298 ANIMAL BIOCHEMISTRY 



the complete saturation of hemoglobin as blood passes through the 

 capillaries. At the same time carbon dioxide, which was transported 

 from the cells to the lungs in the venous blood, largely as sodium 

 bicarbonate, diffuses into the lungs. The dissociation of the bicarbon- 

 ate ion is aided by the slight change in the amphoteric properties of 

 oxyhemoglobin. Oxyhemoglobin is slightly more acid than hemo- 

 globin, so that the bicarbonate ion dissociates into water and carbon 

 dioxide: 



Hb-02 + H3O+ + HCO3- + Na+ -^ Hb-Oo + Na+ + ZHoO + CO2 



There is no net change in the pH of blood as a result of this exchange. 



In tissues of the body the reverse process occurs. Oxyhemoglobin, 

 in a region of low oxygen tension, dissociates and oxygen diffuses 

 across the capillary membrane to the surrounding tissue. Carbon 

 dioxide of the tissue diffuses into the blood and is transported as the 

 bicarbonate ion. 



Nitrogen and the inert gases of air are also absorbed to a small 

 extent. The degree of absorption is limited by their solubility in 

 plasma. There is no change in nitrogen tension on passage through 

 the arterial and venous network, indicating no utilization, whereas 

 oxygen tension drops from 100 to 38 ml. of mercury and carbon 

 dioxide rises from 35 to 43 ml. of mercury. 



Lymph 



In addition to the blood circulatory network there is another 

 system, less well defined in distribution, known as the lymphatic 

 system. This network contains a creamy, colorless liquid called lymph, 

 through which a constant exchange between the blood antl the tissue 

 cells is maintained. Chemically, it is very similar to blood plasma. 

 The suggestion has been made that it probably arises as a residt of 

 seepage of blood plasma through the capillary walls. Experiments 

 using P'^i -labeled serum albumin do not support the above suggestion, 

 so the exact origin of lymph tissue remains in doubt. 



Lymph spaces of all tissues are connected to lymph channels which 

 carry the fluid to ducts. Lymph from the body tissues enters the 

 blood through the thoracic duct, and that from the head enters 

 through a duct in the neck region. 



As stated above, lymph functions in the transport of nutrients and 

 oxygen from the capillaries to the neighboring cells, again through 

 diffusion processes. In addition, lymph functions in fat absorption 

 (see also page 438). Fat is absorbed from the intestinal villi, via 

 the lacteals, and passes by way of the lymphatic system to the blood 



