Exchange surfaces and transport

Surface area, volume and diffusion distance

For geometrically similar shapes, surface area increases with length squared while volume increases with length cubed. Doubling a cube’s side therefore quadruples its surface area but increases its volume eightfold. Its surface-area-to-volume ratio halves. As volume increases, there is more tissue needing supplies and producing wastes, but proportionally less outer surface through which to exchange them.

A low ratio is not the only difficulty. An internal cell may also be far from the outside, making diffusion over that distance too slow for its demand. Folding an exchange surface increases area, thinning a barrier shortens the diffusion path, and ventilation or flowing blood can maintain a concentration or partial-pressure difference.

In a simple diffusion model, rate increases with area and gradient and decreases with diffusion distance, with other factors held constant. If you compare two organs whose barrier thicknesses and gradients also differ, surface area alone cannot explain the result. Oxygen demand also varies with activity and tissue type, so size alone does not specify the required uptake rate.

Gas exchange at the alveoli

At a mammalian alveolus, oxygen moves net from air to pulmonary capillary blood when alveolar oxygen partial pressure is higher. Carbon dioxide moves net the other way when its partial pressure is higher in arriving blood. Thin alveolar epithelium and capillary endothelium create a short diffusion barrier, and many alveoli provide a large total area. The figure separates the layers but simplifies their intervening material.

Ventilation replaces alveolar gas; blood flow brings blood to the surface and takes it away. Haemoglobin binds oxygen reversibly, enabling much more oxygen to be carried than by dissolution in plasma alone. Loading and unloading depend on oxygen partial pressure and haemoglobin properties. Increased carbon dioxide can reduce oxygen affinity in the Bohr effect, assisting unloading at active tissues.

During quiet inspiration, the diaphragm contracts and flattens and the external intercostal muscles help raise the ribs. Thoracic volume increases, lowering pressure within the lungs below atmospheric pressure, so air enters. Quiet expiration mainly follows relaxation and elastic recoil. Forced expiration recruits additional muscles. Oxygen crossing the barrier is diffusion; air moving in bulk during ventilation is a different process.

Countercurrent exchange in fish gills

Gill filaments carry many lamellae, providing a large exchange area with a short route between water and blood. In countercurrent exchange, water and blood flow in opposite directions. At positions along a lamella, water can remain at a higher oxygen concentration than the adjacent blood. This maintains a favourable local gradient over much of the exchange surface.

Do not compare the oxygen-rich water entering one end with blood entering the other end as if they meet at the same location. A countercurrent explanation needs paired values at the same position. Opposite flow direction alone is not the whole mechanism; the maintained local difference is what allows oxygen to continue entering the blood. Ventilation across the gills keeps water moving through this arrangement.

Mammalian double circulation

In mammalian double circulation, the right side of the heart sends blood through the pulmonary circuit to the lungs. Blood returns to the left side, which sends it through the systemic circuit to body tissues. It then returns to the right side. Passing through the heart between circuits allows pressure to be raised again for systemic flow while pulmonary pressures remain lower.

An artery carries blood away from the heart; a vein carries it towards the heart. The pulmonary artery therefore carries relatively deoxygenated blood, while pulmonary veins return relatively oxygenated blood. Vessel names are defined by direction, not by oxygen content. In the figure, blue denotes lower oxygenation; human blood is not blue.

Arterial walls contain elastic tissue and muscle suited to pressure and flow control. Capillaries provide short exchange distances; venous return operates at lower pressure and can be assisted by valves and surrounding muscle activity. During the cardiac cycle, valves open or close in response to pressure differences, limiting backflow. Cardiac output is heart rate multiplied by stroke volume; keep the time and volume units consistent.

Water movement through xylem

Water evaporates from moist mesophyll surfaces and diffuses as vapour through stomata. This lowers water potential in the leaf and draws water from the xylem. Cohesion between water molecules helps transmit tension through the water column; adhesion to vessel walls also contributes. Xylem vessels are largely dead, hollow conduits whose lignified walls resist collapse under tension.

Evaporation drives the transpiration stream. There is no ATP-driven pump inside the dead xylem vessels. Uptake of particular mineral ions by living root cells can involve active transport, but that does not make the entire upward water column an ATP-pumped flow. A potometer estimates water uptake, which is often used as a proxy for transpiration under suitable conditions. It does not directly measure all water lost from leaves.

Humidity, temperature, air movement and stomatal opening affect water loss. Warming a leaf may increase evaporation, but severe water stress can close stomata and restrict water loss. The effect observed therefore depends on the experimental conditions and the plant’s response.

Translocation in phloem

Phloem transports assimilates, especially sucrose, through living sieve-tube elements supported by companion cells. A source supplies assimilate, for example a photosynthesising leaf. A sink uses or stores it, such as a growing root or developing fruit. An organ can change role: a storage organ can become a source when stored material is mobilised.

In the pressure-flow explanation, sucrose loading at a source lowers phloem water potential. Water enters from nearby xylem, raising hydrostatic pressure. Unloading at a sink contributes to a lower pressure, allowing bulk flow along a pressure gradient. Loading in the commonly taught active-loading route depends on metabolism in living cells. Distinguish that energetic dependence from the pressure gradient immediately driving bulk flow.

Translocation can occur in different directions in different sieve tubes because the positions of sources and sinks differ. “Phloem always moves down” therefore fails for a sink above its source. Xylem water flow and phloem assimilate flow have different driving mechanisms.

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Practice for this topic

• Surface area to volume ratio
• Rates and graph gradients
• Units and standard form
• Apply to a case: Gas exchange and circulation: linking measurements
• Practical guide: Dissection: observations and explanations

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Glossary

Partial pressure

Contribution of a gas to pressure; differences help describe gas-exchange direction.

Transpiration

Loss of water vapour from a plant, mainly through stomata.

Translocation

Transport of assimilates through phloem between sources and sinks.