Osmolality Flashcards
Osmolality
Relative water content
Decreased osmolality = relatively more water than solute
Increased osmolality = relatively less water than solute
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?
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
Why is water balance so tightly regulated?
To maintain cellular volume bc cell shrinking & swelling must be avoided
What increases body water content?
Oral: ingesting water or dilute beverages
IV: administration of iV fluids
How does the body sense increase/decrease in water?
Hypothalmic osmoreceptor cells: swell or shrink
What’s the pathway to water excretion?
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)
- inhibition of thirst –> increased water excretion by kidney (goal is to generate water from salt water– remove NaCl and expel dilute urine)
- Inhibition of vasopressin release –> increased water excretion by kidney
What is concentrated urine? Dilute urine? Isothenuric urine?
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
How is dilute urine made in the tubule?
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”
What are the limits of water filtration? i.e. reduced filtration, normal filtration, upper limit of filtration?
Normal is 10-20 L/day
Reduced is 2-4 L/day
20 is the max! Otherwise you get hypoosmotic cells
What determines the limits of filtration/day?
- Volume filtered that reaches diluting segment
- Absence of vasopressin
- Quantity of osmotic particles that need to be excreted
What can decrease water body content?
Sweating (skin)
Secretions in GI tract
Kidney: water not reabsorbed
Lung: water exhaled in air (up to 1L/day)
What’s the mechanism for water conservation after water loss?
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!
How do you form a concentrated urine?
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!
Which common drugs tempers the body’s response to vasopressin?
NSAIDS: bc PG’s enhance vasopressin’s action
What’s the minimum volume you’d expect a person to make a day?
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
What are the possibilities if you have a high Na level but an inappropriately dilute urine?
Vasopressin is not present
Vasopressin is unable to cause urine concentration
What is nephrogenic diabetes insipidus?
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)
Vasopressin
Released from the hypothalamus as a result of cellular shrinking. Can be due to:
- reduced total body water content
- increased extracellular osmolality i.e. from eating salt
What’s the difference between sodium ingestion and water loss?
Both follow same pathway: thirst, vasopressin release
But eating salt results in volume expansion and a dilution of vasopressin
