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Perhapsbecause people breathe almost exclusively with their lungs, respiration is often thought of as takingplace only in specialized organs: if not in the lungs, then in the gills of fishesand crustaceans, the tracheae of insects or the book lungs of spiders. Yetwhenever a relatively thin membrane separates the respiratory medium (the airor water an animal breathes) from living cells or flowing blood, oxygen canenter the cells and the blood and carbon dioxide can leave them. The recentfindings we shall discuss here suggest that the skin serves as an effective andhighly regulated organ of gas exchange in many vertebrates.
Numerous experimental studies in the 1960’s and1970’s examined the partitioning of gas exchange among an animal’s respiratoryorgans: the skin, lungs and gills, if present. For example, by placing plasticmasks over the faces of salamanders, Victor H. Hutchison and his students atthe University of Rhode Island were able to determine what proportion of the animal’stotal oxygen is taken in through the skin and what proportion of the totalcarbon dioxide is eliminated through it. The results of these and many othersuch experiments provide a surprisingly long list of vertebrate skin breathers.
Probably the best-known of these skin breathers arethe amphibians.
For example, some male amphibians engage inelaborate and strenuous courtship rituals that may include repeated bodymovements lasting for hours. Associated with this behavior is a greatlyincreased requirement for oxygen uptake and carbon dioxide elimination.Apparently in response to the increased respiratory burden, parts of the skinin these amphibians gradually become enlarged or develop out growths during thecourtship season. Such surfaces act as accessory gas-exchange organs.
A second adaptation enhancing the capability forcutaneous gas exchange is a reduction in the thickness of the skin to lessenits resistance to the diffusion of respiratory gases. In addition to beingdevoid of such physical barriers as hair, feathers or scales, amphibian skin istypically only between 10 and 50 micrometers (millionths of a meter) thick.Actually the significant morphological factor governing diffusion in this connectionis not the total thickness of the skin but rather the thickness of the“diffusion barrier,” the distance between the respiratory medium outside theskin and the blood flowing through the cutaneous capillaries. Hence anymorphological change that reduces the distance between the bloodstream and therespiratory medium helps to increase gas exchange.
We demonstrated through a simple experiment thatsuch changes are indeed possible within a fraction of an amphibian’s lifetime.Frog larvae were reared in two enclosures, one containing well-aerated waterand the other containing water withapproximately half the oxygen concentration of the aerated water. After fourweeks the diffusion barrier in larvae reared in the well-aerated water was 40micrometers, a typical value. Incontrast, the diffusion barrier averaged only 20 micrometers in animals rearedin the oxygen-poor water. Furthermore, the cutaneous capillary network of theoxygen-deprived larvae was finer and denser. It appears these larvae underwentan acclimatory change that augmented their capacity for cutaneous gas exchange.
Even vertebrates with arelatively thick skin can carry on significant cutaneous gas exchange as long asevolution has led to advantageous placement of the cutaneous capillaries. In thescaly skin of some lizards, for example, the cutaneous capillaries eitherunderlie the skin between the scales or run under the scale’s hinges, where thescale is thinnest. In some snakes the cutaneous capillaries penetrate the scaleitself The dermal scales of fishes are generally covered by a layer of living tissue,an arrangement that pIaces cutaneous capillaries above the diffusion-resistantscale and very close to the respiratory medium.
The partial pressures of oxygen and carbon dioxidein blood within cutaneous capillaries vary according to whether or not theblood is oxygenated. This provides another possible regulatory device. Bycontrolling whether the blood flowing to the skin is primarily oxygenated ordeoxygenated, an organism could presumably regulate the amount of oxygen andcarbon dioxide diffusing through the skin. Cutaneous gas exchange would be mosteffective if only deoxygenated blood flowed to the skin, in the same way thatonly deoxygenated blood flows to the lungs in mammals. The partial pressuredifferentials across the skin of both oxygen and carbon dioxide would belargest in this circumstance. The skin of most vertebrates, however, is nodifferent from any other living tissue: it receives blood only from the majorsystemic arteries, which typically deliver oxygenated blood. Because the differencebetween the oxygen partial pressure of oxygenated blood and that of therespiratory medium is often rather small, oxygen uptake from the environment isnormally limited by the very blood the skin cells need to live. Even under thisseemingly major constraint the skin may nonetheless be important in carbon dioxideelimination because carbon dioxide levels in arterial blood may still beconsiderably greater than they are in the environment. This explains in partwhy carbon dioxide elimination typically exceeds oxygen consumption incutaneous gas ex-change among vertebrates.
Some vertebrates are indeed able to direct afraction of their deoxygenated blood into the systemic arteries and therebyaugment cutaneous gas exchange. Amphibians and reptiles, for example, have anincompletely divided heart that allows deoxygenated blood to flow to the skinwithout first traversing the lungs. Comparative anatomists have traditionallyregarded this arrangement as a primitive and inefficient one. Kjell Johansen ofthe University of Aarhus in Denmark, Fred White of the Scripps Institution of Oceanographyand others have suggested that the opposite is true. They argue that the heartstructure of amphibians and reptiles is actually an important adaptation thatallows these vertebrates to distribute blood where it would best promotegas exchange.
P1：科學家們一直認為皮膚過厚不能呼吸，認為很難通過肺之外的器官呼吸，但是實際上已經有動物做到皮膚呼吸了，而且有兩種方法可以讓這種機制得以實現。
P2：用某種動物做實驗（疑似青蛙），第一個環境是氧氣充足的，第二個環境中的氧氣是另一個環境中的一半，試驗發現，經過一段時間後，在第二個環境中飼養的青蛙的表皮變得比第一個環境中的薄了２０％thickness，因為更薄的表皮有利於用空氣和二氧化碳的交換（即呼吸）。
P3：舉了了一個爬行動物（或者兩棲動物）的例子（疑似為第二種不用肺的呼吸方式：皮膚跟心肺結合起來的調節呼吸），這種動物的心臟是不分左右心房的incompleted dividend heart，這被認為是primitive原始的低等的落後的。一般而言，皮膚只接受deoxegened血液，這種血呼吸效率高，但是一般的動物這類血液比較缺乏，按道理說皮膚就不會參與呼吸了，然而卻不。這是因為，這種被認為原始的發育不全心臟可以把本來要流經動脈的血直接運到表皮附近，這些deoxegened的血會使表皮的呼吸效率更高，因此並非如人們所說這種心臟是低等的。
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发表于 2014-7-12 00:29:24
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