556 VERTEBRATE LIFE AND ORGANIZATION 



Tissue fluid \ .-— -5rrr~->.^^'^ Capillary 



Lymph 

 capillary — ■^ 



Erythrocyte 

 Cell 



eus oF 

 (Z.ndotheIial cell 



Arteriole 



Venule 



eukocyte 



— Bacterium. 

 (bein6 ingested by leuKocyte) 



Figure 27.8. Exchange of materials in a capillary bed. Solutes enter and leave 

 all parts of a capillary. Most of the water leaves at the arterial end and reenters at 

 the venous end. Less than one per cent of the water that leaves the capillaries is 

 returned by the lymphatic system. 



back in. The osmotic pressure remains constant from the arterial to the 

 venous ends of the capillary bed, but blood pressure continues to de- 

 crease. At the arterial end of the capillary bed, blood pressure is greater 

 than osmotic pressure and water is driven out of the capillaries. At the 

 venous end, osmotic pressure is greater than blood pressure, and water 

 is drawn back into the capillaries. Any residual liquid is drained by the 

 lymphatics. 



Venous and Lymphatic Return. The structure of the veins is funda- 

 mentally the same as that of arteries, though a vein is larger and has a 

 much thinner and more flaccid wall than its companion artery. Since 

 they are larger, the veins hold more blood than the arteries, and are an 

 important reservoir for blood. Lymphatic vessels have even thinner walls. 

 Valves present in both veins and lymphatics permit the blood and lymph 

 to flow only toward the heart. It is sometimes easy to demonstrate the 

 valves in the veins on the back of your hand. Push your finger on a vein 

 at the point where several join on the back of your wrist and move your 

 finger distally along the vessel. This will force the blood out of the vein, 

 and you will notice that blood does not reenter this vein from the others 

 at the wrist for valves prevent it from doing so. Remove your finger from 

 the vein and it immediately fills with blood from the periphery. 



Though blood pressure is low in the veins, and lowest in the large 

 veins near the heart, it is still the major factor in the return of blood. 

 Two other factors assist it. One is the fact that the elastic lungs are 

 always stretched to some extent and tend to contract and pull away from 

 the walls of the pleural cavities. This creates a slight subatmospheric or 

 negative pressure within the thoracic cavity, which is greatest during 

 inspiration. The larger veins, of course, pass through the thorax, and 

 the reduction of pressure around them decreases the pressure within 



