Cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective functioning. Enzyme systems of mammals and birds are most efficient only within a narrow range around 37C;a departure of a few degrees from this value seriously impairs their functioning. Even though cells can survive wider fluctuations the integrated actions of bodily systems are impaired. Other animals have a wider tolerance for changes of bodily temperature.
For centuries it has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. Ways of characterizing the difference have become more accurate and meaningful over time, but popular terminology still reflects the old division into warm-blooded and cold-blooded species; warm-blooded included mammals and birds whereas all other creatures were considered cold- blooded.
As more species were studied, it became evident that this classification was inadequate. A fence lizard or a desert iguana-each cold-blooded----usually has a body temperature only a degree or two below that of humans and so is not cold. Therefore the next distinction was made between animals that maintain a constant body temperature, called home0therms, and those whose body temperature varies with their environments, called poikilotherms. But this classification also proved inadequate, because among mammals there are many that vary their body temperatures during hibernation.
第二篇:关于除夕英语作文
To tell you the truth,I dont like the Spring Festival at all. When I was young I liked it very much,because I could had something tasty to eat during the Spring Festival,and I could enjoy very wonderful TV programmes,too,I also could had much free time during which I could did something I liked to do. I dont like it now, there are many reasons.
Firstly,I cant have a good time during the Festival. Every New Years Eve something unpleasant often happens. Once I quarreled with my second brother.
Secondly,I cant relax myself during the Festival. I have to do many things,watch so many TV programmes, meet so many people,all these things make me very tired.
Thirdly,spending one Spring Festival means that I become one year older,I dont like to be old. I want to live longer and be young all the time. Readers, what is your opinions about the Spring Festival?Please tell us,we can share our opinions. Thanks for reading! Goodbye.