GENERAL ZOOLOGY 



concentration results in higher pressure which drives more oxygen through the 

 respiratory membrane into the blood. At the capillary level in the tissues, 

 hemoglobin gives up its oxygen and is known as reduced hemoglobin; this 

 gives the blood a purplish color. 



Gas exchange now occurs between the blood and the cells through the 

 endothelium of the capillaries. The freed oxygen in simple solution diffuses 

 through the lymph into the cells to take part in biological oxidation. The 

 movement of oxygen into the cell and carbon dioxide from it obeys the rules 

 governing gas exchange in the lungs. The pressure gradient for oxygen in 

 the tissues is from blood to cells; the gradient for carbon dioxide is from 

 cells to blood. 



About 92 per cent of the carbon dioxide is carried in the blood in chemical 

 combination, most of it as bicarbonate. The red blood cells contain an 

 enzyme which accelerates the combination of carbon dioxide and water to 

 form carbonic acid. Carbonic acid then dissociates, and about a third of the 

 bicarbonate ions form a salt with potassium in the red blood cell. The other 

 bicarbonate ions diffuse out of the red blood cell and form either a sodium 

 salt or more carbonic acid. In this way, the excreted carbon dioxide is 

 processed by the red blood cells and then transported to the lungs. There it 

 goes into solution and is eliminated by diffusion. 



Excretion 



Continuity of chemical reactions depends on the removal of their end 

 products. Consequently, the waste products of metabolic reactions, or excreta 

 of the body, must be continuously removed from cells in order that metabolic 

 reactions may continue; the waste products diffuse from the cells into the 

 blood stream. The process of removal of waste products of metabolism from 

 the body is called excretion; the important organs of removal are the lungs, 

 skin, excretory organs, and liver. It will be recalled that the waste products 

 of cellular metabolism are carbon dioxide, water, and nitrogenous compounds. 



The excretion of carbon dioxide in the lungs of terrestrial vertebrates has 

 already been discussed. In aquatic animals this waste gas diffuses into the 

 water surrounding the gills. Also, the relatively thin, moist skin of the 

 amphibia excretes considerable amounts of carbon dioxide. 



Water is excreted by a number of organs. Exhaled air is moist because the 

 lungs of air-breathing vertebrates excrete water. In the skin of many mam- 

 mals, including man, there are sweat glands from which watery solutions pass 

 to the outer surface of the body by way of ducts (Fig. 3.25). Sweating, how- 

 ever, is of less importance as an excretory process than it is in the regulation 

 of body temperature (p. 125). In the majority of vertebrates at least 50 per 

 cent of the water is excreted in the urine through the ducts of the kidneys. 



The kidneys in man account for only 0.4 per cent of the total body weight. 

 Yet they handle in 1 day about 180 liters of fluid containing in solution solids 



82 



