Body Fluids Flashcards
What are the average inputs and outputs:
- drinking
- food
- metabolism
- lungs
- skin
- faeces
- urine
1500ml
500ml
400ml
lungs
skin
faeces
urine
Of the outputs, ……… is under the greatest independent control in order to maintain ………… body balance. The others can be highly …………, depending on ……., ………. ….. and …….
urine
water
variable
temperature, metabolic rate and diet
Define a fluid
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
What are the key compartments for body fluids?
intracellular interstitial water, filling the apace between cells, amongst the extracellular matrix Fat (blood) plasma Trasncellular fluid
Define the transcellular compartments
Give examples
separated from the extracellular flood by a membrane
e.g. CSF, peritoneal cavity, pleural cavity, sinovial fluid
What is the approx total body water for a 70kg person?
Which sex is it proportionally greater in?
How is it affect by age?
42 L approx 60% body mass
men (due to fat)
reduces with age
What are the following body fluid compartments in a 70kg person? Intracellular Extracellular - plasma - interstitial fluid - transcellular fluid
28 L
3 L
10 L
1 L
What are the destructive methods of measuring fluid compartments?
- plasma volume: by exsanguination and centrifugation
- total body water: weight a body, then desiccate it, then reweigh
What is the better non-invasive method of measuring fluid compartments?
Inject an insert substance which is known to distribute in a given compartment, and then calcualr Volume of Distribution.
What is the volume of distribution?
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
How can total body water be measured?
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
What markers can be used to measure the following body fluid compartments?
- plasma volume
- extracelullar fluid
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.
What is plasma?
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)
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
a) K+
b) Na+
c) HCO3-
d) Ca2+
e) Cl-
What is the osmolarity of body fluids?
285mOsm.kg-1
What is significance about Ca2+ in the blood?
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.
Why is it necessary to correct Ca2+ for hypoalbuminaemia?
- 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
Define osmole
Give examples
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
Define Osmolality
Define Osmolarity
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
Describe the concept of osmotic pressure
- 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
What is the difference between isotonic and isosmotic?
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.
Describe the how urea is unique in terms of an isotonic solution.
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.
What affects the net movement of water across a capillary?
a balance between hydrostatic force and osmotic pressure
What is oncotic pressure?
the main osmotic forces in capillary beds due to proteins in the plasma
Give an example of the importance of osmotic pressure changes
- fall in plasma albumin
- causes and effects
Caused by liver/renal failure
Effects are oedema and ascites
In which compartment is oedema a critical problem?
How could this be corrected?
Brain
Mannitol
What is mannitol and how does it work?
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
