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Describe the principle of osmosis

Osmosis is the passage of water across a semipermeable membrane down its concentration gradient in pursuit of equilibrium (i.e. no NET movement of water across the membrane). The concentration gradient of water will be inversely proportional to the concentration gradient of the solute. In the absence of other forces (i.e. gravity, pressure differences, or current), the diffusion of water across the membrane will continue until water concentration is equal on both sides of the membrane - it is the relative H2O concentration that drives this equilibrium.However equilibrium will often not occur at equal concentrations of H2O in solution. This occurs when the osmotic pressure is countered by hydrostatic pressure. Equilibrium will occur when these two pressures cancel each other out.Where osmosis does proceed to equal water concentrations, the osmotic pressure difference at the beginning will equal the hydrostatic pressure difference (volume difference) at the end.


What factors influence the rate of diffusion across a membrane?

Under FICK'S LAW OF DIFFUSION, the rate of diffusion of a substance across a membrane is influenced by:Concentration gradient (+ gradient, + rate)Permeability of membrane (+ permeability, + rate)Surface area (+ area, + rate) - more places for diffusionMolecular weight of substance (+ weight, - rate) - bigger substance through same-size hole.Distance/thickness of membrane (+ thickness, - rate)


What is the difference between Osmolarity, Osmolality, Tonicity, and Oncotic Pressure?

Osmolarity and osmolality are used interchangeably in the human body. They describe a solution as it relates to physiological normal (275-295 mOsm in human plasma).Osmolarity relates to the number of particles per Litre of SOLUTION (solution allows for compounds such as NaCl to split into constituents, thereby increasing osmolarity (doubling in this case)).Osmolality relates to the number of particles per Kg of SOLUTION.Oncotic Pressure is a term for osmolarity that derives from the presence of proteins.Tonicity is also called EFFECTIVE OSMOLARITY and describes a particular substance relative to a reference, divided by a plasma membrane. It is thus tailored for medicine and the more valuable concept for clinical use.


What is the osmotic coefficient, and how do you measure osmolarity in non-living substances?

The osmotic coefficient is a measure of how many particles you would expect from a solute in solution, in a real (as opposed to ideal) environment (e.g. NaCl = 2 particles, osmotic coefficient 0.93).When you know the molarity of a solution, you can calculate osmolarity through the coefficient. Eg for 154mM NaCl solution:0.93 x 2 x 154 = 286 mOsm.When molarity is unknown, osmolarity is most commonly measured by the change in freezing point provided by that solute - 1 Osm lowers freezing point by 1.87 degrees.


How do you clinically estimate osmolarity? How does it differ from estimating tonicity?

To clinically estimate osmolarity:2[Na+] + [glucose] + [urea].Tonicity is estimated the same way, but without urea, as it can freely cross the plasma membrane.In clinical situations, glucose will not contribute to tonicity over the longer-term, as it is taken up by cells. It can thus make an initially isotonic substance hypotonic.


What does it mean if a solution is hypo/iso/hyper-tonic(/osmotic)?

HYPOtonic/osmotic solutions have a lower concentration of solutes than the reference - as a result, there will be a net flow of water into the other (reference) solution (this can make blood cells swell).ISOtonic/osmotic solutions have the same concentration of solutes as the reference - there will be no net flow of water between the solutions (blood cells remain the same volume). HYPERtonic/osmotic solutions have a higher concentration of solutes than the reference - as a result, there will be a net flow of water from the other (reference) solution (this can make blood cells shrink).


What is the role of aquaporins in osmosis?

Aquaporins are specialised water channels that control the passage of H2O (and thus semipermeability) across cell membranes. They are tetrameric (4 sub-units, each with a pore), and are controlled by the Antidiuretic Hormone (ADH). They are permeable to H2O at a rate of around 3bn molecules/s, with passage driven by osmotic pressure (concentration gradient).As the 'gatekeepers' of water in the cell, they can permit rapid transfer of water, slow and regulated transfer of water, or effectively prevent transfer of water altogether.


How do proteins affect ionic compositions and intracellular pressure?

The presence of proteins leads to a relative negative charge, and a higher osmolarity inside the cell. Because of protein's negative charge, fewer cations than anions enter the cell. (short version).Long version - Proteins exist in the cytosol and cannot cross into the ECF. As they hold a negative charge, this generates a ‘negative charge bias’ in the cell. Cl- wants to flow down its concentration gradient into the cell, bring a further negative charge. Cations (Na+) will enter the cell to neutralise this charge, increasing osmolarity.The increased osmolarity in the cell draws in more H2O, up to the point where the hydrostatic pressure will equal the osmatic pressure (in reality moderated by Na+/K+-ATPase). GIBBS-DONNAN EQUILIBRIUM is achieved when the electrochemical gradients for each ion are in-balance, with no net flow of ions across the membrane, however the cytosol will hold both many more ions, AND a negative charge, relative to ECF, due to the original 'bias' created by the proteins.The cytosol and ECF will both remain neutral within themselves. However, the difference in total numbers of ions and charges (from ions and proteins) between the ECF and the cytosol will cause both a pressure difference and a electrical potential difference between the cell and the ECF.The DONNAN FACTOR can be calculated as either Na(outside)/Na(inside) or Cl(inside)/Cl(outside).


Explain the role of Na+/K+-ATPase in maintaining cell volume

Na+ wants to flow into the cell down its electrochemical gradient. However, increase Na+ (and thus Cl-, due to charge difference) increases osmolarity, drawing water into the cell. If this were to continue, the cell would explode.Intracellular pressure is reduced by Na+/K+-ATPase, which is activated when the cell swells. This pump (primary active transporter) transports 3Na+ out of the cell while bringing in K+, reducing osmolarity. It is assisted by many other proteins, including K+ channels, which allow K+ to leave the cell, also reducing osmolarity.


What is regulatory volume increase (in osmosis)?

Regulatory volume increase is a process that occurs to minimise and counter-act cell shrinkage in hypertonic solutions. Na+, K+, and Cl- are all taken into the cell immediately via channels and co-transporters, with H2O following osmotically.Longer-term, the cell would accumulate OSMOLYTES to maintain osmolarity.Urea in the ECF can cause an initial cell increase until it crosses the cell membrane, dragging H2O back across.


What is regulatory volume decrease (in osmosis)?

Regulatory volume decrease is a process that occurs in to minimise and counter-act cell swelling in hypotonic solutions. As cell volume increases, the cell would open K+ channels and activate K+/Cl- co-transporter and Na+/K+-ATPase to decrease osmolarity, with H2O following osmotically.


What will happen if you use hyper/iso/hypo-tonic/osmotic solutions in volume replacement therapy?NOTE - need to be able to do the equations in osmosis lecture, from slide 25 on.

Isotonic volume replacement maintains the osmolarity of ECF, and thus will not have any osmotic effect on the cell, leaving cell volume intact and only expanding the ECF. Isotonic is the best way to replace ECF volume, and is normally given at 154mM NaCl.Hypotonic volume replacement (such as pure water) will initially swell the ECF compartment, but will also cause dilution. As a result, it will make the ECF hypotonic, causing H2O to move into the cells, swelling the cells. This is an inefficient way to replace ECF, as most water will go into the ICF.Hypertonic volume replacement will cause the ECF volume to increase at the expense of the ICF.

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