Question

Describe in detail the baroreceptor reflex. Be sure to name and describe all the contributing components, their locations, their specific roles in the reflex. Then give an example of a situation in which blood pressure would suddenly fall and explain how the baroreceptor reflex operates to maintain blood pressure homeostasis. Then give an example of a situation in which blood pressure would suddenly rise and explain how the baroreceptor reflex operates to maintain blood pressure homeostasis. (it’s easiest to choose simple examples like body position changes lying to standing, standing to lying down).

Answer 1

Baroreflex helps in maintaining blood pressure at nearly constant levels. It is one of the body's homeostatic mechanisms works by providing a negative feedback loop which involves the sensory receptors as baroreceptors or stretch receptor chiefly in the aortic arch and carotid sinuses and changing variable as changes in blood pressure which is then transmitted to integration center of medulla oblongata. Decreased blood pressure reflexively decreases activation and causes heart rate to increase and to restore blood pressure levels whereas, increased blood pressure decreases HR and BP. It works less than a fraction of second and so act as key factor in managing postural hypotension by interfering both parasympathetic and sympathetic nerves.ANP and renin-angiotension play role in maintaining BP but it is slower.

When an individual moving from a supine to a standing position, The cardiac output, venous return, stroke volume and arterial and venous pressures are considerably affected by the gravitational force. The effect of gravitational force on thorax, abdomen and leg are equal in supine position since they lie in same horizontal plane and so the venous blood and pressure are equal throughout. Upon standing from supine, the effect of gravity cause higher venous compliance and alters the venous blood volume in the extremities of the leg. This contribute for the increase in venous blood volume VBV and venous blood pressure VBP in the leg while in standing position. Frank-Starling mechanism which states that the stroke volume can be dramatically affected by changes in ventricular preload. The increase in VBV causes shift in the thoracic venous volume and decrease in central venous pressure and so decreases the preload. Decreased preload influence the stroke volume in accordance with the Frank-Starling mechanism and decreases the stroke volume by altering the force of contraction of the cardiac muscle. The reduced pulmonary venous return decreases the left ventricular stroke volume and eventually cause fall in cardiac output and MAP. Sudden change in posture from supine to standing might cause appreciable fall in arterial pressure is termed as “postural or orthostatic hypertension”. This can potentially reduce the central blood flow and person may experience fainting. In order to minimize the MAP fluctuation while in standing position from supine, the baroreceptor mediated tachycardia occurs that is increased in heart rate. This in turn influence the sympathetic mediated increase in systemic vascular resistance, decreases venous compliance, decreased SV and preload.