What is osmolarity?
Concentration of osmotically active particles (cause osmosis) in a solution
What is the unit of osmolarity?
Osmol / L
How do you calculate osmolarity?
no. of osmotically active particles
e.g 150mM NaCl
molar concentration = 150mM, 2 particles, 2x150 = 300 mosmol/l
or 100mM MgCl3 -> 100mM x 3 = 300 mosmol / l
What is the osmolarity of body fluid?
300 mosmol / L
What is the difference between osmolarity and osmolality?
Osmolarity measured in osmol/L
Osmolality measured in osmol/kg
pretty much the same
What is tonicity?
Effect a solution has on cell volume
REFERS TO SOLUTE CONCENTRATION
a) If cell is ISOTONIC to solution, no change in cell volume
b) If cell is HYPOTONIC (i.e lower water content) to solution, INCREASE in cell volume > bursts
c) If cell is HYPERTONIC to solution, DECREASE in cell volume > shrinkage
Can solutions with the same osmolarity have different tonicities?
Cell membranes have different permeabilities
e.g Urea and sucrose are both isotonic to cell
Cell is very permeable to urea, water then follows urea causing lysis
compared to sucrose, which cell membrane is impermeable to, so no change in volume
Which sex has a greater body water content?
Females have a greater fat content, which doesn't contain water
Which two compartments is body water held in?
Intracellular fluid (ICF)
Extracellular fluid (ECF):
ECF divided into plasma and interstitial fluid
Which body compartment has a greater volume?
Intracellular fluid (ICF)
Which compartment of the extracellular fluid has a greater volume?
How can the volume of body compartments be measured?
What is the equation for total body water content?
TBW = ECF + ICF
To calculate volume of distribution:
add known mass of tracer to unknown volume of water
take known volume of water and calculate concentration of tracer
volume of distribution = mass / concentration
What is the main input of water to the body?
Food and drink
What is the main output of water from the body?
On average, how much water is input and output every day?
What changes if there is a water imbalance in the body?
Why can urine production never be turned off?
Toxic metabolites need to leave the body in solution i.e water
Which ions are found mainly
b) outside the cell?
a) K+, Mg2+ negatively charged ions
b) Na+, Cl-
think cells swimming in salty water
What structures separate the
a) plasma from the interstitial fluid
b) the interstitial fluid from the intracellular fluid?
a) Capillary wall
b) Plasma membrane
What is fluid shift?
Movement of water between ICF and ECF by osmotic gradient
Osmolarity ___ as water leaves a compartment.
Water leaves > More solute = INCREASED OSMOLARITY
Water enters > Less solute = DECREASED OSMOLARITY
What happens if the osmotic concentration of the ECF decreases?
Concentration of solute decreases (i.e water has entered ECF)
So osmotic gradient between high (ECF) and low (ICF)
So fluid shift to ICF
If osmotic concentration of the ECF increases, what happens?
Osmotic gradient from high (ICF) to low (ECF)
So fluid shift from ICF to ECF
As osmolarity increases there is a fluid shift (to / from) that compartment.
fluid shift TO that compartment
As osmolarity decreases there is a fluid shift (to / from) that compartment.
fluid shift FROM that compartment
Which organs regulate the composition and volume of the ECF?
How does the osmolarity of the ECF affect blood pressure?
If low, water moves to ICF, so plasma volume decreases, so BP decreases
If high, water moves to ECF, so plasma volume increases, so BP increases