Metabolic alkalosis is an acid-base disorder that occurs when the blood’s pH level rises above the normal range of 7.35 to 7.45. This shift toward alkalinity happens when the body loses too much acid or gains too much base, resulting in an increased concentration of bicarbonate (\(\text{HCO}_3-\)) in the bloodstream. The body strictly regulates this acid-base balance, as even small changes affect physiological functions. Vomiting is a common event that disrupts this balance by causing a substantial loss of the stomach’s acidic contents. This net loss of acid serves as the initial trigger for developing metabolic alkalosis.
The Immediate Loss of Acid from the Stomach
The primary trigger for metabolic alkalosis following vomiting is the direct loss of highly acidic gastric juice. Stomach acid is predominantly hydrochloric acid (HCl), a strong acid produced by specialized parietal cells in the stomach lining. The loss of this acid means the body is losing hydrogen ions (\(\text{H}^+\)).
The production of stomach acid links the secretion of \(\text{H}^+\) into the stomach with the simultaneous addition of bicarbonate (\(\text{HCO}_3-\)) into the bloodstream. This bicarbonate influx, known as the “alkaline tide,” normally occurs after a meal. Once the stomach acid moves into the small intestine, it is neutralized by pancreatic secretions containing bicarbonate, maintaining the body’s overall pH balance.
When vomiting occurs, the stomach contents, including \(\text{H}^+\) and chloride ions (\(\text{Cl}^-\)) from the HCl, are expelled. This prevents the acid from reaching the small intestine, meaning the alkaline tide is never neutralized by pancreatic secretion. The loss of \(\text{H}^+\) and \(\text{Cl}^-\) combined with the uncompensated addition of \(\text{HCO}_3-\) to the blood generates the initial state of metabolic alkalosis. The sustained loss of chloride ions leads to hypochloremia, a major factor in the later maintenance of the alkalosis.
How the Kidneys Maintain Metabolic Alkalosis
While the loss of stomach acid initiates the alkalosis, the condition is sustained because the kidneys fail to excrete the excess bicarbonate. This failure to correct the pH imbalance is due to volume depletion and chloride depletion, making the condition “chloride-responsive.” Vomiting causes a significant loss of water and salt, leading to reduced circulating blood volume. The kidneys prioritize correcting this volume loss over acid-base balance.
Reduced blood volume triggers the activation of the renin-angiotensin-aldosterone system (RAAS), signaling the kidneys to retain sodium and water. To reabsorb sodium, the kidney exchanges it for another ion, often potassium or hydrogen. This process increases the secretion of hydrogen ions into the urine, paradoxically making the urine more acidic despite the body’s alkalosis.
Chloride depletion is key because chloride is normally reabsorbed alongside sodium and is linked to bicarbonate excretion. When chloride levels are low, the kidney cannot effectively excrete excess bicarbonate. Instead, it retains bicarbonate to balance the retained sodium, perpetuating the alkalosis.
Hypokalemia, or low potassium levels, is often present because potassium is lost in the vomitus and its excretion is promoted by increased aldosterone activity. Low potassium levels directly stimulate the kidney to increase bicarbonate reabsorption and hydrogen ion secretion. This further contributes to the maintenance of the metabolic alkalosis.
Symptoms and Treatment of Severe Alkalosis
When metabolic alkalosis is significant, the high blood pH interferes with normal nerve and muscle function, leading to noticeable symptoms. Common signs include muscle cramping, twitching, and weakness due to the effect on calcium levels. In severe cases, neurological symptoms such as confusion, disorientation, or seizures may occur.
The heart is susceptible to severe alkalosis, especially when low potassium is present, which can lead to cardiac arrhythmias. The body attempts compensation by slowing breathing, which retains carbon dioxide to increase blood acidity. This response is often limited, and severe alkalosis may compromise the patient further.
Treatment centers on addressing the underlying factors that maintain the condition: volume and chloride depletion. The most common approach involves administering intravenous fluids containing sodium chloride (saline solution). This infusion restores depleted blood volume and provides the necessary chloride ions.
Once chloride is replenished, the kidneys can excrete the retained bicarbonate, allowing the blood pH to normalize. Potassium chloride is also typically given to correct the associated hypokalemia, helping the kidneys manage acid and base excretion.