Exchange and Mass Transport

Multicellular organisms are too large for diffusion alone to supply all cells with nutrients and remove waste products. They have evolved specialised exchange surfaces and mass transport systems to meet metabolic demands. This topic covers gas exchange in humans, insects, and fish; the mammalian circulatory system; haemoglobin and oxygen transport; and plant transport systems.

Principles of Exchange Surfaces

Efficient exchange surfaces share common features:

Large surface area — maximises the area available for diffusion (e.g., alveoli, villi, root hair cells).
Thin barrier — short diffusion distance speeds up the rate (e.g., alveolar epithelium is one cell thick, ~0.5 µm).
Good blood supply — maintains a steep concentration gradient by rapidly carrying substances away (or delivering them).
Ventilation or movement — maintains concentration gradients (e.g., breathing movements in lungs, countercurrent flow in fish gills).

These features relate to Fick's law: Rate ∝ (surface area × concentration difference) ÷ diffusion distance.

Key Definition: Fick's law states that the rate of diffusion is directly proportional to the surface area and the concentration difference, and inversely proportional to the thickness (diffusion distance) of the exchange surface.

Exam Tip: When explaining how an exchange surface is adapted for efficient exchange, always relate your answer back to Fick's law. For each adaptation, state which factor in Fick's law it affects (e.g., "the thin walls reduce diffusion distance, increasing the rate of diffusion").

Gas Exchange in Humans

Structure of the Lungs

Trachea → bronchi → bronchioles → alveoli.
The trachea and bronchi are supported by C-shaped cartilage rings that keep the airways open during pressure changes.
Goblet cells secrete mucus to trap particles and pathogens; ciliated epithelium sweeps mucus upwards towards the throat (the mucociliary escalator).

Adaptations of Alveoli for Gas Exchange

Adaptation	How It Helps (linked to Fick's law)
Millions of alveoli	Enormous total surface area (~70 m²) — increases SA in Fick's law
Walls one cell thick (squamous epithelium)	Short diffusion pathway (~0.5 µm) — reduces thickness
Dense capillary network	Maintains steep concentration gradient — blood flow carries O₂ away and brings CO₂
Surfactant lining	Reduces surface tension, prevents alveoli collapsing on expiration
Ventilation (breathing)	Refreshes air in the alveoli, maintaining concentration gradient

A diagram of an alveolus would show a thin-walled, roughly spherical sac. The wall is made of a single layer of flattened squamous epithelial cells. A capillary wraps closely around the outside, also made of a single layer of endothelial cells. The total barrier between alveolar air and blood is therefore just two cells thick. Inside the alveolus, a thin layer of moisture (with surfactant) lines the surface. Red arrows would show O₂ diffusing from the alveolus into the blood, and blue arrows would show CO₂ diffusing from the blood into the alveolus.

Ventilation Mechanism

Inspiration: diaphragm contracts (flattens), external intercostal muscles contract (ribs move up and out) → thoracic volume increases → intrapulmonary pressure decreases below atmospheric pressure → air rushes in.
Expiration (at rest): diaphragm relaxes (domes upward), external intercostal muscles relax → thoracic volume decreases → intrapulmonary pressure increases above atmospheric pressure → air is pushed out. Forced expiration additionally uses internal intercostal muscles (ribs pulled down and in) and abdominal muscles.

Pulmonary Ventilation Rate (PVR)

Key Definition: Pulmonary ventilation rate (PVR) is the total volume of air breathed in or out per minute. PVR = tidal volume × breathing rate.

Worked Example 1 — PVR Calculation:

A student at rest has a tidal volume of 0.5 dm³ and a breathing rate of 14 breaths per minute. Calculate the pulmonary ventilation rate.

Solution: PVR = tidal volume × breathing rate PVR = 0.5 dm³ × 14 min⁻¹ = 7.0 dm³ min⁻¹

During exercise, both tidal volume and breathing rate increase to meet the increased demand for oxygen.

Gas Exchange in Insects

Insects use a tracheal system: spiracles (openings) on the body surface lead to tracheae (reinforced with rings of chitin to prevent collapse), which branch into fine tracheoles that penetrate tissues directly.
Gas exchange occurs at the ends of tracheoles, where O₂ dissolves in a thin film of fluid and diffuses directly into respiring cells.
Ventilation is assisted by rhythmic abdominal movements that pump air in and out.
The system limits body size because it relies partly on diffusion over short distances — the maximum body size of insects is constrained by the efficiency of this system.

Gas Exchange in Fish

Fish use gills with a countercurrent flow system.
Water flows over the gill lamellae (secondary lamellae) in the opposite direction to blood flow within the capillaries.
This maintains a concentration gradient along the entire length of the lamella, enabling up to 80% oxygen extraction — far more efficient than a parallel flow system (which would reach equilibrium and only extract ~50%).

A diagram of the countercurrent system would show a gill lamella as a flat plate. Above the lamella, an arrow shows water flowing from left to right with decreasing oxygen concentration (e.g., 100% → 20%). Below, blood flows from right to left with increasing oxygen concentration (e.g., 15% → 85%). At every point along the length, the water has a higher oxygen concentration than the adjacent blood, maintaining a diffusion gradient throughout.

The Circulatory System

Double Circulation in Mammals

Mammals possess a double circulatory system:

Pulmonary circulation — right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium (deoxygenated blood becomes oxygenated).
Systemic circulation — left ventricle → aorta → body tissues → vena cava → right atrium (oxygenated blood delivers O₂ and returns deoxygenated).

flowchart TD
    RA["Right Atrium"] -->|"Deoxygenated blood"| RV["Right Ventricle"]
    RV -->|"Pulmonary artery"| Lungs["Lungs
(Gas exchange)"]
    Lungs -->|"Pulmonary vein"| LA["Left Atrium"]
    LA -->|"Oxygenated blood"| LV["Left Ventricle"]
    LV -->|"Aorta"| Body["Body Tissues"]
    Body -->|"Vena cava"| RA

The advantage is that blood passes through the heart twice per circuit, maintaining high pressure for efficient delivery to tissues. The separation of oxygenated and deoxygenated blood also increases the efficiency of oxygen delivery.

The Heart

Four chambers: right atrium, right ventricle, left atrium, left ventricle.
The left ventricle has a thicker muscular wall to generate higher pressure for systemic circulation (blood must travel further than in the pulmonary circuit).
Atrioventricular (AV) valves (tricuspid on the right, bicuspid/mitral on the left) prevent backflow from ventricles to atria.
Semilunar valves (in the aorta and pulmonary artery) prevent backflow from arteries into ventricles.
The cardiac cycle: atrial systole → ventricular systole → diastole. The heart is myogenic — the heartbeat is initiated within the heart itself.

The Cardiac Conduction System

The sinoatrial node (SAN) in the right atrium acts as the natural pacemaker — it generates electrical impulses that spread across both atria, causing atrial systole.
The impulse reaches the atrioventricular node (AVN), which introduces a slight delay (~0.1 s) to allow the atria to empty fully before the ventricles contract.
The impulse then travels rapidly down the bundle of His (in the interventricular septum) and into the Purkinje fibres, which spread through the ventricular walls, triggering ventricular systole from the apex upwards.

flowchart TD
    SAN["SAN
(Sinoatrial node — pacemaker)
Generates impulse"] --> Atria["Impulse spreads across both atria
→ Atrial systole"]
    Atria --> AVN["AVN
(Atrioventricular node)
Slightly delays impulse ~0.1s"]
    AVN --> BoH["Bundle of His
(in interventricular septum)"]
    BoH --> PF["Purkinje fibres
(spread through ventricular walls)"]
    PF --> VS["Ventricular systole
(from apex upwards)"]

Blood Vessels

Vessel	Structure	Function
Arteries	Thick muscular walls, elastic tissue, narrow lumen, no valves (except semilunar)	Carry blood away from the heart at high pressure; elastic recoil smooths blood flow
Arterioles	Smooth muscle in walls	Regulate blood flow to capillary beds (vasoconstriction/vasodilation)
Capillaries	One endothelial cell thick, very narrow lumen (~8 µm), no muscle	Exchange of substances between blood and tissues; thin walls reduce diffusion distance
Venules/Veins	Thin walls, wide lumen, valves present	Return blood to the heart at low pressure; valves prevent backflow

Tissue Fluid Formation (Starling's Forces)

Key Definition: Tissue fluid is the fluid that surrounds cells in tissues. It is formed from blood plasma that has been forced out of capillaries by hydrostatic pressure.

At the Arterial End of a Capillary

Hydrostatic pressure of the blood (caused by the pumping of the heart) is high at the arterial end of the capillary (~35 mmHg).
This pressure forces water, dissolved ions, glucose, amino acids, and other small molecules out of the capillary through gaps between endothelial cells.
Large plasma proteins (e.g., albumin) cannot pass through and remain in the blood.
These proteins create an oncotic (osmotic) pressure that tends to pull water back in, but at the arterial end, hydrostatic pressure exceeds oncotic pressure, so there is a net outward movement of fluid.

At the Venous End of a Capillary

Hydrostatic pressure has fallen (~15 mmHg) because blood has lost fluid and the capillary bed offers resistance.
Oncotic pressure (~25 mmHg, due to plasma proteins) now exceeds hydrostatic pressure.
There is a net inward movement of fluid — tissue fluid is reabsorbed into the capillary.

Excess Tissue Fluid

Not all tissue fluid is reabsorbed at the venous end; the excess drains into lymph capillaries and becomes lymph.
Lymph is returned to the blood via the lymphatic system, eventually rejoining the blood at the subclavian veins.

Exam Tip: When describing tissue fluid formation, always state the direction of the net pressure at each end. At the arterial end, hydrostatic pressure > oncotic pressure = net outward flow. At the venous end, oncotic pressure > hydrostatic pressure = net inward flow.

Haemoglobin and Oxygen Transport

Haemoglobin is a globular protein with four polypeptide subunits (two α-chains and two β-chains), each containing a haem group with an iron (Fe²⁺) ion that reversibly binds one O₂ molecule. Each haemoglobin can carry up to four O₂ molecules (forming oxyhaemoglobin).
The oxygen dissociation curve is sigmoid (S-shaped) due to cooperative binding: once one O₂ binds, the resulting conformational change increases haemoglobin's affinity for further O₂ molecules. This makes loading at the lungs very efficient.
The Bohr effect: increased CO₂ concentration / decreased pH shifts the curve to the right, reducing haemoglobin's affinity for O₂. This promotes O₂ release in actively respiring tissues where CO₂ is high. CO₂ combines with water (catalysed by carbonic anhydrase in red blood cells) to form carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃⁻. The H⁺ ions cause the Bohr effect.
Foetal haemoglobin (HbF) has a higher affinity for O₂ than adult haemoglobin (HbA) — the dissociation curve is shifted to the left. This enables the foetus to obtain O₂ from maternal blood across the placenta.

Plant Transport

Xylem — Water Transport

Key Definition: Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata in the leaves.

Xylem vessels transport water and dissolved mineral ions from the roots to the leaves. The driving force is transpiration.

The Cohesion-Tension Theory:

Water evaporates from the surface of mesophyll cells into air spaces in the leaf and diffuses out through stomata (transpiration).
This creates a tension (negative pressure) in the leaf, pulling water from the xylem vessels.
Water molecules are attracted to one another by hydrogen bonds (cohesion), forming a continuous column of water from root to leaf.
Water molecules are also attracted to the walls of the xylem vessels (adhesion), which helps support the water column.
As water is pulled up the xylem from the roots, more water enters the root from the soil by osmosis to replace it.

flowchart BT
    Soil["Soil water"] -->|"Osmosis"| Root["Root hair cells"]
    Root -->|"Water enters xylem"| Xylem["Xylem vessels
(continuous water column
held by cohesion & adhesion)"]
    Xylem -->|"Tension pulls
water upward"| Leaf["Mesophyll cells in leaf"]
    Leaf -->|"Evaporation through
stomata (transpiration)"| Atm["Atmosphere"]

Factors affecting transpiration rate:

Factor	Effect
Light intensity	Increases rate — stomata open in light for photosynthesis
Temperature	Increases rate — water molecules have more kinetic energy, evaporate faster
Wind speed (air movement)	Increases rate — moves humid air away from leaf surface, maintaining diffusion gradient
Humidity	Decreases rate — reduces the water potential gradient between leaf and air

The Potometer

Key Definition: A potometer is a piece of apparatus used to measure the rate of water uptake by a leafy shoot, which is used as an estimate of the rate of transpiration.

A diagram of a potometer would show a leafy shoot sealed into a rubber bung at one end of a horizontal capillary tube. The other end of the tube is open to a reservoir of water. An air bubble is introduced into the capillary tube. As the shoot transpires, water is taken up, and the air bubble moves along the tube towards the shoot. The distance moved by the bubble in a given time indicates the rate of water uptake. A scale next to the tube allows measurement.

Important: A potometer measures the rate of water uptake, not strictly transpiration, because some water is used in photosynthesis and other processes. However, ~99% of water absorbed is lost by transpiration, so it is a close estimate.

Xerophyte Adaptations

Key Definition: Xerophytes are plants adapted to survive in dry (arid) conditions by reducing water loss.

Adaptation	How It Reduces Water Loss
Thick waxy cuticle	Reduces evaporation through the leaf epidermis
Sunken stomata	Creates a pocket of humid air, reducing the water potential gradient
Rolled leaves (e.g., marram grass)	Traps moist air inside, reducing the water potential gradient
Reduced number of stomata	Fewer openings for water to escape
Hairy leaves	Trap a layer of still, moist air near the leaf surface
Small/needle-like leaves	Reduced surface area for evaporation
Deep or extensive root systems	Access water deep underground or over a wide area
CAM photosynthesis	Stomata open at night (when cooler) and close during the day

Phloem — Translocation

Key Definition: Translocation is the transport of dissolved organic solutes (mainly sucrose and amino acids) through the phloem from sources (where sugars are produced, e.g., photosynthesising leaves) to sinks (where sugars are used or stored, e.g., roots, growing tips, fruits).

The Mass Flow Hypothesis (Münch, 1930):

At the source (e.g., leaf), sucrose is actively loaded into the phloem sieve tube elements by companion cells using ATP. This occurs via H⁺ co-transport: H⁺ ions are pumped out of the companion cell by a proton pump; they then flow back in through a co-transporter, carrying sucrose with them. Sucrose moves into the sieve tube via plasmodesmata.
The high concentration of sucrose in the sieve tube lowers the water potential, causing water to enter from the adjacent xylem by osmosis.
This increases the hydrostatic pressure at the source end.
At the sink (e.g., root), sucrose is unloaded from the sieve tube (by active transport or conversion into starch). This raises the water potential, and water leaves by osmosis.
The difference in hydrostatic pressure between source and sink causes a mass flow of solution through the sieve tubes from source to sink.

flowchart LR
    subgraph Source["Source (e.g., Leaf)"]
        SL["Sucrose actively loaded
into sieve tube
(lowers water potential)"]
        WI["Water enters from xylem
by osmosis
→ High hydrostatic pressure"]
    end
    subgraph Phloem["Phloem Sieve Tubes"]
        MF["Mass flow of sucrose
solution from source → sink"]
    end
    subgraph Sink["Sink (e.g., Root)"]
        SU["Sucrose unloaded
(raises water potential)"]
        WO["Water leaves by osmosis
→ Low hydrostatic pressure"]
    end
    SL --> WI
    WI --> MF
    MF --> SU
    SU --> WO

Evidence supporting the mass flow hypothesis:

Ringing experiments (removing phloem causes sugars to accumulate above the ring).
Aphid stylets — when an aphid feeds, its stylet punctures a sieve tube. Cutting the aphid away leaves the stylet in place, and sap exudes under pressure.
Radioactive tracers — ¹⁴C-labelled CO₂ fed to a leaf is later found in sugars in the phloem.

Limitations of the hypothesis:

It does not fully explain how sugars can move in different directions in the same sieve tube simultaneously.
Sieve plates appear to obstruct mass flow (though they may function primarily as structural support).

Worked Example 2 — Cardiac output calculation:

A person has a heart rate of 72 beats per minute and a stroke volume of 75 cm³. Calculate the cardiac output.

Solution: Cardiac output = heart rate × stroke volume Cardiac output = 72 × 75 = 5,400 cm³ min⁻¹ (or 5.4 dm³ min⁻¹)

Exam Tip: This formula is provided on the data sheet, but you must know how to use it. Units matter — if stroke volume is given in cm³, the answer will be in cm³ min⁻¹. Convert as needed.

Worked Example 3 — Transpiration rate from a potometer:

A bubble in a potometer moves 45 mm in 5 minutes. The capillary tube has a cross-sectional area of 0.8 mm². Calculate the rate of water uptake.

Solution: Volume of water taken up = distance × cross-sectional area = 45 × 0.8 = 36 mm³ Time = 5 minutes Rate = 36 ÷ 5 = 7.2 mm³ min⁻¹