SM 197: Water + K Balance

Question 1

Osmoregulation: Sensed variable, Sensors, Effectors, Controlled Variable

Answer 1

Sensed variable: Plasma osmolality
Sensors: Hypothalamic Osmoreceptors
Effectors: Thirst + ADH
Controlled Variable: Water Excretion and Intake

Question 2

Disorders of too much and too little water

Answer 2

Water Excess = Hyponatremia

Water Deficit = Hypernatremia

Question 3

How much solute is excreted during an average day?

What are the cutoffs for Oliguria, Anuria, Polyuria?

Answer 3

Average: 600mOsm solute excreted
Oliguria: <0.4L/day
Anuria: <0.1L/day
Polyruia: >2.5-3L/day

Question 4

What is needed for urine concentration? (4 things)

Answer 4

1. Central stimulation for ADH release
2. Functional V2R on Principal Cells of Cortical CD
3. Functional AQP2 in CD Cells
4. Maintained Hypertonic Medullary Interstitium

Question 5

What are the mechanisms for maintaining medullary hyperosmolarity (3)?

Answer 5

1. Countercurrent multiplier (separates water/solute, establishes osmotic gradient)
2. Urea Cycling from CD to TAL (strengthens osmotic gradient)
3. Countercurrent Exchange - Peritubular Capillaries carry away reabsorbed solutes and water without ruining gradient

Question 6

How do osmotic diuretics work? (ex: mannitol)

Answer 6

Freely filtered but not reabsorbed

Increases tubular osmolality = less water reabsorption

Question 7

What stimulates ADH release?
What is the mechanism of ADH?
What are the 2 different types of Diabetes Insipidus? What are the symptoms?

Answer 7

Stimuli: High plasma osmolarity, Volume Depletion
More ADH = trafficking of AQPs to insert into membrane
1. Neurogenic DI - central problem, deficiency in ADH
2. Nephrogenic DI - kidney problem, can’t change in response to ADH (X-linked = V2R mutation, AR/AD = mutation in AQP2, Acquired = Li+ treatment for bipolar disorder inhibits AQP2 expression
Sx: persistent thirst = high water intake + polyuria (if no access to water, susceptible to dehydration)

Question 8

What 4 factors promote cellular intake of K?

Answer 8

1. High Na/K ATPase activity
2. Insulin = more Na/K ATPase
3. Catecholamines = more Na/K ATPase
4. Alkalosis = changes membrane distribution of K

Question 9

What 4 factors promote cellular efflux of K?

Answer 9

1. Exercise = muscle contraction = release of K
2. ECF Hyperosmolarity = osmotic drag
3. Cell breakdown = lysis releases K stores
4. Acidosis

Question 10

What factors promote K secretion?

Answer 10

1. High Na Reabsorption in Cortical CD (Aldosterone)
2. High Tubular Fluid Flow
3. High Bicarb/Anions in Tubular Fluid (Charge Gradient)

Question 11

What factors impair K Secretion?

Answer 11

1. Low Na Reabsorption (ARBs, ACE-i, K-sparing diuretics)

2. Low Tubular Fluid Flow