Topic: Acid-Base Balance
Next Unit: Blood Gases
14 minute read
The hydrogen ion (H+) concentration of the blood is determined by the ratio of the partial pressure of CO2 (PCO2) and the plasma bicarbonate concentration.
In order of potency--influence on lowering pH (making more acidotic)--there are:
- Carbonic acid (H2CO3), from the CO2 produced with each breath (when combined with water). VIA LUNGS.
- Organic acids* (lactic acid, citric acid) from reactions in day-to-day metabolism, and which are further metabolized to neutral products (e.g., glucose), CO2, and water. VIA LIVER.
- Nonvolatile acids, such as sulfuric acids from metabolism of sulfur-containing amino acids in the diet. VIA KIDNEYS.
Since the continuous metabolism of the organic acids (2, above) maintains a stable and low concentration of them, the lion's share of acid-base balance is in addressing the carbonic acid (1, above) and nonvolatile acids (3, above).
Therefore, acid-base balance (buffering) is maintained by:
- normal elimination of carbon dioxide by the lungs (which affects the partial pressure of carbon dioxide--PCO2) and
- normal excretion of nonvolatile acids by the kidneys (which affects the plasma bicarbonate concentration).
The three primary major buffering systems in the body are:
- Carbonic Acid-Bicarbonate Buffer
- Phosphate Buffer System
- Protein Buffer System
The Renal Buffer System represents an overlap of these three systems in that it participates in supplementing all three via excretion or reabsorption of bicarbonates, phosphates, ammonia, and chloride.
CO2 and Buffering (Carbonic Acid-Bicarbonate Buffer)
A byproduct of cellular respiration is carbon dioxide. CO2 combines with water to become carbonic acid (H2CO3). From there, H2CO3 dissociates (loses a hydrogen) to become the base, bicarbonate (HCO3-).
Thus, the source of both acid (carbonic acid) and base (bicarbonate) is CO2.
The buffering system is the balancing act between the acid (carbonic) and the base (bicarbonate). Homeostasis depends on a 20:1 bicarbonate to carbonic acid ratio.
This ratio is maintained by the lungs' blowing off CO2 and the kidneys' excretion or reabsorption of bicarbonate.
Bicarbonate also affects the stomach and duodenum to neutralize gastric acid and stabilize the intracellular pH of epithelial cells.
While in the blood, bicarbonate neutralizes acids in the body, and carbonic acid neutralizes bases. Once bicarbonate reaches the lungs, it is dehydrated back to carbon dioxide and released during exhalation.
Phosphate and Buffering
Besides the respiratory and renal systems, the phosphate buffer system also works to maintain homeostatic pH within the body. The phosphate buffer system is almost identical to the bicarbonate buffer system—where phosphates work—in the intracellular fluid.
The phosphate buffer system operates in the internal fluids of all cells. It consists of dihydrogen phosphate ions as the hydrogen ion donor (acid) and hydrogen phosphate ion as the ion acceptor (base). If additional hydroxide ions enter the cellular fluid, they are neutralized by the dihydrogen phosphate ion. If extra hydrogen ions enter the cellular fluid then they are neutralized by the hydrogen phosphate ion.
Altered pH due to phosphates (in the form of phosphoric acids) is controlled by kidney excretion of phosphate.
Protein buffering is a description of the process wherein protein compounds consume small amounts of acids or bases. For example, hemoglobin is a protein that binds to small amounts of acids in the blood, removing the acid before it changes the blood's pH.
Acid-base balance is also maintained by the renal excretion of the daily acid load (mostly from sulfuric acid generated during the metabolism of sulfur-containing amino acids).
The kidneys can either absorb or excrete bicarbonate levels depending on the body's pH.
Acidosis causes more bicarbonate (base) to be reabsorbed from the renal tubular fluid, thus causing the collecting ducts to secrete more hydrogen (thusly generating even more bicarbonate).
Alkalosis causes the kidneys to excrete more bicarbonate because there is a reduced secretion of hydrogen ions and more ammonia (base) is excreted.
The kidneys also can eliminate H+ via the phosphate buffering system [NEXT].
Ammonia Buffering (Honorable Mention)
Ammonia (NH3), as a buffer, and ammonium ion (NH4+) balance via the renal tubular function. Also, there is a loss of Chloride (Cl-) and gain of bicarbonate (HCO3-) via kidneys.
The Anion Gap
In the body, electrical neutrality is maintained, i.e., Cations = Anions. Metabolic acidosis can be caused by excess bicarbonate loss or an increase in fixed acids, altering the above equation. However, the anion gap does not refer directly to pH but to an electrical charge.
- When fixed acids accumulate (more anions), the anion gap increases. More anions provoke buffering by bicarbonates, causing a deficit in bicarbonate.
- When bicarbonate is lost, however, the anion gap does not change.
When bicarbonate is not indirectly lost (i.e., not via neutralizing acid, as above), but is directly lost (via diarrhea or GI losses), chloride moves in to replace the bicarbonate buffer. The electrical neutrality is maintained, and since the anion "gap" phenomenon is one of electrical charge, there is no anion gap, thanks to chloride.
Lungs excrete CO2, which is in equilibrium with carbonic acid. By-product bicarbonate neutralizes carbonic acid.
Kidneys remove H+ from the body and control the excretion/retention of bicarbonate. Excretes H+ by excretion of phosphates and ammonia.