534 



COLLEGE ZOOLOGY 



RESPIRATORY SYSTEMS 

 AND RESPIRATION 



Respiration is a process made necessary 

 because uninterrupted liberation of energy 

 is indispensable for the continued existence 

 of the cells of the body. Usually animals 

 obtain their energy from the burning of 

 foods. Actual use of oxygen by the cells is 

 called internal (cellular or biologic) respira- 

 tion, as distinguished from external respira- 

 tion, which involves the entrance of oxygen 

 into the lungs and exit of carbon dioxide 

 from them, and the transport of oxygen 

 from the lungs to the cells and of carbon 

 dioxide from the cells to the lungs. Internal 

 respiration is the same for most cells, but 

 external respiration differs among the var- 

 ious groups of animals. 



Respiration in the Protozoa consists prin- 

 cipally in the diffusion of oxygen, dissolved 

 in water, through the surface protoplasm 

 into the body, and of carbon dioxide 

 through the surface protoplasm out of the 

 body. The body surface is very permeable 

 to oxygen and carbon dioxide; oxygen is 

 in higher concentration in the water than in 

 the protoplasm, and carbon dioxide is in 

 higher concentration in the protoplasm than 

 in the water, hence the diffusion of oxygen 

 into the cell and of carbon dioxide out of 

 the cell. In sponges, coelenterates, and many 

 other aquatic animals, respiration occurs di- 

 rectly between the cells in the body wall and 

 the water that bathes them. 



Earthworms and other similar animals do 

 not possess respiratory systems; they take in 

 oxygen and give off carbon dioxide through 

 the moist skin. Oxygen from the air diffuses 

 into the cells in the skin and from these cells 

 into the blood in the capillaries which lie in 

 large numbers just beneath the surface layer 

 of cells. The oxygen is transported by the 

 blood to the various organs, and diffuses from 

 Ihe blood into intercellular spaces, and then 

 into the cells. Carbon dioxide diffuses out 

 of the cells into the blood, which carries it 



to the skin, where it diffuses from the blood 

 into the cells of the skin, and out through 

 the moist surface. 



Insects and certain other types of arthro- 

 pods possess respiratory systems consisting 

 of networks of tubes, the tracheae, that 

 carry air into and out of the body through a 

 number of pairs of openings (spiracles) in 

 the sides of the body segments. From the 

 finest tracheae (tracheoles), gases diffuse 

 into and out of the tissue cells. In these 

 animals the circulatory system is of little 

 importance for purposes of transporting 

 gases. 



In many aquatic animals with circulatory 

 systems, respiration takes place with the aid 

 of gills. For example, the crayfish possesses 

 two rows of gills contained in branchial 

 chambers, one row on each side of the 

 thorax. Water flows into the posterior open- 

 ing and out through the anterior opening 

 of the branchial chamber; and oxygen dif- 

 fuses from this water into and through the 

 cells of the gill filaments, into the blood in 

 the efferent gill channels. Carbon dioxide 

 diffuses out of the blood in the afferent gill 

 channels into the gill filaments and from 

 there out of the body. Respiration is carried 

 on in a similar fashion by fish. Certain so- 

 called lungfishes take in oxygen and give off 

 carbon dioxide by means of lungs as well as 

 gills. 



The amphibians such as the frog carry 

 on respiration through the moist surface of 

 the body, both while in the water and in 

 the air, and also by means of lungs. Other 

 groups of higher vertebrates respire with the 

 aid of lungs. The lungs of the frog are 

 simple, whereas those of the higher verte- 

 brates are more complex. The essential fea- 

 ture of the lungs is the moist epithelial lin- 

 ing with oxygen-containing air in the cavities 

 on one side, and blood in the capillaries on 

 the other side. Oxygen from the lung cavi- 

 ties diffuses into the epithelial cells, and 

 thence into the blood; and carbon dioxide 

 diffuses from the blood into the cells, and 

 thence into the lung cavities. 



