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Respiratory Adaptations Evolution of the respiratory system The advantages of developing air-breathing organs were that fish could stay at the surface ventilating in oxygen rich water using less energy than moving water through the gills whilst swimming. This could supplement brachial gas exchange, but it would only work in warm water if the fish had to linger without swimming. The advantages of having stored air spaces is that fish could submerge with stored oxygen, reducing time spent at the surface and this protecting it from predation. Air-breathing organs (AO) are now thought to have originated from placoderms (ancient armoured fish) and evolved once again in osteoichthyes (bony-fish). Elasmobranches (such as sharks and rays) have placoid scales and large livers to aid buoyancy, therefore AO's confer little selective significance and have either subsequently been lost or never even evolved before in this lineage.
Implications of the evolution of air-breathing The shift of the respiratory pump from the posterior towards the middle of the body freed the tongue and mouth for a diverse array of feeding styles BUT generated a mechanical constraint between coastal ventilation and high-speed locomotion. ADAPTATIONS TO ALTITUDE Oxygen availability at altitude Oxygen availability decreases with altitude. At high altitude, pO2 of vasculature decreases and the oxygen dissociation curve is disrupted as the ability to bind and release oxygen is
Recently, a fossil of a placoderm fish was discovered which appeared to have saclike, paired structures extending dorso-laterally from the midline of the anterior pharynx. The placoderm was a bony armoured fish, and it is thought that these may have been AO's that could have assisted in flotation. Lungs are characteristic of the polypteriformes, lungfish and tetrapods, and the swimbladder is ancestral for all other bony fish. The swimbladder and lungs are homologous, with both forming from pharyngeal pouches 7 and 8. The swimbladder forms from the dorsal side of the pouch and the lungs form from the ventral half and become paired. What is interesting to note is that, during the later stages of development, pharyngeal pouches in elasmobranch embryos disappears. Furthermore, the blood supply to both the swimbladder and the lungs is from the 6th brachial artery; the venous flow enters the heart separately. The only difference is that in the swimbladder, the blood drains into the cardinal veins (there is no separation of oxygenated and de-oxygenated blood) instead of an atria or sinus venosus). Which came first? The lung or the breath?
The breath came first. An air-breathing oscillator must have been present BEFORE AO's as otherwise gills would have only been modified. The likely sequence of event is:
1. Breathing oscillator with motor ventilatory pattern (pre-placoderm)
2. Pharyngeal pouches and 6th brachial artery configured for gas exchange
3. Specialisation of dorsal/ventral gas exchange areas
4. 2 cycle (sarcopterygii) and 4-cycle (actinopterygii) breathing Lungs first appeared in the Silurian. Lungs conferred an advantage during the Devonian period which was thought to be prone to flooding. Lung specialisation acted to turn simple sac lungs and skin gas exchange (with large areas of dead space) into increasingly channelled air passages and decreased dead-space with a large surface area.
affected. At 1500m above sea level, climbers may experience impaired night vision. And 4500m, a feeling of unreality, dizziness and tingling may persist, and even higher is decreased cerebral function. Mount Everest stands at 8850m, and 175 people have died whilst climbing it (and 120 are still there). Sensing changes in Oxygen There are many receptors that act to detect ambient oxygen pressure: o arterial O2-sensing chemoreceptor in the carotid body o thalamus o hypothalamus o pons o medulla The activation of these sites will cause either an increase or a decrease in the respiratory efforts. Adaptations to hypoxic conditions Amazingly, some animals can adapt to life in hypoxic conditions. These will depend on the animal's aerobic needs, as not all oxygen is consumed by ATP-yeilding pathways. However, the will have to cope with reductions in metabolism, the prevention of cellular injury and the maintenance of functional integrity. At high altitudes, reactive oxygen and nitrogen species are formed which can cause oxidative damage to lipids, proteins and DNA.
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