Cutaneous respiration

Although lungs and gills are the primary respiratory organ ¸ the skin can supplement breathing .

Respiration through the skin , referred to as cutaneous respiration , can take place in air , in water, or in both. So many vertebrates are involved in cutaneous respiration .

( Fig – 1 Cutaneous respiration among vertebrates )

• In the European eel and plaice , oxygen uptake through the skin may account for up to 30% of total gas exchange .
• Bats take advantage of cutaneous respiration across their well – vascularized wing membrane to eliminate as much as 12% of their total carbon dioxide waste , but they take up only 1% or 2% of their total oxygen requirement through this cutaneous route ( Fig – 1 ) .
• Sea snakes can supplement up to 30% of their oxygen intake via cutaneous respiration across the skin on their sides and back .
• In fact , in salamanders of the family plethodontidae , adults lack lungs and gills and depend entirely on cutaneous respiration to need their metabolic needs .
• In modern amphibians , the skin is a major respiratory organ , The skin is   moist and the layer of keratin relatively thin , allowing easy diffusion of gases between the environment and the rich supply of capillaries within the integument (Fig – 2 (b) (c) .

( Fig – 2 Adaptation for cutaneous respiration .  Many vertebrates exhibit complex or elaborate specializations that enhance the efficiency of gas exchange through the skin . (a) While still small , this fish larva , Monopterus albus  , occupies the thin layer of water adjacent  to the surface where oxygen levels are relatively high . Its pectoral fins beat , forcing water to flow across its body surface . Blood circulating  through the skin flows in the opposite direction from the water , establishing a countercurrent  exchange between blood and water. (b) In the lake Titicaca frog , Telmotobius  culeus , prominent loose skinfolds on its back and limbs provide extensive surface area for cutaneous respiration . (c) In the male hairy frog , Astylosternus robustus , numerous papillae appear during the breeding season , forming a ruffled supplementary respiratory organ on its sides and hindlimbs .

The newly hatched larva of the teleost fish Monopterus albus , an inhabitant of southeast Asia , uses predominantly cutaneous respiration during its early life . At hatching , the large and heavily vascularized pectoral fins beat in such a fashion as to drive a stream of water backward across the surface of the larva and its yolk sac . Blood in superficial skin vessels courses forward . This establishes a countercurrent exchange between water and blood to increase the efficiency of cutaneous respiration of this larva ( Fig – 2 (a) .