Respiration in Non-Homeotherms - Anatomy & Physiology

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Fish live in a relatively dense and viscous fluid, which has little oxygen carrying capacity. In order to gain sufficient gaseous exchange for survival, fish have developed a highly efficient system which has a large surface area, and utilises energy - the gills.


Gills are in the walls of both sides of the pharynx and are composed of filaments which increase the surface area. When a fish breaths, it takes in water into the mouth, which is then forced out through the gills. A countercurrent exchange system is in place within the gills to make gaseous exchange more efficient.

The gill has several important functions in fish physiology: respiration, nitrogenous excretion and fluid balance regulation.

The anatomic structure of the gills helps in its functions by providing a large and very thin surface. The gill filaments are divided into multiple finger-like projections, the primary lamellae, which in turn are divided into secondary lamellae.

There is usually very little mucus on the gill surface.


In amphibian species, the skin forms the major, and sometimes only respiratory organ. The skin of amphibia is thin, poorly keratinised, highly vascularised and moist to promote diffusion of gases. Aquatic amphibians may also have internal gills and pharyngeal slits. Most amphibia also have lungs for breathing. This contains interconnecting septa which divide the lumen into faveoli, compartments which open into a central chamber within each lung.


Reptiles have a fundamentally different respiratory system to that of mammals in that they lack a bronchial tree. The air in the reptilian respiratory system flows through the following passage:

  1. Air enters via nares
  2. Nasal Cavity
  3. Choana
  4. Glottis (base of tongue)
  5. Trachea
  6. Bronchi
  7. Lungs

The lungs are paired, with respiration only occurring in the cranial region. The respiratory surface is composed of faveoli, honeycomb shaped structures which line the walls. In addition, reptiles have a poorly developed mucocillary escalator, and rely on body positioning to clear mucous from the respiratory system.


Lizards have no diaphragm and a simple, sac-like, paired lungs. The cranial part of lung is the site of respiratory function. The caudal part of the lung forms an air reservoir. Ventilation occurs via expansion and contraction of the ribs. For more information see Lizard and Snake Respiration and Lizard Respiratory System.


The larynx projects medially from the floor of the mouth and is fused with the first few cartilage rings of the trachea. This forms a rigid glottis which is highly mobile and can be displaced laterally to allow breathing during feeding. The tongue is fixed at the rostral point of the oral cavity. Generally, only the right lung is present, or at least functional - colubrids have a vestigial left lung. The lung of snakes is very fragile. Inspiration is an active process, made possible by expansion of the ribs. It occurs approx once every 30 seconds in large, healthy snakes. Expiration is a passive process. Snakes have no vocal cords, but are able to make hissing noises by passing air through the glottis. Aquatic snakes have air sacs which serve as a buoyancy aid. For more information see Lizard and Snake Respiration and Snake Respiratory System.


Chelonians have adapted a dive reflex which enables them to respire anaerobically. They have extremely high levels of bicarbonate in their blood which allows them to buffer the lactic acid produced. Chelonians have an extremely short trachea in relation to their size. They have paired lungs in the dorsal cavity. Intestines etc. lie in the ventral cavity. These are separated by a diaphragmatic septum which has no muscular part. Chelonians possess large spongy lungs which reduce 80% in size when the head and limbs are retracted inside the shell. The lungs have a single intrapulmonary bronchus which radiates in a network of bronchioles and faveoli. Lungs are ventilated by leg movement in terrestrial animals, and have highly developed trunk muscles to facilitate this. Aquatic species are able to respire due to the hydrostatic pressure of water, which forces air in and out of the lungs. In addition, some soft-shelled chelonians are able to absorb oxygen through their shells and skin when submerged.

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