538 VERTEBRATE LIFE AND ORGANIZATION 



He.a.rt 



Vein. 



Lymph 



-A-rt ery 



Cell 



pilla.ry 

 'issue fluid 



Figure 27.1. The fundamental structure of the mammalian circulatory system. 

 Arrows indicate the direction of blood flow. 



through the capillary walls. The tissues are drained by the veins, which 

 return blood to the heart, and by a separate system of lymph capillaries. 

 Lymph capillaries lead to lymph vessels, which pass through lymph 

 nodes, and finally empty into the veins where the venous pressure is 

 lowest, a short distance from the heart. The lymph nodes are an important 

 link in the body's system of defense mechanisms. They produce one kind 

 of white blood cell (lymphocytes), and contain cells that engulf foreign 

 particles. 



228. Blood Plasma 



Blood is one of the tissues of the body. It consists of a liquid com- 

 ponent, the plasma, and several types of formed elements— red blood 

 cells, white blood cells and platelets (Fig. 3.14)— which flow along in it. 

 The plasma is a complex liquid that is in a dynamic equilibrium with 

 the tissue fluid and the fluid within the cells. It is constantly gaining 

 and losing substances, yet its composition is essentially constant. We have 

 seen, for example, how the liver maintains a constant glucose level in 

 the blood despite the heavy intake of glucose from the digestive tract 

 after a meal, and the constant release of glucose to the tissue fluid and 

 cells. Plasma is about 90 per cent water, 7 to 8 per cent soluble proteins, 

 1 per cent salts, and the remaining 1 to 2 per cent is made up of a variety 

 of small organic molecules— urea, amino acids, glucose, lipids, and hor- 

 mones. 



The chief plasma proteins are fibrinogen, albumins and globulins. 

 Other components of the plasma can pass through the semipermeable 

 capillary walls, but the proteins are rather large molecules and remain 



