What you need to know
- 1.6The need for a heart and mass transportThe bulk flow of a fluid, such as blood or air, that carries substances over long distances. to overcome the limitations of diffusionThe net movement of molecules from a region of higher concentration to a region of lower concentration. It is passive..
- 1.8The pulmonary and systemic circuits as context for heart function.
By the end, you should be able to…
- Explain why size and activity increase the need for a transport system.
- Distinguish mass transport from diffusion.
- Trace the two circuits in a mammal and explain their advantages.
Build your understanding
Size creates a transport problem
As an organism becomes larger, its volume increases faster than its surface areaThe total area of a surface. A larger surface area gives a faster rate of diffusion.. Its surface-area-to-volume ratio falls, so there is less exchange surface per unit volume of tissue. Many cells are also far from the external surface. These long diffusionThe net movement of molecules from a region of higher concentration to a region of lower concentration. It is passive. distances make exchange with the environment too slow to meet demand.
Active tissues need a rapid supply of oxygen and respiratory substrateAn organic molecule that is broken down in respiration to release energy, such as glucose, fatty acids or amino acids. and rapid removal of carbon dioxide. Demand matters as well as size: a large, active animal cannot rely on substances diffusing all the way from its body surface to every cell.
Bulk flow for distance, diffusion for exchange
mass transportThe bulk flow of a fluid, such as blood or air, that carries substances over long distances. is the bulk movement of a fluid carrying substances. Contraction of the heart generates pressure differences that drive blood through vessels. Blood carries oxygen, dissolved nutrients and waste products over long distances much faster than diffusion alone could.
Diffusion still operates over the short distances at exchange surfaces. Oxygen diffuses from alveoli into blood, is transported in the circulation, then diffuses from systemic capillaryA tiny vessel whose wall is a single layer of endothelium, giving a short diffusion distance for exchange with tissues. to respiring cells. Blood flow helps maintain concentration gradients by delivering fresh supplies and removing substances that have diffused into the blood. Circulation supports diffusion; it does not replace it.
Two circuits, one complete journey
Mammals have a closed circulation: blood remains within the heart and blood vessels. In a double circulation, blood passes through the heart twice during a complete journey through the lungs and the body.
The right side pumps blood through the pulmonary circulation: heart → lungs → heart. The left side pumps it through the systemic circulation: heart → body tissues → heart. The septumThe wall of tissue that separates the left and right sides of the heart. separates the two sides and prevents mixing of oxygenated and deoxygenated blood within the normal adult heart.
Blood returns to the heart after passing through lung capillaries, allowing its pressure to be raised again before it travels to body tissues. The systemic circuit operates at a higher pressure, supporting rapid delivery around the body. The lower pulmonary pressure suits the delicate lung capillaries and helps limit fluid leakage into the gas-exchange tissues.
NOT: circulation does not replace diffusion at the capillaries.
- Closed circulation: blood stays in heart and vessels
- Septum keeps the two sides separate
- Systemic pressure is raised again after the lungs
- Low surface-area-to-volume ratio makes diffusion alone too slow
Two traps this diagram blocks
- Do not say an animal with a circulation no longer needs diffusion.
- Do not define a double circulation as blood passing through only two chambers.
From the diagram alone
Question
Explain the advantage of raising blood pressure again after blood has passed through the lungs. [3]
Show a model answer
Blood loses pressure as it passes through the lung capillaries. It returns to the heart, and the left ventricle raises the pressure again. That higher pressure drives blood quickly through the systemic circulation to body tissues.
| Process | What moves? | Where it is useful |
|---|---|---|
| Mass transport | A bulk fluid and the substances it carries | Long-distance movement of blood through vessels |
| Diffusion | Net movement of particles down a concentration gradient | Short distances across exchange surfaces |
Make your reasoning visible
Explain the limitation, then the solution
Connect low surface-area-to-volume ratio and long diffusionThe net movement of molecules from a region of higher concentration to a region of lower concentration. It is passive. distance to insufficient supply, then explain how pressure-driven blood flow and short capillary-to-cell distances solve the problem.
Define double circulation accurately
Blood passes through the heart twice per complete circuit. This does not mean it passes through only two chambers; trace the pulmonary and systemic routes separately.
Key explanations
- The two ventricles normally eject the same average volume. The thicker wall means more pressure, not more volume.
Avoid this misconception
The claim: Animals with a circulation no longer need diffusion.
The correction: mass transportThe bulk flow of a fluid, such as blood or air, that carries substances over long distances. moves substances over long distances. Diffusion is still needed between blood and nearby cells and across the lung exchange surface.
Trace the double circulation
Mammals have a double circulation: blood passes through the heart twice on each full trip. Order one trip, starting with blood returning from the body.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Start with deoxygenated blood returning from the body. Put its route in order.
2 of 7 in the right place
Valves and the detailed cardiac cycle are in 1B 4. Some blood takes other routes (for example the hepatic portal vein); only the main circuit is shown.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Explain why an active mammal needs a mass transport system.
Show suggested answer
It has a relatively small surface-area-to-volume ratio and many cells are far from the external surface. Diffusion alone is too slow to supply their high demand for oxygen and substrates. Pressure-driven blood flow rapidly transports these substances to capillaries close to the cells.
Practice question
Retrieval and explanation · suggested answer
Starting at the right ventricle, describe the route taken before blood next returns to the right ventricle.
Show suggested answer
Right ventricle → pulmonary artery → lung capillaries → pulmonary veins → left atrium → left ventricle → aorta → body arteries and capillaries → body veins and venae cavae → right atrium → right ventricle.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A model animal is a cube. Its side length doubles while oxygen demand per unit volume stays constant. How do its surface area, volume and surface-area-to-volume ratio change, and why does this matter?
Show a hint
Surface area is 6l² and volume is l³.
Explore the reasoning
Surface area increases fourfold and volume eightfold. Surface-area-to-volume ratio halves. Total oxygen demand rises eightfold, but external exchange area rises only fourfold. With greater distances to internal cells as well, relying on the surface and diffusion becomes less effective.
Recall this lesson
Lesson recall questions
Question 1 of 5: When diffusion is too slow to meet metabolic ________, a ________ system provides ________ transport.
Why mass transport is needed
Practice question
Retrieval and explanation · suggested answer
When diffusion is too slow to meet metabolic ________, a ________ system provides ________ transport.
Lesson: 1B 1: The Principles of Circulation
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Keeping an organ supplied outside the body
1B · The Principles of CirculationMachine perfusion systems circulate oxygenated blood or nutrient-rich fluid through donated organs. They apply the same transport principles as a circulation while an organ is outside the body.…Specification 1.6Read & use the scienceClose article
Why this matters
Machine perfusion systems circulate oxygenated blood or nutrient-rich fluid through donated organs. They apply the same transport principles as a circulation while an organ is outside the body. Perfusion means supplying an organ with a flowing fluid. Outside the body, a system must provide suitable conditions as well as move that fluid: oxygen delivery and waste removal still matter. A report about extending a preservation window describes a particular method and setting, not a guarantee for every donated organ. The transport lesson explains why circulation is part of the problem. Researchers must then test whether the organ remains suitable for transplantation, rather than judging success from visible flow alone.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.6 understand why many animals have a heart and circulation (mass transport to overcome the limitations of diffusion in meeting the requirements of organisms)
Use the science
Why might circulating oxygenated fluid through an organ be more effective than simply surrounding it with that fluid?
Explore a possible response
Flow through its vessels brings fluid close to internal cells, shortening diffusion distances and helping maintain gradients. Surrounding a large organ leaves its deepest cells far from the external supply.
Explore the source
- NHS Blood and Transplant: machine preservation programmeNHS Blood and Transplant
The scientist who runs a temporary circulation
1B · The Principles of CirculationDuring some heart operations, a clinical perfusionist operates equipment that temporarily supports circulation and gas exchange. This professional monitors the system rather than merely switching on a pump.…Specification 1.6 · 1.8Read & use the scienceClose article
Why this matters
During some heart operations, a clinical perfusionist operates equipment that temporarily supports circulation and gas exchange. This professional monitors the system rather than merely switching on a pump. Blood must reach tissues, acquire oxygen and release carbon dioxide while the surgical team works. Your distinction between mass transport and diffusion explains the division of labour: the machine moves blood over long distances, while exchange still depends on gradients across a thin barrier. The career combines physiology, measurements and rapid communication with other specialists. It also shows why a functioning oxygenator is not enough if blood is not reaching the organs.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.6 understand why many animals have a heart and circulation (mass transport to overcome the limitations of diffusion in meeting the requirements of organisms)
- 1.8 know the cardiac cycle (atrial systole, ventricular systole and cardiac diastole) and relate the structure and operation of the mammalian heart, including the major blood vessels, to its function
Use the science
The oxygenator works normally but the rate of blood flow falls substantially. Explain why tissue oxygen supply can still fall.
Explore a possible response
Oxygen content per volume is not the same as delivery per minute. Lower flow supplies fewer oxygenated red cells and may fail to maintain the tissue diffusion gradient.
Explore the source
- PerfusionRoyal Brompton & Harefield hospitals, NHS