Standing to sitting change is: (AIPG JUN 2009)
|A||Immediate Increase in Venous Return|
|B||Increase in heart rate|
|D||Increased cerebral blood flow|
II. The arterial pressure at head level is 60–75 mm Hg, and the venous pressure is zero.
III. If the individual does not move, 300–500 mL of blood pools in the venous capacitance vessels of the lower extremities, fluid begins to accumulate in the interstitial spaces because of increased hydrostatic pressure in the capillaries, and stroke volume is decreased.
IV. Symptoms of cerebral ischemia develop when the cerebral blood flow decreases to less than about 60% of the flow in the recumbent position.
V. If no compensatory cardiovascular changes occurred, the reduction in cardiac output due to pooling on standing would lead to a reduction of cerebral flow of this magnitude, and consciousness would be lost.So, under normal conditions the cerebral blood flow remains unchanged.
VI. The major compensations on assuming the upright position are triggered by the drop in blood pressure in the carotid sinus and aortic arch.
VII. The heart rate increases, helping to maintain cardiac output. Relatively little venoconstriction occurs in the periphery, but there is a prompt increase in the circulating levels of renin and aldosterone.
VIII. The arterioles constrict, helping to maintain blood pressure. So, when the person sits from a standing posture the venous return increases due to movement of 300-500 ml of blood pooled in the lower extremities towards the heart.