Water, Acids, Bases, and Buffers

• Approximately 60% of our body is water.
• Water is distributed between intracellular and extracellular (interstitial fluids, blood, lymph) compartments.
• Because water is a dipolar molecule with an uneven distribution of electrons between the hydrogen and oxygen atoms, it forms hydrogen bonds with other polar molecules and acts as a solvent.
• Many of the compounds produced in the body and dissolved in water contain chemical groups that act as acids or bases, releasing or accepting hydrogen ions.
• The hydrogen ion content and the amount of body water are controlled to maintain homeostasis, a constant environment for the cells.
  

  
• The pH of a solution is the negative log of its hydrogen ion concentration.
• Acids release hydrogen ions; bases accept hydrogen ions.
• Strong acids dissociate completely in water, whereas only a small percentage of the total molecules of a weak acid dissociate.
• The dissociation constant of a weak acid is designated as K_a.
• The Henderson-Hasselbalch equation defines the relationship between the pH of a solution, the K_a of an acid, and the extent of the acid dissociation.
• A buffer, a mixture of an undissociated acid and its conjugate base, resists changes in pH when either H⁺ or OH^- is added.
• Buffers work best within a range of 1 pH unit either above or below the pK_a of the buffer, where the pK_a is the negative log of the K_a.
• Normal metabolism generates metabolic acids (lactate, ketone bodies), inorganic acids (sulfuric acid, hydrochloric acid), and carbon dioxide.
• Carbon dioxide reacts with water to form carbonic acid.
• Physiological buffers include bicarbonate, phosphate, and the protein hemoglobin.