Mass Transport and Gas Exchange

Why large organisms need mass transport

Small organisms exchange substances directly across their surface. Large organisms have a small surface-area-to-volume ratio and greater demands, so they need specialised exchange surfaces and mass transport systems to move substances efficiently.

Features of an efficient exchange surface: large surface area, thin (short diffusion distance), a steep concentration gradient maintained (by blood flow/ventilation).

Gas exchange examples

• Lungs (mammals): millions of alveoli give a huge surface area; walls one cell thick; surrounded by capillaries; ventilation and blood flow maintain the gradient.
• Fish gills: counter-current flow — blood and water flow in opposite directions, maintaining a concentration gradient along the whole gill so more oxygen is absorbed.
• Insects: a tracheal system delivers air directly to tissues via tracheoles.
• Leaves: gases diffuse through stomata; large internal air spaces.

The circulatory system (mammals)

Mammals have a double, closed circulatory system: blood passes through the heart twice per circuit (a pulmonary circuit to the lungs and a systemic circuit to the body), maintaining high pressure.

• Arteries: thick, elastic, muscular walls; carry blood at high pressure away from the heart.
• Veins: thinner walls, wider lumen, valves to prevent backflow.
• Capillaries: one cell thick, huge surface area — where exchange happens.

The cardiac cycle

The sequence of contraction (systole) and relaxation (diastole):

1. Atrial systole — atria contract, pushing blood into the ventricles.

2. Ventricular systole — ventricles contract; atrioventricular valves close (preventing backflow), semilunar valves open, blood leaves via the aorta/pulmonary artery.

3. Diastole — chambers relax and fill again.

Valves open and close due to pressure differences.

Haemoglobin and oxygen transport

Haemoglobin carries oxygen. The oxygen dissociation curve is S-shaped: haemoglobin loads oxygen where it's plentiful (lungs) and unloads it where it's scarce (tissues). The Bohr effect: more CO₂ (lower pH) shifts the curve right, so haemoglobin releases oxygen more readily to active tissues.

Worked example

Explain how counter-current flow makes fish gills efficient.

• Water and blood flow in opposite directions, so along the whole length of the gill lamellae there is always a higher oxygen concentration in the water than in the blood — maintaining a diffusion gradient and absorbing more oxygen than if they flowed the same way. ✓

Common mistakes

• Forgetting to link exchange-surface features (SA, thin, gradient) to efficiency.
• Confusing arteries (high pressure, away) with veins (valves, towards the heart).
• Getting the Bohr effect backwards — more CO₂ makes haemoglobin release oxygen.

Exam tips

• Learn the features of efficient exchange surfaces and apply them to lungs/gills/leaves.
• Describe the cardiac cycle in terms of pressure changes and valve behaviour.
• Interpret the oxygen dissociation curve and the Bohr shift.

Key facts to remember

• Large organisms need exchange surfaces (large SA, thin, gradient maintained) and mass transport.
• Mammals: double closed circulation; arteries (high pressure), veins (valves), capillaries (exchange); cardiac cycle driven by pressure/valves.
• Haemoglobin loads/unloads O₂ (S-shaped dissociation curve); the Bohr effect (more CO₂ → curve right → more unloading) supplies active tissues.