RESPIRATION 



167 



4. The exchange of oxygen and carbon dioxide between the 

 lymph and the blood plasma. In the capillary regions of all 

 parts of the body except the lungs we have two fluids, the 

 lymph and the blood plasma, containing very different amounts 

 of oxygen and carbon dioxide and separated from each other 

 by the exceedingly thin membrane of the capillary wall. Under 

 such conditions both gases will tend to equalize, and each gas 

 will pass through the membrane from that liquid in which it 

 is more abundant to that 



in which it is less abun- 

 dant; that is to say, the 

 oxygen will pass from the 

 blood plasma in which it 

 abounds to the lymph in 

 which it is scarce ; and 

 the carbon dioxide, in the 

 other direction, from the 

 lymph to the blood plasma 

 (see Fig. 76). Hence the 

 blood enters the veins 

 richer in carbon dioxide 

 and poorer in oxygen than 

 it left the arteries. 



5. The red corpuscle as a carrier of oxygen. The blood 

 plasma under the conditions of temperature and pressure to 

 which it is exposed can hold only a small amount of oxygen, 

 too little to meet satisfactorily the demands of the resting 

 tissues and utterly inadequate for the much greater needs 

 of the working tissues. This difficulty is met and the oxygen- 

 carrying capacity of the blood vastly increased by the peculiar 

 properties of the coloring matter, or pigment, of the red cor- 

 puscles. This substance, known as hemoglobin, readily forms 

 with oxygen a compound (oxyhemoglobiri) whenever the amount 

 of oxygen is high in the medium surrounding it ; if, however, 

 much oxygen is removed from its surrounding medium, the 



FIG. 76. The exchange of oxygen and 



carbon dioxide between the blood and 



the lymph in the tissues 



x 



