6/7 - Integration of salt and water balance Flashcards
What should I know
- Hormonal mechanisms that control fluid and sodium balance
- Know the control and actions of ADH (anti-passing urine) and ang ll
Why do we need a kidney?
- There is always a gap between intake and output of body fluids. The kidney allows you to maintain a large range of fluid intakes and keep osmolality and volume relatively constant
- the kidneys are important for BP, acid base, osm, blood filtering and hormone release
What hormones do the kidneys release?
- renin (RLS for ang ll production)
- erythropoietin to stimulate RBC production and so is important in determining the O2 carrying capacity of the blood
What happens with dehydration summary
- water deficit OR increase in salt
- increase in ECF osmolality
- osmoreceptors sense increase and signal adh secretion from post pit
- increase in adh permeability in distal tubules and collecting ducts
- increased water retention reduced excretion
> feel thirsty when either salt increases or fluid decreases
Do barorecptors influence osmoreceptors?
Yes
Where do you find osmoreceptors?
Osmoreceptors are found in supraoptic and paraventricular nuclei
How do osmoreceptors work?
They sense both changes in osmolality and circulating blood volume and result in adh release.
Inputs to the hypothalamus from the medullary vasomotor center which receive input from the baroreceptors increase adh release in response to a reduced circulating volume.
How do changes in omsol cause a response from osmoreceptors
Small changes in osmol cause small deformations of neurons. In hyperosmotic fluid the size of the neuron changes and opens gap junction channels which causes the neuron to depolarise or repolarise and hence send a signal to the hypothal (if structures are cross-linked)
What is adh released in response to
An increase of less than 1% osmolarity or decreased volume (greater than 10%) of the ECF
What is ADH secretion most sensitive to?
Secretion of ADH in response to increased osmolarity has a LOWER threshold and higher sensitivity/slope than its response to a decreased EC volume
Physiological stimuli to adh secretion
- Increased plasma osmol 1-2%
2. Decreased ECF vol 7-10%
Non-physiological stimuli to adh secretion
- pain, stress
- drugs: nicotine, SSRIs
- Carcinomas
- CNS disorders
- alcohol inhibits ADH secretion
How does ADH work?
Enters the blood/renal interstitial fluid and causes insertion of AQP2 on the tubular lumen causing increased water reabsorption and reduced ECF osmol until the signal decreases (FB loop)
Occurs by ADH stimulating V2 receptor > cAMP > PKA > protein phosp
What is thirst driven by?
High osmol not vol
What senses changes in BV?
Cardio-pulmonary volume receptors - also sends info to osmoreceptors
Where does adh act?
Distal tubule, cortical collecting tubule and outer and inner medullary collecting ducts
What is it called if ADH levels are low
Either central or nephrogenic diabetes insipidus
Central Diabetes Insipidus (neurogenic or pituitary DI)
- inadequate ADH secretion/production
- is a problem with hypothal or pp
- may be due to brain injury, tumor or infection
- rarely hereditary
- can be treated with ADH analogues as receptors work fine
- ADH is absent
Nephrogenic DI
- collecting tubule is unresponsive to ADH
- conc urine cannot be produced
- can be caused by certain drugs i.e. lithium
- less commonly hereditary i.e. congenitial defect in the V2 receptor or Aquaporin2
- is NOT currently treatable
- sufficient ADH is being released by kidney is unresponsive
What receptor does ADH work on?
V2
How to tell apart neurogenic/central diabetes insipidus and nephrogenic?
- is there ADH circulating
- Are they responsive to ADH
- water deprivation test
What is the water deprivation test?
Used to decipher between the 2 types of DI. An ADH analogue is given. If urine osmol increases i.e. there is a response to ADH and more water is reabs then it is central DI
What is SIADH
Syndrome of inappropriate ADH secretion
SIADH
- defect in appropriate ADH secretion
- higher than normal ADH levels and so patient reabsorbs and retains water inappropriately and their ECF omsol decreases.
- if their water intake is not tightly controlled then a hypo-osmolar state can result which is not good
