42L Total Body Fluid
28L Intracellular (67%)
14L Extracellular (33%)
o Membrane Protein Functions: receptors, enzymes, transport, cell recognition, cytoskeleton
o Diffusion – Time required for diffusion increases with the square of the distance. Luckily, most
cells are less than 25-50um from the capillary.
− kT dc
Fick’s Law of diffusion: 6πηr • dx • A
o Diffusion through channels: Dependent on size (less than 0.8nm), charge (filter, NOT chemical
specificity), electrochemical gradient, pressure gradient, hydration energy (larger
electronegativity coefficient causes greater hydration shell).
o Facilitated Diffusion: Does not require energy, shows chemical specificity, can be competitively
inhibited and shows saturation kinetics.
o Active Transport: Shows saturation kinetics, chemical specificity, and competitive inhibition.
Involves the use of energy to move substances against concentration gradient.
o E.g. Na+/K+ pump, 1 ATP, 2 K+, 3 Na+. Functions to increase negativity inside the cell,
and net reduction in particles inside the cell. Can be inhibited by ouabain,
dinitrophenol, O2 deprivation.
o Secondary Active Transport: Powered by concentration gradient, usually an exchange system
Osmotic Pressure – pressure needed to stop the net influx of water into a solution. Depends on the
number of particles, not the type.
Osmosis depends on: the permeability, the concentration gradient, and the pressure gradient.
Osmole – number of particles of an undissociated solute. E.g. NaCl makes 2 osmoles.
- Osmolality of body fluids is approximately 300 mosmoles per kg water
Capillary Hydrostatic Pressure of Plasma – Pc – capillary blood pressure, arterial end about 25 mmHg
while venous end is about 10 mmHg.
Interstitial Fluid Hydrostatic Pressure – Pif – Interstitial Fluid pressure, depends on tissue,
encapsulated organs tend to be (+) and cause reabsorption while others are (-) and cause filtration.
Interstitial Osmotic Pressure – πif – Few proteins in interstitium (5 mmHg)
Plasma Osmotic Pressure – πp – Proteins in plasma (28 mmHg)
Net fluid movement = Kf [ (Pc – Pif) – (πp – πif) ]
+ is filtration - is reabsorption
- lymphatics also play a role!
Edema: (1) tissue damage, increases Kf (2) Decreased plasma protein (malnutrition) lower πp (3)
Blockage of lymphatic system (4) Blockage of venous return increasing Pc
- Potential difference across the plasma membrane is the resting membrane potential
- All cell have an RMP but only some are excitable.
Nersnt Equation – to calculate the equilibrium potential of an ion. (i) membrane must be permeable to
only on ion and (ii) there must be a concentration gradie