Why do gills not work outside water?

Gills are respiratory organs found in aquatic animals like fish, which allow them to extract oxygen from water. They function effectively when the delicate gill filaments are constantly bathed in water, which contains dissolved oxygen. However, when gills are exposed to air, they face several challenges that hinder their ability to function properly:

1. Lack of Oxygen: Air contains a much lower concentration of oxygen compared to water. Water contains about 21% oxygen by volume, while air contains only about 20.95% oxygen. This means that there is significantly less oxygen available for the gills to extract from the air.

2. Structural Collapse: Gill filaments are thin, delicate structures that are supported by water. When exposed to air, they tend to collapse due to gravity and the lack of buoyancy that water provides. This collapse reduces the surface area available for gas exchange, further limiting oxygen uptake.

3. Desiccation: Air is a dry environment compared to water. When exposed to air, the delicate gill tissues lose moisture and become dry and brittle. This process of desiccation damages the gills, making them less efficient at absorbing oxygen.

4. Mucus Production: Gills produce mucus to help protect them from harmful particles and pathogens in water. However, in air, mucus can become thicker and stickier, clogging the delicate gill filaments and further impeding oxygen uptake.

5. Osmoregulation: Aquatic animals use gills not only for respiration but also for osmoregulation, the process of maintaining the proper balance of water and salts in their bodies. In air, this function becomes disrupted, as the gills are unable to regulate the exchange of water and ions effectively.

Due to these challenges, gills are not adapted to function outside of water. When aquatic animals are removed from water, their gills quickly become dysfunctional, leading to hypoxia (oxygen deprivation) and eventually death.