Decreased venous return-diarrhea causes a loss of extracellular fluid volume, which produces a decrease in arterial pressure. the decrease in arterial pressure activates the baroreceptor mechanism, which produces an increase in heart rate when the patient is supine. when she stands up, blood pools in her leg veins and produces a decrease in venous return, a decrease in cardiac output (by the frank-starling mechanism), and a further decrease in arterial pressure.
the further decrease in arterial pressure causes further activation of the baroreceptor mechanism
and a further increase in heart rate.
