METABOLISM AND TRANSPORT IN ANIMALS 



529 



Tissue fluid and lymph 



Materials do not pass directly to the body 

 cells from the capillaries or from the body 

 cells to the capillaries, but they are depend- 

 ent for this service on tissue fluid (Fig. 

 383), which bathes the cells, and by means 



of diffusion and osmosis brings about the 

 exchange between blood and cells. Plasma 

 filters out through the walls of the capil- 

 laries; it contains leukocytes but usually no 

 erythrocytes. In the tissue spaces are minute 

 tubes, the lymphatics; these are part of a 



Leukocyte 



Leukocyte— I 



Wall of capillary 

 Erythrocyte 



Tissue space 



filled with 



tissue fluid 



Blood plasma 



Figure 383. Drawing showing how tissue fluid serves as a medium of exchange between the 

 blood vessels and body cells. Note diapedesis of a leukocyte through the wall of a capillary. 



closed system of tubes (Fig. 386) which fi- 

 nally ends in two large tubes, the right 

 lymphatic duct and the left thoracic duct; 

 these return the lymph, which is tissue fluid 

 that has entered the lymphatic vessels, to the 

 blood by emptying it into veins in the neck 

 region. The lymphatic vessels of the intes- 

 tine are called lacteals (Fig. 380); these ab- 

 sorb fattv substances. Some tissue fluid is 

 returned to the blood stream by reentering 

 the capillaries. 



Spleen 



The spleen is a bean-shaped lymphatic 

 organ lying immediately below the dia- 

 phragm on the left side. It is not essential to 

 life; the body withstands its surgical re- 

 moval successfully. 



It serves as a reservoir for blood and can 

 hold from 1/5 to Vi of all the blood in the 



body. By its contractions and expansions it 

 can regulate the volume of circulating blood 

 to changing needs. In the spleen, also, old 

 red blood corpuscles are destroyed, and 

 white blood cells (lymphocytes) and mono- 

 cytes are produced. 



Circulation in man 

 and other mammals 



The mammalian heart (Fig. 384) consists 

 of 4 chambers, 2 thin-walled atria and 2 

 muscular ventricles. The pulmonary veins 

 bring blood from the lungs to the left 

 atrium; and other veins bring blood from 

 the rest of the body to the right atrium. 

 Blood from the atria flows into the ventricles 

 beneath them, aided by the contraction of 

 the atrial walls. When the ventricles are 

 filled, they begin to contract, and the blood 



