CB8 — Exchange and Transport in Animals
Gas exchange surfaces
Efficient exchange surfaces share four adaptations (Edexcel key checklist):
- Large surface area — more area for diffusion per unit time.
- Thin membrane — short diffusion distance.
- Maintained concentration gradient — constant supply/removal of gases (ventilation + blood flow).
- Good blood supply — rapid transport away from surface.
Alveoli (singular: alveolus): tiny air sacs in the lungs. Adaptations: millions of alveoli → huge surface area (~70 m²); one-cell-thick walls (squamous epithelium + thin capillary wall = ~0.5 µm); surrounded by dense capillary network (maintains gradient); ventilation refreshes air.
Gas exchange in alveoli:
- O₂ diffuses from alveolus (high pO₂) → blood (low pO₂) → combines with haemoglobin → carried to tissues.
- CO₂ diffuses from blood (high pCO₂) → alveolus (low pCO₂) → exhaled.
CP5 — Respiration investigation: measure rate of respiration in organisms using a respirometer. Soda lime absorbs CO₂ produced. As O₂ is consumed, pressure falls → coloured liquid moves along capillary tube. Can compare rates at different temperatures, or between different organisms.
The circulatory system
Humans have a double circulatory system: blood passes through the heart twice per complete circuit.
- Pulmonary circulation: right side of heart → lungs (for oxygenation) → back to left side.
- Systemic circulation: left side of heart → rest of body (to deliver O₂) → back to right side.
Advantage of double circulation: blood is repressurised at the heart between circuits → higher pressure in systemic circulation → faster, more efficient delivery to tissues.
The heart:
- Four chambers: right atrium, right ventricle, left atrium, left ventricle.
- Valves (atrioventricular: bicuspid/mitral on left, tricuspid on right; semilunar in aorta and pulmonary artery) prevent backflow.
- Left ventricle walls thicker than right: must pump blood around the entire body (higher pressure needed).
- Coronary arteries supply the heart muscle with oxygenated blood.
Blood vessels:
| Vessel | Structure | Function |
|---|---|---|
| Artery | Thick muscular walls, narrow lumen, elastic | Carries blood AWAY from heart at high pressure |
| Vein | Thin walls, wide lumen, has valves | Carries blood TO heart at low pressure |
| Capillary | Single cell thick wall | Exchange of substances with tissues |
Blood components
| Component | Structure | Function |
|---|---|---|
| Red blood cells | Biconcave disc, no nucleus, packed with haemoglobin | O₂ transport (Hb + O₂ ⇌ HbO₂) |
| White blood cells | Various; have nucleus | Immune defence (phagocytes, lymphocytes) |
| Platelets | Cell fragments, no nucleus | Blood clotting at wound site |
| Plasma | Watery liquid | Transport of CO₂, glucose, amino acids, urea, hormones, ions |
Haemoglobin + oxygen: Hb + 4O₂ ⇌ HbO₂ (oxyhaemoglobin). High pO₂ (lungs) → Hb loads O₂. Low pO₂ (tissues) → Hb unloads O₂.
Aerobic and anaerobic respiration — CP5
Aerobic respiration (in mitochondria with O₂): $$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O \quad (+ ATP)$$ Complete oxidation of glucose; maximum ATP yield (~38 ATP per glucose).
Anaerobic respiration (no O₂ — in cytoplasm):
- Animals/humans: glucose → lactic acid + ATP (2 ATP per glucose). Produces oxygen debt — lactic acid must be oxidised to CO₂ + H₂O when O₂ becomes available again after exercise. This causes continued heavy breathing after exercise stops.
- Yeast/microorganisms: glucose → ethanol + CO₂ + ATP (fermentation). Used in brewing and bread-making.
Comparison:
| Feature | Aerobic | Anaerobic (animals) |
|---|---|---|
| O₂ required | Yes | No |
| Products | CO₂ + H₂O | Lactic acid |
| ATP yield | High (~38) | Low (2) |
| Location | Mitochondria | Cytoplasm |
CP5 — Investigation of respiration rate: use a respirometer with soda lime (absorbs CO₂) to measure O₂ consumption. OR measure CO₂ production using an indicator (hydrogencarbonate indicator changes colour: red → yellow in CO₂-rich conditions). Variables: temperature (affects enzyme rate), type of organism, substrate.
The heart and cardiovascular disease
Coronary heart disease (CHD): fatty deposits (atheroma/plaque) build up in coronary artery walls → narrows lumen → restricts blood flow → heart muscle deprived of O₂ → heart attack (myocardial infarction) if artery blocked.
Risk factors for CHD: high blood cholesterol (LDL), high blood pressure (hypertension), smoking, poor diet, obesity, physical inactivity, family history.
Treatments: lifestyle changes; statins (reduce cholesterol); angioplasty (inflate balloon in artery + stent); bypass surgery (use vessel from elsewhere to bypass blocked section); artificial valves.
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