Osmoregulation Flashcards
(15 cards)
give a review of fluid compartments
review of fluid compartments:
-body water; 60% body weight
-intracellular water; 40% body weight
-extracellular water; 20% body weight
-60/40/20 rule
-osmosis determines movement of fluid between ICFV and ECFV
explain how changes in plasma [Na] suggests excess or deficit of body water
Changes in plasma [Na] suggest excess or deficit of body water:
-assuming no change in total body Na+, a reduction in water volume will increase Na concentration causing hypernatremia.
-assuming no change in total body Na+, an increase in water volume will reduce Na concentration causing hyponatremia
explain how plasma is the main determinant of ECFV osmolality
-osmolality relates to the number of particles per unit volume of fluid
-osmolality; per kg water
-diference in plasma <2%. Important for high accuracy. changes in either are similar in terms of cause and effect.
-Na+ is the principle cation in the ECFV
-Plasma osmolarity in mOsm L-1 can be estimated from: 2[Na] + 2[K] + glucose + urea
explain plasma and osmolality
-Hypernatremia always means hyperosmolality
-hyponatremia usually means hypo-osmolality body fluids with exceptions
-hypernatremia does not mean too much Na; it means too little water
how is osmolality of ECFV adjusted
-osmoreceptors; sensory receptors located in hypothalamus sense changes in osmolality of ECFV.
-increase in osmolality stimulates; thirst and secretion of vasopressin (ADH)
Explain how ADH (vasopressin) is the osmoregulation hormone
ADH (vasopressin) is the osmoregulation hormone:
-regulates plasma osmolality primarily controlling water excretion and reabsorption
-excretion of water is normally regulated independently of excretion of solute
-this means that the kidney must be able to excrete urine that is either hyperosmotic (retaining water) or hypo-osmotic with respect to ECF.
explain how ADH increases water permeability in the collecting duct and the mechanism of action of ADH in distal tubule and collecting duct
ADH increases water permeability in the collecting duct:
-ADH binds to basolateral receptors on the collecting duct and inserts aquaporins into the luminal membrane –> this increases permeability to water so more water is reabsorbed
explain how ADH secretion is regulated by two physiological mechanisms
1) Under normal conditions, main function of ADH is osmoregulation—-> however when there is a large drop in arterial pressure, this can be a powerful stimulus for ADH release.
2) Osmoreceptors in the hypothalamus monitor the plasma. they send signals to the neurosecretory cells of the hypothalamus to change ADH secretion.
-baroreceptors are activated when there is a large drop in pressure. Have the same effect on neurosecretory cells.
-RAAS is the first line of defence to hypovolemia as it secretes aldosterone which reduces Na+ excretion.
-ADH is the second line of defence + reduces water excretion.
explain volume depletion and thirst and water intake
Volume depletion:
-ADH increases the amount of water being reabsorbs which dilates fluid so osmolality decreases
thirst and water intake:
-thirst is the first and highly defensive line of defence against dehydration;
* hyperosmotic (salty meal)
* hypervolemic (loss of volume) thirst
Explain ADH , water balance and limits of renal excretion
-normally the vasopressin system can maintain plasma osmolarity at about 285mOsmol L-1 in the face of variable water intake.
-kidneys must excrete a significant amount of metabolic waste product in solute form (solute load)
explain abnormalities of water balalnce
abnormalities of water balance:
-water excess; excessive water intake or impairment in renal water excretion]
-water depletion; insufficient water intake or impairment in renal water absorption
explain how continued water intake with failure to suppress ADH can lead to water overload and hyponatremia
-vomiting, diarrhoea
-certain drugs (MDMA, ‘ectasy’ promotes ADH secretion)
-ectopic secretion of ADH
-hypocortisolism
-primary adrenal insufficiency (Addison’s disease)
explain characteristics of Addison’s disease
-loss of cortisol (high levels of CRH which increase vasopressin), androgens and aldosterone.
-high ACTH
-Causes are usually autoimmune
Syndrome of inappropiate ADH secretion:
-excessive ADH reduces urinary excretion of water
results in a state of water excess and;
* low plasma Na+
* low plasma osmolality
* high urine osmolality
major causes:
-tumour- ecotopic production of ADH, small cell carcinoma of the lungs
-CNS disturbance- enhanced ADH release stroke, trauma, infection
-drugs- enhanced release of ADH or response to ADH, carbamezepine, prozac.
explain water depletion
1) water depletion from decreased intake of water can occur in:
-infants
-elderly
-individuals in coma
-individuals with no access to water
2) water depletion from incresed loss of water through the kidney can occur in:
-diabetes mellitus
-impairment in ADH release and/or action
Discuss polydipsia and some causes of diabetes insipidus
Polydipsia (abnormally great thirst); more water being drunk and more urine produced.
some causes of diabetes insipidus:
Central DI:
-lack of secretion of ADH
-genetic mutations
-head trauma
-disease of the hypothalamus/pituitary region
Nephogenic DI:
-impaired response to ADH
-mutation of ADH receptor
-mutation of ADH- dependent H20 channels
-renal disease
-drugs
