What is meant by reverse stress relaxation of circulatory system
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Reverse stress relaxation is a compensatory mechanism in which the blood vessels, especially the veins and venous reservoirs, gradually contract around a reduced blood volume. The smaller volume then fills the circulation better, and pressure moves back toward normal.
Underlying principle (smooth muscle property)
Vascular smooth muscle, like the smooth muscle of other hollow organs, can adjust its tension to the volume it holds. The textbook describes it with the bladder as an example:
- Stress relaxation: A sudden increase in volume stretches the wall and pressure rises sharply. Over the next 15 to 60 seconds the muscle relaxes, and pressure returns almost to its original level, even though the stretch continues.
- Reverse stress relaxation: A sudden decrease in volume makes pressure fall sharply. Over seconds to minutes the muscle contracts, and pressure rises back to or near the original level.
These responses let a hollow organ keep roughly the same internal pressure despite large, sustained changes in volume. (Guyton and Hall Textbook of Medical Physiology, smooth muscle chapter)
Role in the circulation (hemorrhage and shock)
After blood loss, the blood volume is smaller than the vascular capacity. Reverse stress relaxation makes the vessels contract around the remaining blood, which:
- reduces the effective capacity of the vascular bed
- raises the mean circulatory filling pressure
- increases venous return, and so cardiac output and arterial pressure
It is one of the negative feedback mechanisms behind compensated (nonprogressive) shock. The others are baroreceptor reflexes, the CNS ischemic response, renin-angiotensin, vasopressin, catecholamines, and fluid shifts that restore volume. (Guyton and Hall, Nonprogressive Shock section)
Timing
- The sympathetic reflexes and catecholamine release act within about 30 seconds to a few minutes.
- Reverse stress relaxation is slower, like the angiotensin and vasopressin mechanisms. It needs about 10 to 60 minutes to take full effect.
- Restoring blood volume through fluid absorption from the interstitium and gut, and renal salt and water conservation, takes 1 to 48 hours.
Clinical point: In mild shock these mechanisms, including reverse stress relaxation, can overcome the positive feedback influences and the patient recovers. In severe shock they are overwhelmed, and the vicious cycle of progressive shock sets in.
Related concept: Stress relaxation is the opposite process. It occurs when volume is increased, for example by a transfusion. The vessels slowly relax and accommodate the extra volume, so pressure returns toward normal.
A quick PubMed search for recent reviews on this topic returned nothing relevant. This is a standard physiology concept, and I found no recent evidence that changes it.
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