Let’s being by looking at the short-term regulation of rising blood pressure. In response to rising blood pressure, arterial walls are stretched, stimulating baroreceptors in the carotid sinus, aortic arch, and other large arteries of the neck and thorax. As a result, these receptors send off a faster stream of impulses to brain. Let’s observe these afferent nerve impulses traveling to the brain. In response to increased stimulation from the baroreceptors, the brain increases parasympathetic activity and decreases sympathetic activity. This leads to a reduction in heart rate, an increase in the diameter of the arterioles and therefore lower blood pressure, brining blood pressure back down to normal levels. Let’s look at the effect of increased parasympathetic and decreased sympathetic activity on the heart and blood pressure. With increased activity of the vagus nerve, which is part of the parasympathetic nervous system, and decreased activity of the sympathetic cardiac nerve, heart rate is reduced. This leads to lower cardiac output and therefore lower blood pressure. Let’s observe the efferent nerve impulses and their effect on heart rate and blood pressure. Now let’s look at the effect of decreased sympathetic activity on arterioles and blood pressure. Vasomotor fibers are sympathetic nerves that innervate the smooth muscle of blood vessels. With decreased activity of vasomotor fibers, the vascular smooth muscle relaxes, arteriolar diameter increases, and blood pressure is reduced. Let’s recap the short-term regulation of rising blood pressure. In response to rising blood pressure, baroreceptor stretch increasing impulses to the brain. This leads to increased parasympathetic activity and decreased sympathetic activity resulting in slowing of the heart rate and increase in the diameter of the arterioles and consequently a reduction in blood pressure. Now let’s look at the short-term regulation of falling blood pressure. When blood pressure drops suddenly, the baroreceptors are inhibited and they send fewer impulses to the brain. As a result, parasympathetic activity decreases and sympathetic activity increases leading to an increase in blood pressure via three mechanisms. 1. Increased sympathetic impulses to the heart, increase heart rate and contractility. 2. Increased impulses to the blood vessels increase vasoconstriction. 3. Increased sympathetic impulses to the adrenal gland stimulate the release of epinephrine and norepinephrine into the blood stream. These hormones enhance heart rate, contractility, and vasoconstriction. Together these three mechanisms bring blood pressure back up to normal levels as illustrated on the following screens. First, let’s look at the effect of increased sympathetic activity on the heart and blood pressure. Increased sympathetic impulses to the heart and decreased vagus nerve activity lead to increased heart rate and enhanced contractility increasing stroke volume. These changes result in higher cardiac output, which increases blood pressure. Let’s observe the response to declining blood pressure and the effect on heart rate and blood pressure. How does greater sympathetic activity affect arteries and blood pressure? Increased vasomotor fiber activity leads to constriction of vascular smooth muscle. The smaller arteriolar diameter gives rise to increased blood pressure. Now let’s explore the effect of increased sympathetic activity on the adrenal gland and blood pressure. Increased sympathetic impulses to the adrenal gland stimulate the release of epinephrine and norepinephrine into the bloodstream. These hormones increase heart rate, contractility of the heart, and vasoconstriction of blood vessels thus increasing blood pressure. Because they are carried in the blood, these hormones act more slowly than nervous system controls and their effects are more prolonged. Let’s watch the response to falling blood pressure. Here’s a recap of short-term regulation of falling blood pressure. As blood pressure falls, baroreceptors are inhibited resulting in decreased impulses to the brain. Parasympathetic activity is decreased and sympathetic activity is increased leading to increased heart rate and contractility, increased vasoconstriction, and the release of epinephrine and norepinephrine from the adrenal gland. All of these mechanisms contribute to raising blood pressure back up to normal levels.