Body Fluids

Question 1

What are the average inputs and outputs:

• drinking
• food
• metabolism
• lungs
• skin
• faeces
• urine

Answer 1

1500ml
500ml
400ml

lungs
skin
faeces
urine

Question 2

Of the outputs, ……… is under the greatest independent control in order to maintain ………… body balance. The others can be highly …………, depending on ……., ………. ….. and …….

Answer 2

urine
water
variable
temperature, metabolic rate and diet

Question 3

Define a fluid

Answer 3

A substance that deforms under a shear stress. In physiology, the important floods are those in which water, or a fat/lipid, are the solvent

Question 4

What are the key compartments for body fluids?

Answer 4

intracellular
interstitial water, filling the apace between cells, amongst the extracellular matrix
Fat
(blood) plasma
Trasncellular fluid

Question 5

Define the transcellular compartments

Give examples

Answer 5

separated from the extracellular flood by a membrane

e.g. CSF, peritoneal cavity, pleural cavity, sinovial fluid

Question 6

What is the approx total body water for a 70kg person?
Which sex is it proportionally greater in?
How is it affect by age?

Answer 6

42 L approx 60% body mass
men (due to fat)
reduces with age

Question 7

What are the following body fluid compartments in a 70kg person?
Intracellular
Extracellular
- plasma
- interstitial fluid
- transcellular fluid

Answer 7

28 L
3 L
10 L
1 L

Question 8

What are the destructive methods of measuring fluid compartments?

Answer 8

• plasma volume: by exsanguination and centrifugation

- total body water: weight a body, then desiccate it, then reweigh

Question 9

What is the better non-invasive method of measuring fluid compartments?

Answer 9

Inject an insert substance which is known to distribute in a given compartment, and then calcualr Volume of Distribution.

Question 10

What is the volume of distribution?

Answer 10

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
Volume of distribution = amount of drug / plasma conc

Question 11

How can total body water be measured?

Answer 11

In order to find a substance that mixes uniformly with water throughout the body, the best option is to used water!
The water needs to be marked in some way. Usually deuterium or tritium is used to replace the normal isotope of H in the water

Question 12

What markers can be used to measure the following body fluid compartments?

• plasma volume
• extracelullar fluid

Answer 12

Plasma volume:
- labelled proteins injected intravascuarly e.g. Evan’s blue which binds to plasma proteins
Extraceullar Fluid
- Cl-36, thiosulphate/thiocyanate, inulin. None of these are perfect, although thiosulphate/thiocyanate seem to be most accurately. Note that this will not measure transcellular fluid.

Question 13

What is plasma?

Answer 13

The fluid compartment of the blood and usually represents about 55% of the blood by volume. The rest of the volume is occupied by cells (haematocrit)

Question 14

Match up the following ions to their extracellular and intracellular concentrations:
Na+, K+, Ca2+, Cl-, HCO3-
a) OUT 4mmM, IN 140mM
b) OUT 140mM, IN 15mM
c) OUT 25mM, IN 12mM
d) OUT 2.4mM, IN 0.1uM
e) OUT 110mM, IN 4mM

Answer 14

a) K+
b) Na+
c) HCO3-
d) Ca2+
e) Cl-

Question 15

What is the osmolarity of body fluids?

Answer 15

285mOsm.kg-1

Question 16

What is significance about Ca2+ in the blood?

Answer 16

Half of Ca2+ in the circulation is bound to albumin, which means that any change in the albumin concentration will change the total Ca2+ concentration without change the free Ca2+ concentration.
It is the free calcium which is biologically active and thence the more interesting. So, the body regulated the free Ca2+ concentration, rather than the total Ca2+ concentration.

Question 17

Why is it necessary to correct Ca2+ for hypoalbuminaemia?

Answer 17

• because it is the free calcium that is physiologically more important, but total Ca2+ is measured, clinicians need some method for ‘correcting’ the Ca2+ to count for changes in concs of binding sites

Question 18

Define osmole

Give examples

Answer 18

This is a measure of the number of molecules that a compound dissociates into when dissolved in solution. It is very useful in renal physiology for measuring osmotic forces

• 100mmol of NaCl yields 200mOsm in solution, because it dissociates into Na+ and Cl-
• 2.4mmol of CaCl2 yields 7.2mOsm, as each dissociates in Ca2+ and 2 x Cl-
• 4mmol of glucose yields 4mOsm because it doesn’t dissociate

Question 19

Define Osmolality

Define Osmolarity

Answer 19

Osmolality = number of osmoses per unit MASS of the SOLVENT
Osmolarity = number of osmoses per unit VOLUME of the SOLUTION
In physiological ranges, the difference is very small, as the density of water is close to 1kg/L, and the difference between volume of the solvent and the volume of the solution is very small

Question 20

Describe the concept of osmotic pressure

Answer 20

• At the interface between two solutions, molecules exchange because of diffusion
• If the concentration 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 such a new movement is called the ‘osmotic pressure’
• In the context of biological tissues the interface between solution is at a semi-permeable membrane, usually the plasma membrane

Question 21

What is the difference between isotonic and isosmotic?

Answer 21

If two solutions are ISOSMOTIC they share the same osmolality
It a solution is ISOTONIC, then applying the solution to cells will not cause net fluid movement (lysis/shrinkage)
It is possible for a solution to be isosmotic with respect to the intracellular fluid, yet not isotonic. An example of this is urea.

Question 22

Describe the how urea is unique in terms of an isotonic solution.

Answer 22

Urea crosses relatively freely through plasma membranes in cells that express a urea transporter. Hence, an isomolar solution of urea, if applied to cells, is not isotonic because urea will enter the cells, increasing the intracellular osmotic pressure and hence encouraging the water to enter the cells. This causes swelling and ultimately rupture.

Question 23

What affects the net movement of water across a capillary?

Answer 23

a balance between hydrostatic force and osmotic pressure

Question 24

What is oncotic pressure?

Answer 24

the main osmotic forces in capillary beds due to proteins in the plasma

Question 25

Give an example of the importance of osmotic pressure changes

• fall in plasma albumin
• causes and effects

Answer 25

Caused by liver/renal failure

Effects are oedema and ascites

Question 26

In which compartment is oedema a critical problem?

How could this be corrected?

Answer 26

Brain

Mannitol

Question 27

What is mannitol and how does it work?

Answer 27

This is a stable sugar alcohol that, when injected intravascularly, increases the plasma and extracellular space osmolality. By doing so it pulls water from the intracellular and transcellular spaces (such as the CSF)
Hence one of its uses is to decrease intracranial pressure, such as that following intracranial haemorrhage
it is also an osmotic diuretic