water handling Flashcards
normal serum osmolality
280-295 mOsm/kg
Causes of hyponatremia and which is more common
Deficit of ECF sodium or excess of water. Most commonly cased by relative excess of water
Equation for serum osmolality
Sosm (mOsm/kg) = 2 X [Na (mEq/L)] + BUN (mg/dL)/2.8 + Glucose (mg/dL)/18
List Features of a normal diluting system
Normal function of diluting segment, normal delivery of tubular fluid to the distal diluting segment of the nephron and absence of vasopressin (ADH)
Which parts of the nephron are diluting segments
Ascending limb of Loop of Henle and distal convoluted tubule. These regions are impermeable to water and fluid is diluted by reabsorption of NaCl via Na-K-2Cl co transporter (ascending limb) or NaCl cotransporter (distal tubule)
Importance of normal delivery of tubular fluid to distal diluting segments
Although tubular fluid remains isotonic in the proximal tubule, proximal reabsorption is an important determinant of water excretion. Thus, if proximal reabsorption increases and causes decreased distal delivery, the volume of dilute urine excreted will be decreased
Vasopressins functions
Insertion of aquaporins into collecting duct allows for water reabsorption and a more concentrated urine. In absence of vasopressin, collecting duct is impermeable to wate.
Does excessive water intake cause hyponatremia
Only if it exceeds 1L/ hour or if GFR is reduced
Minimum amount of urine produced each day to maintain water balance
0.5L
What 3 features are require for concentrating mechanism
Ability to generate hypertonic interstitium, secretion of ADH and responsiveness of collecting duct to ADH
Describe how a hypertonic interstitium is made in kidneys
Active transport of chloride in the water impermeable thick ascending limb of the loop of Henle both dilutes tubular fluid and makes interstitium hypertonic. This is why water is reabsorbed if collecting duct is made permeable to water
Regulators of ADH release and which is dominant
Changes in serum osmolarity and blood volume. ADH is normally osmoregulatory, but during stress (hypovolemia) becomes a volume regulatory hormone
3 categories of hyponatremia
hypertonic, isotonic, or hypotonic
Serum osmolarity of hyprtonic, isotonic and hypotonic hyponatremia
Hypertonic >300mOsm/Kg, Isotonic 280-300mOsm/Kg, Hypotonic <280 mOsm/kg
Describe Hypertonic hyponatremia and causes
Hyponatremia is due to the shift of water from cells in response to a non-sodium solute (elevated serum osmolality). Common causes of this type of hyponatremia are hyperglycemia and mannitol or glycerol administration. For each increase in serum glucose of 100 mg/dL, serum sodium will fall by about 1.6 mEq/L
Describe isotonic hyponatremia and causes
Normal plasma osmolality. Due to a lab artifact caused by hyperlipidemia or hyperproteinemia (ie. Multiple myeloma). Plasma is 93% water and 7% lipids/proteins so conditions that elevate lipids/proteins will thus decrease water (and Na) in the sodium analysis
Describe Hypotonic hyponatremia and causes
alteration of the ratio between ECF Na and ECF water.
Types of hypotonic hyponatremia
ECF volum can be hypovolemic (Na decreases a lot and water decreases a little), euvolemic (Na stays same but water increases a little) or hypervolemic (Na increases a little but water increases a lot)
What causes hypovolemic hypotonic hyponatremia and is ADH release appropriate, and treatment
ADH release is non-osmotic and “appropriate” to help defend ECF volume. Causes include: GI losses, diuretics, salt-losing nephritis, osmotic diuresis (solutes like glucose increase urine flow) or mineralcorticoid deficiency (increased ADH secretion). Treat with normal saline
What causes hypervolemic hypotonic hyponatremia and is ADH release appropriate and treatment
In congestive heart failure, hepatic cirrhosis, and nephrotic syndrome vascular volume is sensed as decreased (because of low pressure in atrium) despite the overall increase in total body salt and water, so ADH is increased appropriately to the signal. In advanced chronic or acute renal an inability to lose free water because of compromisd GFR leads to hypervolemic hyponatremia. Treat with water/salt restriction, loop diuretics, and inotropes for CHF
Urine sodium levels in heart failure, cirrhosis and nephrotic syndrome
Can be low
What causes euvolemic hypotonic hyponatremia and is ADH release appropriate and treatment
ADH secretion is increased despite the absence of the two physiologic stimuli for its release, thus it is inappropriate. Caused by hyperthyroidism and drugs (nicotine, SSRIs, isoproterenol, etc), adrenal insufficiency (deficiency of glucocorticoids), primary polydispsia (excessive water intake) or syndrome of inappropriate ADH secretion (caused by carcinomas, pulmonary disease or CNS disorders). Treat with hypertonic saline if seizures, otherwise water restriction and ADH antagonists
Signs of hyponatremia
Anorexia, nausea and vomiting occur early. Thereafter altered sensorium develops. Seizures occur with severe or acute hyponatremia. All likely due to cerebral edema
Treatment of hyponatremia
Water restrictions, hypertonic NaCl solution +/- furosemide (to increase free water excretion).
Causes of hypernatremia
Decreased total body Na (total body water loss is»_space; total body salt loss such as GI loss, skin burns or diuretics), Increased total body Na (receiving hypertonic fluid) or normal total body Na (ADH deficiency in central diabetes insipidus or ADH resistance in nephrogenic diabetes insipidus)
Central Diabetes Insipidus description, cause, response to exogenous ADH
An ADH deficiency caused by head trauma, surgery, neoplasms or idiopathic. Kidneys respond to exogenous ADH. Patients are not able to concentrate urine and drink a lot of fluids
Nephrogenic Diabetes Insipidus description, cause, response to exogenous ADH
The renal collecting duct does not respond appropriately to ADH, so exogenous AVP will not change the Uosm significantly. Can be congenital or acquired (from chronic renal failure, hypercalcemia and hypokalemia or drugs such as ethanol)
Signs of hypernatremia
Cell dehydration leads to neuro irritability with twitches, hyperreflexia, seizures, coma and death.
Treatment of hypernatremia
Treatment is directed at restoring serum tonicity to normal and correcting sodium imbalances. Thus, sodium may need to be added or removed while providing water. To calculate how much water to give: Water needed (L) = 0.6 x body weight in kg x [(actual sodium/desired sodium) - 1 ]
