Aquatic Respiration: How Gills Function in Water

Aquatic Respiration: Understanding Gills

Gills are highly effective respiratory structures adapted for life in water. They are laminar expansions formed by the filiform body wall of an animal. If these expansions project outside the body, they are called external gills. If they are housed inside a cavity that communicates with the outside, they are called internal gills.

External Gills: Simpler Respiratory Structures

External gills are evolutionarily the oldest, formed by expansions of the body surface. They occur in some mollusks, insects, and aquatic larvae of amphibians and crustaceans. However, they present several disadvantages:

• They are vulnerable to predators.
• They are easily injured.
• They can make movement difficult.
• Animals possessing these gills often lack a dedicated ventilation system, forcing them to move constantly, wave water, or rely on environmental currents for oxygen exchange.

Internal Gills: Advanced Aquatic Respiration

Internal gills are more complex. Their enclosure within a body cavity led to the emergence of a ventilation mechanism, offering greater protection and improved aerodynamics for the respiratory surfaces. They are found in:

Gills in Bivalve Molluscs

Bivalve molluscs, such as mussels, have gills located in the mantle cavity, which provides a large respiratory surface. Water enters the mantle cavity at the rear ventral side of the body through gaping valves. It then ascends via ciliary action by the gills and palps towards the mouth and gill plates, before descending to exit the loop. This continuous stream facilitates both gas exchange and food filtration.

Fish Gills and Countercurrent Exchange

Fish gills are housed within a gill chamber. Each gill consists of a skeletal axis, known as the gill arch, from which two rows of gill filaments extend. Each filament, in turn, has numerous parallel lamellae.

The blood flow within the gill filaments runs in the opposite direction to the flow of water. This mechanism, known as countercurrent exchange, allows for maximum oxygen extraction from the water.

Fish continuously pump water through their mouth and over the gill arches. During inspiration, the fish's pharyngeal cavity expands, drawing water in through the mouth. When the fish exhale, they close their mouth and the oral valve located behind the teeth, forcing water out over the gills.