Form 4 · Revision notes

Transport in Humans and Animals, revision notes

Complete revision notes for Transport in Humans and Animals: every content standard, the key definitions, comparison tables and a recall checklist for SPM Biology Form 4.

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Overview

Large animals need a transport system to carry oxygen, nutrients and wastes. This chapter covers the human circulatory system, the heart and its beat, blood clotting, blood groups, and the lymphatic system.

It is a diagram-heavy chapter, so labelling the heart and blood vessels accurately matters.

Content standard 10.2 expects detail on what blood itself is made of, not only where it flows. Plasma, a pale yellow liquid, carries dissolved food, wastes, hormones and carbon dioxide; red blood cells carry oxygen bound to haemoglobin; white blood cells defend the body against pathogens; and platelets trigger clotting at a wound.

The cardiac cycle then describes how the heart's chambers contract in sequence, atria first and ventricles a moment later, with valves snapping shut between beats to stop blood flowing backwards, producing the two-part heart sound heard through a stethoscope.

The lymphatic system and its health issues, standards 10.7 and 10.8, are often the least revised part of this chapter yet appear regularly in Paper 2. Lymph vessels drain excess tissue fluid that has leaked from capillaries, filter it through lymph nodes packed with white blood cells, and return it to the bloodstream, so the system supports both fluid balance and defence.

Filariasis, spread by mosquitoes, can block these vessels and cause severe swelling called lymphoedema, while atherosclerosis and hypertension are the circulatory-system health issues most often asked about together with their causes and prevention.

10.1 Types of Circulatory System

Circulatory systems are classified along two independent axes: open versus closed, and single versus double. In an open system, found in insects, blood (haemolymph) leaves vessels and bathes body tissues directly; in a closed system, found in humans and other vertebrates, blood remains inside blood vessels at all times, allowing higher pressure and faster, more controlled delivery.

Humans have a closed, double circulatory system: blood passes through the heart twice for every complete circuit of the body, once through the pulmonary circuit to the lungs and once through the systemic circuit to the rest of the body. This keeps oxygenated and deoxygenated blood almost completely separate and allows the systemic circuit to be driven at much higher pressure than the pulmonary circuit.

10.2 Circulatory System of Humans: Blood and Vessels

Blood itself is composed of plasma, a pale straw-coloured liquid making up over half its volume, in which red blood cells, white blood cells and platelets are suspended. Plasma carries dissolved nutrients, hormones, carbon dioxide and waste products; red blood cells, packed with haemoglobin, transport oxygen; white blood cells defend against pathogens; and platelets trigger clotting at a wound.

Table 3 compares the three types of blood vessel that connect to form the circulatory network. Arteries are built to withstand high pressure as blood surges from the heart, capillaries are built for exchange, and veins are built to return blood at low pressure without it flowing backward.

Feature	Artery	Vein	Capillary
Wall	Thick, muscular and elastic	Thinner, less muscular	One cell thick
Lumen	Narrow	Wide	Very narrow, single blood cell width
Blood pressure	High	Low	Low, allows exchange
Valves	Usually absent (except at heart exits)	Present, prevent backflow	Absent
Direction of flow	Away from the heart	Towards the heart	Between arterioles and venules

10.3 Mechanism of Heart Beat

The heart is myogenic: cardiac muscle can contract rhythmically on its own, without needing a nerve impulse to start each beat. A small patch of tissue called the sinoatrial (SA) node, located in the wall of the right atrium, generates the electrical signal that spreads across both atria, causing them to contract together (atrial systole) and push blood into the ventricles.

The signal then passes to the atrioventricular (AV) node and down specialised fibres into the ventricle walls, triggering ventricular systole slightly later so the ventricles contract only after filling. As the ventricles contract, the atrioventricular valves snap shut to stop blood flowing back into the atria, and the semilunar valves open to let blood into the arteries; during diastole, all four chambers relax and refill.

The nervous system and hormones such as adrenaline can speed up or slow down the rate the SA node fires, but they do not initiate the beat itself.

10.4 Mechanism of Blood Clotting

Clotting is triggered when a blood vessel is damaged and platelets are exposed to the collagen of the broken vessel wall. The platelets clump at the site and release chemicals that, together with a cascade of clotting factors and calcium ions, convert the soluble plasma protein fibrinogen into insoluble fibrin.

Fibrin forms a mesh of fine threads across the wound, trapping red blood cells and platelets to form a clot, which dries into a scab. This seals the wound, stops further blood loss, and blocks the entry point microorganisms would otherwise use, so clotting is both a repair mechanism and a first line of defence.

10.5 Blood Grouping in Humans

The ABO system classifies blood by the antigens on the surface of red blood cells: group A carries antigen A, group B carries antigen B, group AB carries both, and group O carries neither. Plasma carries antibodies against whichever antigen a person's own red blood cells lack, so group O plasma carries both anti-A and anti-B antibodies.

Transfusing incompatible blood causes the recipient's antibodies to bind the donor's antigens, making red blood cells clump together, or agglutinate; the clumps can block small blood vessels and be fatal. The Rhesus (Rh) system adds a second layer of compatibility based on a separate antigen: a person is Rh-positive if red blood cells carry this antigen and Rh-negative if not, and an Rh-negative person can develop anti-Rh antibodies after exposure to Rh-positive blood.

Both the ABO group and the Rh status must be matched before a safe transfusion.

10.6 Health Issues Related to the Human Circulatory System

Atherosclerosis is the gradual build-up of fatty deposits, mainly cholesterol, within the walls of arteries; the deposits narrow the lumen and restrict blood flow, and if a coronary artery becomes blocked, the heart muscle it supplies is starved of oxygen, causing a heart attack. Hypertension is persistently high blood pressure, which strains the heart muscle and the walls of blood vessels over time.

Both conditions are linked to a diet high in saturated fat and salt, low physical activity and smoking, and each can worsen the other, since narrowed arteries force the heart to pump at higher pressure. Reducing these risk factors through diet and lifestyle change is the main biology-level prevention taught at this level.

10.7 Lymphatic System of Humans

The lymphatic system is a network of lymph vessels, lymph nodes and organs such as the spleen that runs alongside the blood circulatory system. Blood pressure in the capillaries forces some plasma out into the spaces between cells, forming tissue fluid; most of this fluid is reabsorbed back into the capillaries, but the excess drains into lymph vessels instead, where it is now called lymph.

Lymph passes through lymph nodes, small swellings packed with white blood cells that filter out pathogens and debris before the cleaned fluid is returned to the bloodstream through large lymph vessels that join the blood system near the heart. Unlike blood, lymph has no central pump; it is moved mainly by the squeezing action of surrounding skeletal muscles and by one-way valves in the lymph vessels that stop it flowing backward.

10.8 Health Issues Related to the Human Lymphatic System

Filariasis is caused by thread-like parasitic worms that are transmitted between people through the bite of an infected mosquito. The worms settle and mature inside lymph vessels, physically blocking the flow of lymph through them.

Blocked lymph vessels can no longer drain excess tissue fluid properly, so fluid builds up in the affected limb, causing the severe, often disfiguring swelling known as lymphoedema, called elephantiasis in advanced cases. Because the disease depends on mosquito transmission, preventing mosquito bites and controlling mosquito breeding sites are the main measures taught for reducing its spread.

Key concepts to master

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.
Capillary structure and exchange, A capillary wall is only one cell thick, giving a short diffusion distance between blood and body cells. This thin wall, together with a narrow lumen that slows blood flow, allows oxygen, nutrients and carbon dioxide to diffuse efficiently between the blood and the surrounding tissue fluid.
Composition of blood, Blood is made of a pale yellow liquid called plasma, which carries dissolved nutrients, wastes and hormones, together with red blood cells that transport oxygen using haemoglobin, white blood cells that defend against pathogens, and platelets that trigger clotting at a wound.
ABO blood group compatibility, Red blood cells carry A and/or B antigens, or neither in group O, while plasma carries antibodies against the antigens a person lacks. Transfusing incompatible blood causes the antibodies to make the donor's red blood cells clump together, or agglutinate, which can block vessels, so blood group must always be matched before a transfusion.
Rhesus factor, The Rhesus (Rh) system is a second blood group system based on the presence, Rh-positive, or absence, Rh-negative, of the Rhesus antigen on red blood cells. An Rh-negative person can develop anti-Rh antibodies after exposure to Rh-positive blood, so Rh status, alongside ABO group, must be checked before a safe transfusion.
Structure of the lymphatic system, The lymphatic system is a network of lymph vessels, lymph nodes and organs such as the spleen. Lymph vessels collect excess tissue fluid that leaks from capillaries, filter it through lymph nodes containing white blood cells that trap pathogens, and return the cleaned fluid, now called lymph, to the bloodstream near the heart.
Health issues of the circulatory system, Atherosclerosis is the build-up of fatty deposits inside artery walls, narrowing them and restricting blood flow; if this blocks a coronary artery it can cause a heart attack. Hypertension, or persistently high blood pressure, strains the heart and blood vessels and is linked to diet, inactivity and smoking.
Health issues of the lymphatic system, Filariasis, caused by thread-like parasitic worms spread by mosquito bites, can block lymph vessels and cause lymphoedema, a painful swelling of the limbs sometimes called elephantiasis. Avoiding mosquito bites and controlling mosquito breeding sites are the main preventive measures against this disease.
Heart rate regulation, The heart is myogenic, meaning it can initiate its own beat without an external nerve signal, though the nervous system and hormones such as adrenaline can speed it up or slow it down. Exercise, emotion, body temperature and fitness level are among the factors that change heart rate to meet the body's changing demand for oxygen.

Quick recall checklist

Can you define and explain Types of circulatory system?
Can you define and explain The heart?
Can you define and explain Mechanism of heartbeat?
Can you define and explain Blood vessels?
Can you define and explain Blood clotting?
Can you define and explain Blood groups & lymphatic system?
Can you define and explain Pulmonary and systemic circuits?
Can you define and explain Stages of the cardiac cycle?
Can you define and explain Capillary structure and exchange?
Can you define and explain Composition of blood?
Can you define and explain ABO blood group compatibility?
Can you define and explain Rhesus factor?
Can you define and explain Structure of the lymphatic system?
Can you define and explain Health issues of the circulatory system?
Can you define and explain Health issues of the lymphatic system?
Can you define and explain Heart rate regulation?

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.