explain effect of gravity on blood pressure max 600 to 900 words
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The shift in blood volume decreases thoracic venous blood volume (CV Vol) and therefore central venous pressure (CVP) decreases. This decreases right ventricular filling pressure (preload), leading to a decline in stroke volume by the Frank-Starling mechanism. On the transition from sitting in a chair to standing, blood is pooled in the lower extremities as a result of gravitational forces. Venous return is reduced, which leads to a decrease in cardiac stroke volume, a decline in arterial blood pressure, and an immediate decrease in blood flow to the brain.In space, where the familiar pull of gravity is missing, the head-to-toe gradient vanishes. Blood pressure equalizes and becomes about 100 mmHg throughout the body. ... Within two to three days of weightlessness, astronauts can lose as much as 22 percent of their blood volume as a result of that errant message.Many parts of the cardiovascular system (including the heart) are influenced by gravity. On Earth, for example, the veins in our legs work against gravity to get blood back to the heart. Without gravity, however, the heart and blood vessels change – and the longer the flight, the more severe the changes.When you stand up, gravity causes blood to pool in your legs and abdomen. This decreases blood pressure because there's less blood circulating back to your heart. Normally, special cells (baroreceptors) near your heart and neck arteries sense this lower blood pressure.Gravitational forces significantly affect venous return, cardiac output, and arterial and venous pressures. To illustrate this, consider a person who is lying down and then suddenly stands up. When the person is lying down (supine position), gravitational forces are similar on the thorax, abdomen and legs because these compartments lie in the same horizontal plane. In this position, venous blood volumes and pressures are distributed evenly throughout the body. When the person suddenly stands upright, gravity acts on the vascular volume causing blood to accumulate in the lower extremities. (Compare the size of veins in the top of your feet while lying down and standing.) Because venous compliance is high and the veins readily expand with blood, most of the blood volume shift occurs in the veins. Therefore, venous volume (Vol) and pressure (VP) become very high in the feet and lower limbs when standingLeft ventricular stroke volume also falls because of reduced pulmonary venous return (decreased left ventricular preload). This causes cardiac output (CO) and mean arterial pressure (MAP) to fall. If arterial pressure falls appreciably upon standing, this is termed orthostatic or postural hypotension. This fall in arterial pressure can reduce cerebral blood flow to the point where a person might experience syncope (fainting).
When a person stands up, baroreceptor reflexes are rapidly activated to restore arterial pressure so that mean arterial pressure normally is not reduced by more than a few mmHg when a person is standing compared to lying down. However, in order to maintain this normal mean arterial pressure, the person who is standing upright has increased systemic vascular resistance (sympathetic mediated), decreased venous compliance (due to sympathetic activation of veins), decreased stroke volume (due to decreased preload), and increased heart rate (baroreceptor-mediated tachycardia). Patients with autonomic nerve dysfunction or hypovolemia will not be able effectively utilize these compensatory mechanisms and therefore will display orthostatic hypotension. Without the operation of important compensatory mechanisms, standing upright would lead to significant edema in the feet and lower legs in addition to orthostatic hypotension. Venous pooling and reduced venous return are rapidly compensated in a normal individual by neurogenic vasoconstriction of veins, the functioning of venous valves, by muscle pump activity, and by the abdominothoracic pump. When these mechanisms are operating, capillary and venous pressures in the feet will only be elevated by 10-20 mmHg, mean aortic pressure will be maintained, and central venous pressure will be only slightly reduced.
When a standing person suddenly changes to the supine position, gravity no longer causes a shift in blood volume from the thoracic compartment to the legs and feet. Therefore, the blood volume in the thoracic (central venous) compartment as blood volume shift away from the legs. This increases preload on the heart, thereby increasing stroke volume, although the resulting increase in cardiac output will be tempered by a reduction in heart rate through vagal activation and sympathetic withdrawal. Sympathetic activation of the systemic vasculature is also reduced, which causes systemic vascular resistance to fall as the resistance vessels dilate. Blood feels gravity, too. On Earth, blood pools in the feet. When people stand, the blood pressure in their feet can be high -- about 200 mmHg (millimeters of mercury). In the brain, though, it's only 60 to 80 mmHg.