Body Fluids L03 Flashcards
how is transcellular fluid different from interstitial fluid?
it’s separated by a membrane e.g. CSF, peritoneal fluid
molecules must pass through this epithelial barrier to enter/leave compartment
what is the total body fluid (tbf) for 70kg person?
42L
what percentage of tbf is in intracellular compartments?
35% (28L) *63% extracellular and 2% transcellular
what % of tbf is in interstitial compartment?
16% (10L) *more than circulating volume
what % of tbf is in plasma?
5% (3L)
what % of tbf is in transcellular compartment?
2%
what is ascites?
accumulation of fluid in the peritoneal cavity, causing abdominal swelling. causes include infection, heart failure, portal hypertension, cirrhosis, and various cancers.
what are the effects of an increase of fluid in the cranium?
-increase in intracranial pressure
> shifting of brain in cavity
> damage to brain tissue
> restriction of blood flow due to closed/damaged cerebral vessels
define the volume of distribution (Vd).
the volume of fluid required to contain the total amount of a substance/drug in the body at the same concentration as present in the plasma
or
Vd = Q/Cp *essentially volume = mass/conc
where Q = amount of drug, Cp = plasma concentration of drug (use plasma to measure conc as easiest compartment to access)
Why can heparin and insulin be used to measure plasma volume?
heparin - large molecule, confined to plasma as cannot pass through endothelium
insulin - strongly bound to plasma proteins, again does not exit circulating plasma
therefore Vd of these drugs = plasma volume
what compartment is Vd of Gentamicin used to measure and why?
extracellular space
very polar molecule so cannot cross membranes into intracellular space (unless there is a protein channel)
Ethanol can be used as a measure of total body volume. It is not an accurate measure, why?
ethanol is both water and fat soluble so some ethanol moves into fat - not accurate measurement of fluid volume in body
How can we measure total body water?
by using water that is ‘marked’ to distinguish it from unmarked water i.e. use radioactive deuterium (H with a neutrons) or tritium (H with 2 neutrons) instead of normal isotope of H in water. Then measure Vd in normal way.
How can we measure plasma volume?
- Vd with substances such as heparin or insulin
- label plasma proteins with dye such as Evan’s Blue (binds to protein) and measure Vd
How do we traditionally measure extracellular fluid?
- 36Cl- (some passes intracellularly)
- inulin (polysaccharide)
- thiosulfate/thiocyanate (most accurate)
*these will NOT measure transcellular volumes
What percentage of blood volume is plasma?
55%
Extracellular concentration of Na+?
140mM
intracellular concentration of Na+?
15mM
extracellular concentration of K+?
4mM
intracellular concentration of K+?
140mM
extracellular concentration of Ca2+?
2.4mM *only half of this is free Ca2+ ions
intracellular concentration of Ca2+?
0.1microM
extracellular concentration of Cl-?
110mM
intracellular concentration of Cl-?
around 4mM
extracellular concentration of HCO3-?
25mM
intracellular concentration of HCO3-?
12mM
extracellular concentration of glucose (fasting conc)?
4mM
Which protein is Ca2+ bound to in plasma?
albumin (half of Ca2+ bound to protein)
What effect will changing albumin concentration (in plasma) have on Ca2+ concentration?
It will change the total Ca2+ plasma concentration without changing the free Ca2+ concentration.
Does the body regulate the total Ca2+ concentration in plasma?
no. it regulates the FREE Ca2+ concentration as it is the free Ca2+ that is biologically active.
How much albumin is normally present in the plasma?
40g/L
Why do physicians use a ‘corrected’ total Ca2+ measurement rather than total Ca2+?
total Ca2+ values do not take into account the changes in free Ca2+ concentration that arise due to changes in albumin concentration (less albumin (pathologies?) = less Ca2+ binding sites = more free Ca2+).
‘corrected’ total Ca2+ does take into account these changes and helps clinicians to interpret results based on normal ranges of free total Ca2+ concentration - if albumin concentration decreases, corrected total Ca2+ concentration shown as increasing.
What is an osmole?
the number of molecules that a compound dissociates into when dissolved in solution.
e.g. 100mM NaCl –> 200mOsm
what is osmolality?
number of osmoles per unit mass of solvent (Osm/kg)
what is osmolarity?
number of osmoles per unit volume of solution (Osm/L)
what is the relationship between osmotic pressure and water potential?
inversely proportional
explain what osmotic pressure is
water moves by osmosis from areas of high water potential to areas of low water potential (across a semi-permeable membrane)
The force (per unit area) required to oppose the movement of water by osmosis is osmotic pressure.
‘the amount of pressure required to oppose osmosis’
What two pressures must be equal to stop the movement of water?
hydrostatic pressure (water pressure) and osmotic pressure
initially osmotic pressure is high (high water potential gradient so lot of force needed to oppose osmosis) and hydrostatic pressure is low (fluid is concentrated).
as water moves, hydrostatic pressure builds up.
when two pressures are equal –> point of equilibrium
define isosmotic
if two solutions are isosmotic, they share the same osmolality
define isotonic
if two solutions are isotonic, there is no net water movement across the membrane
why is a plasma solution of urea isosmotic in relation to intracellular fluid but not isotonic?
urea can readily cross plasma membranes providing that the cell membrane expresses a urea transporter.
Therefore the plasma solution and intracellular fluid may be isosmotic initially but not isotonic. This is because urea will move down its concentration gradient into cells, increasing intracellular osmotic pressure. As a result, water moves by osmosis into the cell (net movement of water - solutions are not isotonic). Once the urea concentration is equal on both sides of the membrane, the hydrostatic pressure balances and the solutions become isotonic/ in equilibrium.
*movement of water can however cause cell swelling and rupture
what is oncotic pressure?
the pressure exerted by plasma proteins, drawing water back into the capillary from tissue fluid.
what are the causes of fall in plasma albumin?
- liver failure - decreased production
- renal failure - increased loss
what are the effects of a fall in plasma albumin?
oedema, ascites
in which compartment is oedema a critical problem?
brain
How can oedema in the brain be treated?
Mannitol - injected intravascularly > increases osmolality of plasma and extracellular space > ‘pulls’ water from intracellular and transcellular spaces (e.g. CSF)
What is mannitol used for?
- osmotic diuretic
- intracranial haemorrhage - decrease intracranial pressure
- inhaled for cystic fibrosis management
What is the osmolarity of extracellular fluid?
around 285mOsm/kg
