Physiology 1 Flashcards
what is osmolarity
the concentration of osmotically active particles in a solution
units of osmolarity
osmol/l or mosmol/l
mosmol/l can be used
for weak salt solutions i.e. body fluids
osmolarity can be calculated if what 2 factors are known
- the molar concentration of solution
2. the number of osmotically active particles present in the solution
e.g 150mM NaCl work out its osmolarity
- molar concentration= 150mM
- number of osmotically activity particles: Na+Cl= 2
osmolarity= 2x150= 300 mosmol/l
osmolarity vs osmolality
osmolality is the concentration of osmotically active particles per kg
units of osmolality
osmol/kg water
for weak salt solution the terms osmolarity and osmolality are
interchangeable
tonicity
is the effect a solution has on cell volume
tonicity of solutions can either be
- hypertonic
- hypotonic
- isotonic
isotonic
the solution has the same osmolarity as the osmolarity of the fluid inside the cell, there will be no net movement of water into or out of the cell
hypotonic
the solution has a lower osmolarity than the osmolarity of the fluid inside the cell, there will be a net movement of water into the cell
hypertonic
the solution has a higher osmolarity than the osmolarity of the fluid inside the cell, there will be a net movement of water out of the cell
although related to osmolarity tonicity
also takes into consideration the ability of a solute to cross the cell membrane
total body water
- 60% of body weight in males
- 50% of body weight in females
total body water exists as 2 major compartments
intra-cellular fluid (67% of total body water) and extra-cellular fluid (33% of total body water)
extra-cellular fluid includes
- interstitial fluid (80% of ECF) which is the fluid in spaces between tissue cells
- plasma fluid (20% of ECF) which is the fluid portion of the blood
- transcellular fluid and lymph fluid is negligible
how do you measure the volume of body fluid compartments
using tracers
how do tracers work
you can determine the volume of distribution of a tracer substance
useful tracers
- total body water= 3H20
- extra-cellular fluid= Inulin
- Plasma= labelled albumin
total body water equals
the extra-cellular fluid plus the intra-celualr fluid therefore, if you use tracers to work out the total body volume and ECF you can calculated the ICF
worked example
- imagine adding a dose of tracer (D=42MG) to a container containing a large and unknown volume of water (V)
- you mix the tracer allowing it to equilibrate
- you then take a small sample volume from the container( (5ml) and measure the concentration of tracer (C) in this sample
- on analysis c= 0.005mg/5ml= 1mg/litre
- so the volume of water in the container= DOSE/ SAMPLE CONC
- VOLUMEN= 42/1= 42 LITRES
Summary for working out volume of distribution
- add a known quantity of tracer X into the body (Qx: mol or mg)
- measure the equilibrium CONCENTRATION of X in the body (X)
- Volume of distribution= Qx/ X
for water balance
inputs have to equal outputs
water input
- fluid intake= 1200ml/day
- food intake= 1000 ml/day
- metabolism= 300 ml/day
water output
insensible loss:
- skin= 350ml/day
- lungs= 350ml/day
sensible losses:
sweat= 100ml/day
faeces= 200ml/day
urine= 1500ml/day
total input and total fluid output
input is 2500ml per day and output is 2500ml per day
water imbalance is manifested as
changes in body fluid osmolarity
water loss in hot weather
- skin= 350ml
- lungs= 250ml
- sweat= 1400ml
- faces= 200ml
- urine= 1200ml
water loss in prolonged heavy exercise
- skin= 350ml
- lungs= 650ml
- sweat= 5000ml
- faeces= 200ml
- urine =500ml
in hot temperatures and during prolonged heavy exercise
water balance is maintained by increasing water ingestion, decreased water excretion is not sufficiency to maintain water balance
ionic component of the intra-cellular fluid
10mM Na+
140mM K+
7mM Cl-
10mM HCO3-
ionic components of the extra-cellular fluid
140mM Na+
4.5mM K+
115 mM Cl-
28 mM HCO3-
ECF and the ICF are separated by
compartments
the cell membrane and membrane transport mechanisms all
cell to maintain internal environments that are different to there surrounding
main ions in the ICF
K+, Mg2+ and negatively charged proteins
main ions in the ECF
Na+, Cl- and HCO3-
Cell membranes
are selectively permeable but despite this the osmotic concentrations of both ECF and ICF and identical (300mosmol/litre)
because changes in the solute concentration lead to
immediate changes in water distribution, the regulation of fluid balance and electrolyte balance are highly intertwined
fluid shift
movement of water between the ICF and ECF in response to an osmotic gradient
Challenges to fluid homeostasis
- Gain or loss of water
- Gain or loss of NaCl
- Gain or loss of isotonic fluid
gain or loss of water
change in fluid osmolarity causes similar changes in the ICF and ECF (both either increase or decrease)
gain or loss of NaCl
Change in fluid osmolarity causes
A) Na+ to be excluded from the ICF
B) osmotic water movements
both of these factors combine to produce opposite changes in ICF and ECF volumes
ECF NaCl gain
increased extra-cellular fluid volume
reduced intra-cellular fluid volume
ECF NaCl loss
reduced extra-cellular fluid volume
increased intra-cellular fluid volume
gain or loss of isotonic fluid
(e.g 0.9% NaCL) causes no change in fluid osmolarity only the ECF volume falls
the kidneys alter
the composition and volume of the ECF, regulation of ECF is vital for long term control of blood pressure
electrolyte balance occurs when
rate of gain= rate of loss
electrolyte balance is important for 2 reasons
- Total electrolyte concentrates can directly alter water balance
- the concentrations of individual electrolytes can affect cell function
Na+ and K+ are particularly important because
- they are major contributors to the osmotic concentration of ECF and ICF respectively
- they directly affect the functioning of cells
sodium balance
more than 90% of the osmotic concentration of the ECF results from the presence of sodium salts
the total amount of sodium in the ECF represents
a balance between 2 factors (input and output)
Na+ is mainly present
in the ECF therefore it is a major determinant of ECF volume (water follows sodium)
Minor fluctuations
in plasma potassium (K+) can have detrimental consequences
potassium plays a key role in
establishing membrane potential
95% of the bodies potassium is
intra-cellular so small leakages or increased cellular uptake may severely affect K+ plasma concentrations leading to
- muscle weakness causing paralysis
- cardiac irregularities causing cardiac arrest
salt balance
for balance to occur input has to equal output
intake of salt
fluids and food= 10.5g/ day
output of salt
- sweat and faeces= 0.5g/day
urine= 10g/day
salt imbalance is manifested as
changes in the extra-cellular fluid volume, regulation of extra-cellular fluid volume is very important for the long term regulation of blood pressure
