140 NUTRITION OF FARM ANIMALS 



with the alkalies. As in the case of oxygen, the amount of 

 carbon dioxid contained in the blood depends upon the partial 

 pressure of this gas in the surrounding medium. Since the ten- 

 sion of the carbon dioxid in the alveolar air is less than that in 

 the blood of the alveolar capillaries, the carbon dioxid passes 

 from the latter to the former. If the air were stationary the 

 process would continue until an equilibrium was reached. 

 Since the air is being continually renewed by breathing, the 

 tension of carbon dioxid in it is kept permanently lower than 

 that in the blood and there is, therefore, a continual passage of 

 carbon dioxid from the blood to the alveolar air. 



It is by means of this tendency to equilibrium that the mech- 

 anism for the regulation of breathing is set in motion. In- 

 creased tissue respiration discharges more carbon dioxid into the 

 blood, where its tension increases. This causes a more rapid 

 diffusion of the gas into the alveolar air and tends to raise its 

 carbon dioxid tension also, so that with an unchanged rate of 

 lung ventilation the carbon dioxid level of both the alveolar 

 air and the blood would be raised. Even a very slight rise in 

 the carbon dioxid tension in the blood, however, as already 

 stated, acts promptly upon the respiratory center and stimu- 

 lates the muscles of breathing, resulting in an increased lung 

 ventilation and consequently a more rapid excretion. At the 

 same -time the rapidity of circulation is increased and in these 

 two ways the level of carbon dioxid tension in the blood and 

 in the alveolar air is maintained very constant. On the other 

 hand, if the lung ventilation be artificially increased, as by 

 artificial respiration or by the use of oxygen, the carbon dioxid 

 excretion may be so facilitated that the amount in the blood 

 falls below the normal and the movements of breathing may be 

 temporarily suspended (apncea). 



198. Excretion of nitrogenous products. The urea and 

 other nitrogenous products of cell action, like the non-nitrog- 

 enous products, pass ultimately into the blood. In its course 

 through the body the blood passes through a capillary system 

 in two bean-shaped organs, the kidneys, indicated by t in Fig. 

 1 8, situated in the abdominal cavity on either side of the spine 

 near the loins. In these organs the urine is being continually 

 secreted, passing thence through the ureters into the bladder 

 from whence it is voided at intervals. 



