Biology of Man 483 



along the lateral surface of the arm and empties into the axillary; the 

 basilic runs along the inner surface and empties into the axillary; the 

 median runs between the other two and connects them. 



The inferior vena cava is formed by the union of the right and left 

 common iliac veins (at about the level of the fifth lumbar vertebra) and 

 accompanies the aorta along the posterior abdominal and thoracic wall, 

 receiving such veins as the renal (kidneys), ovarian or spermatic (go- 

 nads), hepatic (from liver), lumbar (back), phrenic, intercostals (be- 

 tween ribs, etc. ) . The deep veins of the lower extremity follow the 

 arteries. The two superficial veins of the lower extremity are ( 1 ) the 

 great saphenous vein which arises in the medial side of the foot, passes 

 along the medial side of the leg, and empties into the femoral vein and 

 (2) the small saphenous which arises in the lateral side of foot and 

 empties into the popliteal vein. The veins which return the blood from 

 the digestive organs constitute the portal system which detours the blood 

 through the liver and then into the inferior vena cava. This blood is 

 changed in several ways by the liver to prepare it for entrance into the 

 general systemic circulation. 



Capillaries are thin-walled vessels which form a network connecting 

 the arteries and veins. The walls are a single layer of flat endothelial 

 cells. The capillaries are so numerous that one can hardly touch any 

 part of the body without touching capillaries. Through them the ex- 

 change of materials takes place because of their thin walls and the slow 

 movement of the blood within. An average capillary is about 8 microns 

 in diameter. Compare this with the diameter of a red blood corpuscle. 



Functions of the Blood System 



(1) Respiratory — transporting oxygen to the tissues and carbon di- 

 oxide from them; (2) excretory — carrying waste materials from the tis- 

 sues to the organs of excretion; (3) nut/itive — transporting sugars, amino 

 acids, fats, minerals, and vitamins from the digestive system to the body 

 tissues; (4) regulatory — transporting water to and from various organs 

 so that the water content may be fairly constant, equalizing body tem- 

 perature by carrying water throughout the body and giving it off from 

 the vessels near the surface, distributing foods to the endocrine organs 

 (ductless glands) and transporting secretions (hormones) produced by 

 them; (5) protective — defending the body by means of the phagocytic 

 action of certain white blood corpuscles, and the circulation of specific 

 antibodies (antitoxins, etc.) ; (6) the maintenance of the proper acid- 

 alkaline reaction of the various parts of the body. 



