L8. Body fluid compartments Flashcards
How does water move between compartments
Osmotically through all permeable membranes (not the kidneys, ureters and bladder). It flows from low conc to higher conc of osmotically active molecules. 1/3 of all body water is extracellular with 80% being interstitial fluid.
What is the difference between osmolarity and osmolality
Osmolality: number of osmotically active particles per unit weight of solvent. (miliosmoles/kg). This determines the osmotic pressure exerted by a solution across a membrane.
Osmolarity is the number of osmotically active particles per litre of total solution- (miliosmole/ L)
We use osmolality bc they are both very similar for clinical purposes
What is tonicity , how does a cell act in hypo, hypertonic ,isotonic solution
Describes the osmotic pressure a solute exerts across a cell membrane- therefore causing movement of water. It only applies to osmotically active solutes that can’t cross a membrane so its a property of a solution in reference to a particular membrane, unlike osmolality.
in Hypo solution: cells swell- more particles in the cell
in Hyper solution: makes cell shrink
isotonic: water moves at same rate in and out -same size
What is the Gibbs-donnan equilibrium and its relevance to voltage gradients in the cell
Charged particles separated by a semi permeable membrane can fail to distribute evenly across the membrane in the presence of a non diffusible ion- eg. proteins in the cell that are impermeable (-)
This means that the movement of anions down the conc gradient into the cell triggers an electrical gradient causing cations to balance the charge, but this sets up a competing conc gradient for cations to move out of the cell.
Competing electrical and concentration gradients mean that at equilibrium, the side with the negatively charged non diffusible proteins = more negatively charged = voltage gradient
As a consequence of the voltage gradient, more osmotically active molecules in the cell mean what and how does the cell cope with this?
Water flows into the cell. (oncotic pressure). This has risk of bursting so they manage this by pumping out osmotically active Na+ ions using the Na/K ATPase transporter and replacing it by the smaller K+ ion.
The strength of K+ on the inside ( + proteins in the cell) balances the strength of Na+ outside. This means that ICF and interstitial fluid (and plasma) are isotonic
What is the osmolality of the ICF, Interstitial fluid and Plasma
All the same strength of attraction for water (300 mosm/kg) despite different composition making the charges balanced.
eg. ICF has dominance of K+ and - charged protein.
ECF has dominance of Na+ (and Cl-)
How is the tonicity/ osmolality of the ECF controlled and why is it important
1-2% range
The ECF is the solution that the cells are put into so if the osmolality goes up or down this can change whether water is going out of the cells or into them, both harming the brain.
These changes are fixed by adding water or taking away water from the ECF (by the kidney) in correspondence to the amount of Na+ lost or gained to keep the same osmolality.
How does the Kidney regulate water and salt homeostasis
The intake of water and salt from food, drink and metabolism is balanced by the urine and this output and osmolality of the urine varies depending on the intake.
So when input high Na+, there is weight gain (increased ECF volume due to water retention (thirst) in order to maintain constant osmolality until increased salt excretion is able to get going. ECF volume returns to normal if less Na+ is ingested or renal Na+ excretion increases with diuretic.
As ECF volume increases, BP increases and renal Na+ loss increases due to pressure natriuresis- helps to restore ECF to baseline.
What happens when there is increased water intake vs isotonic saline vs glucose infusion (via dextrose)
Free water dilutes the osmolality of the ECF so urine production increases rapidly,
However saline doesn’t change the ECF osmolality so it just increases the ECF volume to replace volume loss with neglible change in urine.
Dextrose infusion has the same osmolality however it is metabolised to water or becomes bound to glycogen so eventually will dilute all compartments
Hypertonic and hypotonic saline infusion are rare
What are the hormones/systems that effect osmolality and ECF volume control and compare the level of regulation between osmolality and ECF volume.
What determines the volume of ECF
Osmolality: Antidiuretic hormone- water reabsorbed or not. Tightly regulated (1-2%)
ECF volume: Renin-angiotensin system and sympathetic ns. Varies continuously (15%)
Because the osmolality of the ECF is kept constant, the amount of na+ in the body determines the volume of the ECF. To maintain constant ECF, amount of Na+ excreted needs to = Na+ input by diet
