Role of the Nervous System in Rapid Control of Arterial Pressure
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 inhibi-tion of parasympathetic vagal inhibitory signals to the heart. Thus, three major changes occur simultaneously, each of which helps to increase arterial pressure. They are as follows:
1. Almost all arterioles of the systemic circulation are constricted. This 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 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, too, increases the arterial pressure.
3. Finally, the heart itself is directly stimulated by theautonomic nervous system, further enhancing cardiac pumping. Much of this is caused by anincrease in the heart rate, the rate sometimes increasing to as great as three times normal. In addition, sympathetic nervous signals have a significant direct effect to increase contractile force of the heart muscle, this, too, 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. This contributes still more to the acute rise in arterial pressure.
Rapidity of Nervous Control of Arterial Pressure. An espe-cially 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 stimula-tion can decrease the arterial pressure to as little as one half normal within 10 to 40 seconds. Therefore, nervous control of arterial pressure is by far the most rapid of all our mechanisms for pressure control.