Fish Respiration System

Respiration in Fish

• Fish gills are composed of thin filaments with a thin epidermal membrane folded into platelike lamellae, richly supplied with blood vessels.
• Gills are located inside the pharyngeal cavity and are covered with a movable flap, the operculum, which provides protection and enables a pumping system for moving water.
• In elasmobranchs, gill slits replace opercular flaps, and water flows out of these slits.
• The branchial mechanism is arranged to pump water continuously and smoothly over the gills, with a countercurrent flow that maximizes oxygen extraction from water.
• Some bony fishes can remove up to 85% of dissolved oxygen from water passing over their gills.
• Very active fishes, such as herring and mackerel, use ram ventilation, swimming forward continuously to force water into their open mouth and across their gills.
• Some fishes can live out of water for varying lengths of time by breathing air, using devices such as lungs, skin, and air chambers.

Osmotic Regulation in Fish

• Freshwater is a dilute medium with a salt concentration much below that of the blood of freshwater fishes.
• Water tends to enter the body osmotically, and salt is lost by diffusion outward across the gills.
• Freshwater fishes are hyperosmotic regulators with defenses against water gain and salt loss, including:
  • Excess water pumped out by the opisthonephric kidneys.
  • Special salt-absorbing cells in the gill epithelium that actively move salt ions from water to the blood.
• These mechanisms allow freshwater fishes to devote only a small part of their total energy to maintaining osmotic balance.
• About 90% of bony fishes are restricted to either a freshwater or a seawater habitat because they are incapable of osmotic regulation in the "wrong" habitat.
• However, some 10% of teleosts are euryhaline, capable of passing between freshwater and seawater habitats with ease.