334 PRESERVATION OF NEUTRALITY 



II. The lungs eliminate CO 2 . The amount eliminated per unit 

 of time is a function of the capacity of the lungs and the rhythm 

 of respiration. The rate and depth of respiration are controlled 

 by the amount of CO 2 in the blood perfusing the respiratory 

 centre in the medulla oblongata. Any increase in the CO 2 of the 

 blood causes an increase in the rate of respiration. Similarly, the 

 process of respiration may be slowed down, till it stops, by de- 

 creasing the amount of CO 2 in the blood. It has been stated that 

 this regulatory action of the medulla is caused not by CO 2 but by 

 the hydrogen ion concentration of the blood, i.e. any acid perfusate 

 will quicken respiration. But, as is obvious from the context, 

 no free acid but CO 2 can occur in the blood of a living animal. 

 Further, careful research has shown that the pH. of blood does 

 not alter, the regulation is so nice. 



III. Excess of base, and acids in combination with bases are 

 eliminated by the kidney. The cells of this organ have a low 

 threshold for such salts. 



IV. The red corpuscles of the blood play an ill-defined part 

 in the preservation of neutrality. In common with other tissue 

 elements, the erthrocytes have a phosphate buffer system. Sodium 

 dihydrogen phosphate, NaH 2 PO 4 , is an acid salt and reacts with 

 bicarbonate, for instance, to form the basic salt Na 2 HPO 4 , 



viz. : NaH 2 PO 4 + Na 2 HCO 3 -Na 2 HPO 4 + H 2 O + CO 2 



acid phosphate. basic phosphate. 



A mixture of these two phosphates such as is found in all tissues 

 obviously will not increase in acidity till nearly all the disodium 

 phosphate has been converted into dihydrogen phosphate, nor 

 will the [H]* markedly decrease till all the dihydrogen phosphate 

 has been converted into the basic salt. 



In addition to this it seems as if the pigment, haemoglobin, took 

 some part in the reaction. Some evidence has been produced 

 which indicates that Hb adsorbs or combines with CO 2 at high 

 CO 2 tensions and parts with it at low tensions, i.e. in the lungs. 

 Be that as it may, the haemoglobin acts as a buffer to a degree of 

 which the plasma proteins are incapable, and promotes the dis- 

 sociation of bicarbonate during diminishing tensions of CO 2 . 

 This property is due to the electrical charge of haemoglobin. 

 In an alkaline medium, haemoglobin carries a positive charge ; 

 the more alkaline the medium, the more acidic is haemoglobin, 

 and conversely, when the blood is taking up CO 2 from the tissues, 

 haemoglobin is least acidic. 



