Water, Solutions, and Diffusion

• The lecture will discuss body fluid compartments.
• It will cover the chemical composition of the body.
• It will cover the properties of water as a biological solvent.
• It will cover the osmotic pressure and tonicity of aqueous solutions.
• It will cover how body water is distributed between the various fluid compartments.
• It will discuss how the volumes of the compartments can be measured.

Body Composition

• The human body consists of the four elements: oxygen, carbon, hydrogen, and nitrogen.
• Oxygen makes up 65% of the body.
• Carbon makes up 18% of the body.
• Hydrogen makes up 10% of the body.
• Nitrogen makes up 3.4% of the body.
• Total minerals make up 3.6% of the body.

Body Mass

• Water accounts for about two thirds of lean body mass.
• Water makes up 65.8% of body mass.
• Protein makes up 13.6% of body mass.
• Fat makes up 17.4% of body mass.
• A normal 70 kg adult holds about 42 liters of water.

Water as a Solvent

• Water is considered the "universal" solvent due to its polarity.
• The electronegativity of oxygen and the non-linear arrangement of atoms in water make it polar.
• The charge difference in water molecules allows them to form hydrogen bonds.
• The water molecule bonds with other water molecules and dipoles in other molecule.
• Hydrogen bonds explain many of water's properties as a solvent.

Classification of Chemical Species

• Chemical species are divided into three classes: hydrophilic, hydrophobic, and amphiphilic.
• Hydrophilic substances readily dissolve in water due to their polar nature
• Examples of hydrophilic substance includes glucose, Na+, ethanol, and many proteins.
• Hydrophobic substances are insoluble in water due to their non-polar nature
• Examples of hydrophobic substance includes fats, waxes, and cholesterol.
• Amphiphilic substances have mixed properties, with one part being polar and the other non-polar.
• Examples of amphiphilic substance includes long chain fatty acids, bile salts, and phospholipids.

Diffusion

• Diffusion is the random dispersion of substances (solutes) in a solution.
• The rate of diffusion depends on the concentration gradient, area available, molecular mass, and diffusion coefficient.
• The diffusion coefficient is a physical constant depending on solute/solvent characteristics and temperature.

Fick's Law of Diffusion

• Fick's Law of Diffusion states: Amount moved = Coefficient x Area x Concentration gradient.
• The equation for Fick's Law of Diffusion is J = - D x A x (dC/dx).
  • J = Amount moved
  • D = diffusion coefficient
  • A = area
  • dC/dx = the concentration gradient
• A is the area over which diffusion can occur
• The negative sign indicates diffusion occurs from high to low concentration areas.
• The diffusion coefficient decreases, and diffusion slows down as molecular size increases.

Osmotic Pressure

• Osmosis is the movement of water ( or other solvent) through a semipermeable membrane.
• The membrane allows the passage of water but not solute particles.
• Hydrostatic pressure needed to stop water flow is called the osmotic pressure.
• Osmotic pressure plays a crucial role in the transport of molecules across membranes.
• Substances in solution exert an osmotic pressure.
• The osmotic pressure of a solution prevents the movement of water into the solution across a semi-permeable membrane.
• Osmotic pressure (π) = MRT.
  • M - Molality
  • R - universal gas constant (8.31JK-1mol-1)
  • T - absolute temperature (310K at normal body temperature)
• Osmotic pressure depends on the number of solute particles per unit volume of solvent, not the chemical makeup.
• Salts separate into their ions, so the osmotic pressure of sodium chloride is twice its molar concentration.

Osmole

• The osmole is the unit of osmotic concentration.
• It is a non-SI unit defining the number of moles of solute that contribute to the osmotic pressure of a solution.
• The total osmotic concentration of a solution equals the sum of the osmotic concentrations of each solute in the solution.

Reverse Osmosis

• Reverse osmosis is a process to purify water which is based on osmotic pressure.
• The water to be purified is placed in a chamber and put under an amount of pressure greater than the osmotic pressure exerted by the water and the solutes dissolved in it
• Water molecules can pass through a semi-permeable membrane in part of the chamber.
• Solute particles remain behind.
• Seawater has osmotic of about 27 atm.
• Reverse osmosis desalinators use pressures of around 50 atm (5,066 kPa) to produce fresh water from sea water.
• The units osmolarity and osmolality are alternatives to measuring osmotic pressure directly.
• Osmolarity is defined as moles of solute particles per litre of solution.
• Osmolality is defined as moles of solute particles per kg of water.
• Clinical medicine generally expresses osmotic pressures of body fluids as osmolality.
• Since body fluids are typically dilute aqueous solutions, the difference between osmolality and osmolarity is negligible.
• One mole of a non-dissociating substance in 1kg of water has an osmolality of 1 Osmol kg-1.
• A 1M solution of glucose has a concentration of 1 Osm and osmolarity of 1 Osmol L-1

Osmolality Of Blood Plasma

• The total osmolality of a solution is the sum of the osmolality due each of the constituents of the solution.
• Blood plasma osmolality is around 300 mOsmol kg-1.
• Principal ions (Na+, K+, Cl-, HCO3-) contribute most of this (around 290 mOsmol kg-1).
• Glucose and other small molecules contribute just under 10 mOsmol kg-1.
• Proteins contribute only about 1 mOsmol kg-1 (less than 0.5% of total plasma osmolality).
• Each kg of plasma has ~ 6.76 g NaCl and 47.4g of albumin.
• Molecular weight of NaCl is 58.4 and albumin is 69,000.
• The osmolality due to NaCI is: 2*(6.76/58.4) = 0.231 Osmol kg-1 or 231 mOsmol kg-1.
• The osmolality due to albumin is: 47.4/69,000 = 0.687 mOsmol kg-1.
• Blood plasma and other body fluids generally have an osmolality around 300 mOsmol kg-1.

Iso-osmotic versus Isotonic

• The tonicity of a solution refers to the influence of its osmotic concentration on cell volume..
• Two solutions with the same osmotic concentration are iso-osmotic.
• Intracellular fluid must have the same osmolality as extracellular fluid, so the volume of cells normally constant.
• Two solutions are said to be isotonic with each other when volume of cells is constant.
• All fluids that are isotonic are also iso-osmotic.
• Not all iso-osmotic solutions are isotonic with cells.
• Iso-osmotic solutions of urea cause cells to swell.
• The two solutions are iso-osmotic with concentrations are the same and also isotonic because the ions cannot move into the cell.
• In 300 mOsm urea, solutions are initially iso-osmotic, but urea diffuses down its concentration gradient into the cell very quickly
• Osmotic concentrations change and becoming lower outside and higher inside the cell.
• Water moves by osmosis from low solute (outside) to high solute (inside).
• A urea solution is not isotonic.
• The volume or pressure get bigger and bigger until such time as the urea and water stop moving or it bursts!

Osmotic Phenomenons

• Blood plasma is isotonic with red blood cells (erythrocytes).
• If blood is diluted with water, red blood cells burst since their flexible membranes cannot withstand the osmotic pressure within.
• Blood is diluted with an isotonic (0.9%) NaCl solution to avoid bursting.
• Cells living in aqueous solutions are hypertonic and subject to a continuous influx of water.
• A pressure builds inside the cell which would rupture it.
• Some ciliates (e.g., Paramecium) and flagellates (e.g., Euglena) can solve this problem.
• These organisms having a special organ, a pulsating vacuole, that actively pumps water out.

Body Water

• Transcellular water makes up 0.8 liters.
• Interstitial water makes up 10.4 liters.
• Plasma makes up 2.8 liters.
• Intracellular water makes up 28 liters.

Body Fluid Volumes

• Volumes of irregularly shaped objects can be measured by dilution methods.
• Concentration = mass/volume so volume = mass/concentration.
• Total body water is measured using 3H2O or 2H2O.
• Plasma volume is measured using the dye Evans Blue.
  • Evans Blue has high affinity for serum albumin.
• Extracellular volume is measured using inulin (NOT insulin).
  • Inulin is a plant polysaccharide.
• For example: 10 ml of a 1% solution (0.1 g or 100 mg) of Evans Blue was injected into a subject's vein.
• For example: After 5 min a blood sample was taken and the plasma was found to have 0.037 mg of dye per ml of blood.
• For example: As volume = mass/concentration, and Plasma volume = 100/0.037 = 2702 ml.
• Total body water = extracellular water + intracellular water.
• Extracellular water = plasma + interstitial water.
• You can then calculate the interstitial and intracellular compartments if knowing the total body water, plasma volume, and volume of the extracellular water.
• If total body water is 43 litres, the plasma volume is 3 litres and the extracellular water volume is 16 litres.
  • Intracellular water volume = 43 - 16 = 27 L
  • Interstitial water volume = 16 - 3 = 13 L

Summary

• Water accounts for 50-60% of total body mass.
• Water is the chief solvent of the body.
• Water is divided between two main compartments: the intracellular fluid and the extracellular fluid.
• The extracellular fluid is made of the plasma and the interstitial fluid.
• The volumes of these compartments can be measured using dilution methods.
• Substances dissolved in water exert an osmotic pressure proportional to their molar concentration.
• Intracellular and extracellular fluids are isotonic with each other.
• Cells swell (or burst) when placed in distilled water and shrink when placed in concentrated salt solutions.

Questions cover hydrogen bonding in water, solubility, diffusion rates, and elemental composition of the human body. Also, it involves calculating body water percentage and predicting the behavior of compounds with polar and non-polar regions.