explain how the structure of muscular arteries, elastic arteries, veins and capillaries are each related to their functions

Transport in Mammals – The Circulatory System (Cambridge AS & A‑Level Biology 8.1‑8.2)

1. Overview of the Human Circulatory System

  • Closed, double‑circulation network: a pulmonary circuit (right heart ↔ lungs) and a systemic circuit (left heart ↔ body).

  • Purpose: transport O₂, CO₂, nutrients, hormones, waste products and heat.

  • Blood is pumped by the heart through a hierarchy of vessels – arteries → arterioles → capillaries → venules → veins – each specially adapted to its role.

1.1. Flow of Oxygen‑Rich and Oxygen‑Poor Blood

Suggested diagram: A simple schematic showing the two circuits.


– Pulmonary circuit: right ventricle → pulmonary artery (deoxygenated) → lungs → pulmonary vein (oxygenated) → left atrium.


– Systemic circuit: left ventricle → aorta (oxygenated) → systemic arteries → capillaries (exchange) → systemic veins → vena cava → right atrium.

2. Major Blood Vessels – Names, Location & Primary Function

VesselCircuitLocation / Direction of FlowPrimary Function
AortaSystemicLeft ventricle → bodyHigh‑pressure conduit that distributes oxygen‑rich blood to all systemic arteries.
Pulmonary arteryPulmonaryRight ventricle → lungsCarries deoxygenated blood to the lungs for gas exchange.
Pulmonary veinPulmonaryLungs → left atriumReturns oxygen‑rich blood from the lungs to the heart.
Vena cava (superior & inferior)SystemicBody → right atriumCollects deoxygenated blood from the systemic circulation and delivers it to the heart.

3. Microscopic Identification of Vessels (Key for Slide Work)

When looking at a stained slide, note the relative thickness of the three tunics (intima, media, adventitia), the presence of elastic laminae, smooth‑muscle nuclei and any valves.

Vessel TypeTransverse Section – What to Look ForLongitudinal Section – What to Look For
Elastic artery (e.g., aorta)Very thick tunica media packed with concentric elastic lamellae; large lumen; internal elastic lamina distinct.Multiple concentric elastic rings; occasional vasa vasorum in outer adventitia.
Muscular (distributing) arteryThick tunica media dominated by smooth‑muscle nuclei; smaller lumen; external elastic lamina present.Prominent smooth‑muscle bundles running parallel to the lumen; fewer elastic layers.
ArterioleThin wall, but still a distinct media with a few smooth‑muscle cells; lumen very small.Few layers of smooth‑muscle; may show pre‑capillary sphincters.
CapillaryVery narrow lumen (≈5–10 µm); wall appears as a single thin line (endothelium + basement membrane).Endothelial cells appear as a thin, continuous sheet; no distinct tunics.
VenuleThin tunica media, relatively thick collagenous tunica adventitia; lumen larger than capillary.Endothelial lining with occasional overlapping cells; no valves.
Vein (large)Thin tunica media, very thick collagen‑rich tunica adventitia; lumen large and often irregular; valves appear as folds of intima.Longitudinal folds of intima forming valves; sparse smooth‑muscle.

4. Heart – Structure Relevant to Blood Flow

  • Four chambers: two atria (receive) and two ventricles (pump).

  • Wall layers (inside out): tunica intima → tunica media (smooth muscle) → tunica adventitia; myocardium (muscular layer) is thickest in the ventricles.

  • Valves: atrioventricular (tricuspid, mitral) and semilunar (pulmonary, aortic) ensure one‑way flow.

  • Conduction system: SA node → AV node → bundle of His → Purkinje fibres, coordinating the cardiac cycle.

5. Blood Composition (Key for Transport)

ComponentKey Structural FeatureMain Function
Plasma (≈55 % of volume)Water‑based solution with proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, hormonesMedium for transport of dissolved substances; maintains osmotic pressure and pH.
Red blood cells (RBCs)Biconcave disc, no nucleus, packed with haemoglobinCarry O₂ (and some CO₂) via haemoglobin.
White blood cells (WBCs)Various shapes; nucleus present; some contain granulesDefence against infection and foreign particles.
Platelets (thrombocytes)Cell‑fragment fragments, no nucleusInitiate clotting to prevent blood loss.

6. Water – The Main Component of Blood & Tissue Fluid

  • ≈90 % of plasma is water, giving blood a high specific heat – essential for temperature regulation.

  • Polarity makes water an excellent solvent for ions, gases and nutrients, facilitating their transport.

  • In interstitial (tissue) fluid, water maintains cell hydration and provides the medium for diffusion between capillaries and cells.

7. Gas Transport and the Haemoglobin System

GasForm of TransportKey Physiological Concepts
Oxygen (O₂)~98 % bound to haemoglobin; ~2 % dissolved in plasmaCo‑operative binding curve; affinity ↑ with high pH, low CO₂, low temperature (Bohr effect opposite).
Carbon dioxide (CO₂)Carbamino‑Hb (≈5 %); bicarbonate ion (HCO₃⁻) in plasma (≈70 %); dissolved CO₂ (≈25 %)Chloride shift (Cl⁻/HCO₃⁻ exchange) in red cells; transport ↑ with lower pH and higher temperature.

8. Blood Pressure and the Cardiac Cycle

  • Systole: Ventricular contraction raises arterial pressure (≈120 mmHg in the aorta).

  • Diastole: Ventricular relaxation lowers pressure (≈80 mmHg).

  • Elastic arteries stretch during systole and recoil during diastole, converting the pulsatile output into a steadier flow.

  • Peripheral resistance – mainly set by the calibre of arterioles and muscular arteries – together with cardiac output determines mean arterial pressure.

9. Structure–Function Relationships of Blood Vessels

9.1. Elastic Arteries (e.g., aorta, pulmonary trunk)

  • Structure: Very thick tunica media rich in concentric elastic lamellae; prominent internal elastic lamina; relatively thin intima.

  • Function: Stretch with each systolic surge, then recoil during diastole, buffering pressure fluctuations and maintaining continuous flow.

  • Microscopic clue: Multiple, tightly packed elastic fibres visible as dark, wavy layers.

9.2. Muscular (Distributing) Arteries (e.g., femoral, brachial)

  • Structure: Thick tunica media dominated by smooth‑muscle cells; fewer elastic fibres; well‑developed external elastic lamina.

  • Function: Contraction and relaxation regulate vessel diameter, controlling regional blood flow and peripheral resistance.

  • Microscopic clue: Rows of nuclei from smooth‑muscle cells give a “striated” appearance.

9.3. Arterioles

  • Structure: Thin wall; tunica media contains only a few smooth‑muscle cells; lumen very small.

  • Function: Principal site of resistance; pre‑capillary sphincters adjust flow into capillary beds.

9.4. Capillaries

  • Structure: One‑cell‑thick endothelial lining supported by a thin basement membrane; no smooth muscle or elastic tissue.

  • Function: Minimal diffusion distance enables rapid exchange of O₂, CO₂, nutrients, waste products and hormones; total surface area ≈ 5 000 m².

  • Microscopic clue: Appears as a single thin line; lumen often invisible without high magnification.

9.5. Venules

  • Structure: Thin tunica media, relatively thick collagenous adventitia; lumen larger than capillaries.

  • Function: Collect blood from capillary beds and begin the low‑pressure return to the heart.

9.6. Veins (including large veins)

  • Structure: Thin tunica media, very thick collagen‑rich tunica adventitia, and valves formed from folds of the intima.

  • Function: Highly compliant walls act as a blood reservoir (≈70 % of total blood volume); valves prevent backflow, ensuring unidirectional return under low pressure.

  • Microscopic clue: Large, irregular lumen; intimal folds (valves) visible in transverse sections.

10. Comparative Summary

Vessel TypeKey Structural FeaturesPrimary Physiological Role
Elastic arteriesThick elastic‑rich tunica media; internal elastic laminaAbsorb pressure surge; maintain continuous flow; dampen pulse pressure.
Muscular arteriesThick smooth‑muscle layer; external elastic laminaRegulate regional blood flow; control peripheral resistance.
ArteriolesThin wall; few smooth‑muscle cellsMajor site of resistance; adjust flow into capillary beds.
CapillariesSingle endothelial cell layer + basement membrane onlySite of exchange of gases, nutrients, wastes and hormones.
VenulesThin media; relatively thick adventitiaCollect blood from capillaries; begin low‑pressure return.
VeinsThin media; thick collagenous adventitia; valvesReturn blood to heart; act as a reservoir; prevent backflow.

11. Key Points to Remember (Exam‑Ready)

  1. Elastic arteries buffer the heart’s pulsatile output, converting it into a steadier flow.

  2. Muscular arteries fine‑tune blood distribution by changing their diameter (vasoconstriction / vasodilation).

  3. Arterioles provide the majority of peripheral resistance; pre‑capillary sphincters regulate entry to capillary beds.

  4. Capillaries’ thin walls and huge total surface area maximise diffusion efficiency.

  5. Veins’ compliance and valves enable low‑pressure, unidirectional return and serve as the main blood reservoir.

  6. Blood pressure = cardiac output × peripheral resistance; arterial elasticity and arteriolar tone are the two major modifiers.

  7. Water’s high specific heat and solvent properties are essential for plasma function and tissue‑fluid balance.

  8. Haemoglobin’s cooperative binding and the Bohr & chloride shifts allow efficient O₂ delivery and CO₂ removal.

12. Quick‑Scan of Syllabus Alignment (Topic 8 – Transport in Mammals)

Syllabus Learning Outcome (LO)Presence in NotesGap / Depth IssueSuggested Fix
8.1 – Name, location & primary function of major vessels (aorta, pulmonary artery, pulmonary vein, vena cava)✔︎ Table with name, location, functionNo explicit visual link of pulmonary vs systemic circuitsAdded schematic description in section 1.1.
8.1 – Recognise arteries, veins, capillaries in microscope slides / draw transverse & longitudinal sections✔︎ Detailed slide‑identification table and key microscopic cluesCould benefit from a concise drawing checklistIncluded “What to look for” columns for transverse & longitudinal sections.
8.2 – Explain how vessel structure relates to function (elastic arteries, muscular arteries, arterioles, capillaries, veins)✔︎ Sections 9.1‑9.6 with structure, function and microscopic cluesArterioles were missing in the original versionAdded a dedicated arterioles subsection.

Suggested diagram for exam practice: Cross‑sectional sketches of an elastic artery, a muscular artery, an arteriole, a capillary, a venule and a vein, each labelled with the relative thickness of tunics and the presence/absence of valves.