2 Body Fluids Flashcards
What is our total water intake daily?
2100ml
How much do we urinate daily?
400-2300ml
Name 5 fluid compartments in the body
intracellular water interstitial water fat plasma transcellular fluid
What is total body water?
42L for a 70 kg person
greater in men than women
decreases with age (sarcopenia)
how much of our total body fluid is extracellular?
Plasma 3L
interstitial 10L
How much of our total body fluid is intracellular?
28L
How much of our total body fluid is transcellular?
1L
Name 4 examples of transcellular compartments
Peritoneal space
CSF
Pleural cavity
synovial fluid
What techniques would we have used in the past to estimate body fluid consumption?
weigh a body, desiccate it, then re-weigh
What is the volume of distribution?
the volume of fluid required to contain the total amount of drug in the body at the same concentration as that present in the plasma
V = Q/C
What kinds of molecules would we use to measure plasma volume?
large molecules (heparin) protein binding molecules (insulin)
anything that will stay in the plasma
What should we do when measuring volume of distribution?
leave the chemical time to equilibrate
What kind of molecule would we use to measure the volume of the extracellular space?
small enough to get across capillary, but too big to get into cells
we can use charged / polar molecules (gentamicin)
Why could you not use tetrahydrocannabinol to measure fat volume?
it is lipophilc yes, but it gives an apparent volume of distribution of 700L
this doesn’t work, as the plasma is not part of fat distribution
How can we measure total body water?
labellling water with deuterium or tritium
What marker could you use for plasma volume?
Evan’s blue (labelled protein injected intravascularly)
What markers could you use for extracellular fluid?
36Cl
Thiosulphate
Thiocyanate
What is Haematocrit?
the measure of the proportion of the blood occupied by cells
(usually around 45%)
Why is haematocrit useful in physiology?
in physiology plasma flow rate is often used over blood flow rate, haematocrit allows us to switch between the two
What is the listed extracellular calcium?
the sum of the bound and free calcium, they are roughly equal
What is the rate of filtration of calcium equal to?
half the listed extracellular volume, so 1.2mM from 2.4 mM, as the kidney does not easily remove protein bound substances
What do we need to watch out for when we get a blood count of calcium?
amount of protein
What is used to calculate the ‘free calcium’ by clinicians?
‘corrected calcium’, which is just a modified form of free calcium
What is normal albumin level?
40g/L
What is osmole?
a measure of the number of molecules that a compound dissociates into when dissolved in solution
How many osmoles would 100mmol NaCl form in solution?
100 Na+ + 100 Cl- = 200mOsm
What is osmole useful for?
measuring osmotic forces
What is osmolality?
The number of osmoles per unit mass of the solvent
Osm.kg-1
What is osmolarity?
the number of osmoles per unit volume of solution
Osm-1
Why do osmolarity and osmolality not differ much?
the density of water is very close to 1kg/L
when you dissolve a solvent in a solute, the volume doesn’t change very much
what is osmotic pressure?
the force per unit area required to oppose a new movement
How long will water keep moving into an area with more water?
until the added hydrostatic pressure resist the flow of incoming water
What is the osmotic pressure equal to for practical purposes?
osmolality
How can we prevent large volume shifts in plasma?
Keeping osmolality constant
What does isosmotic mean?
2 solutions share the same osmolality
What does isotonic mean?
applying the solution to cells (traditionally RBC’s) will not cause net fluid movement
What would happen if a semipermeable membrane separated 2 isotonic soluions?
there would be no net movement across the membrane
If a membrane permeable only to urea and water separated a solution of urea, and a solution of something else, what would happen?
the urea would move into the other compartment, and water would follow
What is the net movement of water across capillary walls a balance between?
hydrostatic force and osmotic pressures
Across the capillary membrane what is the main osmotic pressure due to?
large proteins, as the small ions are in equilibrium
this force is called oncotic pressure
What is the most abundant protein in the blood?
albumin
What might cause a fall in albumin levels?
fall in production - liver failure
increased loss - renal failure
What is the effect of decreased albumin levels?
less of a force keeping water in circulation
water moves out of circulation into other spaces
What might albumin deficiency cause?
peripheral / pulmonary oedema
abdomen swelling (ascites)
brain swelling - pretty bad
How is albumin deficiency treated?
Why?
What other use does it have?
mannitol
inert
transiently increases plasma osmotic pressure, pulling water back
it is cleared by the kidney, taking water with it
an inhaled cystic fibrosis management (dilutes airways by moving water)
