The pH scale measures the acidity or alkalinity of a solution based on the concentration of hydrogen ions (\(\text{H}^+\)). High concentrations of hydrogen ions result in low pH values (acidic), while low concentrations result in high pH values (alkaline or basic). Physiological pH refers to the narrow, optimal range of hydrogen ion concentration required for the cells, tissues, and fluids in the human body to function correctly. This chemical balance is fundamental to sustaining life, as nearly every biological process depends on this level of acidity or alkalinity.
The Precise Range for Human Health
The acidity of human arterial blood is maintained within an exceptionally narrow band between a pH of 7.35 and 7.45. Since 7.0 is chemically neutral, the body’s fluids are slightly alkaline under normal conditions. This small range is biologically significant because the pH scale is logarithmic, meaning a change of one unit represents a tenfold difference in hydrogen ion concentration.
A blood pH below 7.35 is defined as acidemia, while a pH above 7.45 is alkalemia. Even minor shifts outside of this range can rapidly impair bodily functions. The preference for a slightly alkaline environment, rather than a neutral 7.0, is partially due to the need for optimal blood oxygenation and the utilization of biochemical intermediates.
Cellular Importance of pH Stability
The precise control of physiological pH is necessary because the chemical environment directly determines the shape and function of biological molecules. Proteins, including enzymes, are chains of amino acids that must fold into a specific three-dimensional structure to be active. This folding process is highly sensitive to hydrogen ion concentration because it affects the electrical charges on the amino acid side chains.
When the pH moves outside the physiological range, the ionic and hydrogen bonds holding the protein’s structure are disrupted. This disruption leads to denaturation, causing the protein to lose its functional shape. Enzymes, which are biological catalysts, are particularly susceptible, and their ability to speed up chemical reactions is diminished or lost if the pH is not optimal.
Enzyme activity is maximized at a specific pH point, and any deviation reduces efficiency, slowing down metabolism. Furthermore, pH stability is involved in cellular communication and transport across cell membranes. The concentration of hydrogen ions influences the ionization state of molecules, affecting the movement of ions and electrical signaling necessary for nerve conduction and muscle contraction.
Maintaining Balance: Buffer Systems and Organ Function
The body employs multiple layers of defense to manage the hydrogen ion load generated constantly by metabolism, a process known as acid-base homeostasis. The first line of defense is the chemical buffer system, which acts instantly to neutralize excess acid or base. The most significant is the bicarbonate buffer system, found in the extracellular fluid and blood plasma.
This buffer system uses carbonic acid (\(\text{H}_2\text{CO}_3\)) and bicarbonate ions (\(\text{HCO}_3^-\)) to convert strong acids or bases into weaker forms. The bicarbonate component readily accepts excess hydrogen ions, preventing a sharp drop in pH. While highly effective, chemical buffers have a finite capacity and cannot eliminate the problematic substances from the body.
Respiratory System
The second line of defense is the respiratory system, which regulates the concentration of carbon dioxide (\(\text{CO}_2\)). \(\text{CO}_2\), a byproduct of cellular respiration, combines with water to form carbonic acid, making it an acid precursor. The lungs quickly adjust blood \(\text{pH}\) within minutes by altering the rate of breathing.
If the blood becomes too acidic, the respiratory rate increases, causing more \(\text{CO}_2\) to be exhaled and driving the \(\text{pH}\) up. Conversely, if the blood becomes too alkaline, the breathing rate decreases to conserve \(\text{CO}_2\) and lower the \(\text{pH}\). This mechanism rapidly adjusts the volatile acid load in the body.
Renal System
The third and most powerful long-term defense mechanism involves the renal system, or kidneys. The kidneys regulate \(\text{pH}\) over hours to days by excreting fixed acids and managing the body’s supply of bicarbonate. They secrete hydrogen ions into the urine, effectively removing acid from the body.
The kidneys also reabsorb bicarbonate back into the blood or generate new bicarbonate molecules to replenish supplies used by the buffer system. Through the combined efforts of the chemical buffers, the lungs, and the kidneys, the body’s \(\text{pH}\) is maintained within its narrow, life-sustaining range.
When pH Goes Wrong: Acidosis and Alkalosis
When the body’s regulatory systems are overwhelmed, the physiological \(\text{pH}\) moves outside the 7.35 to 7.45 limits, leading to serious medical conditions. Acidosis occurs when the \(\text{pH}\) falls below 7.35, resulting in an overly acidic environment. Symptoms often include fatigue, confusion, and a rapid, deep breathing pattern as the body attempts to compensate by expelling more \(\text{CO}_2\).
Acidosis is classified as either respiratory or metabolic, depending on the cause. Respiratory acidosis occurs when the lungs cannot properly remove \(\text{CO}_2\), often due to conditions like chronic obstructive pulmonary disease (COPD) or hypoventilation. Metabolic acidosis results from an accumulation of fixed acids, such as in uncontrolled diabetes (diabetic ketoacidosis) or kidney disease that impairs acid excretion.
Alkalosis is the opposite condition, defined by a \(\text{pH}\) above 7.45, indicating the blood is too alkaline. This condition can cause symptoms such as tingling in the extremities, muscle twitching, and cognitive impairment. Respiratory alkalosis is caused by hyperventilation, where excessive breathing expels too much \(\text{CO}_2\), reducing the body’s acid content.
Metabolic alkalosis results from the excessive loss of acid, such as through prolonged vomiting or the use of certain diuretics. Both acidosis and alkalosis are life-threatening if severe, demonstrating how closely human health is linked to the maintenance of physiological \(\text{pH}\).