The Body Fluids

Question 1

List the sources of water intake and the volume of water absorbed by each.

Answer 1

1 - Drinking (1500ml).

2 - Food (500ml).

3 - Metabolism (400ml).

Question 2

List the routes of water output and the volume of water lost by each.

Answer 2

1 - Faeces (100ml).

2 - Skin (400ml).

3 - Lungs (400ml).

4 - Urine (1500ml).

Question 3

List 3 key compartments in which body fluids are stored.

Give the volume of water contained within each for a 70kg male.

Answer 3

1 - Intracellular (28L).

2 - Extracellular (14L):

• Interstitial (amongst the extracellular matrix - 10L).
• Intravascular (but only the plasma as the RBCs themselves contribute to the intracellular volume) (3L).
• Transcellular (separated from the extracellular fluid by an epithelial membrane, e.g. CSF - 1L).

3 - In fat.

Question 4

List 4 examples of transcellular fluid.

Answer 4

1 - Cerebrospinal fluid.

2 - Peritoneal fluid.

3 - Pleural fluid.

4 - Sinovial fluid.

Question 5

What is the total body water volume for a 70kg male?

Answer 5

~42L.

Question 6

List 2 examples of destructive methods of measuring fluid compartments.

Answer 6

1 - For plasma volume, exsanguination and centrifugation.

2 - For total body water, weigh a body, dessicate then reweigh.

Question 7

Define volume of distribution.

Give the equation of volume of distribution.

Answer 7

• 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.
• Vd = Q / Cp
• Q = amount of drug, Cp = plasma concentration.
• E.g. with a drug that absorbs well in fats: plasma concentration will be very low (because drug absorbs in fats), so Cp will be very low, so Vd will be large.
• The lowest Vd possible is the volume of the plasma, can be found with a drug that absorbs amazingly in plasma.
• Since the drug distributes in the same way as the compartment in which it dissolves, it’s similar to measuring the compartment directly.

Question 8

How would the volume of distribution differ for a drug that is highly lipid soluble (non-polar) compared to one that is highly water soluble (polar)?

Answer 8

A drug that is highly lipid soluble (non-polar) would have a larger volume of distribution.

Question 9

How can total body water be found using the volume of distribution?

Answer 9

By administering marked water (deuterium or tritium) and measuring the volume of distribution.

Question 10

Which substance is normally used to measure the plasma volume using volume of distribution?

Answer 10

Evan’s blue (a labelled plasma protein).

Question 11

Which substance is normally used to measure the volume of extracellular fluid using volume of distribution?

Answer 11

Thiosulfate / thiocyanate.

Question 12

List the approximate values for the extracellular and intracellular concentrations of Na+, K+, Ca2+, Cl-, HCO3- and glucose.

Answer 12

Na+:

• Extracellular: 140mM.
• Intracellular: 15mM.

K+:

• Extracellular: 4mM
• Intracellular: 140mM

Ca2+:

• Extracellular: 2.4mM
• Intracellular: 0.1uM

Cl-:

• Extracellular: 110mM
• Intracellular: 4mM

HCO3-:

• Extracellular: 25mM
• Intracellular: 12mM

Glucose:

• Extracellular: 4mM
• Intracellular: Depends on the cell.

Question 13

What is the approximate osmolality of the extracellular fluid?

Answer 13

285mOsm kg^-1.

*Equal to the sum of all extracellular osmolalities.

Question 14

What proportion of extracellular calcium is present as free ions?

To which molecule is the rest bound?

Answer 14

• ~50%

- The other half is bound to albumin.

Question 15

Why must Ca2+ levels be corrected in a patient with hypoalbuminaemia?

Answer 15

• In a patient with normal albumin levels, more of the total (measured) Ca2+ will be bound to albumin.
• The amount of free Ca2+ will therefore be greater than normal in a patient with hypoalbuminaemia.
• This is important as only free Ca2+ is physiologically active.

Question 16

What is the equation for corrected Ca2+?

Answer 16

Corrected Ca2+ mM = total Ca2+ mM + 0.02 * (40 - albumin g/L)

Question 17

Define osmole.

Answer 17

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

• E.g. 100mmol of NaCl yields 200mOsm in solution as it dissociates into Na+ and Cl-.

Question 18

What is the difference between osmolarity and osmolality?

Answer 18

• Osmolarity is the number of osmoles per unit volume.

- Osmolality is the number of osmoles per unit mass.

Question 19

Define osmotic pressure.

Answer 19

• The pressure required to oppose osmosis.
• It is therefore a measure of the force of osmosis, which in turn reflects the concentration of the solute.
• It is the inverse of water potential, which is a measure of the potential of water to move from one area to another (high with more dilute solutions).

Question 20

Define hydrostatic pressure.

Answer 20

The pressure exerted by any fluid in a confined space.

Question 21

Define isosmotic.

Answer 21

Sharing the same osmolality.

• Two solutions can be isosmolar even if they contain different types of solutes - it is the amount that matters.

Question 22

Define isotonic.

Answer 22

Sharing the same osmotic pressure.

• If is a solution is isotonic, then applying the solution
  to cells (traditionally red blood cells) will not cause
  net fluid movement.

Question 23

Which molecules are mainly responsible for the osmotic forces controlling water movement in capillaries?

Why do these molecules exert a higher osmotic force than other molecules in the blood?

Answer 23

Proteins, as the smaller ions (which are able to cross capillaries) are in equilibrium.

Question 24

What is the name of the osmotic pressure due to plasma proteins?

Answer 24

The oncotic pressure.

Question 25

What are the two primary forces that determine water movement across solutions?

Answer 25

Hydrostatic pressure (driving) and osmotic pressure (resisting).

Question 26

Why might liver or renal failure cause oedema and ascites?

Answer 26

• Liver failure will result in decreased production of plasma proteins.
• Renal failure will result in an increased loss of plasma proteins.
• This will reduce oncotic pressure in the blood, reducing resistance to osmosis and allowing water to accumulate in tissues.

Question 27

What is mannitol and how does it work as an osmotic diuretic?

Give an example of its use.

Answer 27

• Mannitol is a stable sugar.
• When injected intravascularly, it increases plasma osmolality.
• This results in water movement by osmosis from the intracellular and transcellular spaces into the intravascular space.
• One of its uses is to decrease intracranial pressure following intracranial haemorrhage.