One of the most important functions of nervous control of the circulation is its capability to cause rapid increases in arterial pressure. For this purpose, the entire vasoconstrictor and cardioaccelerator functions of the sympathetic nervous system are stimulated together. At the same time, there is reciprocal inhibition of parasympathetic vagal inhibitory signals to the heart. Thus, the following three major changes occur simultaneously, each of which helps to increase arterial pressure:
1. Most arterioles of the systemic circulation are constricted, which greatly increases the total peripheral resistance, thereby increasing the arterial pressure.
2. The veins especially (but the other large vessels of the circulation as well) are strongly constricted. This constriction displaces blood out of the large peripheral blood vessels toward the heart, thus increasing the volume of blood in the heart chambers. The stretch of the heart then causes the heart to beat with far greater force and therefore to pump increased quantities of blood. This also increases the arterial pressure.
3. Finally, the heart is directly stimulated by the auto nomic nervous system, further enhancing cardiac pumping. Much of this enhanced cardiac pumping is caused by an increase in the heart rate, which sometimes increases to as much as three times normal. In addition, sympathetic nervous signals have a significant direct effect to increase contractile force of the heart muscle, increasing the capability of the heart to pump larger volumes of blood. During strong sympathetic stimulation, the heart can pump about two times as much blood as under normal conditions, which contributes still more to the acute rise in arterial pressure.
Nervous Control of Arterial Pressure Is Rapid. An especially important characteristic of nervous control of arterial pressure is its rapidity of response, beginning within seconds and often increasing the pressure to two times normal within 5 to 10 seconds. Conversely, sudden inhibition of nervous cardiovascular stimulation can decrease the arterial pressure to as little as one-half normal within 10 to 40 seconds. Therefore, nervous control is by far the most rapid mechanism for arterial pressure regulation.
