L

0 50 100 150 200 250 Mean Arterial Pressure (mm Hg)

FIGURE 6 Variations in mean arterial pressure over a 24-hr period in a normal dog before and several weeks after baroreceptor denervation. (Modified from Guyton AC, Hall JE. Textbook of medical physiology, 9th ed. Philadelphia: WB Saunders, 1996.)

venous smooth muscles, particularly in the splanchnic circulation, producing an increase in peripheral vascular resistance and a reduction in unstressed volume, respectively. All of these effects tend to raise blood pressure. The importance of the low-pressure receptors can best be appreciated by denervation studies. When both the arterial baroreceptors and the low-pressure barorecep-tors are denervated, a transfusion of blood will cause MAP to increase more than twice as much as would occur if only the arterial baroreceptors were denervated.

Heart rate can also be affected by stimulation of atrial baroreceptors. This reflex, called the Bainbridge reflex, can increase heart rate by 75%. Receptors eliciting the reflex are located in both the right and left atria. Their afferent fibers are located in the vagus nerves, and the efferent limbs of the reflex are present in both sympathetic and parasympathetic cardiac nerves. The magnitude and even the direction of the reflex are influenced by the prevailing heart rate. When heart rate is low, an increase in atrial pressure tends to increase heart rate, whereas at high basal heart rates increasing atrial volume may actually slow the heart. The Bainbridge reflex is purposeful in that failure of the heart to keep up with venous return will result in elevated atrial pressure and stimulation of the heart. On the other hand, it opposes the baroreceptor reflex under most conditions in which filling pressure to the heart is reciprocally related to cardiac stimulation. Finally, the Bainbridge reflex is extremely selective in that sympathetic activity to the heart is increased with virtually no change in arteriolar and venous smooth muscle tone in the periphery. The one exception is the dilatation of renal arterioles, which promotes fluid loss through enhanced glomerular filtration.

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