Lecture 2 Osmolarity - PDF

Objectives Define osmotic pressure Calculate osmolarity and osmolality Understand terms hyposmotic, hyperosmotic, isosmotic Understand difference between osmolarity and tonicity Understand the effects of osmosis at the level of the cell membrane and the capillary wall Osmolarity (Osmotic...

Objectives Define osmotic pressure Calculate osmolarity and osmolality Understand terms hyposmotic, hyperosmotic, isosmotic Understand difference between osmolarity and tonicity Understand the effects of osmosis at the level of the cell membrane and the capillary wall Osmolarity (Osmotic concentration) The concentration of all solutes in a solution Units of osmolarity - Osm/L (OsmolaRity: Osmoles per LitRe) – Osmolar (like molar concentration (M)) Units of osmolality = Osm/kg – Usually similar value, variation that takes into account only solutes that contribute to a solution's osmotic pressure Some substances may dissociate in solution – 1 mol/L glucose = 1 Osm/L – 1 mol/L NaCl = 2 Osm/L – 1 mol/L CaCl2 = 3 Osm/L Cell membranes are semipermeable barriers Selectively semipermeable – Allow hydrophobic (lipid soluble) substances to cross easily – Allow some small hydrophilic (polar) substances to cross easily – Prevent the free passage of large polar substances e.g. glucose, ions Movement of water across the cell membrane Water is a polar molecule (+/-) Small amount of water passes through lipid bilayer by simple passive diffusion Membrane permeability to water can be increased by presence of aquaporins – Specialised water channel Osmosis Passive transport mechanism – The diffusion of water from an area of high concentration of water molecules (high water potential) to an area of low concentration of water (low water potential) across a partially permeable membrane – High solute concentration = low water concentration – Low solute concentration = high water concentration Osmosis Diffusion of water Must be – Semi-permeable membrane Permeable to water Impermeable to at least one solute Along a concentration gradient Driving force for osmosis is osmotic pressure – i.e. dilute to concentrated Osmosis Solution in the beaker contains 180 g/L sucrose Solution in the “cell” contains 360 g/L sucrose “Cell” membrane is permeable to water but not sucrose What happens? Osmosis Higher concentration of water in the beaker than in the “cell” Water moves into the cell Cell swells Water continues to move into the cell until concentration equalises on both sides of the membrane (~270 g/L sucrose) Osmosis Osmotic pressure = pressure required to prevent osmosis Each “cell” encased in rigid box Pressure exerted on the box = osmotic pressure The greater the solute concentration, the greater the osmotic pressure Body fluid compartments In the body, all compartments are normally in osmotic equilibrium ~300 mOsm Changes in concentration of ICF or ECF results in fluid shift between compartments Comparing Osmolarity You can compare the osmolarity of 2 different solutions using the terms isosmotic, hyposmotic or hyperosmotic A cell is placed in an isosmotic sucrose solution – There is no net water movement and therefore no change in cell volume ICF = 300 300 mmol/L mOsm/L Sucrose Sucrose cannot cross the cell membrane Comparing Osmolarity The “cell” is now placed in a hypoosmotic sucrose solution (100 mmol/L). This creates a gradient for water movement Sucrose cannot cross the cell membrane but water can. Cell swells (oedema). ICF = 300 100 mmol/L mOsm/L Sucrose Comparing Osmolarity Again the “cell” is placed in a hyperosmotic sucrose solution. Again this creates a gradient for water movement. Sucrose cannot cross the cell membrane but water can. Cell shrinks. ICF = 300 400 mmol/L mOsm/L Sucrose Tonicity Describes the behaviour of cells in solutions – Do cells swell, shrink or stay the same volume? Osmolarity is affected by all solutes in a solution, while tonicity is only affected by solutes that cannot cross the membrane A hypotonic solution will cause cells to swell – Less solutes outside cell, so water enters cell, oedema An isotonic solution will cause no change in cell volume – Equal solutes on both sides of membrane A hypertonic solution will cause cells to shrink – More solutes outside cell, so water exits cell, cell shrinks Comparing Tonicity The “cell” is now placed in a 300 mmol/L urea solution (isosmotic). Urea enters the cell and is followed by water. Urea can cross the cell membrane and water follows. Cell swells (oedema). ICF = 300 300 mmol/L mOsm/L Urea Osmotic behaviour of cells RBC placed in isotonic solution – e.g. 0.9 % NaCl (Normal saline NS) RBC placed in hypotonic solution – e.g. water RBC placed in hypertonic solution – e.g. 2% NaCl RBC will burst (haemolysis) if swells > 1.6x This is why you would not give a dehydrated person water intravenously? Osmotic behaviour of cells Hypotonic  Isotonic →→ Hypertonic (Haemolysis) (Shrinkage) How does this relate to cells in the body? Cell membranes are permeable to water – Water passes in & out by osmosis – Movement depends on solute concentration in ECF and ICF Electrolytes contribute to the osmolarity of body fluids ECF and ICF must have same osmolarity Osmosis will occur if water is lost from one compartment Body fluid osmolarity ≈ 270 - 300 mOsm/L Oedema (UK) / Edema (US) Plasma proteins provide the osmotic pressure to keep fluid in blood vessels. Name one ? If plasma proteins are low, fluid leaves the plasma and moves into the tissues Hypoproteinaemia - liver &/or Oedema (swelling) due to loss of kidney disease, malabsorption, fluid from intravascular space to nutritional lack the interstitial fluid Cerebral Oedema Stroke, tumours, trauma etc. Fluid accumulates in the brain Intracranial pressure increases – Hypoxia, damage etc. – Can lead to death Treatment (Osmotherapy) – e.g. hypertonic solution of mannitol i.v. Terms Objectives Define osmotic pressure Calculate osmolarity and osmolality Understand terms hyposmotic, hyperosmotic, isosmotic Understand difference between osmolarity and tonicity Understand the effects of osmosis at the level of the cell membrane and the capillary wall