SOURCE OF TISSUE FLUID 



165 



hour or less, the heart, by successive pulsations, forces into the 

 aorta more blood than that found in the entire body. Harvey's 

 discovery of the circulation of the blood opened up the field of 

 physiology for later investigators. 



Source of tissue fluid. The liquid from the blood plasma is 

 continuously being diffused through the walls of the blood vessels, 

 into the tissue spaces where it is known as tissue fluid. This fluid 

 differs from blood in that it lacks red corpuscles and may have a 

 higher content of waste materials due to 

 the collection of wastes from the cells. 

 White corpuscles can make their way 

 through the cells in the w^alls of the capil- 

 laries and escape into the tissues; there- 

 fore, they may be found in tissue fluid. 



Lymph vessels. Special vessels, similar 

 to capillaries and veins, drain the excess 

 fluid as it collects in the tissues, and return 

 it to the blood. These are called lymph 

 vessels or lymphatics and the fluid in them 

 is now called lymph. The fluid that col- 

 lects in a blister is lymph or tissue fluid. 

 Even the smallest lymphatics are closed off 

 from the tissue spaces by very thin mem- 

 branes. These minute vessels lead to ves- 

 sels of increasing size until they finally 

 unite into two very large ones above the 

 heart. The one on the left, called the 

 thoracic duct, carries the lymph flow from 

 the left side of the head and chest, also 

 from the left arm, abdomen, and the two 

 legs. The one on the right drains the lymph from the right 

 side of the head and chest, and the right arm. They both lead 

 into the venous system from an outlet in the superior vena cava 



Lymph vessels, known as 

 lymphatics, drain the fluid 

 from all the tissues of the 

 body. Ultimately, they lead 

 into a vein of the blood system. 



