Respiration, Part IV
Successful respiration requires a delicate acid-base equilibrium in bodily fluids. H+ concentrations, however meager they may be, significantly impact fluid pH. Furthermore, hydrogen ions are highly reactive and react with many compounds, often disrupting their shape and function.
Hydrogen ion concentrations must be maintained at ~4.0 x 10-8 in order to keep blood pH at its slightly alkaline value of 7.4. Acidosis, caused by accumulation of CO2 in the blood, is a drop in blood pH. Shallow breathing may cause acidosis. The opposite condition, alkalosis, is a rise in pH due to excessive CO2 excretion. Alkalosis is commonly caused by hyperventilation, but is much less serious than respiratory acidosis. We can say that concentrations of CO2 in the blood are directly related to its pH. A pH above 7.7 ([H3O+] = 2.0 x 10-8) or below 7.0 ([H3O+] = 1.0 x 10-7) is incompatible with life.
Somehow the body manages to mediate fluctuating levels of H+ from carbon dioxide and metabolic acids (sulfuric, phosphoric, and other organic acids) to maintain balance – and life! How?
B U F F E R S !
A buffer is a pair of substances which resists changes in hydrogen ion concentrations. This solution contains high concentrations of a weak acid and the salt of that acid. They constitute a:
Conjugate pair, or an acid-base pair differing only by one proton, H+.
The weak acid provides H+ during shortage.
The anion portion of the salt of that weak acid accepts excess H+.
H+ + HCO3– <---> H2CO3 <---> H2O + CO2
The buffer pair HCO3– and H2CO3 maintain pH because:
• HCO3, bicarbonate, “absorbs” hydrogen ions.
• H2CO3, carbonic acid, “leaks” hydrogen ions.
In the red blood cells (RBCs) and in the kidneys, the enzyme carbonic anhydrase accelerates the reaction between carbon dioxide and water (metabolic by-products) to quickly create carbonic acid. Carbonic acid, if you remember, is very unstable; it quickly deteriorates into bicarbonate and hydrogen ions [reaction (4)]. Hemoglobin or other RBC proteins (proteins are excellent buffers! They have carboxyl groups, COOH, that dissociate to COO–) buffer the H+ and the bicarbonate exits the RBC and travels to the lungs, where high partial pressure gradients of hydrogen ions and bicarbonate force the creation of CO2 and H2O.
HCO3 in the plasma comprises an alkaline reserve, or base reservoir, should the blood [H+] rise. The bicarbonate is continually lost as we exhale, so the kidneys play an important role in providing and regulating the body’s bicarbonate. In the kidney filtrate (urine), HCO3 is used to buffer H+; it is flushed away. But at the same time HCO3 is created in kidney tubule cells and reabsorbed to the bloodstream. For each bicarbonate that is flushed away, one is created and reabsorbed.
The phosphate buffer system operates in the extracellular fluid (ECF), but in much lower concentrations. Its function in buffering blood plasma is negligible, but for the purpose of illustrating buffer systems, it is mentioned:
NaH2PO4 acts as a weak acid:
OH– + NaH2PO4 <---> Na2HPO4 + H2O
Na2HPO4 acts as a weak base:
H+ + Na2HPO4 <---> NaH2PO4 + NaCl
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