Transport in Animals — CIE IGCSE Biology Revision Notes

What you'll learn

This topic covers how substances are transported around animal bodies, focusing on the mammalian circulatory system. CIE IGCSE Biology exam questions regularly test heart structure, blood vessel functions, blood composition, and the pathway of circulation. Understanding transport mechanisms is essential for answering 6-mark questions and interpreting data on heart disease.

Key terms and definitions

Circulatory system — a system of blood vessels with a pump and valves to ensure one-way flow of blood around the body.

Double circulation — blood passes through the heart twice during one complete circuit of the body (once through pulmonary circulation, once through systemic circulation).

Plasma — the pale yellow liquid component of blood that transports dissolved substances including nutrients, hormones, carbon dioxide, and urea.

Haemoglobin — the red pigment protein found in red blood cells that binds reversibly to oxygen for transport around the body.

Arteries — blood vessels that carry blood away from the heart, possessing thick muscular walls and elastic fibres to withstand high pressure.

Capillaries — tiny blood vessels with walls one cell thick that form exchange surfaces between blood and tissues.

Lymphocytes — a type of white blood cell that produces antibodies specific to antigens on pathogens.

Coronary arteries — blood vessels that branch from the aorta to supply oxygenated blood to the heart muscle tissue itself.

Core concepts

Why animals need transport systems

Small organisms like flatworms rely on diffusion alone to transport substances because they have a large surface area to volume ratio and short diffusion distances. Larger, more active animals require specialized transport systems because:

Their surface area to volume ratio is too small for diffusion to meet metabolic demands
Diffusion distances from body surface to internal organs are too great
Metabolic rate is high, requiring rapid delivery of glucose and oxygen
Waste products must be removed efficiently to prevent toxicity

The circulatory system in mammals overcomes these limitations by maintaining concentration gradients and reducing diffusion distances through extensive capillary networks.

Single versus double circulatory systems

Fish possess a single circulation where blood flows: heart → gills → body → heart. Blood passes through the heart once per complete circuit. Pressure drops significantly after passing through gill capillaries, limiting oxygen delivery rate to tissues.

Mammals possess double circulation comprising two separate circuits:

Pulmonary circulation: right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium

Systemic circulation: left ventricle → aorta → body organs → vena cava → right atrium

This arrangement maintains high blood pressure in systemic circulation after blood returns from the lungs, enabling faster delivery of oxygen and nutrients to respiring tissues. The left ventricle has a much thicker muscular wall than the right ventricle because it must generate sufficient pressure to pump blood throughout the entire body.

Structure of the mammalian heart

The heart is a muscular pump divided into four chambers:

Atria (upper chambers) — thin-walled chambers that receive blood from veins. The right atrium receives deoxygenated blood from the vena cava; the left atrium receives oxygenated blood from the pulmonary vein.

Ventricles (lower chambers) — thick-walled chambers that pump blood into arteries. The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery; the left ventricle pumps oxygenated blood to the body via the aorta.

Valves prevent backflow of blood:

Atrioventricular valves (tricuspid on right, bicuspid/mitral on left) lie between atria and ventricles
Semilunar valves are located in the pulmonary artery and aorta

Valves open when pressure is higher behind them and close when pressure is higher in front, ensuring one-way flow.

Septum — the wall of muscle separating the left and right sides of the heart, preventing mixing of oxygenated and deoxygenated blood.

The cardiac cycle

The heart beats approximately 70 times per minute at rest. Each cardiac cycle involves:

Atrial systole — atria contract, pushing blood through atrioventricular valves into ventricles
Ventricular systole — ventricles contract, closing atrioventricular valves and forcing open semilunar valves to push blood into arteries
Diastole — heart muscle relaxes, pressure drops, semilunar valves close, and blood from veins fills the atria

The cardiac cycle is controlled by the pacemaker (sinoatrial node) located in the wall of the right atrium, which produces electrical impulses that spread through heart muscle, causing coordinated contraction. Artificial pacemakers can be fitted surgically when natural pacemaker function fails.

Blood vessels: structure and function

Arteries carry blood away from the heart:

Thick walls containing smooth muscle and elastic fibres
Small lumen relative to wall thickness
Withstand and maintain high pressure
Elastic fibres stretch during systole and recoil during diastole, smoothing blood flow
No valves needed (except at heart exit) due to high pressure

Veins carry blood toward the heart:

Thinner walls with less muscle and elastic tissue than arteries
Large lumen relative to wall thickness
Carry blood at low pressure
Contain valves at intervals to prevent backflow
Blood movement assisted by contraction of surrounding skeletal muscles

Capillaries are the site of exchange:

Walls one cell thick (one layer of endothelial cells)
Very small lumen, often allowing only one red blood cell through at a time
Permeable walls allow exchange of substances by diffusion
Extensive networks reduce diffusion distances between blood and cells
Total cross-sectional area is very large, slowing blood flow to allow time for exchange

Composition and functions of blood

Blood consists of plasma (55%) and cells (45%).

Plasma transports:

Glucose from small intestine to respiring cells
Amino acids from small intestine to cells for protein synthesis
Carbon dioxide from respiring cells to lungs
Urea from liver to kidneys for excretion
Hormones from endocrine glands to target organs
Heat energy from respiring tissues (especially liver and muscles) to maintain body temperature
Plasma proteins including antibodies and fibrinogen

Red blood cells (erythrocytes) transport oxygen:

Biconcave disc shape provides large surface area for oxygen diffusion
No nucleus, maximizing space for haemoglobin
Contain haemoglobin, which binds to oxygen in lungs forming oxyhaemoglobin
Flexible membrane allows squeezing through narrow capillaries
In lungs: oxygen + haemoglobin → oxyhaemoglobin
In tissues: oxyhaemoglobin → oxygen + haemoglobin

White blood cells (leucocytes) defend against pathogens:

Phagocytes engulf and digest pathogens by phagocytosis
Lymphocytes produce specific antibodies that bind to antigens on pathogens, clumping them together for destruction
Contain a nucleus and can change shape
Much less numerous than red blood cells

Platelets are cell fragments that assist in blood clotting:

Release chemicals at wound sites
Trigger conversion of soluble fibrinogen protein into insoluble fibrin threads
Fibrin forms a mesh that traps red blood cells, forming a clot
Clots seal wounds, preventing blood loss and pathogen entry

Coronary heart disease

The heart muscle requires its own blood supply via coronary arteries. Coronary heart disease occurs when these arteries become narrowed by fatty deposits (atherosclerosis), reducing blood flow to heart muscle.

Risk factors include:

Diet high in saturated fats and cholesterol
Smoking (carbon monoxide damages artery linings; nicotine increases blood pressure)
Lack of physical exercise
Genetic predisposition
Stress
High blood pressure (hypertension)

Consequences include:

Angina — chest pain during exercise when heart muscle receives insufficient oxygen
Heart attack (myocardial infarction) — coronary artery completely blocked, part of heart muscle dies due to lack of oxygen

Treatments include:

Lifestyle modifications (improved diet, regular exercise, stopping smoking)
Medications (statins to lower cholesterol, aspirin to reduce clotting)
Surgical procedures (bypass surgery, angioplasty with stent insertion)

Worked examples

Example 1: The diagram shows the human heart. Describe the route blood takes from entering the heart from the body until it leaves the heart to return to the body. [4 marks]

Answer: Blood enters the right atrium [1] via the vena cava. Blood passes through the tricuspid/atrioventricular valve into the right ventricle [1]. Blood is pumped to the lungs via the pulmonary artery [1]. Oxygenated blood returns via the pulmonary vein to the left atrium, passes through the bicuspid valve into the left ventricle, then leaves via the aorta [1].

Example 2: Explain why the wall of the left ventricle is thicker than the wall of the right ventricle. [2 marks]

Answer: The left ventricle must generate higher pressure [1] to pump blood all around the body, whereas the right ventricle only pumps blood to the lungs [1].

Example 3: Red blood cells are adapted to their function. Describe two adaptations of red blood cells and explain how each helps them transport oxygen efficiently. [4 marks]

Answer: Biconcave shape provides a large surface area [1] which allows faster diffusion of oxygen into and out of the cell [1]. No nucleus [1] provides more space for haemoglobin to carry more oxygen [1]. (Also accept: contain haemoglobin which binds reversibly to oxygen; flexible membrane allows passage through narrow capillaries.)

Common mistakes and how to avoid them

Mistake: Stating that arteries always carry oxygenated blood and veins always carry deoxygenated blood. Correction: Arteries carry blood away from the heart (pulmonary artery carries deoxygenated blood); veins carry blood toward the heart (pulmonary vein carries oxygenated blood).
Mistake: Confusing the vena cava and aorta in diagrams. Correction: The vena cava brings deoxygenated blood into the right atrium; the aorta carries oxygenated blood away from the left ventricle. Remember the left side of the heart handles oxygenated blood.
Mistake: Writing that valves control the heartbeat. Correction: Valves prevent backflow of blood; the pacemaker (sinoatrial node) controls the heartbeat by producing electrical impulses.
Mistake: Stating that capillaries have thin walls. Correction: Capillary walls are one cell thick (not just thin) — this precise description is required for full marks.
Mistake: Confusing antibodies and antigens. Correction: Antigens are markers on the surface of pathogens; antibodies are proteins produced by lymphocytes that bind specifically to antigens.
Mistake: Writing that red blood cells transport carbon dioxide. Correction: Most carbon dioxide is transported dissolved in plasma; red blood cells primarily transport oxygen via haemoglobin.

Exam technique for Transport in Animals

Describe questions require factual statements without explanation. When describing blood flow through the heart, name all structures in sequence (chambers, valves, vessels) for full marks.
Explain questions require reasoning. Use "because," "so that," or "this allows" to link structure to function. For example, "capillaries have walls one cell thick so that diffusion distance is short, allowing rapid exchange."
Structure-function questions typically award one mark for identifying the structural feature and one mark for explaining how it relates to function. Always make the connection explicit.
Questions worth 4-6 marks often assess understanding of coronary heart disease. Ensure you can describe how fatty deposits narrow arteries, explain risk factors, and discuss consequences including heart attacks.

Quick revision summary

Mammals have a double circulatory system where blood passes through the heart twice per circuit. The four-chambered heart pumps blood through arteries (thick walls, high pressure), capillaries (one cell thick, exchange site), and veins (valves, low pressure). Blood contains plasma (transports dissolved substances), red blood cells (carry oxygen using haemoglobin), white blood cells (destroy pathogens), and platelets (blood clotting). Coronary heart disease results from fatty deposits narrowing coronary arteries, potentially causing heart attacks. Understanding structure-function relationships is essential for exam success.