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This lesson covers the structure and function of blood vessels (arteries, arterioles, capillaries, venules, and veins) and the composition of blood as required by the Edexcel A-Level Biology specification (9BI0). You need to understand how the structure of each vessel type relates to its function, and the roles of the different blood components.
The circulatory system uses three main types of blood vessel, each with a structure precisely adapted to its function.
Arteries carry blood away from the heart under high pressure.
| Structural feature | Function |
|---|---|
| Thick muscular wall with a thick layer of smooth muscle (tunica media) | Withstands high blood pressure; smooth muscle can constrict (vasoconstriction) or relax (vasodilation) to regulate blood flow |
| Thick layer of elastic tissue | Stretches and recoils with each heartbeat, smoothing the pulsatile blood flow into a more continuous flow (the Windkessel effect) |
| Relatively narrow lumen compared to wall thickness | Maintains high blood pressure |
| Endothelium (tunica intima) — smooth inner lining | Reduces friction and turbulence |
| No valves (except in the aorta and pulmonary artery — semilunar valves) | Blood flows under sufficient pressure that backflow is prevented |
| Tough outer layer (tunica adventitia) | Prevents over-expansion and provides structural support |
Arterioles are smaller branches of arteries that lead into capillary beds.
Capillaries are the smallest blood vessels, and they are the site of exchange between blood and body tissues.
| Structural feature | Function |
|---|---|
| Extremely narrow diameter (~7–10 µm, just wide enough for a single red blood cell) | Brings blood into close contact with tissue cells; slows blood flow to allow time for exchange |
| Wall just one cell thick (single layer of endothelial cells) | Very short diffusion pathway for O₂, CO₂, glucose, etc. |
| No smooth muscle or elastic tissue | Not needed — pressure is low in capillaries |
| Tiny gaps (pores) between endothelial cells | Allow plasma and dissolved substances to leak out, forming tissue fluid (but red blood cells and most proteins are too large to pass through) |
| Huge total cross-sectional area (vast capillary networks) | Slows blood flow; maximises time for exchange |
Key Definition: Tissue fluid — fluid that leaks out of capillaries due to the hydrostatic pressure of the blood. It bathes the cells and allows exchange of substances between blood and cells. Most tissue fluid is reabsorbed at the venous end of capillaries; the remainder enters the lymphatic system.
Veins carry blood back to the heart under low pressure.
| Structural feature | Function |
|---|---|
| Thin walls with less smooth muscle and elastic tissue than arteries | Blood is at low pressure, so thick walls are not needed |
| Wide lumen (relative to wall thickness) | Reduces resistance to blood flow, helping blood return to the heart |
| Valves (semi-lunar pocket valves) | Prevent backflow of blood — essential because blood pressure in veins is low |
| Smooth endothelium | Reduces friction |
Blood pressure in veins is very low, so veins rely on several mechanisms to return blood to the heart:
Exam Tip: A classic exam question asks you to compare and contrast arteries, veins, and capillaries. Use a table format and always link the structural features to functions.
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Direction of blood flow | Away from heart | Between arterioles and venules | Towards heart |
| Blood pressure | High | Low (drops across capillary bed) | Very low |
| Wall thickness | Thick | One cell thick | Thin |
| Lumen diameter | Relatively narrow | Very narrow (~7–10 µm) | Wide |
| Elastic tissue | Abundant | None | Little |
| Smooth muscle | Thick layer | None | Thin layer |
| Valves | No (except semilunar in aorta/pulmonary artery) | No | Yes (throughout) |
| Blood velocity | Fast, pulsatile | Slow | Slow to moderate |
Tissue fluid is formed at the capillary beds and is the medium through which substances are exchanged between blood and cells.
Not all tissue fluid is reabsorbed at the venous end. The excess drains into lymphatic capillaries and becomes lymph. Lymph is eventually returned to the blood via the thoracic duct, which empties into the subclavian vein near the heart.
Exam Tip: When explaining tissue fluid formation, always refer to hydrostatic pressure and oncotic pressure, and state the direction of net fluid movement at each end of the capillary.
Blood is a connective tissue consisting of cells (and cell fragments) suspended in a liquid matrix called plasma.
Plasma makes up approximately 55% of blood volume. It is a pale yellow liquid containing:
| Component | Function |
|---|---|
| Water (~90% of plasma) | Solvent; carries dissolved substances |
| Plasma proteins (albumin, globulins, fibrinogen) | Albumin maintains oncotic pressure; globulins include antibodies; fibrinogen is involved in blood clotting |
| Glucose, amino acids, lipids | Nutrients transported to cells |
| Urea | Waste product transported from the liver to the kidneys |
| Hormones | Chemical messengers transported to target organs |
| Ions (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻) | Maintain osmotic balance, pH buffering, nerve function |
| Dissolved gases (O₂, CO₂) | Small amounts dissolved in plasma |
White blood cells are part of the immune system. There are several types:
| Type | Function |
|---|---|
| Neutrophils (~70% of WBCs) | Phagocytosis — engulf and digest bacteria; first responders to infection |
| Lymphocytes (~25% of WBCs) | B lymphocytes produce antibodies; T lymphocytes destroy infected cells |
| Monocytes | Develop into macrophages in tissues; phagocytosis |
| Eosinophils | Involved in parasitic infections and allergic responses |
| Basophils | Release histamine; involved in inflammatory responses |
When a blood vessel is damaged, the following cascade occurs:
Exam Tip: Blood clotting is an example of a positive feedback mechanism — the initial formation of thrombin accelerates further thrombin production, rapidly amplifying the response. Know the key steps: platelets → clotting factors → prothrombin → thrombin → fibrinogen → fibrin.
| Component | Key features |
|---|---|
| Arteries | Thick walls, elastic tissue, narrow lumen, carry blood from heart at high pressure |
| Capillaries | One cell thick, tiny diameter, pores, site of exchange |
| Veins | Thin walls, wide lumen, valves, carry blood to heart at low pressure |
| Tissue fluid | Formed by ultrafiltration at capillary beds; excess → lymphatic system |
| Red blood cells | Biconcave, no nucleus, packed with haemoglobin |
| White blood cells | Immune defence (phagocytosis, antibody production) |
| Platelets | Cell fragments; essential for clotting |
| Plasma | Liquid matrix; transports dissolved substances |
A thorough understanding of blood vessel structure, tissue fluid formation, and blood composition is essential for the Edexcel exam — expect comparison questions and application to disease contexts.
This material occupies the vascular and haematological core of Edexcel 9BI0 Topic 7 (Run for your life — Exchange and Transport). The specification requires candidates to relate the wall structure of arteries, arterioles, capillaries, venules and veins to their respective functions, to explain tissue-fluid formation by net filtration at the arteriolar end and net reabsorption at the venular end of a capillary in terms of hydrostatic and oncotic pressures, and to describe the composition of blood (plasma, erythrocytes, leucocyte subtypes, platelets) together with the basic clotting cascade. Synoptic with lesson 4 (heart structure) — arterial wall design exists to absorb the systolic pressure pulse generated there; with lesson 5 (cardiac cycle) — peak ventricular pressure (~120 mmHg) is the source of the arteriolar hydrostatic pressure that drives filtration; with lesson 7 (haemoglobin) — erythrocytes are the cellular vehicle for the O2-carrying pigment whose dissociation kinetics are studied next; with Topic 6 (immunity) — the leucocyte subtypes named here (neutrophils, lymphocytes, monocytes) are the cellular agents of innate and adaptive responses; and with Topic 8 (kidney/nephron) — Starling-force balance at the systemic capillary is the same physical reasoning applied to ultrafiltration at Bowman's capsule. Refer to the official Pearson Edexcel 9BI0 specification document for the exact wording of the relevant statements.
Question (8 marks):
Figure 1 (not shown) summarises pressures at the two ends of a systemic capillary in resting skeletal muscle:
| End of capillary | Hydrostatic pressure (mmHg) | Oncotic pressure (mmHg) |
|---|---|---|
| Arteriolar | 35 | 25 |
| Venular | 15 | 25 |
(a) Calculate the net filtration pressure at the arteriolar end and the net reabsorption pressure at the venular end. State the direction of net fluid movement at each end. (3)
(b) Explain, with reference to the data, why approximately 90% of filtered fluid is reabsorbed back into the capillary while the remainder leaves the tissue by a different route. (3)
(c) A patient with severe liver disease has a plasma albumin concentration only 40% of normal. Predict and explain the effect on tissue-fluid balance. (2)
Solution with mark scheme:
(a) M1 (AO2) — Arteriolar end: net pressure =35−25=+10 mmHg outwards Rightarrow net filtration out of the capillary.
M1 (AO2) — Venular end: net pressure =25−15=+10 mmHg inwards Rightarrow net reabsorption into the capillary.
A1 (AO1) — Hydrostatic pressure falls along the capillary because of frictional resistance to flow; oncotic pressure stays approximately constant because plasma proteins do not leave the vessel.
A common pitfall is treating the two pressures as additive rather than opposing — examiners want the subtraction, with the sign giving the direction.
(b) M1 (AO1) — Filtration and reabsorption rates are not exactly equal: a small net excess (~10%) of filtered fluid remains in the interstitium.
M1 (AO2) — This excess enters lymphatic capillaries as lymph and is returned to the bloodstream via the thoracic duct into the subclavian vein.
A1 (AO3) — The lymphatic system therefore has to carry the small per-capillary residue from every tissue in the body; if it fails (lymphoedema) tissue fluid accumulates and oedema results.
(c) M1 (AO2) — Reduced plasma albumin lowers oncotic pressure (from ~25 mmHg towards ~15 mmHg), so reabsorption at the venular end falls.
A1 (AO3) — Net filtration now exceeds net reabsorption at most capillaries; fluid accumulates in the tissues, producing oedema (a hallmark of advanced liver failure).
Total: 8 marks (M5 A3).
Question (6 marks): Compare and contrast the structure of an artery, a capillary and a vein, and explain how each structure is matched to its function.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that arteries have a thick tunica media of smooth muscle and elastic tissue, capillaries are a single endothelial layer with a basement membrane, and veins have a thin wall with a wide lumen and valves |
| 2 | AO1.2 | Linking the artery's elastic tissue to recoil between heartbeats (the Windkessel effect) maintaining diastolic flow |
| 3 | AO2.1 | Linking the capillary's single-cell-thick wall to a short diffusion pathway and large surface area for exchange |
| 4 | AO2.1 | Linking the vein's wide lumen to low resistance and its valves to prevention of backflow under low pressure |
| 5 | AO2.7 | Stating that arteriolar smooth muscle is the main site of resistance and controls distribution of cardiac output between tissues |
| 6 | AO3.1 | Synthesis: the three vessel types form a graded pressure-and-exchange continuum — arteries deliver under pressure, capillaries exchange at low pressure and high surface area, veins return at low pressure with assistance from valves and skeletal-muscle pumping |
Total: 6 marks (AO1 = 2, AO2 = 3, AO3 = 1). Edexcel vessel-comparison questions of this type reliably split AO marks roughly 30/50/20 across AO1/AO2/AO3.
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