Why many animals have a heart and circulation
Every living cell needs a supply of oxygen and glucose and must get rid of carbon dioxide and other wastes.
In a single-celled organism, diffusion across the cell surface does the whole job.
In a large animal, most cells are millimetres or even metres away from the surface, and diffusion over that distance is far too slow.
This section explains how a heart and circulation solve that problem.
Topic 1 is AS content and can be set on any of the three A level papers; most of its questions on the heart, blood vessels and cardiovascular disease have appeared on Paper 2, with health statistics and carbohydrates also turning up on Paper 1, and synoptic questions on Paper 3.
None of the papers is safe to ignore.
You need to understand why many animals have a heart and circulation (mass transport to overcome limitations of diffusion in meeting the requirements of organisms).
Topic 1 is taught through lifestyle, the heart and cardiovascular disease (CVD), so this first idea, why a heart is needed at all, sets up everything that follows: what the blood carries, how vessels and the heart move it, and what happens to cells when the flow fails.
The problem: diffusion is fast only over short distances
Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, caused by their random movement.
It needs no energy from the cell, and over a few micrometres it is very quick.
The trouble is that the time diffusion takes rises with the square of the distance.
Ten times the distance takes about a hundred times as long; a thousand times the distance takes about a million times as long.
A molecule that crosses one cell in a fraction of a second would take hours to diffuse a centimetre.
Size makes this worse.
As an animal gets bigger, its volume (and so the amount of respiring tissue) rises faster than its surface area, so it has a smaller surface area to volume ratio.
A large animal has less surface per unit volume through which substances could diffuse in, and its innermost cells are a long way from that surface.
A large, active animal also has a high metabolic rate, so its cells use oxygen and glucose quickly and produce carbon dioxide quickly.
So three things limit diffusion as the only supply route in a large, active animal:
- the distance from the surface (or from the gut or gas exchange organ) to the cells is too great;
- the surface area to volume ratio is too small;
- the demand of active cells for oxygen and glucose is too high.
The solution: mass transport
Mass transport is the bulk movement of a fluid, carrying many substances along together, in one direction, driven by a pressure difference.
A heart is a muscular pump that creates the pressure.
Blood vessels are the tubes that carry the flow to within a short distance of every cell.
Diffusion is still used, but only over the last few micrometres: from the capillary to the cell, or from the alveolus into the capillary.
That is the key idea to carry into an exam answer.
The heart and circulation do not replace diffusion; they bring the blood close enough to every cell that diffusion only has to work over a very short distance.
Blood carries, among other things:
- oxygen from the gas exchange surface to respiring cells, and carbon dioxide back;
- glucose, amino acids, fatty acids and vitamins from the gut to the tissues;
- urea from the liver to the kidneys;
- hormones from glands to their target cells;
- antibodies and white blood cells;
- heat from active tissues to the skin and the rest of the body.
Most of these travel dissolved in the plasma, which is mostly water.
Why water is such a good transport medium is the subject of the next section.
Not every animal needs one
Small or flat animals manage without a heart.
A sea anemone's body wall is only two cell layers thick round a water-filled cavity, so every cell is close to sea water.
A flatworm is so thin that every cell is near its surface.
Insects do have a heart, but it pumps blood at low pressure through an open circulation, where blood bathes the organs directly in a body cavity; oxygen reaches their cells through a separate system of air-filled tubes (tracheae), so the blood does not have to deliver it quickly.
Vertebrates have a closed circulation: blood stays inside vessels all the time, so it can be kept at high pressure and directed to the tissues that need it most.
Single and double circulation
The diagram below compares the circulation of a fish with that of a mammal.
In a fish, blood passes through the heart once per circuit: heart to gills to body to heart.
Squeezing through the gill capillaries drops the pressure, so blood reaches the body slowly.
In a mammal, blood passes through the heart twice per circuit.
The right side of the heart pumps deoxygenated blood to the lungs (the pulmonary circulation); it returns to the left side, which pumps the now oxygenated blood round the body at high pressure (the systemic circulation).
This has two advantages:
- blood can be pumped to the lungs at a lower pressure, so the delicate alveolar capillaries are not damaged and there is time for gas exchange;
- blood returning from the lungs is pumped again, so it reaches the body at high pressure and flows fast, which meets the high oxygen demand of a warm-blooded, active animal.
Used again in: Topic 7 (Run for your Life), where the heart rate and stroke volume are raised to deliver more oxygen to muscles during exercise, and where you learn how the heartbeat is started and controlled.
The Paper 3 article
Paper 3 ends with a 30-mark question on a scientific article that Pearson releases eight weeks before the exam.
Candidates who study the article closely, work out the biology behind each paragraph and read up on any unfamiliar terms do best.
Each part of that question hooks onto an everyday phrase in a numbered paragraph and asks you for specification content, so practise turning everyday phrases into the terms used in these notes.
A practice article on the circulation appears in the blood vessels section of this unit.
- One mark per marking point, and the scheme usually lists more points than there are marks, so any correct points up to the total score.
- Examiners ALLOW stated alternatives, IGNORE vague or irrelevant additions and DO NOT ALLOW named wrong words.
- A contradiction cancels the point: two different answers to one point earn nothing.
- Only the answer on the answer line is marked. A correct calculated answer with no working gains full marks; with working shown, a wrong answer can still earn marks carried forward from your own earlier value.
- Asterisked items (6 or 9 marks) are marked by levels: coverage of every strand, linked reasoning and a logical structure decide the level.
- Determine needs a number made from the data (a difference, a fold change or a percentage). Deduce states a conclusion that goes beyond the data, then the evidence for it.
- Comment on gives a judgement from the data, not a critique of the method unless one is asked for. Compare and contrast needs at least one similarity and one difference, and every sentence names both things.
- Mass transport: the bulk movement of a fluid, with the substances it carries, in one direction, driven by a pressure difference.
- Diffusion: the net movement of molecules or ions from a region of higher concentration to a region of lower concentration.
- Closed circulation: blood is pumped round the body inside blood vessels.
- Double circulation: blood passes through the heart twice in each complete circuit of the body, once in the pulmonary circulation and once in the systemic circulation.
Explain why a large, active mammal cannot rely on diffusion alone and needs a heart and a circulation.
(3)
Full-mark answer (an explanation that makes reference to three of the following):
- Diffusion is too slow over the large distances from the surface to the cells inside a large animal (1).
- Large animals have a small surface area to volume ratio, so their body surface cannot supply enough oxygen and glucose by diffusion (1).
- The heart pumps blood through vessels, giving mass transport of substances (1).
- So blood is brought close to every respiring cell, where diffusion only has to work over a short distance (1).
- Active animals have a high metabolic rate and need a rapid supply of oxygen to respiring cells and rapid removal of carbon dioxide (1).
Weaker answer: "The heart pumps blood to carry oxygen around the body."
This scores 0 or at most 1.
"Pumps blood" alone restates what a heart is, and "oxygen around the body" is vague: say oxygen is delivered to respiring cells or tissues.
There is no mention of diffusion, distance or surface area to volume ratio, which is the reason.
Describe the route taken by blood in the double circulation of a mammal, starting in the right ventricle.
(3)
Full-mark answer:
- The right ventricle pumps deoxygenated blood along the pulmonary artery to the lungs, where it is oxygenated (1).
- Oxygenated blood returns along the pulmonary vein to the left atrium and left ventricle (1).
- The left ventricle pumps it along the aorta to the body; deoxygenated blood returns along the vena cava to the right atrium (1).
Weaker answer: "Blood goes from the heart to the lungs and back, then to the body and back."
This scores 1 at most: it never names a chamber or vessel, and it starts at "the heart" when the stem starts in the right ventricle.
Start exactly where the stem starts.
Mudskippers are fish that spend long periods out of water, gulping air into a mouth lined with many capillaries.
Their circulation is single.
Deduce why mudskippers are less active than a mammal of the same mass.
(2)
Full-mark answer:
- In a single circulation, blood loses pressure passing through the capillaries of the gas exchange surface before going to the body, so it reaches the body slowly (1).
- So oxygen is delivered to respiring muscle cells more slowly, limiting aerobic respiration and the rate of activity (1).
Weaker answer: "Fish are cold-blooded so they do not need as much energy."
This misses the circulation point the stem sets up and earns nothing.
In any "why do animals need a heart" answer, name diffusion and its limit (distance, surface area to volume ratio, demand), then the solution (mass transport, pumped by the heart, through vessels to every cell).
Every noun should be precise: "respiring cells", not "the body"; "mass transport", not "carries things".
- "Large animals have a small surface area." They have a large surface area but a small surface area to volume ratio. The ratio is the point, but it is tempting to shorten it.
- "The heart replaces diffusion." Diffusion still happens at every exchange surface; mass transport cuts the distance diffusion has to cover.
- "To pump blood round the body." True, but it earns nothing by itself because it restates the stem. Add what the pumping achieves and why diffusion alone is not enough.
- "Oxygen around the body." Too vague: oxygen is delivered to respiring cells, and carbon dioxide is removed from them.
Check your understanding
1. Which of the following best explains why a large, active mammal needs a mass transport system? (1)
- A it has a large surface area to volume ratio, so it loses heat quickly
- B it has a small surface area to volume ratio, and its cells are too far from the surface for diffusion to supply them
- C diffusion cannot occur across the walls of its capillaries
- D its cells carry out anaerobic respiration only Total [1]
2. The common octopus (Octopus vulgaris) is an active marine predator with a closed circulation and three hearts. The beadlet anemone (Actinia equina) lives attached to rocks; its body wall is two cell layers thick round a cavity filled with sea water. Explain why the octopus needs a heart and circulation but the anemone does not. (3) Total [3]
3. Compare and contrast the single circulation of a fish with the double circulation of a mammal. (3) Total [3]
4. A student wrote: "A heart is needed so that oxygen can get around the body." Explain why this answer would earn little credit, and give a better version. (2) Total [2]
5. Insects have an open circulation. Their heart is a long tube that pumps blood at low pressure into a body cavity, where it bathes the organs. Oxygen reaches insect cells through air-filled tubes called tracheae, which branch to every tissue. (a) State what is meant by mass transport. (1) (b) Explain why an insect can be very active even though its circulation is at low pressure. (2) (c) Deduce one advantage to a mammal of having a closed circulation, compared with the open circulation of an insect. (2) Total [5]
6. Which of the following substances is not carried from one organ to another by mass transport in the blood plasma of a mammal? (1)
- A urea
- B glucose
- C insulin
- D cellulose Total [1]
Answers
1. The only correct answer is B. A is not correct because a large mammal has a small surface area to volume ratio, and heat loss is not why it needs mass transport. C is not correct because diffusion does occur across capillary walls; that is how substances leave the blood. D is not correct because mammals respire mainly aerobically, which is why they need a supply of oxygen.
2. An explanation that makes reference to three of the following:
- the octopus is large / thick, so many of its cells are far from the surface and diffusion over that distance is too slow (1)
- it has a small surface area to volume ratio (1)
- it is an active predator with a high metabolic rate / high demand for oxygen for aerobic respiration in its muscles (1)
- its hearts pump blood through vessels, so mass transport brings oxygen close to every respiring cell (1)
- every cell of the anemone is close to sea water, so diffusion over a short distance is fast enough, and it is inactive with a low demand (1) IGNORE "the octopus is bigger" unqualified.
3. An answer that makes reference to the following:
- similarity: both are closed circulations / both pump blood through vessels with a heart (1)
- difference: in a fish, blood passes through the heart once per circuit, whereas in a mammal it passes through the heart twice (1)
- difference: in a fish, blood goes to the body at low pressure after the gills, whereas in a mammal it is pumped again after the lungs, so it reaches the body at high pressure (1) ALLOW a mammal has separate pulmonary and systemic circulations, whereas a fish has one circuit.
4. An explanation that makes reference to the following:
- "around the body" is vague / does not say where the oxygen goes or why diffusion is not enough (1)
- better: the heart pumps blood by mass transport to bring oxygen close to respiring cells, because diffusion over large distances is too slow (1)
5. (a) The bulk movement of a fluid, carrying substances, in one direction, driven by a pressure difference (1). (b) An explanation that makes reference to the following:
- oxygen reaches the cells through the tracheae, not in the blood (1)
- so the blood does not have to deliver oxygen rapidly, and low pressure is enough to carry nutrients and wastes (1) (c) An answer that makes reference to two of the following:
- blood stays in vessels, so it can be kept at a higher pressure (1)
- so it flows faster / delivers oxygen and glucose to respiring cells more rapidly (1)
- blood can be directed to the tissues that need it most, by narrowing or widening vessels (1)
6. The only correct answer is D. A is not correct because urea is carried from the liver to the kidneys in the plasma. B is not correct because glucose is carried from the gut and liver to the tissues in the plasma. C is not correct because insulin is a hormone carried from the pancreas to its target cells in the plasma.