Water Metabolism Flashcards
Osmolality, calculated osmolality, osmolar gap
Concentration of osmotically active molecules
Calculated Osm = 2x Na + Urea + Glucose
Osmolar gap = Measured - Calculated
- -> <10 is normal
- -> >10 = extra osmotically active substances in blood e.g. alcohol, sugars such as mannitol or sorbitol, lipids, proteins
Tonicity and osmoles
Concentration of effective osmoles
- determines water movement across cells
Effective osmoles = Na, K, glucose (in diabetic)
Ineffective osmoles = urea, alcohol (but still contributes to osmolality)
Distribution of body water and daily requirements
Total body water = 60% body weight
- 2/3 in ICF, 1/3 in ECF
- within ECF: 3/4 interstitial, 1/4 intravascular
Minimum water intake = 1100 ml/day
Minimum urine output = 500 ml/day (0.5 ml/kg/day)
Dehydration vs Hypovolemia
Dehydration = loss of free water without Na (hypotonic loss)
- cause increased Osm and hyperNa
- early response: thirst; late response: BP and HR
- manage by giving free water e.g. water, D5
Hypovolemia = isotonic loss of Na and water
- caused decreased ECF volume (ECV)
- early response: raise HR and decrease BP; late response: thirst
- manage by giving NS
Regulation of blood volume loss (Na)
- -> decrease low pressure baroreceptor activation = decrease ANP
- -> decreased CO = increase RAAS (chemoreceptors) and SNS (increase PVR)
- -> stimulate osmoreceptors (later response) = thirst, ADH
absolute vs relative hypovolemia
- actual loss of fluid e.g. GI bleed, diuresis, burns
- relative e.g. 3rd space loss/ nephrotic syndrome, capillary leak in sepsis, sequestration in IO
Regulation of free water loss (water)
- -> increase ADH
- -> stimulate hypothalamic thirst centre (outside BBB)
- -> shift of water from ICF to ECF
Functions of ADH
Respond to Osm (1% change) and blood volume (10% change)
- V1: vasoconstriction
- V2: increase AQ-2 channels at DCT and collecting ducts for water reabsorption
Minimal ADH if Osm <280, maximal if >295
Activated in severe hypovolemia (preserve vol before Osm)
Mechanism of thirst
Increased Osm –> +ve osmoreceptors –> hypothalamic thirst centre outside BBB –> +ve median preoptic nucleus
IV fluids
Normal saline (0.9% NaCl) - isotonic --> stays in ECF
D5 (5% dextrose)
- isotonic in bag but quickly metabolised in body to leave free water –> goes to ECF and ICF
- good for dehydration
Colloids (e.g. gelofusin)
- isotonic but oncotic pressure slowly distributes in body
- 50% intravascular and 50% interstitial
- good blood volume expanders
Diabetes insipidus - causes, manifestations, investigation, treatment
Causes:
- cranial –> commonly primary hypothalamic disease; tumour, vascular
- nephrogenic –> familial, hypoK, hyperCa, Li toxicity
Manifests as:
- polyuria with dilute urine
- polydipsia
- euvolemic, normal Na and Osm (unless insufficient water intake – hyperNa and dehydrations)
Investigation:
- fluid deprivation test
Treatment:
- treat underlying cause!
- cranial: DDAVP
- nephrogenic: thiazide (increase Na excretion) and amiloride (decrease K loss)
Fluid deprivation test procedure and interpretation
Test ability to concentrate urine
- supervised water deprivation for 8 hrs then give DDAVP
- measure Uosm
- caution: stop if BW decrease by >3%
Uosm >750 = normal
300-750 = primary polydipsia (down regulation of ADH)
<300 = DI
==> >50% increase after DDAVP = cranial DI
==> <50% increase after DDAVP = nephrogenic DI
SIADH
Causes:
- malignancy e.g. SCLC, Ca prostate
- lung pathologies e.g. TB, pneumonia
- encephalitis
- surgery, drugs e.g. SSRI
- lots of causes! – very common
Manifests as:
- inappropriately concentrated urine in presence of hypotonic blood
- euvolemic (aldosterone response), hypotonic hypoNa (dilutional and depletion at later stage)
Investigation:
- Uosm:Sosm
- UNa
Treatment:
- fluid restriction:1200ml/day
- salt PO (IV rare)
Diagnosis of SIADH
Diagnosis of exclusion!
- Uosm > 100 or Uosm>Sosm
- euvolemic with UNa>40
- ruled out renal, cardiac or endocrine causes
- not on diuretics
- improves with fluid restriction
DDx: cerebral salt wasting (hypovolemic)
