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Diffusion is the term applied to the spreading or scattering of molecules of gases or liquids. When two gases or liquids are brought into contact, the continual movement of the molecules will soon produce a uniform mixture, irrespective of their density. The process of diffusion occurs even when two substances are separated by a thin membrane.
In body, diffusion of body fluids, electrolytes and other substances takes place through the pores of capillary and cell membranes. The rate of diffusion varies according to the size of molecules, the concentration of the solution and the temperature of the solution. Smaller molecules move more quickly than the large ones. Molecules move from a solution of higher concentration to a solution of lower concentration. Increase in the temperature increases the rate of diffusion. Osmosis
The diffusion of water molecules (solvent) through a permeable membrane, from an area of lesser solute concentration to an area of greater solute concentration, is known as osmosis. If copper sulphate solution (coloured) and water are separated by a permeable membrane, the water molecules will pass through the pores of the membrane to the copper sulphate solution, thereby raising its level. The copper sulphate in the solution exerts a pressure known as ‘osmotic pressure’. It is due to this osomotic pressure that the water flows to the salt side rather than in the reverse direction.
Osmotic pressure is the drawing force of water exerted by the solute particles. The solute particles may be crystalloids (salts) or colloids (protein particles). When the concentration of the solute on one side of the membrane becomes greater than the other side, the osmotic pressure and the attraction for water will increase. Water will flow towards the solution of greater concentration until the concentration gradient disappears.
The principle of osmosis can be applied clinically in the administration of intravenous fluids. Usually, the solutions given as intravenous infusions, are isotonic, having the same concentration (osmolality) as blood plasma. This prevents sudden shifts of fluids and electrolytes from the vascular bed to the interstitial fluids. In some cases, hypertonic solutions, which have a greater concentration of solutes than plasma are advisable, (e.g., 50% glucose). This may be given to reduce cerebral oedema. The high concentration of glucose temporarily draw fluid from the interstitial spaces of brain tissue into the blood compartment. The use of hypotonic solutions are rare. They may be used in cases of subcutaneous infusions to help in the quick absorption of fluids. Both hypertonic and hypotonic solutions are to be used cautiously for the intravenous infusions. The hypertonic solutions can destroy the blood cells by shrinking them as they give off water and the hypotonic solution can haemolyse the blood cells as they take in more water than the cell membrane can support.
The osmotic pressure is greatest in the blood vessels due to the presence of plasma proteins. The protein concentration is greater than that of the interstitial fluid. The protein molecules are large and do not readily pass across the capillary membrane. This greater colloid osmotic pressure of plasma maintains the intravascular fluid volume. The plasma proteins work somewhat like a sponge, holding the water within the vessels and sucking back that water which escapes from the blood vessels. Hydrostatic pressure
Hydrostatic pressure is the pressure exerted by a fluid within a closed system. Counterbalancing the osmotic pressure of the plasma, which attracts fluid into the vascular system, is the hydrostatic pressure of the blood flowing through the capillaries which pushes fluid aut of the vascular system. Thus the hydrostatic pressure of the : -: -1 is the force exerted by the blood against the vascular walls. It is also referred to as filtration force. The principle involved in the hydrostatic pressure is that, fluids move, from the area of greater pressure to the area of lesser pressure. For this reason, the fluid moves out of the blood vessels. The net movement of fluids from the vascular space to the tissue space and vice versa depends upon two forces- the hydrostatic pressure, which forces the fluids out of the blood vessels and the osmotic presure which draws fluid into the blood vessels. Normally fluid moves out of capillaries at the arterial end where the hydrostatic pressure exceeds the colloid osmotic pressure. At the venous end of capillaries, fluid is drawn from the interstitial compartment into the vascular compartment, where the osmotic pressure is greater of the two.
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