The measure of acidity or alkalinity, known as pH, is central to internal stability. pH is a measure of hydrogen ion concentration, where a lower number indicates higher acidity and a higher number indicates greater alkalinity. Blood pH must be maintained within a very narrow, slightly alkaline range of 7.35 to 7.45, as even small deviations impair bodily functions. Maintaining this range is complicated because normal metabolism constantly produces acidic byproducts, such as carbon dioxide and lactic acid. The body employs a multi-layered defense system to neutralize these acids and preserve the function of proteins and enzymes, which are sensitive to pH changes.
The Body’s Immediate Chemical Buffer Systems
The first and fastest line of defense involves chemical buffer systems in the blood and cells. A buffer is a substance that rapidly absorbs or releases hydrogen ions, resisting a change in pH. This mechanism operates instantaneously, preventing dramatic fluctuations when acid or base is introduced.
The most significant extracellular buffer is the bicarbonate buffer system, composed of carbonic acid and bicarbonate ions. When excess acid enters the bloodstream, bicarbonate ions quickly bind to free hydrogen ions, converting the strong acid into the much weaker carbonic acid. Conversely, if a base is added, carbonic acid dissociates to release hydrogen ions, which neutralize the base.
Other buffers contribute, including phosphate buffers, which are important inside cells and in the urine. Protein buffers, such as hemoglobin, also play a role by binding to or releasing hydrogen ions. These systems stabilize the pH until the slower organ systems can take over.
Rapid Adjustment Through the Respiratory System
The respiratory system is the second line of defense, providing rapid adjustment within minutes. The lungs control the body’s supply of carbonic acid by regulating the amount of carbon dioxide (CO2) in the blood. CO2 combines with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate.
The brain’s respiratory center monitors the concentration of hydrogen ions and CO2 in the blood and cerebrospinal fluid. If the blood becomes too acidic, the respiratory rate and depth increase, a process called hyperventilation, to expel more CO2. This removal of CO2 shifts the chemical balance, effectively pulling hydrogen ions out of the circulation and raising the blood pH back toward the normal range.
Conversely, if the blood becomes too alkaline, breathing is suppressed (hypoventilation) to retain more CO2. The retained CO2 increases the concentration of carbonic acid and hydrogen ions, which lowers the pH. While fast-acting, the respiratory system cannot eliminate non-volatile acids, such as lactic acid, limiting its capacity to managing CO2-related acids.
Long-Term Stability Maintained by the Kidneys
The kidneys are the third and slowest mechanism for long-term pH stability, taking hours to days to fully respond. They are the only organs capable of eliminating non-volatile acids, produced from the metabolism of proteins and phospholipids. The renal system acts by controlling the two main components of the bicarbonate buffer system: hydrogen ions and bicarbonate ions.
One primary function is the secretion and excretion of excess hydrogen ions into the urine. Specialized cells in the kidney tubules transport hydrogen ions out of the blood and into the filtrate, where they are buffered by substances like phosphate and ammonia. Excreting these fixed acids removes the daily acidic load the body produces.
The second function is the reabsorption and generation of new bicarbonate ions, the body’s primary base reserve. Nearly all filtered bicarbonate is reabsorbed back into the circulation to replenish the buffer system. During prolonged acidosis, the kidneys can synthesize new bicarbonate from sources like the amino acid glutamine, boosting buffering capacity.
What Happens When pH Regulation Fails
When the buffers, lungs, and kidneys are overwhelmed, the body enters a state of acid-base imbalance, threatening physiological function. Acidosis occurs when the blood pH falls below 7.35, indicating an excess of acid or a deficit of base. This state can lead to fatigue and confusion, and in severe cases, depress the nervous system and impair heart function.
The opposite condition, alkalosis, occurs when the blood pH rises above 7.45 due to a base excess or acid deficit. Alkalosis can cause lightheadedness, muscle twitching, and spasms. Both conditions are dangerous because they disrupt the structure of enzymes, preventing them from catalyzing the chemical reactions necessary for metabolism. Without intervention, failure in pH regulation can progress to severe organ dysfunction, coma, or death.