describe the functions of the main blood vessels of the pulmonary and systemic circulations, limited to pulmonary artery, pulmonary vein, aorta and vena cava

Learning Outcomes (Cambridge IGCSE/A‑Level Biology – Topic 8)

• Describe the structure and function of the pulmonary artery, pulmonary vein, aorta and vena cava.
• Explain how the heart’s chambers link the pulmonary and systemic circulations.
• Compare elastic arteries, muscular arteries and veins, including the three capillary types.
• State the role of plasma (especially albumin) in maintaining osmotic pressure.
• Apply the Starling forces to the formation of tissue fluid and lymph.
• Quantify the main modes of O₂ and CO₂ transport and explain the Bohr, Haldane and chloride‑shift mechanisms.
• Interpret a simple practical that demonstrates the Windkessel effect of the aorta.

The Circulatory System (Topic 8)

1. Overview of the Pulmonary and Systemic Circuits

The heart acts as a double pump:

• Right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium (pulmonary circuit).
• Left atrium → left ventricle → aorta → systemic arteries → capillaries → veins → vena cava → right atrium (systemic circuit).

Deoxygenated blood leaves the right side of the heart, is oxygenated in the lungs, and returns to the left side to be distributed throughout the body.

2. Key Blood Vessels

• Pulmonary artery
• Pulmonary vein
• Aorta
• Vena cava (superior & inferior)

3. Functions of the Main Vessels

Pulmonary Artery

• Transports deoxygenated blood from the right ventricle to the lungs.
• Only artery that normally carries low‑oxygen blood.
• Divides into right and left pulmonary arteries → arterioles → capillary networks that surround each alveolus.

Pulmonary Vein

• Returns oxygen‑rich blood from the lungs to the left atrium.
• Four pulmonary veins (two per lung) are the only veins that carry highly oxygenated blood.

Aorta

• Largest elastic artery; receives oxygenated blood from the left ventricle.
• Branches into the ascending aorta, aortic arch, thoracic aorta and abdominal aorta, supplying all systemic tissues.
• Elastic laminae stretch during systole and recoil during diastole – the “Windkessel” effect – which smooths the pulse pressure and maintains continuous flow.

Vena cava (Superior & Inferior)

• Collect deoxygenated blood from the systemic circulation and deliver it to the right atrium.
• Thin walls, large lumen, and one‑way valves (especially in the limbs) prevent back‑flow.

4. Structural Differences Between Vessels

• Elastic arteries (aorta, pulmonary trunk) – thick tunica media with abundant elastic fibres; designed to absorb and release the high pressure pulse.
• Muscular arteries (e.g., femoral, brachial) – relatively thicker tunica media with many smooth‑muscle cells; regulate regional blood flow by vasoconstriction/vasodilation.
• Veins – thin tunica media, large lumen, valves; act as capacitance vessels storing up to 70 % of total blood volume.
• Capillaries – single endothelial cell layer; site of exchange between blood and tissues. Three structural types:

  • Continuous – most of the body; tight junctions limit leakage, allowing selective diffusion of gases, nutrients and wastes.
  • Fenestrated – kidneys, intestinal villi, endocrine glands; pores (≈ 60–80 nm) speed up exchange of water, ions and small molecules.
  • Discontinuous (sinusoidal) – liver, spleen, bone marrow; large gaps permit passage of plasma proteins, cells and large lipoproteins.

5. Plasma – The Transport Medium

• ≈ 90 % water → high specific heat capacity; buffers temperature changes during exercise or cold exposure.
• Solvent for nutrients, ions, hormones, waste products and gases (O₂, CO₂).
• Plasma proteins:

  • Albumin – maintains colloid osmotic (oncotic) pressure, drawing water back into capillaries at the venous end.
  • Globulins – transport lipophilic substances and act as antibodies.
  • Fibrinogen – essential for clot formation.

6. Formation of Tissue Fluid (Starling Forces)

• Arterial end of capillary: hydrostatic pressure (≈ 35 mm Hg) > plasma‑colloid osmotic pressure (≈ 25 mm Hg) → net filtration of plasma into the interstitial space.
• Venous end of capillary: hydrostatic pressure falls (≈ 15 mm Hg) while oncotic pressure remains ≈ 25 mm Hg → net re‑absorption of most filtered fluid.
• The small volume that remains becomes interstitial fluid; excess is collected as lymph and returned to the circulation via the lymphatic system.

7. Transport of Oxygen and Carbon Dioxide

Oxygen Transport

• ≈ 98 % of O₂ is bound to haemoglobin (Hb) inside red blood cells (4 O₂ molecules per Hb).
• ≈ 2 % is dissolved directly in plasma (contributes to the measured arterial PO₂).
• O₂‑dissociation curve is sigmoidal because of cooperative binding; a right‑shift (Bohr effect) occurs in active tissues where ↑ CO₂, ↓ pH and ↑ temperature reduce Hb affinity and promote O₂ release.

Carbon Dioxide Transport

• ≈ 70 % of CO₂ is carried as bicarbonate (HCO₃⁻) formed by carbonic anhydrase in red blood cells:
  

  CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

• Chloride shift (Hamburger phenomenon):
  

  Cl⁻ (plasma) ↔ HCO₃⁻ (RBC)
  

  Bicarbonate leaves the erythrocyte in exchange for chloride ions, preserving electrical neutrality.

• ≈ 20 % binds directly to the globin chains as carbamino‑haemoglobin (Hb‑CO₂).
• ≈ 10 % remains dissolved in plasma.
• Haldane effect: deoxygenated Hb has a higher affinity for CO₂, enhancing CO₂ uptake in peripheral tissues and its release in the lungs.

8. Practical Application – Demonstrating the Windkessel Effect

Measure pulse pressure (systolic – diastolic) at rest and after 5 minutes of moderate exercise (e.g., jogging). The aorta’s elastic recoil maintains a relatively high diastolic pressure, so the increase in pulse pressure is less than the rise in systolic pressure alone would suggest. This simple experiment links the aorta’s structure to its physiological function.

9. Summary Table

10. Histology Tips – Identifying Vessels in Slides

• Arteries – thick tunica media with numerous, dark‑staining smooth‑muscle nuclei; often a distinct internal elastic lamina.
• Veins – thin tunica media, large, often collapsed lumen; endothelial cells appear flattened; valves appear as thin, slit‑like structures.
• Capillaries – single endothelial cell layer; lumen may be barely visible. Fenestrated capillaries show small pores; sinusoidal capillaries have a discontinuous basal lamina and large gaps.