lec 7 - water movement

where water is gained or lost from 

always from ECF irst - transitional compartment 

total body water 

adult male ~60% 
adult female ~55%
of body weight 

total body water distributions 

ECF = 33% of TBW
ICF = 67% of TBW
interstitial fluid = 75% ECF 
plasma 20% ECF 
transcellular fluid 5% ECF 

osmotic equilibrium

water moves freely between the ICF and ECF comparments
the volume and composition of body fluid compartments are highly regulated homeostatic variables 
deviation in fluid homeostasis results in illness and may be life-threatening 

net diffusion of water across a membrane

diffusional movement of water from a region of higher water concentratio to an area of lower concentration 
change in water concentration due to difference in solute concentration 

driving force for water movement 

passive movement
cell membranes cannot move water against a gradient
driving force is the difference in osmotic pressure across the membrane 
in certain tissues (e.g. capillaries, glomerulus) hydrostatic pressures also influences water movement (not for cells) 

osmotic pressure equation (JH2O)

JH2O = Lp (Δπ - ΔP)
Lp = hydraulic conductivity (how easily does water cross membrane) 
Δπ = osmotic pressure difference 
ΔP = hydrostatic pressure difference (not for cells)

osmotic pressure

the pulling force acting on water 
water moves by osmosis down its concentration gradient - where there are low solutes/a lot of water -> where there are many solutes/less water 
osmosis continues until the solute/water concentrations in both compartments are equal

measuring osmotic pressure 

force is applied to exactly oppose osmosis 
force is applied to compartment B (hydrostatic pressure) until osmosis stops 
means of quantitatively measuring the osmotic driving force in an artificial system 
animal cell membranes cant withstand hydrostatic pressure gradients, so dont measure osmotic pressure experimentally 

osmotic pressure - van't Hoff equation 

π = RTCi
π = osmotic pressure (Pa) 
R = gas constant (8.314 JK-1mol-1)
T = temperature in K (310.15K/37 degrees in biological systems)
C = total solute concentration (molL-1)
i = number of solute molecules formed by dissociation 

osmolarity - calculation

molarity (molL-1) x particles formed from dissociation = osmolarity (osmolL-1)
so a particle that dissociates e.g. NaCl has to be multiplied by 2 becuase it disociates into 2 ions, but things that don't dissociate are multiplied by 1 

hyposmotic solutions

<275 mOsmL-1
fewer solute molecules 
lower osmorality
higher water concentration 

isosmotic solutions 

~300mOsmL-1 
similar number of solute molecules 
similar osmolarity 
similar water concentration

hyperosmotic solutions 

>310 mOsmL-1 
more solute molecules 
higher osmolarity 
lower water concentration

diffusion of water across the lipid bilayer 

water is a small molecule so can directly diffuse through lipid bilayer 
relatively slow (low hydraulic conductivity) and unregulated 
all biological membranes are somewhat permeable to water, but some cell membranes (e.g. red blood cells, proximal renal tubule) have very high hydraulic conductivities 
aquaporins increase the conductivity of the membrane 

aquaporins 

non-gated tunnel
small membrane bound water pores 
100 fold increase in hydaulic conductivity - highly selective for water over ions 
selectivity determined by asparagine residues (prevents ion passage)
AQP2 (in distal nephron) is under hormonal control
difference in osmotic pressure still provides the driving force 

tonicity - what 

describes the volume change of a cell

isotonic solution

if the cell in the solution does not change size/volume at equilibrium = the solution is isotonic 
no net movement of water -> no change in cell size 

hypertonic solution

if the cell loses water and shrinks at equilibrium then the solution is hypertonic 
net movement of water out of the cells -> cells shrink 

osmolarity vs tonicity

osmolarity:
either solution or the cell itself
chemical measure of solutes in solution
canbe calculated or measured (mOsmL-1)
tonicity:
describes only the solution
experimentally determined - what does the solution do to cell volume 
determined by osmolarity and solute permeability i.e. whether the solute in solution can enter the cell

IV solutions 

the tonicity not the osmolarity is what is important
0.9% NaCl
5% glucose 
both isosmotic 

IV solutions - NaCl

isosmotic 150mmolL-1 NaCl is isotonic (Na+ and Cl- are both non-penetrating solutes)

IV solutions - glucose 

at equilibrium in a healthy person IV osmostic glucose is effectively hypotonic
in isolated red blood cells, glucose is considered a non-penetrating solute 

omslarity vs tonicity