Water 



Urea 



CO. 



Oxygen 



^jSjE^yWWWB 





Protein etc 



BETWEEN THE BLOOD AND THE LYMPH 



From the blood within the capillary, water, salts, food and oxygen pass out by os- 

 mosis. From the surrounding lymph, carbon dioxide, urea and water pass into the 

 blood. White corpuscles squeeze through the walls of the capillaries, between 

 the cells 



their carbon dioxide, urea, and other wastes. The lymph and the blood com- 

 municate by osmosis through the walls of the smallest blood vessels (see 

 illustration above), and by way of definite connections between lymph 

 tubes and certain large blood vessels. 



Like plasma, lymph consists chiefly of water and carries practically the 

 same kinds of substances in suspension and in solution, although in smaller 

 quantities. In addition, the lymph has floating in it many white corpuscles. 

 It thus resembles blood lacking red corpuscles. The lymph has been com- 

 pared in its composition to the ocean, in- which life may have originated, 

 and from which so many one-celled organisms obtain their supplies directly. 

 The lymph is an internal ocean from which all the cells of the many-celled 

 animal obtain their supplies. 



Clotting of Blood When blood gets out of the blood vessels, it usu- 

 ally coagulates, or becomes thickened. The clotting is itself a solidifying of 

 a certain protein in the plasma known as fibrinogeti — that is, "fibrin-maker". 

 The process is started by any injury to the lining of a blood-vessel or by 

 contact of the blood with a foreign substance. The platelets then break 

 down and discharge a special enzyme. This acts upon another substance in 

 the blood and produces the actual clotting agent, thrombin, which solidifies 

 the fibrinogen into fibrin. 



If we let blood clot in a glass vessel, we can see the mass of fibers detach 

 itself from the walls of the vessel, as the threads shrink and the clot floats at 

 last in a clear, almost colorless or slightly yellowish liquid, called a serum. 



187 



