Section Three

• About 60% of the body mass is water
• 2.2 lbs equates to 1 kg
• volume: amount/concentration
• Extracellular water is approximately 1/3 of total body water in a normal person
• intracellular water is approximately 2/3 of total body water
• Within the extracellular fluid, it can further be subdivided into plasma and interstitial fluid. Plasma is about 1/5 of the ECF whereas the interstitial fluid is about 4/5 of the ECF
• Plasma: 1/5 of the ECF
• Interstitial Fluid: 4/5 of the ECF
• 40% of your weight is in the ICF
• 20% of your weight is in the ECF
• Facilitated diffusion: specifically permit diffusion of water, but exclude ions and other substacnes
• Aquaporins are selective: only allow the movement of water
• Aquaporins are found in all organisms from bacteria to mammals
• Constitutive aquaporins: In membrane all of the time (AQP1). This means they are not exocytosed
• Recruitable aquaporins: Move in and out of cell in response to hormones (AQP2). Hormones usually signal in response to change in homeostasis
• Nephrogenic diabetes insipidus is caused by mutations in the AQP2 gene. With this disease, individuals are unable to concentrate urine
• With Nephrogenic Diabetes insipidus:

  1. Dehydration triggers the release of ADH
  2. ADH binds to ADH receptor and triggers AQP2 exocytosis
  3. AQP2 inserted on collecting duct side and help reabsorb water back into the body
• Osmosis: The passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration
• Molarity can be calculated by moles of solute/L of solution
• One millimolar (mM) is equal to 0.001 mole per liter
• One mole is 6.02 x 10^23 particles
• The concentration of pure water is 55.5 M
• There is a decrease in water concentration as solute concentration increases
• Osmolarity: Measures the number of osmoles of solute particles per unit volume of solution
• One osmole = One mole of solute particles
• 1 Osm (osmol/L) equals one mole of solute particles in 1 liter of solution
• Molarity: Measures the number of moles of solute per unit volume of solution
• One M glucose solution will equal 1 Osm solution as glucose will stay as one particle due to dissolving properties
• 1 M NaCl solution will equal 2 Osm solution as it will split into separate Na+ and Cl- ions
• Osmotic Pressure: The pressure that must be applied to the solution to prevent the net flow of water
• Tonicity: The ability of a solution to cause water movement
• Between 300 and 200 mOsm, 200 mOsm has more water
• Hypertonic Solution: More solutes than ICF, less water in ECF. The cell will shrink
• Isotonic Solution: No change in cell volume, no net movement of water
• Hypotonic Solution: More water in ECF than in ICF; building osmotic pressure; the cell will swell
• If the ECF is hypertonic, water will move out of the cell, causing the cell to shrink
• If the ECF is isotonic, then water will move in equilibrium
• If the ECF is hypotonic, then water will move into the cell and the cell will swell and possibly burst
• The purpose of normal saline is to expand the ECF when used. Normal saline contains 0.9% NaCl and is isotonic solution
• Hypotonic saline is used to increase water in cell and is given when cells are dehydrated.