318 THE CHEMISTRY OF ANIMAL PHYSIOLOGY 



The carbon dioxide in the veins is under greater pressure 

 than the carbon dioxide in the inspired air. Hence the 

 plasma loses carbon dioxide and sodium bicarbonate breaks 

 up. The partial pressure of the oxygen dissolved in the 

 plasma is less than that of the oxygen in the lungs. Hence 

 more oxygen is dissolved by the plasma and consequently 

 a combination of oxygen with haemoglobin takes place. 



Chemically one volume of oxygen produces one volume of 

 carbon dioxide, but in the case of animal respiration the 

 amount of carbon dioxide evolved is normally less than the 

 amount of oxygen absorbed. This is because some oxygen 

 is used to oxidize the hydrogen of fat to water and to form 

 waste products from proteins like urea. Therefore it does 

 not appear as carbon dioxide. The ratio of carbon dioxide 

 to oxygen is called the respiratory quotient. 



232. Lymph. — The tissues of the body are all bathed in a 

 liquid called lymph, or tissue fluid, which serves to bring 

 nutrient material in direct contact with the tissue cells and 

 to carry waste products away from the tissue cells. The 

 various tissues are all in a constant state of building up and 

 tearing down. Changes are constantly taking place. New 

 cells are forming and old ones wearing out. The blood 

 serves as the fluid which carries nutrient material from 

 one part of the body to another and transports waste 

 products of metabolism for elimination. It is carried within 

 the walls of the blood-vessels — veins, arteries, and capillaries. 

 Water, soluble compounds, and leucocytes can pass through 

 the walls of the capillaries, and this fluid, which is practically 

 blood plasma, constitutes the lymph. 



Not only are the tissues bathed in lymph but the tissue 

 spaces unite to form lymph vessels which are provided with 

 valves at frequent intervals to prevent the fluid from flowing 

 backward. These vessels permeate the body in every direc- 

 tion in a network, and combine sooner or later into the 

 thoracic duct (Fig. 82, th.d.), a large lymph vessel running 

 through the left side of the centre of the body and emptying 

 into the venous system at the left side of the base of the 

 neck. At intervals along the lymph vessels are enlargements 

 called lymph glands which serve among other things as a 

 principal source of white corpuscles. 



