Transport Mechanism of Electrolyte

1. Osmosis
2. Diffusion
3. Active transport (Na and K pumb)
4. Filtration
5. Phagocytosis
6. Pincytosis

Substances are transported between cellular and extracellular fluids between biological membranes. These transport mechanisms are mentioned above.

Osmolarity – refers to the concentration of active particles per unit of solution. Two opposing forces exist with in the vascular compartment. These are:

1. Hydrostatic pressure of the blood which forces fluid out through semi permeable membrane
2. Osmotic pressure of the blood protein (colloid osmotic pressure) – which is pulling or holding force opposing the flow of fluid across the vascular membrane

When blood enter the arteriol and the capillaries hydrostatic pressure is greater than osmotic pressure and fluid filters out of the vessels. The movement of fluid out of the vessel is facilitated also by negative hydrostatic pressure – sucking fluid from plasma and the osmotic pressure in the interistissual space.

The result of the force that promotes the movement of fluid through the capillary is the sum of positive out ward pressure from within the capillaries and the negative hydrostatic pressure and the osmotic pressure in the interstissual spaces. E.g. Intracapillary hydrostatic pressure (ICHP), plasma osmotic pressure (POP). Negative
interstissual hydrostatic pressure (Int.-H.P) Interstissual Osmotic Pressure (Int.O.P)

At arterial end of capillaries, there is outward force =
CHP – POP + Int.H.P – Int.O.P
30 - 28 + 6 – (-5.3) = 13.3

At the venous end: POP – CHP + Int.H.P – Int.O.P
28 – 10 + 6 – (-5.3)
= 6.7

In extracellular fluid the principal osmotic forces are exerted by sodium and chlorine ions. Potasium, magnesium and phosphorous are mainly responsible for osmotic pressure within the cells.

Effect of osmosis as applied to different extracellular solute concentration will give isotonic, hyper tonic and hypotonic solution.

When all contributions to osmolality are summed the total serum osmolality ranges from 275 mosm/kg to 290 mOsm/kg.

Solutions can be categorized according to how their osmolality compared with that of extracellular fluids. When the osmolality is the same as extracellular fluid, a solution is lebelled isoltonic. Such a solution remains within extracellular compartment. One third is distributed to the vascular space and two thirds to the interstissual space.

A fluid with a lower or higher osmolality is lebelled hypotonic or hypertonic respectively. Hypotonic fluids are distributed in proportion of 1/3 to the extracellular compartment and ⅔ of intracellular compartment. They are associated with cell swelling. When hypertonic fluids are added to the vascular space, the extracellular
osmolality becomes greater than that of intracellular fluid. As a result water moves from the intracellular to extracellular compartment and cells shrink.