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
coming . . . .
All material © Copyright - Kerry Brandis, 2001