Form 4 · Common mistakes

Transport in Humans and Animals, common mistakes

The mistakes SPM students make on Transport in Humans and Animals, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Believing an open circulatory system also confines blood within vessels at all times.	Students focus on memorising 'closed = humans' without understanding what 'open' means in insects.	In an open circulatory system, such as in insects, the blood (haemolymph) leaves the vessels and bathes body tissues directly, rather than remaining enclosed within vessels as it does in a closed system like the human circulatory system.
Assuming the SA node needs a signal from the brain before it can fire.	Students confuse the nervous system's role in adjusting heart rate with initiating each individual heartbeat.	The SA node fires and initiates each heartbeat on its own, without waiting for a signal from the brain; the nervous system and hormones such as adrenaline only speed up or slow down how often it fires.
Describing the AV valves as opening during ventricular systole.	Students assume all heart valves behave the same way at the same point in the cycle.	The atrioventricular valves close during ventricular systole to stop blood flowing back into the atria, while the semilunar valves in the arteries open at the same moment to let blood leave the ventricles.
Thinking platelets alone are sufficient to form a blood clot, without any other factor involved.	Diagrams of clotting sometimes show platelets prominently, making them seem like the whole mechanism.	Platelets trigger the clotting process by releasing chemicals at the wound site, but the clot itself only forms once clotting factors and calcium ions convert fibrinogen into fibrin, which traps blood cells in a mesh.
Writing that a person with blood group AB carries no antigens on their red blood cells.	Students sometimes reverse the logic of group AB, confusing it with group O.	Group AB red blood cells carry both A and B antigens, which is why group AB plasma contains neither anti-A nor anti-B antibodies, making group AB the universal recipient for red cell transfusions.
Believing Rh-negative and Rh-positive refer to two different antibodies a person is born with.	Students confuse the presence of an antigen with the presence of an antibody.	Rh-positive and Rh-negative describe whether the Rhesus antigen is present on a person's red blood cells, not whether they already have anti-Rh antibodies; an Rh-negative person only develops anti-Rh antibodies after being exposed to Rh-positive blood.
Assuming atherosclerosis only affects arteries supplying the limbs.	Students sometimes picture atherosclerosis only in the context of leg circulation.	Atherosclerosis can develop in any artery, but when it narrows a coronary artery supplying the heart muscle itself, the resulting blockage can cause a heart attack, which is why it is examined together with the heart.
Describing tissue fluid as identical to blood plasma.	Both are pale, watery liquids, so students assume they contain exactly the same components.	Tissue fluid is formed when blood pressure forces plasma out of the capillaries into the spaces between cells; it lacks the large plasma proteins and blood cells that remain inside the capillaries, so it is similar to plasma but not identical to it.
Thinking lymph nodes produce lymph.	The similar names of 'lymph node' and 'lymph' suggest to some students that one produces the other.	Lymph nodes do not produce lymph; they filter lymph as it passes through, using white blood cells to trap and destroy pathogens, while lymph itself is formed from excess tissue fluid that drains into lymph vessels.
Assuming filariasis can be prevented mainly by treating the water supply.	Students sometimes confuse the prevention measures for a mosquito-borne disease with those for a waterborne one.	Because filariasis is transmitted through mosquito bites rather than through water, prevention centres on avoiding mosquito bites and controlling mosquito breeding sites, such as removing stagnant water where mosquitoes lay eggs.

How to avoid these mistakes

Label the heart and write the path of blood through both circuits.
Compare the three blood vessels in a table (wall, lumen, pressure, valves).
Learn the steps of blood clotting and the ABO transfusion rules.
Draw and label the full path of blood through both circuits, starting and ending at the right atrium.
Make a table of the four blood components and one function of each.
Practise explaining, in order, why incompatible ABO or Rhesus blood causes agglutination.

Frequently asked questions

Why is the human circulatory system called a double circulation?

Blood passes through the heart twice for each complete circuit of the body: once through the right side to the lungs (pulmonary circulation) and once through the left side to the rest of the body (systemic circulation). This keeps oxygenated and deoxygenated blood separate and maintains high pressure to the body.

What is the difference between arteries and veins?

Arteries carry blood away from the heart, usually at high pressure, and have thick, muscular, elastic walls. Veins return blood to the heart at low pressure, have thinner walls and a wider lumen, and contain valves to stop blood flowing backwards. Most arteries carry oxygenated blood and most veins deoxygenated blood, except the pulmonary vessels.

How does blood clot at a wound?

When a blood vessel is damaged, platelets gather and release chemicals. With clotting factors and calcium, these convert the soluble protein fibrinogen into insoluble fibrin threads, which form a mesh that traps blood cells and seals the wound, stopping bleeding and blocking entry of microorganisms.