1. What is the concentration of water in pure water?
1 L water weighs 1000 g. Gram molecular wt water = 18 grams
(1000g/L)/(18grams/mole) = 55.5 moles/L or 55.5 molar
2. What is the water concentration of a 1 M NaCl solution?
1 M NaCl --> 2 osmoles. Each osmole displaces approximately 1 mole of water.
Therefore the water concentration is (55.5-2)moles/L, or 53.5 molar.
3. Distinguish between... Tonicity of a particular solution is defined by
whether steady-state cell volumes change when they are placed in the solution. Osmolality
is determined by the number of particles, i.e. ions or molecules, in solution. An
"isotonic" solution is one in which cells neither swell nor shrink in the steady
state. An "isosmotic" solution (with respect to plasma) is one that has the same
osmolality as plasma. The tonicity of a solution depends only on the osmolality of
impermeant solutes, whereas the osmolality depends on the concentrations of all solutes. A
solution containing 300 mOsm/kg water of urea is isosmotic with plasma, because both have
the same osmolality, but it is not isotonic because urea permeates cell membranes easily.
Cells placed in a 300 mOsm/kg urea solution will swell as urea and water enter.
4. The edema of congestive heart failure... The central venous pressure is the
pressure in the right atrium and thoracic venae cavae. Heart failure can be defined as any
abnormality in the pumping action of the heart that reduces its ability to perform
external work. Central venous pressure rises as cardiac output declines. The mean arterial
blood pressure may stay normal because of compensatory reflexes (homeostatic control), and
only decline in the late stages of heart failure. Applying the Starling concept of
capillary filtration and reabsorption of fluid, normal arterial pressure and increased
venous pressure will decrease reabsorption of fluid at the venous ends of capillaries and
hence cause edema.
5. A 70 kg male has a body fluid osmolality... This question was drawn from
Berne and Levy, 3rd Ed., pp 756ff. Addition of 290 mMoles of NaCl to the ECF adds 580 mOsm
to the ECF. We assume NaCl is restricted to the ECF--i.e. does not cross cell membranes.
The osmolality of the ECF will increase, which will cause net water flow from the ICF to
the ECF. The flow of water increases osmolality of the ICF. Net water flux ceases when the
ECF and ICF have the same effective osmolality.
Calculations:
Initial conditions:
Initial total body water (TBW): 70 kg x 0.6 = 42 kg or 42 L waterInitial ICV
volume: 42 x 2/3 = 28 L
Initial ECF volume: 42 x 1/3 = 14 L
Initial total body osmoles: 42 kg x 290 mOsm/kg = 12180 mOsm
Initial ICF osmoles: 28 kg x 290 mOsm/kg = 8120 mOsm
Initial ECF osmoles: 14 kg x 290 mOsm/kg = 4060 mOsm
Final conditions:
Final osmolality = (new total body osmoles)/(TBW) (12180 mOsm + 580 mOsm)/(42kg water)
= 303.8 mOsm/kg water
Final ICF volume = ICF osmoles/(new osmolality)
8120 mOsm/(303.8 mOsm/kg water) = 26.7 kg or 26.7 L
Final ECF volume = TBW - ICF = 42 - 26.7 = 15.3 L
(Could also have been calculated as follows:)
Final ECF volume = (new ECF osmoles)/(new osmolality)
(4060mOsm + 580mOsm)/(303.8 mOsm/kg water)
4640/303.8 = 15.3 kg water or 15.3 L
Note that addition of NaCl to the ECF has altered the fractions of the TBW
contained in the ECF and ICF. The two compartiments no longer contain 1/3 and 2/3 of the
TBW.
6. A 70 kg male has a body fluid osmolality... This is also taken from examples
in Berne and Levy, 3rd Ed. Addition of isotonic saline to the ECF does not change the
effective osmolality of the ECF. There is, therefore, no driving force for net water
movement. The two liters of saline remains in the ECF. The initial conditions are just as
calculated for question 5. The final conditions are:
