140 GENERAL ZOOLOGY 



completely and suddenly, all of the oxygen being given off. The 

 pressure of oxygen in saturated arterial blood is equivalent to the 

 pressure of 75 mm. of mercury, and since the oxygen tension 

 of the tissues is zero, oxygen is readily given off to the tissues as 

 the blood circulates through the systemic capillaries. Returning 

 to the lungs with its oxygen tension reduced to 37.6 mm., the 

 blood, in passing through the pulmonary capillaries, readily 

 absorbs oxygen from the alveolar air, which has an oxygen 

 tension of about 100 mm. 



As a result of these properties the blood can absorb oxygen 

 from the lungs and give it off to the tissues with great rapidity. 

 At the same time, the absorption of oxygen is independent of 

 small variations in atmospheric pressure. 



The blood collects waste products of metabolism from the 

 tissues and these leave the body through an excretory organ, such 

 as the kidney, through skin as sweat (in mammals), and also 

 through respiratory surfaces of the body and through the 

 alimentary canal. 



Carbon dioxide is an important gaseous waste product of cells. 

 It is about 20 times as soluble in blood plasma as in oxygen. In 

 the blood, part of it is held in solution in the plasma and the 

 remainder in two forms of chemical combination: (1) in the form 

 of sodium carbonate, and (2) in combination with blood proteins, 

 including hemoglobin, forming a compound similar to oxyhemo- 

 globin in the readiness with which it dissociates. The tension of 

 carbon dioxide in the arterial blood entering the capillaries is 

 about 35 mm., while that of the lymph and tissues is about 50 to 

 70 mm., so that there is a ready movement of carbon dioxide 

 from the tissues to the blood. The carbon dioxide tension 

 of the blood when it reaches the lungs is 42.6 mm. while that of 

 alveolar air is 35 mm., as a result of which carbon dioxide leaves 

 the blood. 



The remaining excretory products are mostly in the form of 

 solids in solution. In the dissolved state these substances are 

 carried from the tissues by the blood to various excretory sur- 

 faces, the details of which will be discussed in connection with 

 the general function of excretion. 



Temperature Regulation. — About 90 per cent of the plasma of 

 vertebrate blood is water. Some of the water that is excreted is 

 water absorbed as such and some of it is metabolic water formed 



