Biological importance of Osmotic pressure

29.11.12

Osmosis takes place due to difference in hydrostatic pressure of water and solution separated by a semi permeable membrane (permeable only to water)

Apply an external pressure to the side containing solution

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

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

Measurement of Osmotic Pressure

Osmosis when two solutions with different hydrostatic and osmotic pressure are

separated by a semi permeable membrane

The difference in net hydrostatic pressures of two solutions is equal to difference in osmotic pressures of two solutions

Water is pulled into the solution with relatively higher pulling tendency (concentrated solution). Osmosis takes place

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

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 blood plasma

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

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/ultrafiltration and reabsorption/osmosis at arterial and venous end of capillary is the tissue fluid formation

Starling Equation

The Starling equation is an equation that illustrates the role of hydrostatic and oncotic forces (the so-called Starling forces) in the movement of fluid across capillary membranes

Tissue Fluid Formation

Tissue Fluid Formation

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

Edema

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

Venous obstruction or 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

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 and 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 is 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

The force with which surface molecules are held is called the surface tension of the liquid

It is the force acting perpendicularly inward on the surface layer of a liquid to pull its surface molecules towards the interior of the fluid

It keeps the surface like a stretched membrane, and hence keeps the contact area minimum

Surface Tension

Water striders use surface tension to walk on the surface of pond. The surface of the water behaves like an elastic film: the insect's feet cause indentations in the water's surface. Its tiny mass and geometry of its legs allow it to be supported by the high surface tension of water

Surface Tension

Formation of drops occurs when a mass of liquid is stretched. Water adhering to the tap gains mass until it is stretched to a point where the surface tension can no longer bind it to the tap. It then separates and surface tension forms the drop into a sphere. If a stream of water were running from the tap, the stream would break up into drops during its fall. Gravity stretches the stream, then surface tension pinches it into spheres

Surface tension at liquid-air interface: A soap bubble is a thin film of soapy water enclosing air that forms a hollow sphere. Surface tension causes a bubble to assume the smallest surface area to contain a given volume -- resulting in the spherical shape

Liquid-solid interfaceBeading of rain water on the surface of a waxy surface, such as a leaf. Water adheres weakly to wax and strongly to itself, so water clusters into drops. Surface tension gives them their near-spherical shape, because a sphere has the smallest possible surface area to volume ratio

Liquid-Liquid interfaceSeparation of oil and water (in this case, water and liquid wax) is caused by a tension in the surface between dissimilar liquids.

Surface tension at interfaces

Reference book

Chapter 3: Human Physiology: From Cells to Systems By Lauralee Sherwood

Chapter 1 :An Introduction to Med. Biophysics  by Prakash

Chapter 10: Biophysics by P. S. Mishra