SOME BASIC CHEMISTRY
Understanding how breathing influences acid-base balance requires some basic knowledge of chemistry. Carbon dioxide and bicarbonates play a very important role in buffering body acids and in the maintenance of electrolyte balance.
Electrolytes are substances that dissociate into positively or negatively charged particles, ions, when dissolved in aqueous (water) solutions such as body fluids, i.e., they ionize when placed in water, e.g., NaCl → Na+ + Cl־. Positively charged ions (+) are known as cations, e.g., hydrogen (H+), sodium (Na+), potassium (K+), calcium (Ca+2), and magnesium (Mg+2). Negatively charged ions (־) are known as anions, e.g., chloride (Cl־), bicarbonate (HCO3־), and phosphate (HPO4־). Body fluids are maintained electrically neutral Cations and anions add up to the same total ionic charge, i.e., milliequivalents (mEq/L) of cations are equal to the mEq/L of anions.
Acids donate and bases accept hydrogen ions, H+. An acid always contains a hydrogen ion, and can donate it to another substance. A base is a compound that can accept a hydrogen ion from another substance. Acids and bases ionize in aqueous solution (water), i.e., they dissociate into ions. Acids dissociate, to one degree or another into cations (H+) and anions (B־), where B־ is the conjugate base of the acid because it accepts the H+.
Strong acids fully dissociate. This means that the acid gives up all of its hydrogen ions which then remain in the solution: HB (strong acid) ↔ H+ + B־, wherein HB is no longer, or minimally, present, e.g., (hydrochloric acid),
HCL ↔ H+ + CL־ (hydrogen and chloride ions).
Weak acids only partially dissociate. This means that the acid gives up much fewer of its hydrogen ions.
HB (weak acid) ↔ (HB+) + H+ + B־, wherein substantial HB remains present , e.g., (carbonic acid),
H2CO3 ↔ (H2CO3) + HCO3־ + H+ (bicarbonate and hydrogen ions).
Buffer systems prevent radical pH changes. A body buffer system usually consists of a weak acid and its conjugate base, e.g., carbonic acid (H2CO3) and sodium bicarbonate (NaHCO3). Here is an example of buffering:
STEP 1: Lactic acid (CH3CHOHCOOH) fully dissociates into lactate ions (CH3CHOHCOO־ ) and hydrogen ions (H+):
CH3CHOHCOOH → CH3CHOHCOO־ + 2H+.
STEP 2: The H+ are then buffered by sodium bicarbonate (NaHCO3, a base), resulting in sodium lactate (CH3CHOHCOONa, a salt) and carbonic acid (H2CO3, a weak acid): CH3CHOHCOO + 2H+ + NaHCO3 → CH3CHOHCOONa + H2CO3.
Lactic acid is now buffered. It has been replaced with carbonic acid, a weaker acid than lactic acid.
STEP 3: The H+ are then UTILIZED (used for making something new) and the HCO3־ are RESTORED (returned to the system for buffering newly formed acids). The H+ are utilized to form water through oxidation of sodium lactate (breaks down into H2O and CO2), or to form glucose through gluconeogenesis.
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