Neorohormonal Control of Blood Pressure and Volume

In this post we'll be talking about how tha autonomic nervous system can control blood pressure. I'll also discuss the arterial and atrial baroreceptor reflexes.

The short term regulaiton of blood pressure involves the heart and blood vessels regulating cardiac output and total peripheral resistance. The long term regulation of blood pressure involves the kidneys which regulate blood volume.

The Arterial Baroreceptor Reflex

The arterial baroreceptor reflex  consists of several components:

• Detectors: these are arterial baroreceptors which are located in the walls of the carotid sinus and aortic arch. They contain nerve endings which are sensitive to stretch (distension) of the arterial wall by pressure. These baroreceptors monitor the pressure of every systolic ejection. 
• Integrator: this is the neural control of cardiovascular function. The parasympathetic nervous system regulates heart rate while the sympathetic nervous system predominates during exercise. The sympathetic nervous system increases heart rate and contractility of cardiac muslce. This leads to an increase in cardiac output.
• Effectors: in most tissues, all blood vessels except capilaries and precapillary sphincters are innervated by the sympathetic nervous system. With skeletal muscle as the exception, sympatheitc stimulation of small arteries generally increases resistance and decreases flow. The sympathetic stimulation of veins causes the volume of the vessels to decrease. This increases venous return to the heart.

If arterial baroreceptors detect a decrease in Mean Arterial Pressure they decrease the level of  parasympathetic activity. This leads to an increase in the frequency of action poteintial firing, which leads to an increase in heart rate. The baroreceptors may also cause an increase in sympathetic nervous system activity. This may increase contractility which leads to an increase in stroke volume. It may also cause an increase in venomotor tone, decreasing compliance and increasing venous pressure. This leads to an increase in end diastolic volume and an increase in stroke volume. An increase in sympathetic activity may increase vasocontriction which may increase total peripheral resistance. An increase in heart rate, stroke volume and total peripheral resistance will result in an increase in mean arterial pressure.The arterial baroreceptor reflex is essential for the normal moment-to-moment stability of mean arterial pressure but has little effect on the long term level of blood pressure. 

The Atrial Baroreceptor Reflex

The atrial baroreceptor reflex has atrial volume receptors which are stretch receptors in the walls of the atria. They detect the volume of blood in each atria which indirectly monitors total blood volume. The atrial baroreceptor reflex has the same integrators and effectors as the arterial baroreceptor reflex.

The atrial volume receptors decrease in activity in response to a decrease in blood volume. This detected by the central nervous system. This may affect the hypothalamus which may cause the animal to feel thirsty, this leads to an increased water intake. The central nervous system may also communicate with the pituitary which could cause an increased amount of antidiuretic hormone to be released, this leads to a decrease in urine production. The sympathetic nervous system may also be affected causing the kidney to release more renin which decreases the excretion of Na+. In addition, the sympathetic and parasympathetic nervous systems could be stimulated causing a similar response that was seen in the arterial baroreceptor reflex. The increase in water intake and the decrease in sodium excretion and urine flow act to minimise the effects of a decrease in blood volume and pressure.