Metabolic acidosis is primarily characterized by an imbalance in the body's acid-base homeostasis, specifically involving a reduction in serum bicarbonate (HCO3–) levels, either due to excessive production of nonvolatile acids or an inability of the kidneys to excrete enough acid. This condition disrupts the acid-base balance, leading to a decrease in blood pH. The causes of metabolic acidosis can range from increased acid production to impaired renal acid excretion or loss of bicarbonate through the gastrointestinal or renal systems. Regardless of the cause, the fundamental issue is that the body's ability to neutralize acids through bicarbonate is overwhelmed, leading to acidemia.
Acid Production
Under normal metabolic conditions, acid production in the body is tightly regulated. The average adult produces approximately 1 mmol of acid per kilogram of body weight per day, and this increases slightly in infants and children. However, in pathological states, this acid production can increase dramatically. For example, conditions like lactic acidosis or diabetic ketoacidosis can cause a surge in organic acid production due to abnormal metabolic processes. Additionally, ingestion of certain toxic substances such as methanol or ethylene glycol can lead to the formation of organic acids as these chemicals are metabolized in the body. This excessive acid production overwhelms the body's buffering systems, leading to a significant drop in bicarbonate levels and subsequent metabolic acidosis.
Renal Regulation of Acid-Base Balance
The kidneys play a vital role in maintaining acid-base balance by reabsorbing filtered bicarbonate and synthesizing new bicarbonate to neutralize endogenous acid loads. On a daily basis, the renal tubules reabsorb approximately 4,500 mmol of bicarbonate. The majority of this reabsorption (80-85%) occurs in the proximal convoluted tubule (PCT), facilitated by sodium-hydrogen exchangers (NHE3) and proton-translocating ATPases (H+-ATPase). Carbonic anhydrase enzymes (CA II and CA IV) in the PCT promote the breakdown of carbonic acid (H2CO3) into bicarbonate and hydrogen ions, which are then secreted into the lumen.
The bicarbonate that escapes reabsorption in the PCT is further reabsorbed in the thick ascending limb of the loop of Henle and the collecting ducts. In normal physiological states, very little bicarbonate is lost in the urine, with the urine pH typically below 6.0 during fasting. However, defects in any part of this renal process, such as mutations in the SLC4A4 gene or deficiencies in carbonic anhydrase, can impair bicarbonate reabsorption, leading to conditions like proximal renal tubular acidosis (RTA).
Generation of New Bicarbonate
In addition to reabsorbing filtered bicarbonate, the kidneys generate new bicarbonate through processes occurring in both the proximal tubule and collecting ducts. A significant portion of new bicarbonate is produced via ammonium (NH4+) excretion. The breakdown of glutamine in the PCT generates ammonia (NH3) and bicarbonate, with NH3 eventually being secreted into the urine as NH4+. This mechanism is tightly regulated by various factors, including the pH of the body and the production of glucocorticoids, both of which increase during states of metabolic acidosis. This process helps the kidneys neutralize excess acid and restore bicarbonate levels in the bloodstream.
Another mechanism for generating new bicarbonate involves the excretion of titratable acids. Protons secreted into the renal lumen combine with phosphate ions (HPO42–) to form dihydrogen phosphate (H2PO4–). The amount of bicarbonate produced by this mechanism depends on the urine pH and the quantity of phosphate excreted. However, this process is limited, as the urine pH cannot fall below 4.5-5.0, and phosphate excretion is not highly adaptable.
Regulation of Acid Secretion in the Collecting Ducts
The secretion of protons in the collecting ducts occurs through H+-ATPases and H+/K+-ATPases, which are found on the luminal surface of intercalated cells. Bicarbonate generated in this process is transported back into the bloodstream via the anion exchanger AE1. A second bicarbonate transport mechanism involves pendrin, a Cl-/HCO3– exchanger localized to the apical membrane of beta-intercalated cells, which plays a role in bicarbonate secretion into the lumen. The activity of pendrin decreases during states of metabolic acidosis, thus favoring proton secretion over bicarbonate excretion.
The rate of proton secretion in the collecting duct is influenced by a variety of systemic and local factors, including angiotensin II, calcium-sensing receptors, extracellular calcium levels, pH, and partial pressure of CO2 (pCO2). Disturbances in these regulatory systems can reduce acid excretion and lead to metabolic acidosis, as seen in conditions like distal renal tubular acidosis (RTA), where impaired proton secretion or reduced ammonium production leads to insufficient acid removal from the body.
Consequences of Impaired Renal Function
In individuals with chronic kidney disease (CKD), the kidneys’ ability to excrete acid is impaired, leading to a decrease in ammonium production. As CKD progresses, the decline in glomerular filtration rate (GFR) results in a reduced ability to maintain acid-base homeostasis. This leads to a gradual buildup of acids in the bloodstream, contributing to chronic metabolic acidosis. While the kidneys attempt to compensate by increasing bicarbonate reabsorption and acid excretion, these mechanisms become less effective as renal function deteriorates.
| Site or mechanism | What happens | When it fails |
|---|---|---|
| Proximal convoluted tubule | Reabsorbs 80–85% of filtered bicarbonate via sodium-hydrogen exchangers (NHE3) and H+-ATPase; carbonic anhydrase II and IV break down carbonic acid | SLC4A4 mutations or carbonic anhydrase deficiency impair reabsorption → proximal renal tubular acidosis |
| Thick ascending limb and collecting ducts | Reabsorb the bicarbonate that escapes the proximal tubule, so little is lost in urine (fasting urine pH usually below 6.0) | — |
| Ammonium excretion | Glutamine breakdown in the proximal tubule generates ammonia and new bicarbonate; ammonia is secreted into urine as NH4+, and the process is upregulated by low pH and glucocorticoids | Reduced ammonium production leaves excess acid in the body, as in distal renal tubular acidosis |
| Titratable acid excretion | Secreted protons combine with phosphate to form H2PO4–, generating new bicarbonate | Inherently limited — urine pH cannot fall below 4.5–5.0 and phosphate excretion is not highly adaptable |
| Collecting duct proton secretion | H+-ATPases and H+/K+-ATPases on intercalated cells secrete protons; bicarbonate returns to blood via the anion exchanger AE1 | Impaired proton secretion causes distal renal tubular acidosis |
| Pendrin | Cl–/HCO3– exchanger on beta-intercalated cells that secretes bicarbonate into the lumen; its activity falls during acidosis, favoring proton secretion | — |
| Systemic and local regulators | Angiotensin II, calcium-sensing receptors, extracellular calcium, pH, and pCO2 set the rate of collecting duct proton secretion | Disturbance of these systems reduces acid excretion and leads to metabolic acidosis |
| Chronic kidney disease | Falling glomerular filtration rate with reduced ammonium production | Acids accumulate gradually; compensatory reabsorption and acid excretion become less effective as renal function declines |
