Acidosis and alkalosis describe the abnormal conditions that result from an imbalance in the pH of the blood caused by an excess of acid or alkali (base). This imbalance is typically caused by some underlying condition or disease.
Normal blood pH must be maintained within a narrow range, typically 7.35-7.45, to ensure the proper functioning of metabolic processes and the delivery of the right amount of oxygen to tissues. Acidosis refers to an excess of acid in the blood that causes the pH to fall below 7.35, and alkalosis refers to an excess of base in the blood that causes the pH to rise above 7.45. Many conditions and diseases can interfere with pH control in the body and cause a person's blood pH to fall outside of healthy limits.
Normal body functions and metabolism generate large quantities of acids that must be neutralized and/or eliminated to maintain blood pH balance. Most of the acid is carbonic acid, which is created from carbon dioxide (CO2) and water. CO2 is produced as the body uses glucose (sugar) or fat for energy. Lesser quantities of lactic acid, ketoacids, and other organic acids are also produced.
The lungs and kidneys are the major organs involved in regulating blood pH.
- The lungs flush acid out of the body by exhaling CO2. Raising and lowering the respiratory rate alters the amount of CO2 that is breathed out, and this can affect blood pH within minutes.
- The kidneys excrete acids in the urine, and they regulate the concentration of bicarbonate (HCO3-, a base) in blood. Acid-base changes due to increases or decreases in HCO3- concentration occur more slowly than changes in CO2, taking hours or days.
Both of these processes are always at work, and they keep the blood pH in healthy people tightly controlled.
Buffering systems that resist changes in pH also contribute to the regulation of acid and base concentrations. The main buffers in blood are hemoglobin (in red blood cells), plasma proteins, CO2, bicarbonate, and phosphates.
The absolute quantities of acids or bases are less important than the balance between the two and its effect on blood pH (see Figure 1, below).
Acidosis occurs when blood pH falls below 7.35. It can be due to increased acid or decreased base:
- Increased acid production within the body
- Consumption of substances that are metabolized to acids
- Decreased acid excretion
- Increased excretion of base
Alkalosis occurs when blood pH rises above 7.45. It can be due to decreased acid or increased base:
- Electrolyte disturbances caused by, for example, prolonged vomiting or severe dehydration
- Administration or consumption of base
- Hyperventilation (with increased excretion of acid in the form of CO2)
Any disease or condition that affects the lungs, kidneys, metabolism or breathing has the potential to cause acidosis or alkalosis. The normal balance between acid and base can be visualized in Figure 1.
Illustration of Acid-Base Balance
- The blood's pH is normally between 7.35 and 7.45.
- The body's goal is a constant balance between incoming/produced acids and bases (faucet on) and eliminated acids and bases (drain open).
- Imbalances lead to acidosis (acid sink overflow) or alkalosis (base sink overflow).
- Balance can be restored by increasing elimination (faster draining) and/or by decreasing flow (slowing down drippy faucet).
Acid-base disorders are divided into two broad categories:
- Those that affect respiration and cause changes in pH due to changes in CO2 concentration are called respiratory acidosis (low pH) and respiratory alkalosis (high pH). Respiratory acid-base disorders are commonly due to lung diseases or conditions that affect normal breathing.
- Disorders that affect metabolism and cause changes in pH due to either increased acid production or decreased base are called metabolic acidosis (low pH) and metabolic alkalosis (high pH). Metabolic acid-base disorders may be due to kidney disease, electrolyte disturbances, severe vomiting or diarrhea, ingestion of certain drugs and toxins, and diseases that affect normal metabolism (e.g., diabetes).