The concept of pH is a chemical measurement that determines the acidity or alkalinity of a fluid based on the concentration of hydrogen ions present. The scale ranges from 0 (acidic) to 14 (alkaline), with 7 being neutral. In a biological context, maintaining a precise acid-base balance is fundamental because most life processes are sensitive to changes in this environment. The body constantly produces acids as a byproduct of metabolism, necessitating a complex regulatory system to prevent harmful pH fluctuations. A stable blood pH is maintained within an extremely tight range to ensure that all cellular and molecular functions proceed normally.
Defining Normal Blood pH
The numerical range considered normal for arterial blood pH is narrow, spanning only from 7.35 to 7.45. This range indicates that human blood is naturally slightly alkaline, rather than neutral like pure water (pH 7.0). The body maintains this slightly basic state because it represents the optimal environment for biochemical reactions.
Even small deviations outside of this 0.1 pH unit range can quickly disrupt cellular function and lead to serious health complications. For instance, a blood pH below 6.8 or above 7.8 is generally considered incompatible with life and can rapidly lead to death.
The Role of pH in Physiological Function
Maintaining the pH between 7.35 and 7.45 is important because nearly every biological process relies on proteins, which are directly affected by acidity or alkalinity. Enzymes, which catalyze chemical reactions, function optimally only within this specific, narrow pH range.
When the pH shifts too far, excess hydrogen ions disrupt the electrical charges on the amino acid side chains, changing the enzyme’s three-dimensional shape. This structural change, known as denaturation, can destroy the enzyme’s active site, preventing it from binding to target molecules and halting the chemical reaction.
The acid-base balance also directly influences oxygen transport via red blood cells. Hemoglobin, the protein responsible for carrying oxygen, changes its affinity for oxygen in response to pH fluctuations, a phenomenon known as the Bohr effect.
When the blood becomes slightly more acidic, such as in active muscle tissue producing carbon dioxide and lactic acid, hemoglobin releases oxygen more readily to the surrounding cells. Conversely, in the lungs, where carbon dioxide is exhaled and the blood becomes slightly more alkaline, hemoglobin’s affinity for oxygen increases, allowing it to bind and transport a fresh supply.
The Body’s pH Control System
The body employs three distinct, overlapping systems to ensure blood pH remains stable.
Chemical Buffer Systems
Chemical buffer systems act instantly to resist changes in hydrogen ion concentration by either absorbing or releasing them. The bicarbonate buffer system is the most significant, using bicarbonate ions to neutralize excess acid and carbonic acid to neutralize excess base. This chemical system works within seconds to minimize immediate pH shifts.
Respiratory Regulation
The respiratory system regulates the blood’s concentration of carbon dioxide (CO2). Since carbon dioxide combines with water to form carbonic acid, controlling CO2 levels directly controls the amount of acid in the blood.
When blood acidity rises, the brain signals the lungs to increase the rate and depth of breathing (hyperventilation) to rapidly expel more CO2 and raise the pH. Conversely, slower, shallower breathing retains CO2 to lower an excessively high pH.
Renal Regulation
The kidneys provide the third system, which is the slowest to respond (taking hours to days), but is the most potent and long-lasting mechanism. The kidneys maintain balance by selectively conserving or excreting hydrogen ions and bicarbonate ions into the urine.
If the blood is too acidic, the kidneys excrete hydrogen ions while reabsorbing bicarbonate, the body’s primary buffer. This control allows the kidneys to manage the metabolic component of the acid-base balance and regenerate used buffers.
When pH Goes Wrong Acidosis and Alkalosis
When the body’s control systems are overwhelmed, the blood pH moves outside the normal range, leading to two primary pathological states: acidosis and alkalosis.
Acidosis
Acidosis occurs when the blood pH drops below 7.35, indicating an excess of acid or a depletion of base. This condition is categorized as respiratory acidosis if it results from inadequate CO2 excretion due to lung failure. It is metabolic acidosis if caused by the accumulation of non-carbonic acids, such as the ketoacids seen in uncontrolled diabetes.
Alkalosis
Alkalosis occurs when the blood pH rises above 7.45 due to a deficiency of acid or an excess of base. Respiratory alkalosis is triggered by hyperventilation, often seen during panic attacks, which causes excessive CO2 to be blown off.
Metabolic alkalosis may result from the excessive loss of stomach acid due to prolonged vomiting or ingesting too much alkaline substance. Both extreme acidosis and alkalosis can lead to severe symptoms, including confusion, fatigue, and ultimately, life-threatening seizures or coma as the central nervous system fails.