Fluid Compartments Flashcards
What is a fluid?
A substance that deforms under a sheer stress
What are the key types of fluid?
- Intracellular water
- Interstitial water (filling the spaces between cells amongst the extracellular matrix)
- Fat
- Plasma
- Transcellular fluid (separate by interstitial water by a membrane)
For a 70kg person, how much is the total body water?
42L
How much of the 42L is found in each fluid type?
Plasma 3L
Interstitial fluid 10L
Transcellular fluid 1L
Intracellular 28L
What is peritoneal fluid?
fluid in abdomen
What is a haematocrit?
A measure of the proportion of the blood occupied by cells (usually 45%)
When you separate blood in a centrifuge, what order do the substances separate? (heaviest first)
RBCs
WBCs
plasma
What is the role of HCO3^- in the body?
PH regulation
What are the two ways that Ca^2+ ions exist in the body?
Half are free
Half are bound to plasma or tissue proteins
What tends to match the conc of anions in the body?
The conc of cations
Is there more extracellular or intracellular HCO3^-?
Extracellular
How can the total Ca^2+ conc be altered without changing the free Ca^2+ conc?
-Half Ca^2+ in circulation is bound to albumin which means that changing the albumin concentration changes the total Ca2+ conc
What are the destructive methods of measuring fluid compartments?
- plasma volume by exsanguination and centrifugation (not ethical)
- Total body water by weighing a body and then desiccating it then reweighing it
Explain a good way of measuring fluid body compartments
-inject a substance which is known to distribute in a given compartment and then calculate the volume of distribution (Vd)
Vd = Q/Cp
Q= amount of drug Cp= plasma conc of drug
What is the Vd of heparin?
3.5L
What does insulin bind to?
Plasma proteins
Why is ethanol a good substance for Vd?
It has a high water and fat solubility
Why is gentamicin a good substance for Vd?
Small enough to get into capillaries but not cross into cells
Also strongly polar
What units can Vd sometimes be measured in?
L/Kg so just times by Kg to work out per L
How do you measure the amount of total body water?
- You need a substance that mixes uniformly with water, use marked water deuterium or tritium
- calculate using Vd equation
- larger volume of water in body = lower conc of deuterated water
What is the marker used to measure plasma volume/
Labelled proteins injected intravascularly with Evan’s blue (binds to plasma proteins)
What are the markers used to measure extracellular fluid?
36Cl^- (although some passes intracellularly)
thiosulphate/thiocyanide
inulin
None of these are perfect but thiosulphate/thiocyanide works best
(can’t measure transcellular liquid)
What is an osmole?
A measure of the number of moles that a compound dissociates into when dissolved in the solution
What’s the difference between osmolality and osmolarity?
Osmolality= number of osmoses per unit mass of the solvent (Osm.kg^-1) Osmolarity= number of osmoles per unit volume of the solution (Osm.l^-1)
Explain osmotic pressure
-At surface between two solutions molecules are exchanged due to diffusion
-If the conc of any species is different on either side of the interface there will be a net movement of molecules from one side to the other
-In the case of water, the force (per unit area) required to oppose the movement is osmotic pressure
(interface is usually a semi-permeable membrane)
(amount of pressure required to oppose osmosis)
What is the equation to calculate osmotic pressure?
osmotic pressure = nCRT
nC= osmolality (0.28osm.kg^-1) R= ideal gas constant (0.082L.atmmol^-1.K^-1) T= temp (310K)
What is osmotic pressure directly proportional to?
osmolality
What happens if two solutions are isosmotic?
share the same osmolality
What is an isotonic solution?
A solution that when applied to cells there will be no net fluid movement
What is an example of a solution thats isosmotic with respect to intracellular fluid but not isotonic?
Urea because it can pass freely through plasma membranes but is not isotonic because when it enters cells it increases the intracellular osmotic pressure and hence causes water to enter the cells
what causes the main osmotic forces in capillaries?
Due to proteins (oncotic pressure)
What does a fall in plasma albumin cause?
liver failure, protein malnutrition, renal failure
What is mannitol and what is its role as a drug?
- A stable sugar alcohol
- When injected intravascularly increases the plasma and extracellular space osmolality
- This pulls water from intracellular space and transcellular spaces
- Used to decrease intracranial pressure
- also an osmotic diuretic
