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
