Regulation of osmolarity Flashcards
what hormone controls water regulation
ADH - vasopressin (anti diuretic hormone
What stores ADH
posterior pituitary gland where it is then released into the blood stream
Where is ADH synthesised
hypothalamus
What controls ADH secretion
Plasma osmolarity
The higher the plasma osmolarity, the higher the rate of discharge of ADH secreting hormone and therefore greater release of ADH from the posterior pituitary
What mediates the change in the neuronal discharge of ADHSH - ADH secreting hormone
Osmoreceptors in the anterior hypothalamus
What is the effect of increased osmolarity on osmoreceptors
Increased plasma osmolarity means the water leaves the cell to try to dilute the plasma and achieve equilibrium which causes the cell to shrink which activates a stretch sensitive ion which causes increased ADH secretion due to increased neuronal discharge from the ion channels
What does a change in the volume of an osmoreceptor do
Causes a change in the neuronal discharge by the osmoreceptor
What is required to cause an increase in ADH with regards to increased osmolarity
An increase in tonicity - increase in non-permeable particles which cause an osmotic drag to balance them ie sodium chloride being followed by water
An example of something that can increase osmolarity but not tonicity is urea so ADH would not be released
What does a higher concentration of solute require to be excreted
Would require higher volumes because there is a max concentration of urine at 1200-1400 mOsmoles/l
What is the effect of ADH at the collecting duct
ADH binds to vasopressin recepotrs which activates Cyclic AMP (cAMP) secondary messenger signals - the effect of the signals makes storage vesicles bind to the luminal membrane - the cell inserts aquaporin channels into the apical membrane
What does the aquaporin and ADH allow
It lets water move from the collecting duct lumen, into the collecting duct cell which is against the gradient, and then the water moves to the interstitial fluid to reach equilibrium due to the high osmolarity of the interstitial fluid - the vasa recta then reabsorbs the water
What is the effect of maximal ADH
More water leaves the collecting duct lumen and gets reabsorbed so a small volume of highly concentrated urine is produced
What happens in the absence of ADH
the collecting ducts are impermeable to water so the water cannot leave the collecting duct lumen and a large volume of dilute urine is excreted
What is the function of urea
Urea has a tendency to move down it’s concentration gradient and leave the collecting duct to move to the interstitium - this is enhanced by the effect of ADH - urea plays an important part in the production of concentrated urine
Where in the collecting duct is it most permeable to urea
The medullary tips
What is the function of urea in an anti-diuresis (saving water)
The urea is retained in order to save water and reinforce the medullary gradient at the thin ascending loop of Henle by moving from the collecting duct to the interstitium - the urea is then reabsorbed by the vasa recta
Why is it important that urea is reabsorbed after a diuresis is completed
because if the urea stayed in the interstitium it would exert an osmotic effect which would limit water entering the interstitium
What is the relationship between ADH and ECF
Inversely proportional
Increase in ECF reduces ADH secretion
Decrease in ECF increased ADH secretion
What are low pressure receptors and where are they located
Have a sense of the fullness of the circulation (volume receptors) which are located in the L and R atria and major veins
What are high pressure receptors and where are they located
they measure high pressures during systole and are located at the carotid and aortic arch baroreceptors
What happens when there is a moderate decrease in ECF volume
The atrial receptors (low pressure) are affected and they have an increased discharge which leads to ADH release increasing
If volume changes enough to affect MBP (mean arterial blood pressure) what happens
Carotid and aortic receptors (high pressure) receptors cause an increase in ADH secretion
They are very important in haemorrhage where there is a rapid drop in blood volume and MBP
Even when moving from lying down to standing up causes an ADH release
What cells are ADH secreting
Neurons which release multiple inputs to determine the ADH concentration
What is the affect of alcohol on ADH
decreases ADH which decreases aquaporin channels in the collecting duct so more water is excreted and therefore a higher volume of a more diluted urine is secreted
