state that the mammalian circulatory system is a closed double circulation consisting of a heart, blood and blood vessels including arteries, arterioles, capillaries, venules and veins

Topic 8 – Transport in Mammals: The Circulatory System

Learning Objectives (Cambridge IGCSE / A‑Level)

State that the mammalian circulatory system is a closed double circulation consisting of a heart, blood and blood vessels (arteries, arterioles, capillaries, venules, veins).

Describe the structure and function of each component, including elastic vs. muscular arteries and vein valves.

Explain the cardiac cycle with correct sequence, timing and isovolumetric phases.

Identify the main constituents of blood, give quantitative data (plasma‑cell proportions, Hb concentration) and explain the role of haemoglobin in O₂/CO₂ transport.

Discuss the Bohr shift and chloride shift and their importance for gas exchange.

Explain mechanisms of capillary exchange (diffusion, bulk flow, trans‑cytosis) and the factors influencing them.

State the basic quantitative relationships governing blood pressure and flow (Poiseuille’s law, pressure gradients, MAP).

Outline the principal regulatory mechanisms of circulation (autonomic control, baroreceptor reflex, venous return).

Link circulatory concepts to other syllabus topics (protein synthesis, immunity, cell cycle).

1. Closed Double Circulation

Closed system: Blood remains within a continuous network of vessels; it does not leave the vascular system under normal conditions.

Double circulation: Two separate, non‑mixing loops:
1. Pulmonary circuit – carries de‑oxygenated blood from the heart to the lungs and returns oxygen‑rich blood to the heart.
2. Systemic circuit – distributes oxygen‑rich blood to all body tissues and returns de‑oxygenated blood to the heart.

Suggested diagram: schematic of the mammalian heart showing the pulmonary and systemic circuits together with the major vessel types (labelled “pulmonary circuit” and “systemic circuit”).

2. Main Components

2.1 Heart – muscular pump

Feature	Details / Function
Chambers	Right atrium, right ventricle, left atrium, left ventricle.
Valves	Atrioventricular (tricuspid, mitral) – prevent back‑flow into the atria. Semilunar (pulmonary, aortic) – prevent back‑flow into the ventricles.
Wall layers	Epicardium (outer), myocardium (muscular, contractile), endocardium (inner, smooth).
Cardiac‑cycle timing (typical adult at rest)	Atrial systole – ~0.10 s Isovolumetric contraction – ~0.05 s Ventricular ejection – ~0.30 s Isovolumetric relaxation – ~0.08 s Ventricular diastole (rapid filling + atrial systole) – ~0.45 s (Total cardiac cycle ≈ 0.8 s → ~75 bpm)

Cardiac‑cycle flow‑chart (with isovolumetric phases)

Right atrium fills (venous return) → atrial systole pushes blood through the tricuspid valve.

Right ventricle begins contraction → isovolumetric contraction (all valves closed).

Pulmonary semilunar valve opens → ventricular ejection into pulmonary artery.

Blood is oxygenated in pulmonary capillaries → returns via pulmonary veins to left atrium.

Left atrium contracts → left ventricle fills.

Left ventricle contracts → isovolumetric contraction → aortic valve opens → ventricular ejection into aorta.

Diastole: all chambers relax, valves close, coronary arteries fill.

2.2 Blood – transport medium

Component	Key Features / Function
Plasma (≈55 % of blood volume)	≈90 % water; electrolytes; plasma proteins (albumin, globulins, fibrinogen); nutrients, hormones, waste; maintains osmotic pressure (~7 kPa).
Red blood cells (RBCs) – ~45 % of volume (≈84 % of cellular fraction)	Biconcave disc, no nucleus, ~5 × 10¹² cells L⁻¹; contains ~270 million Hb molecules per cell; primary O₂ carrier.
Haemoglobin (Hb)	Tetrameric protein (α₂β₂); concentration ≈150 g L⁻¹; O₂‑binding capacity ≈1.34 mL O₂ g⁻¹ Hb (≈20 mL O₂ dL⁻¹ blood).
White blood cells (WBCs)	Leukocytes (neutrophils, lymphocytes, monocytes, eosinophils, basophils) – immune defence; total ≈4–11 × 10⁹ L⁻¹.
Platelets (thrombocytes)	Cell‑fragment fragments; essential for clot formation (coagulation cascade); ≈150–400 × 10⁹ L⁻¹.

2.3 Blood Vessels – conduits and exchange sites

Vessel type	Structural features	Primary function
Elastic arteries (e.g., aorta, pulmonary trunk)	Thick tunica media with numerous elastic lamellae; large lumen.	Carry blood away from the heart; dampen pulsatile pressure via elastic recoil.
Muscular arteries (e.g., brachial, femoral)	Prominent smooth‑muscle layer, fewer elastic fibres.	Distribute blood to regions; regulate flow by vasoconstriction/dilation.
Arterioles	Diameter 10–100 µm; many smooth‑muscle cells; often terminate in precapillary sphincters.	Major resistance vessels; control entry into capillary beds.
Capillaries	Single endothelial layer + basement membrane; pores (fenestrated) or diaphragms (continuous).	Site of exchange – diffusion of gases, nutrients, wastes; bulk flow (Starling forces).
Venules	Thin walls, few smooth‑muscle cells; larger than capillaries.	Collect blood from capillaries; begin low‑pressure return to the heart.
Veins (large)	Thin tunica media, large lumen, abundant connective tissue; contain bicuspid valves (two leaflets) to prevent back‑flow.	Return blood to the heart; act as capacitance vessels (store ≈70 % of total blood volume).

3. Path of Blood Through the Double Circulation

De‑oxygenated blood → right atrium (via superior & inferior vena cava).

Right atrium → right ventricle (tricuspid valve).

Right ventricle → pulmonary artery (pulmonary semilunar valve) → lungs.

Gas exchange in pulmonary capillaries → oxygenated blood → pulmonary veins → left atrium.

Left atrium → left ventricle (mitral valve).

Left ventricle → aorta (aortic semilunar valve) → systemic arteries.

Systemic arteries → arterioles → capillary beds (exchange).

Capillaries → venules → veins → superior/inferior vena cava → right atrium (cycle repeats).

4. Gas Transport & the Haemoglobin Cycle

O₂ transport
- ≈1.34 mL O₂ g⁻¹ Hb at 37 °C, pH 7.4 (≈20 mL O₂ dL⁻¹ blood).
- Cooperative binding gives a sigmoidal O₂‑Hb dissociation curve.
- In tissues O₂ unloads because of lower PO₂ and the Bohr effect (↓pH, ↑CO₂ → ↓Hb affinity).

CO₂ transport
- ~10 % dissolved in plasma.
- ~20 % as carbamino compounds (mainly with Hb).
- ~70 % as bicarbonate (HCO₃⁻) formed in RBCs:
  1. CO₂ + H₂O ⇌ H₂CO₃ (catalysed by carbonic anhydrase).
  2. H₂CO₃ ⇌ H⁺ + HCO₃⁻.
  3. HCO₃⁻ exits the RBC via the chloride shift (Cl⁻ enters to maintain electroneutrality).
  4. In the lungs the reverse reactions occur, releasing CO₂ for exhalation.

5. Capillary Exchange Mechanisms

Diffusion – driven by concentration gradients of O₂, CO₂, glucose, ions.

Bulk flow (filtration & reabsorption) – described by Starling’s equation:
Jv = Lp S [(Pc – Pi) – σ(πc – πi)]
where P = hydrostatic pressure, π = oncotic pressure, L_p = hydraulic conductivity, S = surface area, σ = reflection coefficient.
- Arterial end: high P_c (~35 mm Hg) → net filtration.
- Venous end: low P_c (~15 mm Hg) → net reabsorption.

Trans‑cytosis – vesicular transport of large proteins (e.g., immunoglobulins) across the endothelium.

6. Blood Pressure, Flow & Simple Mathematics

Systolic pressure ≈ 120 mm Hg; diastolic pressure ≈ 80 mm Hg.

Mean arterial pressure (MAP) ≈ ⅓ (systolic + 2 × diastolic) ≈ 100 mm Hg.

Poiseuille’s law (laminar flow):
Q = (ΔP π r⁴) / (8 η L)
where Q = flow rate, ΔP = pressure difference, r = vessel radius, η = blood viscosity, L = length.
- Flow is proportional to the fourth power of radius – a small change in r produces a large change in resistance.
- Explains why arteriolar constriction markedly raises systemic resistance.

7. Regulation of Circulation

Autonomic nervous system
- Sympathetic → ↑ heart rate, ↑ contractility, vasoconstriction (α₁‑receptors).
- Parasympathetic → ↓ heart rate (vagus nerve, muscarinic receptors).

Baroreceptor reflex – stretch receptors in carotid sinus and aortic arch sense MAP; afferent signals to the medulla adjust sympathetic/parasympathetic outflow to keep pressure stable.

Venous return mechanisms
- Muscle pump – skeletal‑muscle contraction squeezes veins.
- Respiratory pump – intrathoracic pressure changes during breathing.
- Valves in veins (bicuspid) prevent back‑flow.

Clinical relevance (quick notes)
- Hypertension – chronic elevation of MAP; risk factor for heart disease.
- Atherosclerosis – plaque narrows arterial lumen → resistance ↑ (∝ r⁻⁴).
- Valve disorders (e.g., mitral stenosis) reduce cardiac output.

8. Links to Other Syllabus Topics

Protein synthesis (Topic 6) – globin chains of haemoglobin are synthesized on ribosomes; mutations affect O₂ affinity.

Cell cycle & erythropoiesis (Topic 5) – red blood cells are produced in bone‑marrow; erythropoietin from kidneys regulates production.

Immunity (Topic 11) – white blood cells circulate in blood; antibodies (immunoglobulins) are transported in plasma.

9. Quick Review Checklist (Action‑oriented)

Checklist Item	Do
Define “closed double circulation”.	State that blood stays within vessels and that there are separate pulmonary and systemic loops.
List heart chambers, valves, wall layers and timing.	Draw a labelled heart diagram and add a timing box for the cardiac cycle.
Outline the cardiac cycle.	Write the sequence including isovolumetric contraction and relaxation with approximate durations.
Describe blood composition with percentages.	Complete the table of plasma, RBCs, WBCs, platelets and haemoglobin; note plasma ≈ 55 % and cells ≈ 45 %.
Explain O₂ and CO₂ transport, Bohr & chloride shifts.	Sketch the haemoglobin cycle and write the relevant equilibrium equations.
Identify structural differences between vessels.	Fill in the vessel‑structure table (elastic vs. muscular arteries, vein valves).
Calculate MAP and a simple flow rate using Poiseuille’s law.	Use given pressure and radius values in the formulas.
Summarise the main regulatory mechanisms.	Write a short paragraph linking baroreceptors, autonomic control and venous return.

10. Suggested Further Reading / Practical Skills

Microscope slide of a capillary bed – observe endothelial fenestrations.

Heart‑rate experiment: effect of standing, lying, and deep breathing on pulse (autonomic regulation).

Blood‑pressure measurement with a sphygmomanometer – identify systolic and diastolic values.

Calculate the effect of a 10 % change in arterial radius on resistance (use Poiseuille’s law).

Read Chapter 12 of the Cambridge A‑Level Biology textbook (Circulatory System) for deeper discussion of the Bohr effect and clinical cases.

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