8.
What are the humoral factors that participate in regulation of blood flow, and what are their actions?
Regulation of Heart Rate and Myocardial Performance
Cardiac output is defined as the quantity of blood pumped by the heart each minute. Cardiac output may be varied by a change in the heart rate or the volume of blood ejected from either ventricle with each heartbeat; this volume is called the stroke volume. Mathematically, cardiac output (CO) can be expressed as the product of heart rate (HR) and stroke volume (SV):
Equation 18.1
Thus to understand how cardiac activity is controlled, consider how the heart rate and stroke volume are regulated. Heart rate is regulated by the activity of the autonomic nervous system to modulate the intrinsic cardiac pacemaker. Stroke volume is determined by myocardial performance (which is determined by cardiac cell contractility) and by the hemodynamic loads on the heart. All of these determinants are interdependent, inasmuch as a change in one determinant of cardiac output almost invariably alters another.
Nervous Control of the Heart Rate
Although certain local factors, such as temperature changes and stretching of tissue, can affect the heart rate, the autonomic nervous system is the principal means by which the heart rate is controlled.
The average resting heart rate is approximately 70 beats per minute in normal adults, and it is significantly faster in children. During sleep, the heart rate decreases by 10 to 20 beats per minute. It may increase during emotional excitement, and during muscular exercise, it may increase to rates well above 150 beats per minute. In well-trained athletes, the usual resting rate is only approximately 50 beats per minute.
Both divisions of the autonomic nervous system tonically influence the cardiac pacemaker, which is normally the sinoatrial (SA) node. The sympathetic nervous system enhances automaticity, whereas the parasympathetic nervous system inhibits it. Changes in heart rate usually involve a reciprocal action of these two divisions of the autonomic nervous system. Thus the heart rate ordinarily increases with a combined decrease in parasympathetic activity and increase in sympathetic activity; the heart rate decreases with the opposite changes in autonomic neural activity.
Parasympathetic tone usually predominates in healthy, resting individuals. When a resting individual is given atropine, a muscarinic receptor antagonist that blocks parasympathetic effects, the heart rate generally increases substantially. If a resting individual is given propranolol, a β-adrenergic receptor antagonist that blocks sympathetic effects, the heart rate usually decreases only slightly (
Fig.
18.1 ). When both divisions of the autonomic nervous system are blocked, the heart rate of young adults averages approximately 100 beats per minute. The rate that prevails after complete autonomic blockade is called the intrinsic heart rate.
•Fig. 18.1 Effectsoffourequaldosesofatropine(muscarinicrecep (β-adrenergicreceptorantagonistthatblockssympatheticeffects)ontheheartratesof10healthyyoungmen.Inhalfthetrials,atropine wasgivenfirst(top curve); intheotherhalf,propranololwasgivenfirst(bottom curve). (RedrawnfromKatonaPG,etal.J Appl Physiol. •Fig. 18.2 Changesinheartrateevokedbystimulation(horizontal 1982;52:1652.) bars) ofthevagus(A) andsympatheticnerves(B). (ModifiedfromWarnerHR,CoxA.J Appl Physiol. 1962;17:349.)