Not all animals have lungs. Many animals live in environments where lungs would be efficient enough for survival. Gills are another type of respiratory system, which are very efficient at removing oxygen from water: there is only 1/20 the amount of oxygen present in water as in the same volume of air. Gills greatly increase the surface area for gas exchange and they occur in a variety of animal groups including arthropods (including some terrestrial crustaceans), annelids, fish, and amphibians.
Gills are typically comprised of a gill arch, which contain veins and arteries that supply blood flow to the attached gill filaments. The arches are a rigid stucture which provide support and protection for the attached lamellae. There are usually two types of filaments, which are attached, primary and secondary.
Primary lamellae (or gill filaments) extend perpendicular from the gill arch. The filaments are located close together arranged in rows extending from both sides of the gill arches. With usually 4 gill arches side by side per side of the fish (Graham, 1997) the filaments form a “sieve through which the ventilatory water must pass.” (Evans, 1998) Each primary lamellae house an efferent and afferent blood vessel, which supplies the secondary lamellae.
Secondary lamellae extend vertically from the primary lamellae (or filaments) and are placed closely together forming small channels for water to flow through. Each secondary lamella is made up of two sheets of epithelial cells with pillar cells that hold them apart. These pillar cells form small tunnels within each secondary lamellae that act as channels for blood to perfuse through. Pillar cells are used to help regulate gas exchange across the secondary lamellae surfaces. The pillar cells have the ability to expand or contract, increasing or decreasing the size of the blood flow tunnels. This allows more or less blood to perfuse through the tunnels, it also increases or decreases the channel size between two secondary lamella allowing more or less water to perfuse through them. In water with high oxygen content the pillar cells will expand allowing more blood to rush through the lamellae to pick up oxygen while at the same time slowing the amount of high oxygenated water that flows through the channels in order to prevent the fish from getting too much oxygen. In waters of low oxygen content the pillar cells will contract widening the water flow channels to allow more water to perfuse through, while at the same time allowing less blood to move through the lamellae, for it can only pick up as much oxygen that is present in the water. Water flows through these lamellae channels in one direction while blood flows in the opposite direction through the epithelial cells. This creates a countercurrent flow that maximizes oxygen transfer.
The total number of lamellae constitutes the total surface area of the gills available for gas transfer. The number of lamellae per animal is correlated with their size and activity, the larger and more active the animal the more lamellae it will have. (Evans 1998)
Gills provide a one-way flow for oxygen to perfuse over them. This one way flow increases their efficiency since there is not much mixing of oxygenated and deoxygenated water directly over the gills and there is no “dead air space” such as the trachea in which oxygenated and deoxygenated water can get mixed.
Thr physiology of fishes. David Evans 1998