Control Abnormalities Body Water Flashcards
What happens if you add more water to a Na+ solution?
→ Hyponatremia
→ Concentration decreases
What happens if you remove water from a Na+ solution?
→ Hypernatremia
→ Concentration increases
What does osmolarity relate to?
→ Number of particles per unit volume of fluid
What does osmolality relate to?
→ Number of particles per unit weight of fluid
What is the equation for estimating plasma osmolarity?
→ 2[Na+] + 2[K+] + [glucose] + [urea]
What is normal plasma osmolality?
→ 275-295 mOsm kg-1
What does hypernatremia mean?
→ hyper osmolality
→ too little water
What does hyponatremia mean?
→ hypo-osmolality
→ too much water
What happens during diabetes mellitus?
→ Diabetes mellitus the glucose concentration rises
→ It contributes to the osmolality
→ high glucose concentration is filtered into the kidney tubule
What are osmoreceptors?
→ Sensory receptors located in the hypothalamus sense changes in osmolality of ECFV
What does an increase in osmolality stimulate?
→ Thirst
→ Secretion of vasopressin
How does ADH regulate plasma osmolality?
→Controlling water excretion and reabsorption
What does concentrated urine tell you about ADH levels?
→ ADH relatively high
Describe the loop of henle
→ Tubular fluid is iso-osmotic in the PCT
→ along the descending limb there is active reabsorption of Na+
→ Water moves out passively
→The loop has very concentrated interstitial fluid
→ in the ascending loop there is dilution of the tubular fluid as there is reabsorption of NaCl
→ at the DCT the tubular fluid is the most dilute
What is the effect of ADH on the collecting duct?
→ ADH binds to basolateral receptors
→ more water channels inserted in the luminal membrane
What is the mechanism ADH uses?
1) ADH binds to the receptor V2 on principal cells
2) Receptor activates cAMP
3) inserts AQP2 into apical membrane
4) V1 receptors on vascular smooth muscle - vasoconstriction but only significant with very high ADH levels- increases systemic BP
What is the function of ADH under normal conditions?
→ Osmoregulation
What does a large drop in arterial pressure cause?
→ release of ADH
Why is ADH secreted during hypovolaemia?
→ retention of water to increase blood volume
What happens to plasma osmolality during severe haemorrhage?
→ Loss of BP is sufficient to stimulate ADH
→ decrease in plasma osmolality
→ ADH effect is always dilutional (adds pure water)
What are the two systems activated during hypovolaemia?
→ RAAS occurs first
→ ADH secretion happens later
What is the first line of defense against dehydration?
→ Thirst
What kind of thirsts are there?
→ hyperosmotic
→ hypovolemic
What is hyperosmotic thirst?
→ after eating a lot of salt
What is hypovolemic thirst?
→ After losing a lot of blood
What is the metabolic waste product in the kidney excreted as?
→ in a solute form
→ called the solute load
How do you estimate the solute load?
→ body weight x 10
How do you work out how much water is needed to excrete the solute load?
→ divide the urine concentration by the solute load
What is water excess (hyponatremia)?
→ excessive water intake
→ impairment in renal water excretion
What is water depletion (hypernatremia) ?
→ Insufficient water intake
→ Impairment in renal water reabsorption
What are situations where water intake is continued but ADH is suppressed?
→ vomiting , diarrhoea → Ecstasy → Ectopic secretion of ADH (SIADH) → Hypocortisolism → Primary adrenal insufficiency ( Addisons)
What is missing in people with Addisons?
→ Loss of cortisol
→ Loss of aldosterone
What is the effect of Addisons?
→ Lack of sodium retention
→ Water is lost with it
→ hyponatremia due to water intake
What are the major causes of SIADH(syndrome of inappropriate antidiuretic hormone secretion )?
→ TUMOR - ectopic production of ADH
→ CNS disturbance - trauma, stroke, infection
→ DRUGS - carbamazepine, prozac
What are signs of SIADH?
Low plasma sodium
→ Low plasma osmolality
→ High urine osmolality
How can water depletion from decreased intake of water occur?
→ Infants
→ Elderly
→ Individuals in a coma
→ Individuals with no access to water
How can water depletion from increased loss of water through the kidney occur?
→ Diabetes mellitus
→ Impairment in ADH release
What are nephrogenic causes of diabetes insipidus?
→ Mutation of ADH receptor
→ Mutation of ADH dependent H2O channels
→ Renal disease
→ Lithium
What are central causes of diabetes insipidus?
→ genetic mutations
→ head trauma
→ disease of the hypothalamus /pituitary region
What is the ECF divided into?
the interstitial compartment and the plasma compartment.
Movement between the two is controlled by starling’s forces
What happens when pure water was added to ECF?
initially the concentration in ECF will be diluted but then this creates osmotic gradient between ICF and ECF resulting in movement of water into ICF.
How is the osmolarity of the ECF adjusted?
adding or removing water, not solute
What happens as a defence against dehydration with ADH?
→Net water loss increases ECF osmolarity
→Changes detected by osmoreceptors in anterior hypothalamus
→Project to magnocellular neurons of paraventricular and supraoptic nuclei of hypothalamus
→PVN and SON neurons release ADH from their axon terminals in posterior pituitary
What is the normal range for ADH?
Normal range 285-295 mOsm/kg
Above this range small changes in osmolality produce large changes in ADH secretion
What happens to stimulate thirst in defence against dehydration?
Changes detected by osmoreceptors in anterior hypothalamus
→Strong desire to drink when plasma osmolality ≥295 mOsm/kg
→Large (10-15%) drops in blood volume/pressure
→Angiotensin 2 acting on hypothalamus in significant drop of blood volume
What other receptors reduce thirst?
Oropharyngeal and upper gastrointestinal receptors reduce thirst on drinking- unless osmolarity is brought back within controlled range, thirst sensation returns
What does principle of electroneutrality dictate?
dictates that a molar equivalent number of anions must be present
→Mainly Cl-, significant amount HCO3-, small contribution from other inorganic and organic anions
Causes of hypernatremia in gain of sodium…
→Iatrogenic- mistakes in i.v
→Excess ingestion (rare)
→Excess mineralocorticoid activity
What is diabetes insipidus?
a condition characterized by large amounts of dilute urine and increased thirst.
What happens in severe volume depletion?
the retention of water will lead to hyponatraemia
Why is ADH less effective to retain water than RAAS?
ADH promotes retention of pure water which will distribute between ICF and ECF so effect is weaker
What happens in hypervolemic hyponatremia?
→Can occur when total sodium is increased, but total water increased more
→Example: congestive heart failure
→RAAS ‘thinks’ body is hypovolemic
→Na/water retention (volume expansion)
Volume expansion is ineffective because perturbed Starling forces (excess capillary filtration): oedema
→If low volume signals activate ADH hyponatremia will ensue
→If continued intake of water then hyponatremia
