Osmolality Flashcards

1
Q

Osmolality

A

Relative water content

Decreased osmolality = relatively more water than solute

Increased osmolality = relatively less water than solute

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2
Q

What is the relationship between cellular volume and extracellular osmolality i.e. if you are in a desert? if you eat something salty like pizza?

A

If you’re in a desert: at first, extracellular volume decreases –> increase in solute conc extracellularly –> water flows out of cells –> cells shrink

If you eat pizza: your volume stays the same but you have higher conc of extracellular solute –> water leaves the cell –> cell’s size decreases, solute concentration increases

Body’s response to both: osmo receptors in hypothalmus make you thirsty –> pituitary glands release vasopressin –> goes to blood vessels & down to kidney –> tells kidney to conserve H2O

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3
Q

Why is water balance so tightly regulated?

A

To maintain cellular volume bc cell shrinking & swelling must be avoided

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4
Q

What increases body water content?

A

Oral: ingesting water or dilute beverages

IV: administration of iV fluids

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5
Q

How does the body sense increase/decrease in water?

A

Hypothalmic osmoreceptor cells: swell or shrink

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6
Q

What’s the pathway to water excretion?

A

Senses cell swelling on osmole receptors of hypothalamus (either a reduced extracellular volume OR an increase in osmolality – usually coulped to reduced total body water content)

  1. inhibition of thirst –> increased water excretion by kidney (goal is to generate water from salt water– remove NaCl and expel dilute urine)
  2. Inhibition of vasopressin release –> increased water excretion by kidney
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7
Q

What is concentrated urine? Dilute urine? Isothenuric urine?

A

Concentrated >285 mOsm, up to 1200 mOsm (maximally concentrated urine)

Dilute = < 285 mOsm, specific gravity = 1.001-1.005

Isothenuric = 285 mOsm, specific gravity = 1.010

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8
Q

How is dilute urine made in the tubule?

A

Occurs in the thick ascending limb & distal tubule by reabsorping Na

Must escape reabsorption - collecting duct must be impermeable to water = NO VASOPRESSIN can be around

Formation of “free water”

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9
Q

What are the limits of water filtration? i.e. reduced filtration, normal filtration, upper limit of filtration?

A

Normal is 10-20 L/day

Reduced is 2-4 L/day

20 is the max! Otherwise you get hypoosmotic cells

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10
Q

What determines the limits of filtration/day?

A
  1. Volume filtered that reaches diluting segment
  2. Absence of vasopressin
  3. Quantity of osmotic particles that need to be excreted
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11
Q

What can decrease water body content?

A

Sweating (skin)

Secretions in GI tract

Kidney: water not reabsorbed

Lung: water exhaled in air (up to 1L/day)

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12
Q

What’s the mechanism for water conservation after water loss?

A

Concentration of plasma, ECV and ICV decreases –> shrinking of hypothalamic osmoreceptor cells –>

(1) thirst –> ingestion of water
(2) Vasopressin release –> decreased water excretion by kidney (concentrated urine)

Both restore water content!

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13
Q

How do you form a concentrated urine?

A

Tubule is surrounded by a hyperosmotic interstitium (medulla)

It draws out water from the collecting duct –> urine becomes concentrated & water is conserved

Progressively more concnetrated moving from cortex to tip of medulla: urea and NaCl helps create this hyperosmotic part of the medulla

You remove water!

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14
Q

Which common drugs tempers the body’s response to vasopressin?

A

NSAIDS: bc PG’s enhance vasopressin’s action

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15
Q

What’s the minimum volume you’d expect a person to make a day?

A

Depends on their diet, but assuming the average person excretes 600 mOsms waste/day –> if it were in a maximally concentrated urine (1200)= 500 ml/24 hours

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16
Q

What are the possibilities if you have a high Na level but an inappropriately dilute urine?

A

Vasopressin is not present

Vasopressin is unable to cause urine concentration

17
Q

What is nephrogenic diabetes insipidus?

A

When the body can’t respond to vasopressin:

(1) vasopressin receptor mutation (AVPR2) = congenital, x-linked recessive
(2) Aquaporin receptor mutation (AQP2) = autosomal (recessive and dominant forms)

18
Q

Vasopressin

A

Released from the hypothalamus as a result of cellular shrinking. Can be due to:

  1. reduced total body water content
  2. increased extracellular osmolality i.e. from eating salt
19
Q

What’s the difference between sodium ingestion and water loss?

A

Both follow same pathway: thirst, vasopressin release

But eating salt results in volume expansion and a dilution of vasopressin