174 The Vertebrate Organ Systems 



region of the spinal cord in the chest accelerates. The action of these 

 nerves is delicately balanced, and results in very accurate control of 

 the heart rate. These nerves regulate the heart rate, but they in turn are 

 partially controlled by certain reflexes (automatic responses). One im- 

 portant reflex center is in the aorta. When this artery is stretched due 

 to increased blood volume, it results in a stimulation of the depressor 

 center in the brain, and then in a decreased heart rate. Within the 

 carotid arteries are other centers which act in a similar way. Another 

 reflex which originates in the right auricle and main veins emptying into 

 this auricle results in an increased heart rate. This is apparently due 

 to stretching of the fibers of these structures. Thus exercise which 

 brings about an increased volume of blood returning to the heart also 

 causes an increase in heart rate. 



Within the brain itself, the nerve center may indirectly afifect either 

 the inhibitory or excitatory nerve supply of the heart. Excitement, for 

 example, will increase the heart rate ; fear may slow it. 



Certain chemical substances also affect the heart. Within the 

 body, the hormones, thyroxin from the thyroid, and adrenaline from 

 the adrenal medulla both increase heart rate. 



The heart rate also varies with age and condition of health of the 

 person. It usually is considered that about 72 per minute is normal 

 for an adult human being. At birth, the heart beats about 140 times 

 a minute, in young adults about 90, and in older persons about 75. 

 In general, very small mammals have a much higher rate than do large 

 ones. The heart of an elephant beats only 25 times per minute while 

 a mouse's contracts about 700 times. 



In poikilothermic vertebrates, the heart rate is affected by temper- 

 ature changes. Increased temperatures result in a more rapid rate while 

 lowered ones result in a decreased rate. 



Blood Pressure and Rate of Flow. — The heart is constantly 

 pumping the blood against the resistance offered by the blood vessels. 

 This resistance is least in the large vessels and greatest in the capillary 

 bed. This difference is correlated with the enormous increase in cross- 

 sectional area provided by capillaries over that of the arteries which 

 supply them. Thus the rate of flow in a large artery may be as much 

 as 100 times greater than that in a capillary. Some typical rates of flow 

 in human beings are as follows : 



Large artery (during systole) 320 mm. /second 



Large artery (during diastole) 150 mm./second 



