Question

Discuss how the kidneys and the carotid body/aortic arch baroreceptors interact to regulate mean arterial pressure both on a minute-by-minute and long-term basis. Be sure to include the mechanisms that are involved in maintaining blood pressure homeostasis.

Answer 1

Baroreceptors are specialized stretch receptors located within thin areas of blood vessels and heart chambers that respond to the degree of stretch caused by the presence of blood. They send impulses to the cardiovascular center to regulate blood pressure. Vascular baroreceptors are found primarily in sinuses (small cavities) within the aorta and carotid arteries. The aortic sinuses are found in the walls of the ascending aorta just superior to the aortic valve, whereas the carotid sinuses are located in the base of the internal carotid arteries. There are also low-pressure baroreceptors located in the walls of the venae cavae and right atrium.

When blood pressure increases, the baroreceptors are stretched more tightly and initiate action potentials at a higher rate. At lower blood pressures, the degree of stretch is lower and the rate of firing is slower. When the cardiovascular center in the medulla oblongata receives this input, it triggers a reflex that maintains homeostasis.

Arterial baroreceptors also influence secretion of posterior pituitary hormones. Decreased ABP sensed by arterial baroreceptors increases vasopressin secretion from the posterior pituitary, with readily understandable influences on cardiovascular homeostasis: increased fluid retention by the kidneys and increased arterial vasoconstriction. Increases in ABP momentarily inhibit the activity of vasopressin-secreting cells (a response that has been used as a defining characteristic of magnocellular vasopressin neurons), but this effect does not appear to be sustained long enough to have any physiological impact. Arterial hypotension also causes secretion of the other major posterior pituitary hormone, oxytocin. While the physiological significance of hypotension-evoked oxytocin secretion is not clear, recent studies indicate that this hormone promotes renin secretion from the kidney, at least in rats. In contrast to vasopressin-secreting cells, increases in ABP appear to have no effect on oxytocin-secreting cells.