Explain how structure and function are related in arteries, veins and capillaries.

Cambridge IGCSE Biology 0610 – Transport in Humans

Learning Objective

Explain how the structure of arteries, veins, capillaries (and hepatic vessels) determines their specific functions in the circulatory system, and relate these structures to the operation of the heart and the composition of blood.

9.1 – Definition of the Circulatory System (core)

The circulatory system is a closed pump‑vessel network that transports blood, nutrients, gases, hormones and waste products between the heart, lungs, kidneys and all body tissues.

Overview of the Cardiovascular Unit

• Heart – muscular pump that generates the pressure needed to move blood.
• Blood vessels – three main types (arteries, veins, capillaries) plus specialised hepatic vessels; they form a continuous circuit.
• Blood – liquid connective tissue that carries cells, plasma proteins and dissolved substances.

9.2 – The Heart (core)

A four‑chambered organ that receives de‑oxygenated blood from the body and pumps oxygen‑rich blood to the lungs and systemic tissues.

• Chambers
  • Right atrium (RA) – receives de‑oxygenated blood from the body.
  • Right ventricle (RV) – pumps blood to the lungs via the pulmonary artery.
  • Left atrium (LA) – receives oxygenated blood from the lungs.
  • Left ventricle (LV) – pumps blood to the systemic circuit via the aorta.
• Valves (prevent back‑flow)
  • Atrioventricular (AV) valves – tricuspid (right) and mitral/bicuspid (left).
  • Semilunar valves – pulmonary (right) and aortic (left).
• Major vessels attached to the heart
  • Aorta – carries oxygenated blood away from the left ventricle.
  • Superior & inferior vena cava – return de‑oxygenated blood to the right atrium.
  • Pulmonary artery – carries de‑oxygenated blood from the right ventricle to the lungs.
  • Pulmonary vein – carries oxygenated blood from the lungs to the left atrium.
  • Renal artery & renal vein – illustrate the link between the heart and the kidneys.
• Cardiac cycle (one complete heartbeat)
  • Atrial systole – atria contract, filling the ventricles.
  • Ventricular systole – ventricles contract; AV valves close (producing the “lub” sound) and semilunar valves open.
  • Diastole – ventricles relax; semilunar valves close (producing the “dub” sound) and AV valves open.

9.3 – Blood Vessels – Structure and Function (core)

Arteries

Carry blood away from the heart under high pressure.

• Structure
  • Three tunics: tunica intima, thick tunica media (abundant smooth muscle & elastic fibres), and tunica externa.
  • Relatively small lumen.
  • Elastic laminae between tunics give stretch‑recoil ability (Windkessel effect).
• Function
  • Elastic recoil maintains blood pressure between heartbeats.
  • Smooth‑muscle tone regulates diameter → controls regional blood flow.
  • High pressure and small lumen propel blood rapidly through the systemic circuit.

Veins

Return blood to the heart under low pressure.

• Structure
  • Thin walls; tunica media contains less smooth muscle than arteries.
  • Large, compliant lumen.
  • One‑way valves (leaflets) especially in the limbs.
• Function
  • Valves prevent back‑flow, ensuring unidirectional movement toward the heart.
  • Large lumen reduces resistance, allowing a greater volume of blood to be stored (venous reservoir).
  • Muscle pump – skeletal‑muscle contraction squeezes veins, assisting upward flow.

Capillaries

Sites of exchange between blood and tissues.

• Structure
  • Single layer of endothelial cells (≈ 5 µm thick).
  • Diameter 5–10 µm – just wide enough for red blood cells to pass in single file.
  • Basement membrane may be continuous, fenestrated or sinusoidal depending on tissue.
• Function
  • Thin wall minimises diffusion distance for O₂, CO₂, nutrients and wastes.
  • Very large total surface area (> 4 000 m²) maximises exchange efficiency.
  • Permeability varies with type:
    • Continuous – low permeability (skin, skeletal muscle).
    • Fenestrated – moderate permeability (kidney glomeruli, intestinal villi).
    • Sinusoidal – high permeability, allowing passage of large proteins and cells (liver, spleen, bone‑marrow).

Hepatic Vessels (supplementary requirement)

Students must be able to identify these vessels in diagrams and explain their role in the portal circulation.

• Hepatic artery – high‑pressure artery that supplies oxygen‑rich blood to the liver.
• Portal vein – low‑pressure vein that brings nutrient‑rich blood from the gastrointestinal tract and spleen to the liver.
• Hepatic vein – drains de‑oxygenated blood from the liver to the inferior vena cava.

9.4 – Blood – Composition and Function (core)

• Plasma (≈ 55 % of blood volume)
  • Water (≈ 90 %) – solvent for nutrients, hormones, waste.
  • Plasma proteins:
    • Albumin – maintains osmotic pressure.
    • Globulins – immune functions.
    • Fibrinogen – clotting precursor.
  • Electrolytes, nutrients (glucose, amino acids), gases (O₂, CO₂).
• Formed elements (≈ 45 % of blood volume)
  • Red blood cells (RBCs) – contain haemoglobin; transport O₂ and CO₂.
  • White blood cells (WBCs) – lymphocytes, neutrophils, monocytes, eosinophils, basophils; part of immune defence.
  • Platelets (thrombocytes) – cell‑fragment fragments that initiate clot formation.
• Key Functions of Blood
  • Transport of gases, nutrients, hormones and waste.
  • Regulation of body temperature and pH.
  • Defence against infection (WBCs, antibodies).
  • Clotting to prevent blood loss (fibrinogen → fibrin mesh).

Supplementary Blood Topics (9.4 – supplement)

• Lymphocytes – B cells (antibody production) and T cells (cell‑mediated immunity).
• Phagocytes – neutrophils and macrophages, engulf pathogens.
• Clotting cascade – role of calcium, vitamin K‑dependent clotting factors.

Practical / Experimental Skills (AO3)

• Measuring pulse rate (radial or carotid) and heart rate.
• Using a sphygmomanometer to determine systolic and diastolic blood pressure.
• Capillary refill test – assessing peripheral circulation.
• Microscopic examination of a blood smear to identify RBC shape, WBC types and platelet count.
• Demonstrating vessel constriction/dilation with a rubber‑tube model and adjustable pressure source.

Comparison of Arteries, Veins, Capillaries and Hepatic Vessels

Key Points to Remember

1. Arterial walls are thick and elastic to withstand high systolic pressure and to provide the Windkessel effect, which keeps blood flow continuous.
2. Veins have thin walls, large compliant lumens and one‑way valves; they act as a low‑pressure reservoir and rely on skeletal‑muscle contraction to return blood to the heart.
3. Capillaries are only one cell thick, giving a very short diffusion distance and a massive total surface area; their permeability is adapted to the needs of the tissue (continuous, fenestrated, sinusoidal).
4. Hepatic vessels illustrate how arteries and veins can work together in a specialised portal system.
5. Blood composition (plasma, red cells, white cells, platelets) underpins the transport, defence and clotting functions of the circulatory system.
6. Practical skills – pulse measurement, blood‑pressure reading, capillary refill test, blood‑smear microscopy and simple vessel‑model experiments – help students link structure to function.