554 VERTEBRATE LIFE AND ORGANIZATION 



will be its contraction. This capacity of the heart to adjust its output 

 per stroke to the volume of blood delivered to it is known as Starling's 



"law of the heart." 



The heart of a normal adult man, who is not exercising, sends 

 about 70 ml. of blood per beat out into the aorta. At the normal rate 

 of 72 beats per minute, this is a total output of 5 liters per minute, 

 which is approximately equivalent to the total amount of blood in the 

 body. A similar observation made in 1628 by William Harvey helped 

 to lead him to the conclusion that the blood recirculates. Until that 

 time it was believed that blood was continually produced in the liver, 

 pumped to the tissues, and consumed. Harvey's calculations showed 

 that the amount of blood pumped by the heart each hour was much 

 more than could possibly be produced and consumed. He made the cor- 

 rect inference that the blood must recirculate, even though he could not 

 see the microscopic capillaries that connect arteries and veuis. 



Although a large volume of blood flows through the cavities of the 

 heart, this blood does not provide for the metabolic needs of the heart 

 musculature. A pair of coronary arteries arise from the base of the arch 

 of the aorta and supply capillaries in the heart wall. This capillary bed 

 is drained ultimately by a coronary vein that empties into the right 

 atrium. Obviously any damage to the coronary vessels, the plugging of 

 one of the larger arteries by a thrombus, for example, could have serious 

 consequences, for the heart muscles cannot function without a continu- 

 ing supply of oxygen and food. 



The Arteries. Arteries are lined with endothelium and have a rela- 

 tively thick wall containing elastic connective tissue and smooth muscles. 

 The walls of the larger arteries are richly supplied with elastic tis- 

 sue. The force of each ventricular systole forces blood into the arteries 

 and stretches them to accommodate it. During diastole, the elastic recoil 

 of the arterial walls keeps the blood moving, ff they were rigid pipes, 

 the arteries would deliver blood to the tissues in spurts that coincided 

 with ventricular systole. The blood would pound like steam rushing 

 into empty radiator pipes. The elasticity of the large arteries transforms 

 what would otherwise be an intermittent ffow into a steady flow. 



The smaller arteries, and especially the arterioles preceding the 

 capillaries, contain a relatively large amount of smooth muscle, and 

 they are concerned with regulating the supply of blood to the various 

 organs. Vasodilator and vasoconstrictor nerves supply these muscles, 

 causing them to relax or contract. If a region of the body becomes very 

 active, its small arteries enlarge and the blood flow through them is 

 increased. If an area is not particularly active, its small arteries constrict 

 and blood flow is reduced. In this way a maximum use is made of the 

 volume of blood available. 



As the arteries extend to the tissues, they branch and rebranch. 

 Each time their lumen becomes smaller but the total cross sectional area 

 of all of the branches increases greatly. The velocity of blood flow, there- 

 fore, decreases, for the blood, like a river widening out and flowing into 

 a lake, is moving into an area that grows larger and larger. The mean 

 blood pressure is also decreased continually because of the friction of 



