222 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



especially important in returning blood from the legs against the pull of 

 gravity. If a person stands still for some time, the blood in the veins is not 

 circulated properly and the feet and legs swell. If the same person is walking, 

 the contractions of muscles force the blood onward and no such swelling 

 results. Breathing greatly aids venous flow. When the chest muscles and 

 diaphragm contract, the space in the chest cavity is increased, and the pressure 

 within it is lowered to such an extent that air enters the lungs freely and blood 

 enters the right auricle. 



Vasomotor Control. When one part of the body, e.g., the skeletal 



Vein spread open, cups Section of vein showing 



hanging from wall. valves closed preventing 



backward flow. 



2- n**^»\t.iffff„ffffi 



Toward 

 heart 



^i2. 



2> ^ 



heart 



^^ 



Valves open 



Frc. 12.21. The valves of the veins which prevent a backward flow of blood. 

 They were used by William Harvey ( 1578-1657) in his argument that the blood 

 continually circulates through the body in one direction. 



muscles, the brain, the stomach, is especially active, it receives an extra amount 

 of blood. The walls of arterioles contain smooth muscle innervated by two 

 sets of nerves. An increase in the number of impulses in one set of these nerves 

 (vasoconstrictors) causes the muscles in the walls of the arterioles to contract, 

 decreasing the size of the vessels, and lessening the blood supply. An increase 

 in the impulses in the other set (vasodilators) causes the muscles to relax and 

 increases the size of the arterioles and the consequent flow of blood within 

 them. Ordinarily, these muscles are partially contracted, due to a balance of 

 the impulses in both sets of nerves. 



Chemical Control. Arterioles are also affected by carbon dioxide and 

 epinephrine. When muscles are very active, e.g., as in running or sawing 

 wood, their highly increased output of carbon dioxide acts on the smooth 

 muscle of the arterioles, causing them to relax. Arterioles are enlarged and the 

 blood supply to the hard-working muscles is increased. Epinephrine relaxes 

 the muscles in the walls of arterioles in skeletal muscles, but contracts those 

 in the internal organs such as the stomach and intestine. 



William Harvey and the Circulation of Blood 



For upwards of 2000 years human blood was believed to ebb and flow in 

 the vessels like the tides of the sea. Capillaries were unknown, because of the 



