Fluid Compartments

Question 1

What is a fluid?

Answer 1

A substance that deforms under a shear stress

Question 2

What is a phsiologically important fluid?

Answer 2

Fluids in which water or fat/lipids are the solvent.

Question 3

What are the key types of fluid?

Answer 3

• 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 4

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

Answer 4

42L

Question 5

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

Answer 5

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

Question 6

What is peritoneal fluid?

Answer 6

fluid in abdomen

Question 7

What is a haematocrit?

Answer 7

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

Question 8

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

Answer 8

RBCs
WBCs
plasma

Question 9

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

Answer 9

PH regulation

Question 10

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

Answer 10

Half are free

Half are bound to plasma or tissue proteins

Question 11

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

Answer 11

The conc of cations

Question 12

Is there more extracellular or intracellular HCO3^-?

Answer 12

Extracellular

Question 13

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

Answer 13

-Half Ca^2+ in circulation is bound to albumin which means that changing the albumin concentration changes the total Ca2+ conc without changing the free Ca2+ ion concentration

Question 14

What are the destructive methods of measuring fluid compartments?

Answer 14

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

Question 15

Explain a good way of measuring fluid body compartments

Answer 15

-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 16

What is the Vd of heparin?

Answer 16

3.5L

Question 17

What does insulin bind to?

Answer 17

Plasma proteins

Question 18

Why is ethanol a good substance for Vd?

Answer 18

It has a high water and fat solubility

Question 19

Why is gentamicin a good substance for Vd?

Answer 19

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

Question 20

What units can Vd sometimes be measured in?

Answer 20

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

Question 21

How do you measure the amount of total body water?

Answer 21

• 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 22

What is the marker used to measure plasma volume?

Answer 22

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

Question 23

What are the markers used to measure extracellular fluid?

Answer 23

36Cl^- (although some passes intracellularly)
thiosulphate/thiocyanide
inulin

Question 24

Which of the markers for extracellular fluid works best?

Answer 24

thiosulphate/thiocyanide works best

can’t measure transcellular liquid

Question 25

What is an osmole?

Answer 25

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

Question 26

What’s the difference between osmolality and osmolarity?

Answer 26

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 27

Explain osmotic pressure

Answer 27

-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 28

What is the equation to calculate osmotic pressure?

Answer 28

Osmotic pressure = nCRT

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

Question 29

What is osmotic pressure directly proportional to?

Answer 29

Osmolality

Question 30

What happens if two solutions are isosmotic?

Answer 30

share the same osmolality

Question 31

What is an isotonic solution?

Answer 31

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

Question 32

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

Answer 32

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 33

what causes the main osmotic forces in capillaries?

Answer 33

Due to proteins (oncotic pressure)

Question 34

What does a fall in plasma albumin cause?

Answer 34

liver failure, protein malnutrition, renal failure

Question 35

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

Answer 35

• 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

Question 36

What is a symptom of liver failure, protein malnutrition, renal failure?

Answer 36

Oedema