Cambridge International AS & A Level Biology 9700: Transport in mammals – Revision Notes

These revision notes cover Topic 8, Transport in mammals, in the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: sections 8.1 The circulatory system, 8.2 Transport of oxygen and carbon dioxide and 8.3 The heart. It is AS Level content, examined on Papers 1 and 2, and Paper 4 can draw on it. For full explanations and worked examples, use the Transport in mammals study guide.

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8.1 The circulatory system

Key definitions

• Closed circulation: blood stays inside vessels.
• Double circulation: blood passes through the heart twice per complete circuit (pulmonary + systemic).
• Vessel sequence: heart → artery → arteriole → capillary → venule → vein → heart.
• Tissue fluid: fluid that leaks from capillaries and bathes cells; it supplies oxygen and nutrients and removes carbon dioxide and waste.

The four named vessels

Vessel	Blood	Direction
Pulmonary artery	Deoxygenated	Right ventricle → lungs
Pulmonary vein	Oxygenated	Lungs → left atrium
Aorta	Oxygenated	Left ventricle → body
Vena cava	Deoxygenated	Body → right atrium

Structure → function

Vessel	Key structure	Why
Elastic artery	Many elastic fibres in wall	Stretches when blood surges in, recoils between beats; smooths pressure
Muscular artery	Thick smooth muscle layer	Constricts or dilates to control flow to organs
Vein	Thin wall, wide lumen, valves	Low pressure; little resistance; valves stop backflow
Capillary	Wall one endothelial cell thick, gaps between cells	Short diffusion distance; lets plasma out to form tissue fluid

Recognising cells

Cell	Nucleus	Other features
Red blood cell	None	Biconcave disc, pale centre
Neutrophil	Lobed	Granular cytoplasm
Monocyte	Large, kidney-shaped	Largest white cell
Lymphocyte	Large, round, fills most of cell	Thin rim of cytoplasm

Water: two properties only

• Solvent action → glucose, ions, amino acids and urea are carried dissolved in plasma.
• High specific heat capacity → stable blood temperature; heat carried around the body.

Method in steps: tissue fluid formation

1. Arterial end: hydrostatic pressure high → plasma forced out through gaps between endothelial cells.
2. Cells and most plasma proteins stay in the blood (too large).
3. Venous end: hydrostatic pressure lower; plasma proteins keep blood water potential lower → water returns by osmosis.
4. Excess drains into lymph vessels.

Plan diagram rules

Tissue-layer outlines only; no individual cells; no shading; clear continuous lines; layers in the correct proportions; label lines ruled and not crossing.

8.2 Transport of oxygen and carbon dioxide

Equations to learn

CO₂ + H₂O → H₂CO₃        (catalysed by carbonic anhydrase)
H₂CO₃ → H⁺ + HCO₃⁻
H⁺ + Hb → HHb             (haemoglobinic acid)
CO₂ + Hb → carbaminohaemoglobin

Method in steps: carbon dioxide in a respiring tissue

1. CO₂ diffuses into the red blood cell.
2. Carbonic anhydrase speeds up formation of carbonic acid, which dissociates to H⁺ and HCO₃⁻.
3. H⁺ binds to haemoglobin → haemoglobinic acid → oxyhaemoglobin releases O₂; blood pH is buffered.
4. HCO₃⁻ diffuses out into the plasma; Cl⁻ moves in (chloride shift) to keep the charges balanced, so HCO₃⁻ removal can continue.
5. Some CO₂ binds to haemoglobin as carbaminohaemoglobin; a little dissolves in plasma.

Role of plasma: carries most of the carbon dioxide, as hydrogencarbonate ions in solution, plus a small amount of dissolved CO₂.

The oxygen dissociation curve

• Axes: percentage saturation of haemoglobin (y) against partial pressure of oxygen, pO₂ (x).
• Shape: sigmoid. The first O₂ binds with difficulty; haemoglobin then changes shape so the next O₂ molecules bind more easily; the curve flattens near full saturation.
• Lungs (high pO₂): near-full saturation → loading.
• Respiring tissues (low pO₂): on the steep part → a small drop in pO₂ gives a large release of O₂.

Percentage saturation = (oxygen carried ÷ maximum oxygen that could be carried) × 100.

The Bohr shift

High pCO₂ → more H⁺ → haemoglobin’s affinity for O₂ falls → curve shifts right → more O₂ unloaded in active tissues. In the lungs, CO₂ is lost → higher affinity → loading.

Must-know distinctions

• Haemoglobinic acid (H⁺ + haemoglobin) vs carbaminohaemoglobin (CO₂ + haemoglobin) vs oxyhaemoglobin (O₂ + haemoglobin).
• Chloride shift (ion movement across the red blood cell membrane) vs Bohr shift (movement of the dissociation curve).
• Right shift = lower affinity = more unloading. Left = higher affinity.

Small worked reminders

• Unloading from saturations. Maximum 20.0 cm³ O₂ per 100 cm³; lungs 96%, tissue 60%. Released = 20.0 × (0.96 − 0.60) = 7.2 cm³ per 100 cm³. Subtract the percentages first, then multiply once.
• Reading a shifted curve. If, at the same pO₂, the curve with higher pCO₂ gives a lower saturation, it lies to the right. Quote both saturations and the pO₂ you read them at.
• Unit check. 1 mm = 1000 µm. Convert the image measurement to the unit the answer asks for before dividing by the magnification.

Must-know distinctions: fluids

Fluid	Where	Contains
Blood plasma	Inside blood vessels	Water, dissolved solutes, plasma proteins
Tissue fluid	Around cells	Water and small solutes; very little protein; no red blood cells
Lymph	Lymph vessels	Excess tissue fluid that did not return to the capillaries

8.3 The heart

Structure checklist

• Four chambers: right and left atria, right and left ventricles; septum between the two sides.
• Atrioventricular valves: tricuspid (right), bicuspid or mitral (left), held by tendons attached to papillary muscles.
• Semilunar valves: at the base of the aorta and pulmonary artery.
• Coronary arteries: on the outer surface, supplying heart muscle.

Wall thickness

Comparison	Thicker	Reason
Atria vs ventricles	Ventricles	Pump blood out through a circulation, not just into the next chamber
Left vs right ventricle	Left	Pumps through the whole body against greater resistance; right pumps only to the nearby lungs at lower pressure

Method in steps: reading a pressure graph

1. Find where the atrial and ventricular lines cross: AV valve opens (atrium higher) or closes (ventricle higher).
2. Find where the ventricular and arterial lines cross: semilunar valve opens (ventricle higher) or closes (artery higher).
3. Heart rate = 60 ÷ length of one cycle in seconds.

Must-know distinctions: the cycle

• Systole = contraction; diastole = relaxation. Say which chambers.
• Atrioventricular valves close when ventricular pressure rises above atrial pressure; they stop backflow into the atria.
• Semilunar valves close when arterial pressure rises above ventricular pressure; they stop backflow into the ventricles.
• The SAN starts each beat; the AVN delays it; Purkyne tissue conducts it through the ventricles.

Control sequence

SAN (right atrium wall, pacemaker) → excitation across both atria → atria contract → non-conducting layer blocks direct spread → AVN delay (atria finish emptying) → Purkyne tissue down the septum → ventricles contract from the base upwards. Nervous and hormonal control are not required.

Quick self-test

1. Name the vessel that carries oxygenated blood into the heart.
2. State two structural differences between an elastic artery and a muscular artery wall.
3. A white blood cell has a lobed nucleus and granular cytoplasm. Identify it.
4. In a photomicrograph at ×3000, a neutrophil measures 36 mm across. Calculate its actual diameter in µm.
5. Explain why plasma proteins do not normally appear in tissue fluid.
6. State the role of carbonic anhydrase.
7. Explain the importance of the chloride shift.
8. A sample of blood carries 15.6 cm³ O₂ per 100 cm³; its maximum is 20.8 cm³ per 100 cm³. Calculate its percentage saturation.
9. What happens to the dissociation curve when pCO₂ rises, and why is this useful?
10. One cardiac cycle lasts 0.6 s. Calculate the heart rate.
11. Explain why the AVN delays the wave of excitation.
12. Which valves close at the start of ventricular systole?

Answers

1. Pulmonary vein.
2. Elastic artery: a larger proportion of elastic fibres. Muscular artery: a thicker layer of smooth muscle (relative to elastic tissue).
3. Neutrophil.
4. 36 mm = 36 000 µm; 36 000 ÷ 3000 = 12 µm.
5. They are too large to pass through the gaps between endothelial cells.
6. Catalyses the reaction of carbon dioxide with water to form carbonic acid.
7. Cl⁻ moves into the red blood cell as HCO₃⁻ moves out, balancing the charge so hydrogencarbonate can keep leaving and CO₂ can keep being converted.
8. 15.6 ÷ 20.8 × 100 = 75%.
9. It shifts right (Bohr shift); haemoglobin’s affinity for oxygen falls, so more oxygen is unloaded in actively respiring tissues.
10. 60 ÷ 0.6 = 100 beats per minute.
11. So the atria finish contracting and emptying into the ventricles before the ventricles contract.
12. The atrioventricular valves (tricuspid and bicuspid).

Where marks are usually lost

• Writing that the pulmonary artery carries oxygenated blood.
• Saying veins have “no muscle” rather than a thinner muscle layer.
• Omitting “gaps between endothelial cells” when explaining how tissue fluid forms.
• Stating that the chloride shift moves chloride out of the red blood cell.
• Confusing haemoglobinic acid with carbaminohaemoglobin.
• Describing the Bohr shift as a shift to the left, or omitting “lower affinity”.
• Saying the SAN is in the left atrium, or that the AVN “starts” the heartbeat.
• Linking valve opening to contraction rather than to a pressure difference.
• Forgetting to convert mm to µm before using magnification.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, Cambridge University Press & Assessment – Topic 8, Transport in mammals (sections 8.1–8.3).