Aquatic respiration

Aquatic respiration is gas exchange by organisms in water, using oxygen dissolved in water or air accessed from the surface. It includes diverse structures and behaviors shaped by oxygen availability and the challenge of extracting gas from water.

Countercurrent exchange: A transfer process in which two fluids flow in opposite directions, maintaining a gradient along their shared interface. Opposing water and blood flows let fish gills extract oxygen efficiently.
Dissolved oxygen: Molecular oxygen present in water rather than as a separate gas phase. It is the oxygen source for organisms that breathe underwater.
Fish respiration: Gas exchange in fish, usually through gills that take up dissolved oxygen from water. Fish illustrate the specialized use of gills for underwater oxygen uptake.
Pulmonary respiration: Gas exchange between air and blood in lungs. Lungs extract oxygen from air, which contains far more available oxygen per volume than water.
Cutaneous respiration: Gas exchange across an organism’s external body surface. Many aquatic animals absorb oxygen directly through permeable skin rather than gills.
Henry's law: A gas’s equilibrium concentration in a liquid is proportional to its partial pressure above the liquid at a given temperature. It describes how oxygen enters water from the atmosphere.
Amphibian respiration: Gas exchange in amphibians through combinations of lungs, skin, and gills that vary with species and life stage. Amphibians often combine aquatic and aerial respiratory routes.
Branchial respiration: Gas exchange through gills, typically between water and blood. It names the gill-based route that contrasts with lung-based breathing.
Diffusion: The net movement of particles from regions of higher concentration to lower concentration. Oxygen crosses respiratory surfaces down its concentration gradient.
Oxygen–hemoglobin dissociation curve: A graph showing how hemoglobin’s oxygen saturation varies with oxygen partial pressure. It links oxygen uptake at respiratory surfaces to delivery through blood.

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