The hemoglobin of red-blooded animals carries oxygen; human blood 

 carries 60 times as much oxygen as dissolves in an equal volume of water. 



The oxygen-carrying substance in animals with blue blood is hemo- 

 cyanin. 



In all animals with blood, external respiration is the gas exchange be- 

 tween the blood and the outside; internal respiration is the gas exchange 

 between the blood and the living cells. 



Insects breathe by means of tracheae, or air-tubes, which open to the sur- 

 face and reach the fluids in all parts of the body. Body movements compress 

 and release these tubes, setting up air movements. 



In some animals respiration takes place by osmosis through a moist skin 

 or through gills, which are specialized skin outgrowths within which blood 

 circulates and around which the oxygen supply moves. 



Other animals breathe by means of lungs. Fresh air is brought into the 

 lungs, and stale air is exhaled, by muscular movements. Dissolved gases 

 pass into and out of the blood by osmosis through living membranes of the 

 lungs and through walls of blood vessels. 



EXPLORATIONS AND PROJECTS 



1 To find the relation of air to plants and animals living in water, place 

 small fish from the aquarium in two vessels, one containing ordinary tap water 

 and the other tap water which has been cooled in a closed flask after the air has 

 been removed by boiling. Compare results and note conclusions. 



2 To show that dissolved gases diffuse through a membrane, prepare two 

 8-ounce widemouthed bottles as model cells (see page 88), one containing plain 

 water, and the other water through which carbon dioxide has been bubbled for 

 about fifteen minutes. Invert the two bottles, after the membranes have been 

 securely fastened, in two dishes containing water. On the following day add a 

 few drops of pinJ{^ phenolphthalein solution to each vessel. If the indicator loses its 

 color, the water has become acid from carbon dioxide dissolved in it.^ Compare 

 results and note conclusions. 



3 To find out whether oxidation is accompanied by a loss in weight, compare 

 the dry weight of equal quantities of corn or wheat grains before and after 

 germination. Account for the results. 



4 To study the structure of the respiratory tract, obtain a haslet from the 

 butcher. Blow air into the trachea and note the expansion of the lung tissue. 

 Compress the trachea and bronchial tubes. What holds them so rigid? Open one 

 side of the trachea and of the main branch of the bronchial tube within one of the 

 lungs, to show the many little openings through which small tubes carry air to 

 and from the larger tubes. 



^Red cabbage extract (see page 199) can also be used as an indicator. 



212 



