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Word Count: 560
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 120 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
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