Internal transport is the specification section that ties the whole body together. The blood, the heart and the vessels form the delivery network that supplies every cell with oxygen and nutrients and removes every waste product. The edexcel igcse human biology exam tests this with consistent depth.
This section spans two topics: blood and circulation, and heart disease and treatment. Together they cover the composition of blood, the role of plasma, tissue fluid, red blood cell adaptations, blood groups, white blood cells, clotting, blood vessels, the circulatory plan, the structure of the heart, and a substantial block on cardiovascular disease and its treatments. A solid understanding of edexcel igcse human biology internal transport is essential for success on the 4HB1 specification, because questions on this section routinely require you to link anatomy, physiology and clinical applications. These edexcel igcse human biology revision notes address both topics in full, with the worked examples and precision the mark scheme rewards.
Blood composition
Blood is a fluid tissue made up of four components:
| Component | Structure | Function |
|---|---|---|
| Red blood cells (erythrocytes) | Biconcave disc shape, no nucleus, packed with haemoglobin | Transport oxygen from the lungs to the tissues |
| White blood cells (phagocytes) | Larger than red blood cells, with a lobed nucleus | Engulf and digest pathogens by phagocytosis |
| White blood cells (lymphocytes) | Large nucleus that fills most of the cell | Produce antibodies that bind to antigens on pathogens |
| Platelets | Cell fragments, no nucleus | Involved in blood clotting at wound sites |
| Plasma | Straw-coloured liquid | Transports dissolved substances throughout the body |
What plasma transports
The specification names five substances carried by plasma:
- Carbon dioxide: Waste product of respiration, transported from cells to the lungs for exhalation
- Digested food: Glucose, amino acids, fatty acids and glycerol from the small intestine to cells throughout the body
- Urea: Waste product of protein metabolism, produced in the liver, transported to the kidneys for excretion
- Hormones: Chemical messengers from endocrine glands to their target organs
- Heat energy: Distributed from active organs (such as the liver and muscles) to the rest of the body, helping to maintain core temperature
Tissue fluid
As blood passes through capillaries, the pressure forces plasma (minus the large proteins) out through the capillary walls into the spaces between cells. This fluid is called tissue fluid. It bathes the cells and delivers oxygen and nutrients directly to them. Waste products (CO2, urea) pass from the cells into the tissue fluid and then back into the capillaries.
Not all tissue fluid returns directly to the capillaries. The excess drains into the lymphatic system, a network of vessels that eventually returns the fluid to the blood near the heart.
Red blood cell adaptations
Red blood cells are adapted for oxygen transport in several ways:
- Biconcave disc shape: Increases the surface area to volume ratio, allowing faster diffusion of oxygen in and out
- No nucleus: More space for haemoglobin, so each cell can carry more oxygen
- Packed with haemoglobin: The iron-containing protein that binds reversibly with oxygen. In the lungs, haemoglobin binds oxygen to form oxyhaemoglobin (Hb + O2 → HbO2). In the tissues, oxyhaemoglobin releases oxygen (HbO2 → Hb + O2)
- Flexible: Can squeeze through narrow capillaries
ABO blood groups
The specification requires understanding of the ABO blood group system and its importance in transfusions. Blood groups are determined by the antigens on the surface of red blood cells and the antibodies in the plasma:
| Blood group | Antigens on red blood cells | Antibodies in plasma | Can receive blood from |
|---|---|---|---|
| A | A | Anti-B | A, O |
| B | B | Anti-A | B, O |
| AB | A and B | Neither | A, B, AB, O (universal recipient) |
| O | Neither | Anti-A and Anti-B | O only (universal donor) |
If a patient receives blood with antigens that match their antibodies, the donated red blood cells will agglutinate (clump together), which can block blood vessels and be fatal. This is why blood typing before transfusion is essential.
White blood cells and immunity
Phagocytosis: Phagocytes detect pathogens, engulf them by wrapping their cell membrane around the pathogen, and digest them using enzymes inside the cell.
Antibody production: Lymphocytes recognise specific antigens on the surface of pathogens. They produce antibodies that are complementary to those antigens. Antibodies bind to the antigens, marking the pathogen for destruction (by phagocytes), causing agglutination, or neutralising toxins.
Blood clotting
When a blood vessel is damaged, platelets gather at the wound site and trigger a cascade of enzyme reactions. The key step is the conversion of the soluble protein fibrinogen (dissolved in plasma) into insoluble fibrin threads. These threads form a mesh over the wound, trapping red blood cells and forming a clot (scab). Clotting prevents further blood loss and blocks the entry of pathogens.
Blood vessels
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Wall thickness | Thick, muscular, elastic walls | Thinner walls with less muscle and elastic tissue | Wall is one cell thick |
| Lumen | Narrow (relative to wall thickness) | Wide | Very narrow (one red blood cell wide) |
| Valves | No valves (except in the aorta and pulmonary artery) | Valves present to prevent backflow | No valves |
| Blood pressure | High | Low | Decreasing |
| Function | Carry blood away from the heart under high pressure | Return blood to the heart under low pressure | Exchange of substances between blood and tissues |
The pulse is the rhythmic expansion of an artery wall caused by each heartbeat. You can feel it in arteries that pass close to the surface, such as the radial artery in the wrist.
The circulatory system
The human circulatory system is a double circulation: blood passes through the heart twice on each complete circuit of the body.
- Pulmonary circulation: Right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary vein → left atrium
- Systemic circulation: Left ventricle → aorta → body organs → vena cava → right atrium
The specification names three specific organ circuits:
- Lungs: Pulmonary artery (deoxygenated blood) and pulmonary vein (oxygenated blood)
- Liver: Hepatic artery (oxygenated blood), hepatic portal vein (blood from the intestines carrying absorbed nutrients), hepatic vein (blood leaving the liver)
- Kidneys: Renal artery (oxygenated blood with urea) and renal vein (blood with reduced urea)
Structure and function of the heart
The heart is a muscular pump with four chambers: right atrium, right ventricle, left atrium and left ventricle. The septum separates the left and right sides, preventing oxygenated and deoxygenated blood from mixing.
Blood flows through the heart in one direction, maintained by valves:
- The atrioventricular valves (between atria and ventricles) prevent backflow from ventricles to atria.
- The semilunar valves (in the aorta and pulmonary artery) prevent backflow from arteries to ventricles.
The left ventricle has a thicker wall than the right ventricle because it must pump blood at higher pressure to the entire body (systemic circulation), while the right ventricle only pumps blood the short distance to the lungs.
Heart disease and treatment
Coronary heart disease (CHD) occurs when the coronary arteries (which supply the heart muscle with oxygenated blood) become narrowed by atherosclerosis: the build-up of fatty deposits (atheroma) on the inner walls. This restricts blood flow, reducing the oxygen supply to the heart muscle. A complete blockage causes a heart attack (myocardial infarction), where part of the heart muscle dies.
Causes and prevention
- Diet: High intake of saturated fat and cholesterol contributes to atherosclerosis. A balanced diet low in saturated fat reduces risk.
- Exercise: Regular aerobic exercise strengthens the heart and improves circulation.
- Smoking: Carbon monoxide and nicotine increase heart disease risk. Stopping smoking reduces it.
- Stress: Chronic stress raises blood pressure and heart rate.
Treatments
- Stents: A small mesh tube inserted into a narrowed coronary artery to hold it open, restoring blood flow.
- Artificial hearts: Mechanical devices that replace or assist the heart temporarily, often while a patient waits for a transplant.
- Heart transplants: A diseased heart is replaced with a healthy donor heart. Problems include donor shortage, immune rejection (the recipient's immune system attacks the donor organ), and the need for lifelong immunosuppressant drugs.
- Statins: Drugs that lower blood cholesterol levels by reducing cholesterol production in the liver, slowing the progression of atherosclerosis.
- Plant stanol esters: Substances found in some foods (or added as supplements) that reduce cholesterol absorption from the gut, lowering blood cholesterol levels.
- Beta-blockers: Drugs that reduce heart rate and blood pressure by blocking the effects of adrenaline on the heart. Used to treat heart failure and angina (chest pain caused by reduced blood flow to the heart).
Blood pressure
Systolic pressure is the pressure in the arteries when the heart ventricles contract (the higher number in a blood pressure reading). Diastolic pressure is the pressure when the ventricles relax (the lower number). A reading of 120/80 mmHg means systolic is 120 and diastolic is 80.
Hypertension (high blood pressure) increases the risk of stroke, heart attack and kidney damage. It is managed by lifestyle changes (diet, exercise, reducing salt and alcohol) and medications such as ACE inhibitors, which work by blocking the enzyme that produces angiotensin II (a hormone that narrows blood vessels), allowing blood vessels to relax and blood pressure to fall.
Monoclonal antibodies
Monoclonal antibodies are identical antibodies produced by clones of a single lymphocyte. The production process involves:
- Injecting an animal with the target antigen to stimulate lymphocytes.
- Extracting the lymphocytes that produce the desired antibody.
- Fusing these lymphocytes with tumour cells to create hybridoma cells that divide rapidly and produce large quantities of the same antibody.
- Growing the hybridoma cells in culture and harvesting the monoclonal antibodies.
Monoclonal antibodies can be used to detect and treat diseases such as cancer. They can be designed to bind to specific antigens on cancer cells, either marking them for destruction by the immune system, delivering toxic drugs directly to the cancer cells (reducing side effects on healthy tissue), or blocking signals that tell cancer cells to divide.
Self-check questions
- Name two substances transported by plasma.
- Explain how tissue fluid is formed from blood in the capillaries.
- State three adaptations of red blood cells for oxygen transport.
- A person with blood group B needs a transfusion. Which blood groups can they safely receive?
- Describe the process of blood clotting at a wound site.
- Explain why the left ventricle has a thicker muscular wall than the right ventricle.
- State two risk factors for coronary heart disease.
- Describe how monoclonal antibodies are produced.
These edexcel igcse human biology notes on internal transport cover all specification points across both sub-topics. On the Green Bridge CBT platform, you can practise internal transport edexcel igcse questions and igcse 4hb1 internal transport revision sets under timed conditions. For the full breadth of the course, explore the edexcel igcse human biology explained series and the complete bank of edexcel igcse human biology practice questions.
Edexcel IGCSE Human Biology internal transport revision notes: blood composition, heart structure, blood vessels, heart disease and monoclonal antibodies.
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