Control And Abnormalities Of Body Water Flashcards
How is volume regulation maintained in the body?
Retaining sodium (and water with it)
What is osmoregulation?
Preventing shifts of water between the ECF and ICF
How is the osmolarity of the ECF adjusted?
Adding or removing water in the body (thirst mechanism or kidney)
What is the only site of regulated water loss?
Renal tubule
What happens to the urine if there’s a water excess?
Large volume of dilute urine
What happens to the urine if there’s a water deficit?
Small volume of concentrated urine
What are the two ways the kidney osmoregulate?
Concentrate interstitial fluid in the medulla, dilute urine in ascending limb and distal tubule
What is the threshold for urine concentration?
50 - 1200 mOsm/L
What are the concentration difference between the interstitial fluid and tubular fluid in the proximal tubule?
Isotonic
Where do you get the max concentration of both the interstitial and tubular fluid?
Bend of the loop of henle
Where do you get the maximum dilution of the tubular fluid?
Distal tubule
What is the osmoregulatory hormone?
ADH
What does lack of ADH in the collecting duct mean?
It’s impermeable to water and therefore maximally dilute urine is secreted
What urine osmolality is meant to signify the presence of ADH?
> 100mOsm/Kg
At maximum ADH levels what is the limit of water reabsorption set by?
The osmolality of the medullary interstitial fluid
What is ADH’s mechanism of action?
Vasopressin V2 receptors act on principal cells and get them to make and insert aquaporin 2 channels into the membrane
What do V1 receptors on vascular smooth muscle lead to?
Vasoconstriction but only significant with very high ADH levels
How does a net water loss change ECF osmolality?
Increase
What is the normal ECF osmolality range?
285-295 mOsm/Kg
Where are changes in osmolality detected?
Osmoreceptors in the anterior hypothalamus
Where do the osmoreceptors project to?
Magnocellular neurons of the paraventricular and supraoptic nucleus of the hypothalamus
Or thirst centres
What do the paraventricular and supraoptic nucleus release?
ADH
Where do the paraventricular and supraoptic nuclei release ADH from?
The axon terminus in the posterior pituitary glands
What is the threshold for ADH release?
280-285 mOsm/Kg
Above the range, what do small changes in osmolality lead to?
Large changes in ADH secretion
When do you get a strong desire to drink?
Plasma osmolality > 295 mOsm/Kg
What else can thirst be stimulated by?
Large drops in blood volume/ pressure and angiotensin 2 acting on hypothalamus
What is the main determinant of ECF osmolality?
Plasma [Na+]
What formula do you use to estimate plasma osmolarity?
2[Na+] + 2[K+] + [glucose] + [urea]
What is hypernatraemia?
Too little water (too much sodium)
What are the causes of hypernatraemia?
Gain of sodium or loss of water (more common)
How can you gain sodium?
Iatrogenic, excess ingestion, excess mineralocorticoid activity
What does iatrogenic mean?
Unintended side effect of medical treatment
What can cause a loss of water?
Extrarenal losses (dehydration/infection)
Renal losses (osmotic diuresis/ diabetes insipidus)
What is osmotic diuresis?
Impaired tubular fluid dilation meaning the osmotic gradient is off so water reabsorption is decreased
What causes diabetes insipidus?
Lack of effective ADH- either due to a failure of secretion (central) or a lack of renal response (nephrogenic)
What does hypoosmotic hypernatraemia signify?
Water excess
When does pseudohyponatraemia occur?
When some other solute conc is high enough that sodium has to be reduced to maintain electro neutrality
What can cause syndrome of inappropriate ADH secretion?
CNS damage/ disease and ectopic ADH production by tumour
What does syndrome of inappropriate ADH secretion cause?
Hyponatraemia and high urine osmolarity
Which is more powerful- low volume/ pressure or osmotic signals?
Low volume/ pressure
What happens in kidneys in congestive heart failure?
RAAS -> sodium/ water retention (volume expansion) -> ineffective because dodgy starling forces -> oedema
