enough oxygen to interior cells by diffusion alone. The same logic applies to every substance a cell needs: glucose from the intestine, hormones from endocrine glands, waste products heading to the kidneys. A large body is simply too big for diffusion to cover the distances involved in any useful timeframe.
The numbers make this concrete. Diffusion of oxygen across 1 μm (one cell width) takes about 1 millisecond. Diffusion across 1 mm takes about 1 second. Diffusion across 1 cm takes about 100 seconds. Diffusion across 1 metre — the distance from your small intestine to your brain — would take approximately 11 days. A transport system replaces slow passive diffusion with rapid bulk flow, reducing that 11-day journey to a few seconds.
Two evolutionary solutions — same problem, radically different results
The fundamental distinction between transport systems is whether the transport fluid stays inside vessels at all times, or whether it leaves vessels and bathes tissues directly.
Transport fluid (haemolymph) is pumped by a tubular heart into the body cavity (haemocoel), where it directly bathes organs and tissues. There are no blood vessels beyond the heart — haemolymph pools around organs and slowly drains back through openings called ostia.
Transport fluid (blood) is pumped by a chambered heart and remains inside a continuous network of vessels (arteries → capillaries → veins) at all times. Exchange with tissues occurs only at thin-walled capillaries.
Four components, each with a distinct job
Blood is not a simple fluid — it is a complex tissue. A pinhead-sized drop contains approximately 5 million red blood cells, 10,000 white blood cells, and 250,000 platelets, all suspended in plasma. Each component carries different cargo and serves a distinct function.
~5 million per mm³ · ~120-day lifespan
Biconcave disc shape — no nucleus at maturity. Small and flexible — can squeeze through capillaries narrower than the cell itself.
~7,000–10,000 per mm³ · Variable lifespan
Larger than RBCs, with visible nuclei. Multiple subtypes with different immune roles. Can exit blood vessels to reach infection sites (diapedesis).
~250,000 per mm³ · ~10-day lifespan
Cell fragments derived from megakaryocytes in bone marrow. No nucleus. Smallest formed element in blood.
~55% of blood volume · 90% water
Straw-coloured liquid matrix suspending all blood cells. Contains an enormous variety of dissolved substances in transit.
Blood leaving the heart is not the same blood returning — IQ3 in action
One of IQ3's core questions asks: "How does the composition of the transport medium change as it moves around an organism?" Every tissue blood passes through takes something from it or adds something to it — and the changes are predictable and measurable.
Five errors that cost marks in HSC exams every year
These misconceptions appear regularly in HSC scripts. Each reveals a conceptual gap — examiners identify and penalise them specifically.
Activities
A standard blood smear shows a stained sample viewed under a light microscope. Using your knowledge of blood components, answer the following questions.
For each vessel, predict whether O₂, CO₂, glucose, and urea are at HIGH or LOW concentration compared to blood in the aorta, and give a one-line reason.
| Vessel | O₂ | CO₂ | Glucose | Urea |
|---|---|---|---|---|
| Pulmonary vein lungs → heart |
||||
| Hepatic portal vein intestine → liver (post-meal) |
||||
| Renal vein kidneys → heart |
"Compare open and closed circulatory systems in animals. In your answer, describe the structure of each system, identify an animal example for each, and explain one advantage of the closed system over the open system." (5 marks)
Use: whereas / in contrast / however. Five distinct marking points: define open → define closed → example each → one structural difference → functional advantage explained.
Assessment
Select the best answer — feedback shown immediately
1. Which vessel carries deoxygenated blood at high pressure away from the heart?
2. Which of the following correctly explains why insects can survive with an open circulatory system while large vertebrates cannot?
3. A mature red blood cell has no nucleus. Which of the following is the most significant functional consequence of this?
4. Blood sampled from the hepatic portal vein two hours after a large carbohydrate meal would show which composition compared to blood in the aorta?
5. Which of the following is a correct similarity between open and closed circulatory systems?
6. Explain why large multicellular animals require specialised transport systems, referring to the SA:V ratio principle. 3 MARKS
7. Describe the structure and function of red blood cells. Explain how two structural features are specifically adapted to maximise oxygen transport. 4 MARKS
Two features × two marks each: structure → function link.
8. Compare the composition of blood in the pulmonary artery and the pulmonary vein, and explain the changes that occur between these two vessels. 3 MARKS
1. C — The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs at high pressure. Pulmonary vein carries oxygenated blood back to the heart; the aorta carries oxygenated blood at high pressure away from the left ventricle; the vena cava is a vein carrying blood toward the heart.
2. D — Insects' tracheal system delivers O₂ directly to cells independently of haemolymph. This means haemolymph only needs to transport nutrients and waste — tasks achievable at low pressure. Large vertebrates cannot do this because tracheal diffusion becomes too slow over large body distances.
3. A — The primary benefit is maximised haemoglobin content, and the direct trade-off is no DNA → no cell division → fixed ~120-day lifespan. Flexibility comes from the biconcave disc shape and elastic cell membrane, not from nucleus absence.
4. B — Post-meal, glucose absorbed from the small intestine enriches the hepatic portal vein → higher glucose. Intestinal cells are metabolically active during absorption, consuming O₂ and producing CO₂ → lower O₂ and higher CO₂ than the aorta.
5. A — Both open and closed systems have a pumping organ: a tubular heart in insects, a chambered heart in mammals. Haemoglobin is absent from insect open systems; closed vessels are only in closed systems; open systems cannot supply large active bodies.
As body size increases, volume increases proportionally faster than surface area, causing the SA:V ratio to decrease. This means the outer body surface area relative to the volume of cells needing supply becomes progressively smaller.
Diffusion over distances greater than a few millimetres is extremely slow — diffusing across 1 metre would take approximately 11 days. Interior cells of a large organism would receive oxygen and nutrients far too slowly to sustain life by diffusion from the body surface alone.
A specialised transport system solves this using bulk flow — the heart pumps blood under pressure through a vessel network, delivering oxygen and nutrients to within diffusion distance of every cell in seconds rather than days.
Feature 1 — Biconcave disc shape: RBCs have a flattened biconcave disc shape — depressed on both faces. This increases the surface area to volume ratio compared to a sphere of equivalent volume, exposing more haemoglobin molecules to O₂ at the cell membrane and reducing the maximum diffusion distance from membrane to any interior haemoglobin molecule. Both effects accelerate O₂ loading at the lungs and unloading at body tissues.
Feature 2 — No nucleus at maturity: Mature RBCs eject their nucleus during development, freeing approximately 40% more internal volume for haemoglobin. Each mature RBC contains approximately 280 million haemoglobin molecules — maximising O₂-carrying capacity per cell. The trade-off is that without a nucleus, the cell cannot produce proteins, divide, or repair damage, limiting its lifespan to approximately 120 days before breakdown and replacement by new cells from bone marrow.
The pulmonary artery carries blood from the right ventricle to the lungs — it contains low O₂ (haemoglobin largely unloaded from previous systemic circulation) and high CO₂ (accumulated from cellular respiration throughout the body).
The pulmonary vein carries blood from the lungs back to the left atrium — it contains high O₂ (haemoglobin fully saturated) and low CO₂.
These changes occur because in the alveolar capillaries, O₂ diffuses from alveolar air (high O₂ partial pressure) into the blood (low O₂ partial pressure), binding to haemoglobin. Simultaneously, CO₂ diffuses from blood (high CO₂ partial pressure) into alveolar air (low CO₂ partial pressure) and is exhaled. Both movements are driven by concentration gradients maintained by continuous ventilation refreshing the alveolar air.
Tick when you've finished all activities and checked your answers.