Disorders of Water Metabolism

Question 1

1. List the components of water balance (both input and output).

Answer 1

intake is managed by GI absorption and thirst (often modified by ADH)
65% of water that is filtered is reabsorbed in the PCT
ascending limb is the most important limb in nephron for water balance although no water is reabsorbed here
distal tubule has more Na reabsorption

collecting duct is important for last bit of water reabsorption, largely governed by ADH

Question 2

2. Explain what controls the secretion of antidiuretic hormone. Discuss physiologic stimuli.

Answer 2

the main controller at higher volumes is osmolality

at very low volumes ADH is triggered by strongly by the perceived drop in pressure

Question 3

3. Explain the interaction between circulating volume and plasma osmolality.

Answer 3

hypo-osmotic circulating volume will cause water to travel to the ICF compartment

hyper-osmotic circulating volume will cause water to travel into the ECF compartment, specifically the intravascular compartment

Question 4

4. Explain the interaction between circulating volume and plasma osmolality.

Answer 4

osmolality of is inversely proportionate to ICF volume

osmolality is a window into ICF volume

most of the tonicity in molecules that contribute to osmolality is represented by Na+ an therefore Na+ is a reasonable approximation of osmolality (glucose and urea are disregarded in MOST cases)

Question 5

5. Describe the clinical conditions that are associated with excess or decreased antidiuretic hormone secretion.

Answer 5

diabetes insipidus
central DI (not make ADH): neurosurgery, trauma, various infection(affecting the posterior pituitary)

nephrogenic DI: receptor (lithium, hypokalemia, hypercalcemia, interstitial diseases to the kidney)

differentiated by water deprivation test

Question 6

6. Compare and contrast a water diuresis and a solute diuresis.

Answer 6

aldosterone will cause solute diuresis and solutes will be followed by water, this can be encouraged by blocking of Na channels by diuretic agents

water diuresis occurs when aquaporins are not inserted into luminal side of the collecting ducts, largely controlled by ADH

Question 7

Define osmolality.

Answer 7

osmolality= total osmoles/ total water

Question 8

Name the two major disorders of water balance.

Answer 8

hyponatremia (too much water)

hypernatremia (too little water)

Question 9

What is the normal range of serum [Na+]?

Answer 9

145-135, usually symptoms and treatment occur below 130 or above 150 (meq/L)

Question 10

What is the action of ADH binding in the kidney

Answer 10

ADH binds with V2 receptors to cause insertion of AQP in the luminal side

Question 11

Describe non-physiologic stimuli for ADH release.

Answer 11

any disease of the CNS

lung disease

nausea or pain

Question 12

What detects blood osmolarity in order to regulate ADH levels?

Answer 12

hypothalamic chemoreceptors

remember, ADH and osmolality move in the same direction when the body is working normally

Question 13

What are the clinical and laboratory signs of sodium imbalance?

Answer 13

clinical: confusion, seizure, coma, death

lab(+++++): serum and urine osmolality, serum Na

Question 14

What are the clinical and laboratory signs of ECF fluid balance?

Answer 14

clinical (+++) orthostatic hypotension, tachycardia, low BP, dry mucous membranes, weight loss
lab: hemoconcentration, high uric acid, urine Na and Cl

Question 15

When working with a patient who is hyponatremic, what are 3 important questions to remember?

Answer 15

1. What is the serum osmolality?
2. What is the ECF volume status
3. In hyponatremia with normal ECF, is the kidney behaving normally?

Question 16

In a hyponatriumic state, what would account for hyperosmotoic or isosmotic serum?

Answer 16

high serum osmolaltiy and low serum Na implies water is being dragged from ICF (ie. glucose in blood)

if there is significant volume of plasma occupied by a substance other than water, plasma sodium will appear low

Question 17

In a patient with hyponatremia with normal ECF volume, what is likely the cause if the kidney is working normally vs. not (increased ADH).

Answer 17

if kidney is working appropriately, the patient is likely drinking lots of water, if the patient has inappropriate secretion of ADH, they will urinate frequently, a dilute urine

Question 18

How would you treat different causes hyponatremia based on ECF status?

Answer 18

low ECF, give salt and water
normal ECF, restrict water intake
high ECF, remove salt and water (diuretics)

Question 19

In patients with hypernatremia, what would be the cause of low, normal and high ECF?

Answer 19

low ECF- loss of more water than salt: diarrhea, burns/sweat, renal (diuretics)