LECTURE 112-3-2012

Importance of Osmosis and Osmotic Pressure

Osmotic pressure

The pressure of a solution which just stops osmosis is the osmotic pressure

Osmotic pressure is the excess of the pressure required to equalize water activities in the two compartments

The term osmotic pressure is used for a solution and not for a solvent

All non penetrable solutes in a solution exerts oncotic pressure

Factors on which osmotic pressure depends

Van’t Hoffs Equation

According to Van’t Hoff, osmotic pressure (π) depends on the molar concentration (C) of the solution and the temperature T

Where and R is the gas constant

The osmotic pressure doesn’t depend on the nature of the dissolved substance; it depends only on the concentration, temperature and molecule weight.

Osmotic pressure is higher when concentration difference is higher or temperature is higher and the molecular weight is lower

Osmolarity/Osmolality

Osmotic pressure depends mainly on the molar concentration or molarity of a solution

To describe the total number of osmotically active particles per litre of solution term osmolarity is used

Molecules like glucose do not dissociate in water, and only contribute one particle per molecule. NaCl does dissociate in water (into Na+ and Cl-), so there are two particles per molecule in solution.

 OsM of 1 M glucose =1 OsM OsM of 1 M NaCl = 2 OsMSo, two solutions can have the same molarity but different osmolarities.

The higher the osmolarity, the greater the osmotic pressure of the solution.

The osmotic pressure of an ionic solution is where I is the numberof ions formed by dissociation per molecule

 

Importance of Osmosis and Osmotic Pressure

Oncotic pressure of blood plasma

Formation of tissue fluid

Regulation of cell volume

Oncotic pressure of blood plasma

Some 90% by weight of plasma is water and about 8% is plasma proteins (albumin, globulins, fibrinogens)

Blood plasma is an aqueous solution containing different ions (Na+, K+, Ca2+…), small non dissociated molecules (glucose, amino acids) and proteins- macromolecules (albumin, globulin etc). Each type of molecules contributes with its own osmotic pressure, the sum representing the colloid-osmotic pressure or oncotic pressure of plasma

πplasma = ∑ πmol + ∑ πions+ ∑ πproteins Oncotic pressure of plasma usually tends to pull water into the

circulatory system Albumin is the major contributor to oncotic pressure of plasma

because it has the lowest molecular weight of the major plasma proteins and its concentration is almost double that of globulin

In addition albumin binds with Cl- ions which causes attraction and retention of cations in the vascular compartments and repulsion of diffusible anions

The total oncotic pressure of an average capillary is about 28 mmHg with albumin contributing approximately 22 mmHg of this oncotic pressure.

Throughout the body, dissolved compounds have an osmotic pressure. Because large plasma proteins cannot easily cross through the capillary walls, their effect on the osmotic pressure of the capillary interiors will, to some extent, balance out the tendency for fluid to leak out of the capillaries. In other words, the oncotic pressure tends to pull fluid into the capillaries.

Tissue Fluid Formation

Filtration takes place at the arterial end of capillary because hydrostatic pressure of blood overcomes the oncotic pressure of plasma proteins

Reabsorption takes place at the venous end of capillary because hydrostatic pressure of blood falls below the oncotic pressure of plasma proteins

Net result of this filtration and reabsorption process is the tissue fluid formation. The balancing of filtration/ultrafiltration and reabsorption/osmosis at arterial and venous end of capillary is referred to as the Starling Equilibrium

Removal of tissue fluidTo prevent a build up of tissue fluid surrounding the cells in the tissue, the lymphatic system plays a part in the transport of tissue fluid. Tissue fluid can pass into the surrounding lymph vessels, and eventually ends up rejoining the blood

Tissue Fluid Formation

Tissue Fluid Formation

Edema

If the ultrafiltration is excessive, the volume of interstitial fluid increases. When it becomes clinically detectable, it is called edema

Venous obstruction, erect posture and plasma protein deficiency can lead to edema

In conditions where plasma proteins are reduced, e.g. from being lost in the urine (proteinuria) or from malnutrition, there will be a reduction in oncotic pressure and an increase in filtration across the capillary, resulting in excess fluid buildup in the tissues

Fluid reabsorption into capillaries increases after haemorrhage. Such response helps to restore blood volume

Excessive reabsorption also occurs during decreased venous pressure and dehydration

Maintenance of Cell Volume

The determinants of cell volume are the total number of osmotically active particles within the cell and the osmolarity of the extracellular fluid.

The cell has a considerable quantity of impermeant solutes i. e. proteins ans organic phosphates whereas the interstitial fluid is relatively devoid of these.

Hence there exists a colloid osmotic gradient across the cell and this would draw fluid into the cell. This effect of cell macromolecules if offset by the Na+-K+ pump

3 positive ions (Na+) are pumped out of the cell (towards ECF)

for every 2 positive ions (K+) pumped into the cell (towards

ICF). This means that there is more positive charges leaving

the cell than entering it.

As a result, positive charge builds up outside the cell compared

to inside the cell. The difference in charge between the outside

and inside of the cell limits the fluid flow into the cell

About 90% of the osmotic pressre of extracellular fluid is due

to sodium ions

Maintenance of Cell Volume

Maintenance of body fluid osmolality by Kidney

Kidney maintains the optimum osmolality of body fluid by

regulating the volume of body fluids

When water intake is low or when water is lost through

diarrhea or perspiration, the kidney conserves water by

producing a small volume of urine which is hypertonic

When water intake is high, the kidney excretes a large

volume of hypotonic urine.

Kidney maintains normal osmolality by regulating excretion

of water and sodium chloride within a narrow range

Reverse Osmosis

Reverse osmosis is a membrane based filtration method that

removes many types of large molecules and ions from solutions

by applying pressure to the solution when it is on one side of a

selective membrane.

If an external pressure is applied on a concentrated solution,

this pressure is distributed evenly throughout the solution

If the applied pressure is higher than the osmotic pressure

water will flow towards the other side of the membrane leaving

solute behind

This technique is used for purification of water

Reverse Osmosis