Form 4 · Worked answers

Transport in Humans and Animals, worked answers

Fully worked answers for Transport in Humans and Animals, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Labelling the heart and tracing the path of blood.
Comparing arteries, veins and capillaries in a table.
Explaining blood clotting or blood group compatibility.
Tracing the path of blood through the pulmonary and systemic circuits, naming each chamber and vessel in order.
Explaining why a blood transfusion between incompatible ABO or Rhesus groups is dangerous.
Describing the structure and role of the lymphatic system in returning tissue fluid and defending the body.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Explain why the heart is described as myogenic, and state the role of the sinoatrial node.

[4]

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The heart is myogenic because cardiac muscle can initiate and maintain its own rhythmic contraction without an external nerve impulse starting each beat. The sinoatrial node, a small patch of tissue in the wall of the right atrium, generates the electrical signal that spreads through the atria and triggers each heartbeat, acting as the heart's natural pacemaker.

myogenicsinoatrial nodepacemakerelectrical signalright atrium

Explain why the wall of a capillary being one cell thick is important for its function.

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A capillary wall only one cell thick gives a very short diffusion distance between the blood inside the capillary and the surrounding tissue fluid and body cells. This short distance allows oxygen, glucose and other dissolved substances to diffuse quickly and efficiently in and out of the blood, which is essential since exchange, not transport, is the capillary's main function.

one cell thickshort diffusion distanceexchangeoxygentissue fluid

A patient with blood group A is accidentally given a transfusion of group B blood. Explain what happens as a result.

[6]

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The patient's own plasma contains anti-B antibodies, since their red blood cells carry the A antigen rather than B. When group B blood, carrying the B antigen, is transfused, the patient's anti-B antibodies bind to the antigens on the donor's red blood cells, causing the red blood cells to clump together, a reaction called agglutination. The clumps can block small blood vessels, restricting blood flow and potentially proving fatal, which is why ABO group must always be checked before a transfusion.

anti-B antibodiesB antigenagglutinationclumpincompatible transfusion

Describe the sequence of events that converts fibrinogen into a blood clot after a wound.

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When a blood vessel is damaged, platelets are exposed to the collagen of the vessel wall and gather at the site, releasing chemicals that activate a series of clotting factors. In the presence of calcium ions, these clotting factors convert the soluble plasma protein fibrinogen into insoluble fibrin. Fibrin threads form a mesh across the wound that traps red blood cells and platelets, forming a clot that seals the wound and dries into a scab.

plateletsclotting factorscalcium ionsfibrinogenfibrinclot

Explain how excess tissue fluid is returned to the blood by the lymphatic system.

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Blood pressure in the capillaries forces some plasma out into the spaces between cells, forming tissue fluid; most of it re-enters the capillaries, but the excess drains into nearby lymph vessels, where it becomes lymph. Lymph is moved along by the squeezing of surrounding skeletal muscles and by one-way valves that prevent backflow, and eventually passes through lymph nodes before rejoining the bloodstream through large lymph vessels near the heart.

tissue fluidlymph vesselslymphskeletal musclelymph nodesnear the heart

A person diagnosed with filariasis develops severe swelling in one leg. Explain how the disease causes this swelling.

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Filariasis is caused by thread-like parasitic worms transmitted by mosquito bites that settle and mature inside the lymph vessels. As the worms grow, they physically block the affected lymph vessels, preventing lymph, and the tissue fluid that would normally drain into it, from being carried away properly. Fluid then accumulates in the surrounding tissue of the limb, producing the severe swelling known as lymphoedema.

filariasisparasitic wormsmosquitolymph vessels blockedlymphoedema

Phrasing that earns marks

Types of circulatory system: Humans have a closed, double circulatory system: blood passes through the heart twice per full circuit.
The heart: Four chambers (two atria, two ventricles) with valves that keep blood flowing one way.
Mechanism of heartbeat: The heart muscle contracts (systole) and relaxes (diastole), controlled by the SA node.
Blood vessels: Arteries carry blood away from the heart at high pressure; veins return it; capillaries allow exchange.
Blood clotting: Platelets and clotting factors turn fibrinogen into fibrin, forming a clot that seals a wound.
Blood groups & lymphatic system: ABO and Rhesus groups determine safe transfusions; the lymphatic system returns tissue fluid and helps defence.
Pulmonary and systemic circuits: In pulmonary circulation, the right ventricle pumps deoxygenated blood through the pulmonary artery to the lungs, where it picks up oxygen and returns through the pulmonary vein to the left atrium. In systemic circulation, the left ventricle pumps oxygenated blood through the aorta to the rest of the body, and deoxygenated blood returns through the vena cava to the right atrium.
Stages of the cardiac cycle: During atrial systole the atria contract and push blood into the ventricles; during ventricular systole the ventricles contract, closing the atrioventricular valves and forcing blood into the arteries; during diastole all chambers relax and fill with blood. The valve closures produce the familiar 'lub-dub' heart sound heard through a stethoscope.

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.