a change in posture alters the hydrostsatic pressure in the blood vessel affecting TRANSMURAL pressure and also on venous capacity.
hydrostatic pressure :- in supine or prone posture all blood vessels are approximatelyat the level of the heart.when standing there is increase in arterial and venous pressure below the level of the heart and decrease in arterial and venous pressure above the level of heart.
there is reflex compensation due to decrease in end diastolic volume caused by sudden standing,it is detected by atrial stretch receptors and the decrease in mean arterial blood pressure is detcted by carotid baroreceptors.
so it leads to:-
1 increse in heart rate and myocardial contractility
2 vasoconstriction in the skeletal musculature,skin ,kidneys and gut increasing total peripheral resistance
3 venocostriction in all veins , but in particular in those of skin and gut.
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