Nervous Regulation of arterial pressure in animals
Baroreceptor reflex
Arterial blood pressure is monitored by pressure sensitive nerve endings known as baroreceptors; Carotid sinus and aortic body located in the carotid bodies and aortic arch are sensitive to stretch of the arterial wall. The frequency of impulses generated from baroreceptors is proportional the BP.
They send afferent impulses through sinus and aortic nerves (Buffer nerves) to vasomotor areas of the CNS which monitor beat to beat changes in BP.
The CNS reflexly alters cardiac output via sympathetic and parasympathetic divisions of ANS altering the vascular resistance to keep blood pressure at a set point by either vasoconstriction or vasodilatation.
The vasomotor area of the brain responds to an increase in BP by decreasing sympathetic activity and enhancing parasympathetic activity. Heart rate and force of contraction are decreased and stroke volume is decreased.
Sympathetic inhibition causes vasodilatation of arterioles results in decreased peripheral resistance. All of these effects restore arterial BP to normal. A decrease in BP produces opposite effect to those considered above.
Atrial volume receptor reflex
This reflex is initiated by stretch/ volume receptors located in the walls of the left and right atria which are sensitive to change in atrial blood volume (increase or decrease).
When blood volume is decreased due to haemorrhage and dehydration the CNS receives fewer impulses from the volume receptors and CNS reflexly increases the sympathetic activity to heart and blood vessels, but reflexly decreases parasympathetic activity.
The volume receptor reflexly stimulates the thirst center of the hypothalamus to increase blood volume by drinking of water. It also activates renin-angiotensin-aldosterone system to conserve sodium along with water. ADH secretion is also enhanced to facilitate water reabsorption from the renal tubule.
Bain bridge reflex
Bain bridge reflex operates when the venous return in increased.
Psychogenic responses
Two important psychogenic responses are the- Defense-alarm reaction and Vaso-vagal syncope.
Defense alarm reaction
Defense alarm reaction is known as fight or flight and emotional response to a threatening situation such as physical injury or trauma. It involves increased sympathetic and decreased parasympathetic activity which includes increased heart rate and stroke volume, vasoconstriction in kidneys, splanchnic organs and skin and vasodilatation in coronary vessel and skeletal muscles and increased BP. There is enhanced secretion of ADH, angiotensin II and ACTH, retention of water and sodium ions to improve blood volume.
Vaso vagal syncope
Vaso vagal syncope occurs in response to certain emotional situation in which some animals faint due to decrease in BP. It involves a decrease in sympathetic activity and increase in parasympathetic activity causing vasodilatation and decrease in TPR. Heart rate and cardiac output decreases with a drop in BP resulting in inadequate blood flow to brain and fainting.
Chemoreceptor reflex
Carotid bodies and aortic bodies contain chemoreceptors. The excess CO2 or H+ from the body fluids or lack of oxygen or blood supply to these receptors stimulate the chemical sensory impulses through sinus and aortic nerves to vasomotor centre and BP is reflexly elevated to eliminate excess quantity of CO.