Fluid Physiology
5.8 Interaction between Volume & Osmolality in Control


Changes in blood volume or blood pressure have major effects on osmoreceptor function. These are illustrated in Figure 5.4. The threshold and the sensitivity of the osmoreceptor can both be altered. Overall, the change that occurs is such that it would tend to rapidly correct the disturbance (provided of course that the kidney was able to respond to [ADH] changes normally). Experiments in humans show that blood volume changes do not have much effect until the change in blood volume is of the order of 7 to 10%. Changes in blood pressure have similar effects and this presumably is sensed via the carotid baroreceptor mechanism.

The change with hypovolaemia increases the ADH response to a given level of osmolality. This causes renal water retention to assist with correcting the hypovolaemia but this occurs at the expense of maintaining the normal plasma osmolality. Hypovolaemia impairs water excretion & tends to cause hypotonic hyponatraemia.

Fig 5.4 Effect of Changes in Plasma Volume on the Plasma [ADH] & Plasma Osmolality Relationship


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All material Copyright - Kerry Brandis, 2001