state that water is the main component of blood and tissue fluid and relate the properties of water to its role in transport in mammals, limited to solvent action and high specific heat capacity

Transport in Mammals – Overview

In mammals the circulatory system is a closed, double‑circulation network that transports gases, nutrients, hormones and waste products between the external environment and every cell. The fluid media – blood and tissue (interstitial) fluid – are > 90 % water, so the physicochemical properties of water largely determine how transport occurs.

1. The Circulatory System (Cambridge Syllabus 8.1)

• Closed double circulation – blood never leaves the vascular system; it passes through two separate circuits:

  • Pulmonary circuit: heart → lungs → heart.
  • Systemic circuit: heart → body tissues → heart.

• Major vessels and their functions

  Vessel	Direction of flow	Physiological role
  Pulmonary artery	Right ventricle → lungs	Carries de‑oxygenated blood to the lungs for gas exchange.
  Pulmonary vein	Lungs → left atrium	Returns oxygen‑rich blood from the lungs to the heart.
  Aorta	Left ventricle → systemic arteries	Distributes oxygenated blood to all body tissues.
  Vena cava (superior & inferior)	Systemic veins → right atrium	Collects de‑oxygenated blood from the body and returns it to the heart.

• Recognising & drawing the three basic vessel types

  • Arteries – thick tunica media, elastic or muscular; carry blood away from the heart under high pressure.
  • Veins – thinner tunica media, large lumen, valves in limbs; carry blood back to the heart under low pressure.
  • Capillaries – one‑cell‑thick endothelium + basement membrane; site of exchange between blood and tissue fluid.

2. Composition of Blood

2.1 Water – the dominant constituent

• ≈ 90 % of plasma is water.
• ≈ 99 % of interstitial (tissue) fluid is water.
• Because water is the main component, its two key properties – excellent solvent ability and high specific heat capacity – dictate how transport functions.

3. Plasma Composition (Key Solutes)

• Electrolytes – Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻ (osmotic balance & pH control).
• Glucose – main energy source for cells (≈ 5 mmol L⁻¹ fasting).
• Plasma proteins

  • Albumin (≈ 60 % of plasma protein) – maintains colloid osmotic pressure.
  • Globulins – transport of hormones, metal ions, immune functions.
  • Fibrinogen – clot formation.

• Hormones, vitamins, waste products – dissolved and carried to target tissues or excretory organs.

4. Tissue (Interstitial) Fluid Formation

1. Ultrafiltration at capillaries: hydrostatic pressure forces plasma water and small solutes through the endothelial wall into the interstitial space.
2. Reabsorption: plasma oncotic pressure (mainly albumin) draws water back into the capillary.
3. The resulting tissue fluid has essentially the same ionic composition as plasma but contains far less protein (≈ 0.2 % of plasma protein).

5. Structure of Blood Vessels (Cambridge Syllabus 8.1)

6. Gas Transport – Oxygen and Carbon Dioxide

• Oxygen (O₂)

  • ≈ 98 % bound to haemoglobin (Hb) in RBCs; 2 % dissolved in plasma.
  • Haemoglobin has four heme groups – each can bind one O₂ molecule.
  • Bohr effect: lower pH or higher CO₂ reduces Hb’s affinity for O₂, aiding release at active tissues.

• Carbon dioxide (CO₂)

  • Transported in three forms:

    1. ≈ 7 % dissolved in plasma.
    2. ≈ 23 % as bicarbonate ion (HCO₃⁻) – formed by carbonic anhydrase inside RBCs.
    3. ≈ 70 % bound to haemoglobin as carbamino compounds.

  • Chloride shift (Hamburger phenomenon): as HCO₃⁻ leaves RBCs, Cl⁻ enters to maintain electroneutrality.
  • In the lungs the reactions reverse, allowing CO₂ to be expelled.

7. The Heart – Anatomy and Cardiac Cycle (Syllabus 8.1)

7.1 Structural Overview

• Four chambers: right atrium, right ventricle, left atrium, left ventricle.
• Valves ensure unidirectional flow:

  • Atrioventricular (tricuspid & mitral) valves.
  • Semilunar (pulmonary & aortic) valves.

7.2 Cardiac Cycle

1. Atrial systole – atria contract, completing ventricular filling.
2. Ventricular systole

   • Isovolumetric contraction – pressure rises, all valves closed.
   • Ventricular ejection – semilunar valves open, blood expelled into pulmonary artery (right) or aorta (left).

3. Ventricular diastole

   • Isovolumetric relaxation – pressure falls, all valves closed.
   • Ventricular filling – AV valves open, blood flows from atria.

7.3 Conduction System (Electrical Control)

• SA node (sino‑atrial) – natural pacemaker; initiates impulse.
• AV node (atrioventricular) – delays impulse to allow ventricular filling.
• Bundle of His → right & left bundle branches → Purkinje fibres – rapid conduction throughout ventricles, producing coordinated contraction.

8. How the Properties of Water Enable Efficient Transport

9. Summary

Water constitutes the overwhelming majority of both blood plasma and tissue fluid, providing the medium in which nutrients, gases and wastes are dissolved and carried. Its polarity makes it an unrivalled solvent, while its high specific heat capacity protects mammals from rapid temperature fluctuations. Combined with the specialised structure of arteries, veins and capillaries, the composition of plasma, the gas‑carrying capacity of red blood cells, and the rhythmic pumping action of the heart, these features enable the highly efficient transport system required for mammalian life.