IB Biology HLTransport in Animals & PlantsPaper 1 & 2~10 min read
Circulatory Systems
A fish pushes its blood through the gills and then straight on to the rest of the body. By the time it gets there the pressure has almost gone. A mammal sends blood back to the heart in between, gets a second push, and can therefore run.
📚 What you need to know
Both fish and mammals have a closed circulatory system: blood stays inside blood vessels.
Bony fish have single circulation. Blood passes through the heart once per complete circuit and the heart has two chambers.
In a fish, blood enters the heart from the body, is pumped to the gills where it is oxygenated, and then travels on to the rest of the body.
Mammals have double circulation. Blood passes through the heart twice per circuit and the heart has four chambers.
The right side pumps deoxygenated blood to the lungs – the pulmonary circulation.
Blood returns to the left side, which pumps oxygenated blood around the body at high pressure – the systemic circulation.
Advantages of double circulation: oxygenated and deoxygenated blood stay separate, blood reaches the body at high pressure, and the lungs receive blood at lower pressure so their delicate vessels are not damaged.
Single circulation in a bony fish
A fish heart has just two chambers: one atrium to receive blood and one ventricle to pump it. The route is a single loop.
The blood that reaches the body has already been through one capillary bed, so it arrives slowly and at low pressure.
Double circulation in a mammal
A mammal splits the journey in two, and gives the blood a fresh push in the middle. The four-chambered heart is really two pumps side by side, kept apart by the septum.
The septum down the middle of the heart is what keeps the two circuits separate. Without it the two bloods would mix.
Feature
Single circulation (bony fish)
Double circulation (mammal)
Times through the heart per circuit
Once
Twice
Heart chambers
Two
Four
Oxygenated and deoxygenated blood
Pass through the same chambers at different times
Kept completely separate by the septum
Pressure reaching the body
Low, because pressure was lost in the gill capillaries
High, because the blood is pumped again by the left ventricle
Pressure reaching the gas exchange organ
The same as the pressure leaving the heart
Lower, from the thinner-walled right ventricle
Overall rate of delivery
Slower
Faster, supporting a higher metabolic rate
Why two circuits are better
No mixing. Blood going to the body is fully oxygenated, so cells receive as much oxygen as possible for aerobic respiration.
High pressure to the body. The left ventricle gives the blood a second push after it has crossed the lung capillaries, so it reaches every tissue quickly, even the toes and the brain.
Lower pressure to the lungs. The right ventricle has a thinner wall and generates less pressure. Alveolar capillaries are extremely delicate, and full systemic pressure would damage them and force too much fluid out into the alveoli.
The third point is the one that separates strong answers. Everyone writes “high pressure is good”. Ask yourself why the two ventricles are different thicknesses, and the answer is that the lungs need a gentler pump than the body does.
Why fish get away with it. Fish are supported by water, are usually not warm-blooded and have a lower metabolic rate, so they need less oxygen per second. A single circulation delivers enough. A mammal that maintains its body temperature needs far more, and could not manage on one push.
Worked examples
WE 1
Comparing the two systems
Compare single circulation in a bony fish with double circulation in a mammal. (4 marks)
Point 1: both
Both are closed systems, with blood contained in vessels at all times.
Point 2: times through the heart
In a fish blood passes through the heart once per circuit; in a mammal it passes through twice.
Point 3: chambers
A fish heart has two chambers, a mammal heart has four, so a mammal keeps oxygenated and deoxygenated blood separate.
Point 4: pressure
In a fish, blood loses pressure at the gills and travels on slowly; in a mammal it returns to the heart and is pumped to the body at high pressure.
Once versus twice, two chambers versus four, low versus high pressure to the body“compare” means give both sides of each point, and it is worth naming one similarity too
WE 2
The advantage of a septum
Explain the advantage to a mammal of keeping oxygenated and deoxygenated blood separate. (2 marks)
Point 1: what separation achieves
Blood leaving the left side of the heart is fully oxygenated, because it has not been diluted by returning deoxygenated blood.
Point 2: why it matters
Cells therefore receive blood with the highest possible oxygen concentration, supporting a fast rate of aerobic respiration and a high metabolic rate.
Undiluted oxygen supply to every tissuelink separation to aerobic respiration, not just to “efficiency”
WE 3
Two different pressures
Suggest why the wall of the right ventricle is thinner than the wall of the left ventricle. (3 marks)
Point 1: distance
The right ventricle pumps blood only as far as the lungs, while the left ventricle pumps it all the way around the body.
Point 2: pressure needed
Less muscle is needed to generate the lower pressure required for the short pulmonary circuit.
Point 3: protecting the lungs
Lower pressure also protects the delicate capillaries around the alveoli from damage and stops too much fluid being forced into the air spaces.
Shorter journey, gentler pump, safer lungsthe third point is the one most students miss
💡 Exam tips
Say once or twice through the heart per complete circuit – that is the definition being tested.
Name the two circuits: pulmonary to the lungs, systemic to the body.
Remember the right side handles deoxygenated blood and the left side oxygenated blood.
Use the word closed when describing either system.
For advantages, give all three: separation, high systemic pressure, low pulmonary pressure.
Link fast delivery to a high metabolic rate, which is what mammals need it for.
⚠ Common mistakes
Saying fish have an open circulatory system. They are closed; insects are the open example.
Counting the fish heart as having one chamber. It has two: atrium and ventricle.
Saying oxygenated and deoxygenated blood mix in a fish. They do not mix; the blood simply passes through the heart at one point in the circuit.
Getting left and right the wrong way round on a diagram – the heart is labelled as if it were in your body facing you.
Saying the lungs need high pressure to work well. They need lower pressure to avoid damage.
Writing “double circulation means two hearts”. It means one heart with two separate pumps.
Up next: The Mammalian Heart – the chambers, valves and vessels behind those two pumps, in detail.
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