Exchange surfaces: active recall

For similarly shaped organisms, volume increases faster than external surface area as linear size increases. Metabolic demand is associated with living tissue, while exchange depends on available surface and gradients. Internal transport and specialised surfaces can help meet demand; size alone does not specify an organism’s exact metabolic rate.

A larger area and steeper concentration difference favour a greater diffusion rate, while a longer diffusion path slows it under comparable conditions. Explain which structural or physiological feature changes each factor. A thin barrier does not automatically maintain a gradient without movement or consumption of the exchanged substance.

Many alveoli provide a large total surface. Thin alveolar and capillary barriers give a short diffusion path. Ventilation refreshes alveolar air and blood flow carries gases to and from the surface, helping maintain differences in oxygen and carbon dioxide partial pressure.

Contraction of the diaphragm and external intercostal muscles increases thoracic volume. Lung expansion lowers alveolar pressure below atmospheric pressure, so air flows in. Air is moved by a pressure difference; oxygen subsequently crosses the gas-exchange surface by diffusion rather than being actively transported into blood.

No. Quiet expiration largely involves relaxation of inspiratory muscles and elastic recoil, reducing lung volume and raising alveolar pressure. Forced expiration can recruit additional muscles. Distinguish the normal resting mechanism from a forced breathing manoeuvre.

Water and blood move in opposite directions across lamellae. This can maintain an oxygen partial-pressure difference favouring movement from water into blood along much of the exchange surface, allowing diffusion throughout. With concurrent flow, the streams approach equilibrium sooner and the driving difference becomes smaller.

Filaments bear many thin lamellae, providing a large exchange area with a short path to circulating blood. Water movement and blood flow help maintain oxygen gradients. The functional explanation needs both surface structure and continued flow, rather than merely stating that gills are folded.

Air enters through spiracles and moves through tracheae to fine tracheoles near tissues. Oxygen generally reaches cells without being carried there by blood in the mammalian manner. Diffusion and, in some insects, ventilatory movements contribute; spiracle regulation also affects water loss.

Stomata connect the atmosphere with internal air spaces. These spaces provide routes to moist mesophyll surfaces, and relatively short paths help gases diffuse to cells. Guard-cell control changes stomatal opening. The same openings also permit water vapour loss, creating a trade-off.

Opening changes the available pathway for diffusion. Carbon dioxide moves down its gradient as photosynthesis consumes it internally, while water vapour can move out. Guard-cell changes regulate access; they do not actively carry each carbon dioxide molecule through the pore.

Villi and epithelial microvilli increase surface area, and the epithelial barrier is thin. A blood supply helps carry absorbed soluble products away; lacteals receive many absorbed lipid products. Particular substances use particular transport pathways, so a large surface alone does not explain their absorption mechanism.

Without renewed air or water and continued blood or tissue transport, nearby concentrations can approach equilibrium. The driving difference for diffusion then falls even though surface area and barrier thickness have not changed. Structural adaptations and mechanisms maintaining gradients work together.