Jinny123看过来，本月阅读寂静第10篇无脊椎动物呼吸的一些背景知识

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Frogs have a three-chambered heart, consisting of two atria and a singleventricle. Blood leaving the ventricle passes into a forked aorta, where theblood has an equal opportunity to travel through a circuit of vessels leadingto the lungs or a circuit leading to the other organs. Blood returning to theheart from the lungs passes into one atrium, while blood returning from therest of the body passes into the other. Both atria empty into the singleventricle. While this makes sure that some blood always passes to the lungs andthen back to the heart, the mixing ofoxygenated and deoxygenated blood in the single ventricle means the organs arenot getting blood saturated with oxygen. Still, for a cold-blooded creaturelike the frog, the system works well. Humans and all other mammals, as well asbirds, have a four-chambered heart with two atria and two ventricles.Deoxygenated and oxygenated blood are not mixed. The four chambers ensureefficient and rapid movement of highly oxygenated blood to the organs of thebody. This has helped in thermal regulation and in rapid, sustained musclemovements.

The main disadvantage is that theamphibians' oxygen-rich blood is mixed with the blood from the tissues. Inthe heart of mammals the blood from the lungs is pumped to the rest of the bodyand the blood coming back from the tissues is pumped to the lungs.

Reptiles and Amphibians

Reptiles and amphibians both have lungs and exchangegases in the capillaries like mammals, but there are some differences in howthey ventilate their respiratory systems. Reptiles do not typically breathe thesame way as mammals since many reptiles lack a diaphragm. Without it they relyon muscles used in locomotion to ventilate their lungs.

Amphibians are capable of buccalpumping to push air into the lungs. This begins by muscles pulling airthrough the mouth or nose into a buccal cavity. Throat muscles then pump andmove the floor of the mouth up in a way that Is visible from the outside. Thisforces air out of the mouth and into the lungs. Look at this frog's throat constantly moving.

Apart from their capillaries, amphibians can also perform gas exchange directlythrough their highly vascularized skin. This means that their skin has lots ofblood vessels going through it. Since the blood vessels are close to theirpermeable skin surface, diffusion can take place right through the skin. Infact, some salamanders have no lungs at all, and they get all of their oxygenthrough their skin. The take home message is never get in a breath holdingcontest with a salamander. We wouldn't recommend a staring contest, either.

In the adultstage, amphibians (especially frogs) lose their gills 鱼鳃and develop lungs. They have a heart that consists ofa single ventricle心室and two atria心房. When the ventricle starts contracting, deoxygenated blood is pumpedthrough the pulmonary 肺部的artery 动脉to the lungs.Continued contraction then pumps oxygenated blood around the rest of the body.Mixing of the two bloodstreams is minimized by the anatomy of the chambers

Breathing in amphibians

The lungs in amphibians are primitive compared to that of the amniotes.They possess few internal septa and large alveoli, giving the lung a honey-combappearance. There is a slow diffusion rate of oxygen into the blood.Ventilation is accomplished by buccal pumping. However, most amphibians areable to exchange gasses with the water or air via their skin. To enablesufficient cutaneous respiration, the surface oftheir highly vascularized skin must remain moist in order for the oxygen todiffuse at a sufficient rate. Because oxygen concentration in the waterincreases at both low temperatures and high flow rates, aquatic amphibians in thesesituations can rely primarily on cutaneous respiration, as in the Titicacawater frog and hellbender salamanders (Figure 2). In air, where oxygen is moreconcentrated, some small species can rely solely on cutaneousgas exchange, most famously the plethodontid salamanders which have neither lungs nor gills. Many aquatic salamanders and all tadpoles havegills in their larval stage, with some (such as the axolotl) retaining gills asaquatic adults.

http://library.thinkquest.org/C003758/Development/amphibian.htm

ranmelanie

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