Ventilation in Insects, Fish and Mammals

Ventilation renews air or water at exchange surfaces, helping maintain gas gradients as exchange occurs. For a gas crossing between air or water and blood, compare its partial pressures, the pressures due to that gas. Insect tracheoles instead deliver gases close to tissue cells. Ventilation's main role is medium renewal; it does not require creating extra exchange area.

During mammalian inhalation, the diaphragm contracts and flattens, increasing chest volume. The lungs expand and pressure in them falls below atmospheric pressure. Air enters from higher to lower total pressure. This bulk airflow is different from diffusion: oxygen and carbon dioxide cross the alveolar barrier down their own partial-pressure gradients.

During quiet exhalation, the diaphragm relaxes and elastic recoil reduces lung volume. Lung pressure rises above atmospheric pressure, so air leaves. At a pause with open airways, lung pressure equals atmospheric pressure and bulk airflow stops. Equal total pressures do not imply equal oxygen partial pressures: oxygen can still diffuse into blood if its partial pressure is higher in alveolar air.

Fish ventilation moves water over gill lamellae. Blood inside a lamella flows opposite to the water in countercurrent exchange. During normal oxygen uptake, water's oxygen partial pressure exceeds that of adjacent blood along the lamella. Oxygen can therefore enter blood throughout its length. Compare neighbouring water and blood, not the total oxygen carried by each stream.

With parallel flow, water and blood move in the same direction. As oxygen enters blood, the two oxygen partial pressures approach each other and the gradient declines. Countercurrent flow maintains a favourable local difference along the lamella and permits greater oxygen extraction in an otherwise comparable model. Parallel flow has no fixed halfway point where its gradient must disappear.

Insects admit air through spiracles into branching tracheae and fine tracheoles near cells. Less active insects may obtain enough oxygen through diffusion without active pumping. Many active insects also use body movements to compress and expand parts of the tracheal system. Pressure differences move air through it. Some insects have air sacs that help this movement; neither air sacs nor active pumping occur in every insect.