Fluid Compartments Flashcards

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1
Q

What is a fluid?

A

A substance that deforms under a shear stress

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2
Q

What is a phsiologically important fluid?

A

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

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3
Q

What are the key types of fluid?

A
  • Intracellular water
  • Interstitial water (filling the spaces between cells amongst the extracellular matrix)
  • Fat
  • Plasma
  • Transcellular fluid (separate by interstitial water by a membrane)
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4
Q

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

A

42L

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5
Q

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

A

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

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6
Q

What is peritoneal fluid?

A

fluid in abdomen

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7
Q

What is a haematocrit?

A

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

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8
Q

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

A

RBCs
WBCs
plasma

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9
Q

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

A

PH regulation

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10
Q

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

A

Half are free

Half are bound to plasma or tissue proteins

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11
Q

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

A

The conc of cations

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12
Q

Is there more extracellular or intracellular HCO3^-?

A

Extracellular

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13
Q

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

A

-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

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14
Q

What are the destructive methods of measuring fluid compartments?

A
  • plasma volume by exsanguination and centrifugation (not ethical)
  • Total body water by weighing a body and then desiccating it then reweighing it
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15
Q

Explain a good way of measuring fluid body compartments

A

-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
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16
Q

What is the Vd of heparin?

A

3.5L

17
Q

What does insulin bind to?

A

Plasma proteins

18
Q

Why is ethanol a good substance for Vd?

A

It has a high water and fat solubility

19
Q

Why is gentamicin a good substance for Vd?

A

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

20
Q

What units can Vd sometimes be measured in?

A

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

21
Q

How do you measure the amount of total body water?

A
  • 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
22
Q

What is the marker used to measure plasma volume?

A

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

23
Q

What are the markers used to measure extracellular fluid?

A

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

24
Q

Which of the markers for extracellular fluid works best?

A

thiosulphate/thiocyanide works best

can’t measure transcellular liquid

25
Q

What is an osmole?

A

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

26
Q

What’s the difference between osmolality and osmolarity?

A
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)
27
Q

Explain osmotic pressure

A

-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)

28
Q

What is the equation to calculate osmotic pressure?

A

Osmotic pressure = nCRT

nC= osmolality (0.28osm.kg^-1) 
R= ideal gas constant (0.082L.atmmol^-1.K^-1) 
T= temp (310K)
29
Q

What is osmotic pressure directly proportional to?

A

Osmolality

30
Q

What happens if two solutions are isosmotic?

A

share the same osmolality

31
Q

What is an isotonic solution?

A

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

32
Q

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

A

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

33
Q

what causes the main osmotic forces in capillaries?

A

Due to proteins (oncotic pressure)

34
Q

What does a fall in plasma albumin cause?

A

liver failure, protein malnutrition, renal failure

35
Q

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

A
  • 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
36
Q

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

A

Oedema