 Dear students, in this topic we shall discuss the functional anatomy of gills of fishes. The gills are effective respiratory organs for gas exchange in water. They have a thin, moist and highly vascularized epithelial layer which permits gas exchange through water medium. Now we shall discuss the anatomy of fish gills. Every gill has two rows of dorsoventrally flattened filaments which are sported by one gill arch. Each gill arch has two flattened gill filaments. Each filament has an upper and lower row of vascular folds of epithelium, which are called pharyngeal lamellae. These pharyngeal lamellae form the respiratory portion of the gill. Under the gill arch, there is a thin sheet of epithelial cells, while the inner wall is formed by pillar cells. These pillar cells have a lot of collagen fibers which spores them. The bronchial artery supplies blood to the gills. This bronchial artery breaks into capillary bed in pharyngeal lamellae. Lamellae of successive filaments are in close contact in a row forming a sieve-like structure. The entire gill looks like a water-flow structure. Lamellae make slit-like channels which flow water from the channels. These channels are 0.02 to 0.05 mm in width and 0.2 to 1.6 mm in length. Because of this small size, water flows in very thin sheets between the lamellae. The result is that the diffusion distance between the water and the blood flowing in the capillary bed is reduced. When the water and the blood flow in the capillary bed is reduced, the maximum gas exchange is possible.