Bertha is 81 years old and was admitted to hospital after contracting community-acquired pneumonia. She had been bedridden for 3 days, so her nurse arranged for a physiotherapist to assist her out of bed to help her slowly regain her mobility
Bertha decided not to wait for the physiotherapist, and after arising in the morning, she eased herself out of the bed and stood up. Suddenly Bertha's vision dimmed and she felt light-headed and dizzy. A passing nurse saw her fall back to the bed and rushed to help her. The nurse comforted Bertha, and then suspecting orthostatic hypotension, went to find a sphygmomanometer to check her blood pressure.
Prolonged bed rest decreases plasma levels and vasomotor tone, which can both lead to orthostatic hypotension. How do changes in vascular resistance and radius affect blood flow? Assuming Bertha is otherwise healthy, how does her heart activity change in an attempt to compensate for the orthostatic hypotension she experienced?
Considering the venous circulation, how is blood from the lower extremities returned to the heart?
Why did Bertha's capillary fluid pressure (or hydrostatic pressure) change when she moved from a lying to standing position?
According to the law of Laplace, the greater the pressure change between two ends of a vessel, the greater the blood flow. A weaker pressure differential decreases blood flow. A very small change in vessel radius has a powerful effect on the movement of blood, and the relationship is inversely proportional: a decrease in vessel radius increases blood flow, and an increased radius decreases flow. Because blood flow is determined by PVR and CO, Bertha's heart rate (and therefore CO) would increase to compensate for the hypotension she experienced when she stood up.
Venous circulation is maintained by a variety of mechanisms. Valves exist in the veins of the lower extremities to resist retrograde flow and pooling. Skeletal muscle lies in close proximity to veins and helps to move venous blood. Finally, low intrathoracic and intra-abdominal pressures assist to "pull" venous blood from the lower extremities to the heart.
Capillary fluid pressure (or hydrostatic pressure) is influenced by gravity and is increased by the weight of blood in the vascular column during standing. When recumbent, blood vessels lie at the level of the heart, and hydrostatic pressure is therefore low.
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