water handling

Question 1

normal serum osmolality

Answer 1

280-295 mOsm/kg

Question 2

Causes of hyponatremia and which is more common

Answer 2

Deficit of ECF sodium or excess of water. Most commonly cased by relative excess of water

Question 3

Equation for serum osmolality

Answer 3

Sosm (mOsm/kg) = 2 X [Na (mEq/L)] + BUN (mg/dL)/2.8 + Glucose (mg/dL)/18

Question 4

List Features of a normal diluting system

Answer 4

Normal function of diluting segment, normal delivery of tubular fluid to the distal diluting segment of the nephron and absence of vasopressin (ADH)

Question 5

Which parts of the nephron are diluting segments

Answer 5

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)

Question 6

Importance of normal delivery of tubular fluid to distal diluting segments

Answer 6

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

Question 7

Vasopressins functions

Answer 7

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.

Question 8

Does excessive water intake cause hyponatremia

Answer 8

Only if it exceeds 1L/ hour or if GFR is reduced

Question 9

Minimum amount of urine produced each day to maintain water balance

Answer 9

0.5L

Question 10

What 3 features are require for concentrating mechanism

Answer 10

Ability to generate hypertonic interstitium, secretion of ADH and responsiveness of collecting duct to ADH

Question 11

Describe how a hypertonic interstitium is made in kidneys

Answer 11

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

Question 12

Regulators of ADH release and which is dominant

Answer 12

Changes in serum osmolarity and blood volume. ADH is normally osmoregulatory, but during stress (hypovolemia) becomes a volume regulatory hormone

Question 13

3 categories of hyponatremia

Answer 13

hypertonic, isotonic, or hypotonic

Question 14

Serum osmolarity of hyprtonic, isotonic and hypotonic hyponatremia

Answer 14

Hypertonic >300mOsm/Kg, Isotonic 280-300mOsm/Kg, Hypotonic <280 mOsm/kg

Question 15

Describe Hypertonic hyponatremia and causes

Answer 15

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

Question 16

Describe isotonic hyponatremia and causes

Answer 16

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

Question 17

Describe Hypotonic hyponatremia and causes

Answer 17

alteration of the ratio between ECF Na and ECF water.

Question 18

Types of hypotonic hyponatremia

Answer 18

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)

Question 19

What causes hypovolemic hypotonic hyponatremia and is ADH release appropriate, and treatment

Answer 19

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

Question 20

What causes hypervolemic hypotonic hyponatremia and is ADH release appropriate and treatment

Answer 20

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

Question 21

Urine sodium levels in heart failure, cirrhosis and nephrotic syndrome

Answer 21

Can be low

Question 22

What causes euvolemic hypotonic hyponatremia and is ADH release appropriate and treatment

Answer 22

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

Question 23

Signs of hyponatremia

Answer 23

Anorexia, nausea and vomiting occur early. Thereafter altered sensorium develops. Seizures occur with severe or acute hyponatremia. All likely due to cerebral edema

Question 24

Treatment of hyponatremia

Answer 24

Water restrictions, hypertonic NaCl solution +/- furosemide (to increase free water excretion).

Question 25

Causes of hypernatremia

Answer 25

Decreased total body Na (total body water loss is&raquo_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)

Question 26

Central Diabetes Insipidus description, cause, response to exogenous ADH

Answer 26

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

Question 27

Nephrogenic Diabetes Insipidus description, cause, response to exogenous ADH

Answer 27

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)

Question 28

Signs of hypernatremia

Answer 28

Cell dehydration leads to neuro irritability with twitches, hyperreflexia, seizures, coma and death.

Question 29

Treatment of hypernatremia

Answer 29

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 ]