Fluid Compartments

Question 1

What is a fluid?

Answer 1

A substance that deforms under a sheer stress

Question 2

What are the key types of fluid?

Answer 2

• Intracellular water
• Interstitial water (filling the spaces between cells amongst the extracellular matrix)
• Fat
• Plasma
• Transcellular fluid (separate by interstitial water by a membrane)

Question 3

For a 70kg person, how much is the total body water?

Answer 3

42L

Question 4

How much of the 42L is found in each fluid type?

Answer 4

Plasma 3L
Interstitial fluid 10L
Transcellular fluid 1L
Intracellular 28L

Question 5

What is peritoneal fluid?

Answer 5

fluid in abdomen

Question 6

What is a haematocrit?

Answer 6

A measure of the proportion of the blood occupied by cells (usually 45%)

Question 7

When you separate blood in a centrifuge, what order do the substances separate? (heaviest first)

Answer 7

RBCs
WBCs
plasma

Question 8

What is the role of HCO3^- in the body?

Answer 8

PH regulation

Question 9

What are the two ways that Ca^2+ ions exist in the body?

Answer 9

Half are free

Half are bound to plasma or tissue proteins

Question 10

What tends to match the conc of anions in the body?

Answer 10

The conc of cations

Question 11

Is there more extracellular or intracellular HCO3^-?

Answer 11

Extracellular

Question 12

How can the total Ca^2+ conc be altered without changing the free Ca^2+ conc?

Answer 12

-Half Ca^2+ in circulation is bound to albumin which means that changing the albumin concentration changes the total Ca2+ conc

Question 13

What are the destructive methods of measuring fluid compartments?

Answer 13

• plasma volume by exsanguination and centrifugation (not ethical)
• Total body water by weighing a body and then desiccating it then reweighing it

Question 14

Explain a good way of measuring fluid body compartments

Answer 14

-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

Question 15

What is the Vd of heparin?

Answer 15

3.5L

Question 16

What does insulin bind to?

Answer 16

Plasma proteins

Question 17

Why is ethanol a good substance for Vd?

Answer 17

It has a high water and fat solubility

Question 18

Why is gentamicin a good substance for Vd?

Answer 18

Small enough to get into capillaries but not cross into cells
Also strongly polar

Question 19

What units can Vd sometimes be measured in?

Answer 19

L/Kg so just times by Kg to work out per L

Question 20

How do you measure the amount of total body water?

Answer 20

• 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

Question 21

What is the marker used to measure plasma volume/

Answer 21

Labelled proteins injected intravascularly with Evan’s blue (binds to plasma proteins)

Question 22

What are the markers used to measure extracellular fluid?

Answer 22

36Cl^- (although some passes intracellularly)
thiosulphate/thiocyanide
inulin
None of these are perfect but thiosulphate/thiocyanide works best
(can’t measure transcellular liquid)

Question 23

What is an osmole?

Answer 23

A measure of the number of moles that a compound dissociates into when dissolved in the solution

Question 24

What’s the difference between osmolality and osmolarity?

Answer 24

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)

Question 25

Explain osmotic pressure

Answer 25

-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)

Question 26

What is the equation to calculate osmotic pressure?

Answer 26

osmotic pressure = nCRT

nC= osmolality (0.28osm.kg^-1)
R= ideal gas constant (0.082L.atmmol^-1.K^-1)
T= temp (310K)

Question 27

What is osmotic pressure directly proportional to?

Answer 27

osmolality

Question 28

What happens if two solutions are isosmotic?

Answer 28

share the same osmolality

Question 29

What is an isotonic solution?

Answer 29

A solution that when applied to cells there will be no net fluid movement

Question 30

What is an example of a solution thats isosmotic with respect to intracellular fluid but not isotonic?

Answer 30

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

Question 31

what causes the main osmotic forces in capillaries?

Answer 31

Due to proteins (oncotic pressure)

Question 32

What does a fall in plasma albumin cause?

Answer 32

liver failure, protein malnutrition, renal failure

Question 33

What is mannitol and what is its role as a drug?

Answer 33

• 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