Respiration
A few gas exchange mechanisms across species are across the body surface, gills, trachea, and lungs. Ventilation is the process of bringing oxygenated water or air into contact with a gas exchange surface.
All respiratory organs share certain common features. Those features are moist surfaces in which gases dissolve and diffuse, high surface area for gas exchange, extensive blood flow, and a thin, delicate structure.
Water-breathing vs Air-breathing
There are different challenges to gas exchange when it comes to water-breathing vs air-breathing.
Aquatic animals have less available oxygen, and the oxygen availability fluctuates when there are temperature changes in the water. Moving dense water over their respiratory membranes takes more effort than moving air. Water also removes heat from the gill surface and can create osmotic movement.
Terrestrial animals deal with desiccation of their respiratory membranes.
Partial Pressure Gradients in Gas Exchange
Gases diffuse down pressure gradients in the lungs and other organs as a result of differences in partial pressure.
Partial pressure: the presence exerted by a particular gas in a mixture of gases.
A gas diffuses from a region of higher partial pressure to a region of lower partial pressure.
In the lungs and tissues, oxygen and carbon dioxide diffuse from where their partial pressures are greater to where their partial pressures are lower.
Respiratory Media
Respiratory medium: source of oxygen
Animals can use air or water as a source of oxygen.
In a given volume, there’s less oxygen available in water than air.
Obtaining oxygen from water requires greater efficiency than obtaining it from air.
Gills in Aquatic Animals
Gills: outfoldings of the body that create a large surface area for gas exchange
Ventilation moves the respiratory medium over the respiratory surface. Aquatic animals move through water or move water over their gills for ventilation. Fish gills use a countercurrent exchange system, where blood flows in the opposite direction to water passing over the gills (blood is always less saturated with oxygen than the water).
External Gills
External gills vary in appearance but they all have a a large surface area with extensive projections. They may exist in one body area or be scattered over a large area.
Limitations:
- Unprotected and subject to damage
- Energy is required to wave gills back and forth
- Appearance and motion may attract predators
Internal Gills
Internal gills are covered by operculum.
Gill arches: main support structure
Filaments: branch off of gill arches
Lamellae: branch off of filaments
Blood vessels run the length of the filaments. Oxygen-poor blood travels through the afferent vessel and oxygen-rich blood travels through the efferent vessel.
Countercurrent exchange of water and blood flow maximizes diffusion into blood.
Lungs
Lungs: an infolding of the body surface
The circulatory system transports gases between the lungs and the rest of the body. The size and complexity of lungs correlate with an animal’s metabolic rate. With a few exceptions, all air-breathing terrestrial vertebrates use lungs. Lungs may be filled via positive or negative pressure.