Acid-Base Disorders Explained

Acid-base disorders are disturbances in the normal balance between acid production, carbon dioxide removal, bicarbonate regulation and hydrogen ion excretion. They are common in emergency medicine, intensive care, renal disease, respiratory disease, endocrine emergencies and sepsis.

This article is a foundation article. It explains the core ideas behind acidosis, alkalosis, respiratory disorders, metabolic disorders and compensation. It deliberately avoids becoming a detailed ABG interpretation guide. Once the physiology is clear, arterial blood gas interpretation becomes much easier and much less mechanical.

What Is pH?

pH is a measure of hydrogen ion concentration. Hydrogen ions are what make a solution acidic. When hydrogen ion concentration rises, pH falls. When hydrogen ion concentration falls, pH rises.

Because pH uses a logarithmic scale, small numerical changes represent large physiological changes. A fall from pH 7.40 to 7.20 is not a minor laboratory curiosity; it reflects a major increase in hydrogen ion concentration and may signal severe illness.

State	pH value	Meaning
Normal arterial pH	7.35-7.45	Normal acid-base range
Acidaemia	< 7.35	Blood pH is lower than normal
Alkalaemia	> 7.45	Blood pH is higher than normal

The Core Acid-Base Balance

The most important relationship in acid-base physiology is the balance between bicarbonate and carbon dioxide. Blood pH depends mainly on the ratio between HCO3- and CO2.

If bicarbonate falls or carbon dioxide rises, pH tends to fall. If bicarbonate rises or carbon dioxide falls, pH tends to rise. This simple idea explains why acid-base disorders are divided into metabolic and respiratory disorders.

Normal ABG Values

Variable	Normal adult arterial value	Main meaning
pH	7.35-7.45	Overall acidity or alkalinity of blood
PaCO2	35-45 mmHg	Respiratory component; reflects ventilation
HCO3-	22-26 mmol/L	Metabolic component; reflects bicarbonate buffer and renal handling

Normal Acid-Base Physiology

The body continuously produces acids during normal metabolism. This happens even in healthy people at rest. Every cell uses energy, produces carbon dioxide, metabolises proteins and fats, and generates substances that must be buffered, converted or excreted.

The main acid sources include:

• Carbon dioxide, produced by aerobic metabolism and carried in blood as part of the bicarbonate buffer system
• Lactic acid, produced when anaerobic metabolism increases, such as in shock, seizures or severe hypoxia
• Ketoacids, produced during diabetic ketoacidosis, starvation or alcoholic ketoacidosis
• Sulfuric acid, produced from metabolism of sulfur-containing amino acids
• Phosphoric acid, produced from phosphate-containing compounds

The body must therefore do two things continuously: buffer acids immediately and remove acids from the body. Failure of either process can lead to acidosis or alkalosis.

Why pH Must Remain Stable

Normal enzyme activity, cellular metabolism, cardiac contractility and neurological function depend on a stable internal environment. When pH moves too far in either direction, proteins change shape, ion channels behave differently, vascular tone changes and cardiac rhythm becomes unstable.

Severe acidosis can cause:

• Reduced myocardial contractility
• Arrhythmias
• Hypotension
• Reduced responsiveness to catecholamines
• Altered consciousness

Severe alkalosis can cause:

• Neuromuscular irritability
• Paraesthesia, cramps and tetany due partly to reduced ionized calcium; see Calcium Homeostasis Explained and Hypocalcemia Explained
• Seizures
• Arrhythmias
• Reduced cerebral blood flow in severe respiratory alkalosis

How The Body Regulates pH

The body defends pH using three major systems. They act at different speeds and in different ways:

Defence system	Speed	Main role
Chemical buffers	Immediate	Bind or release hydrogen ions to reduce sudden pH change
Lungs	Minutes	Regulate carbon dioxide through ventilation
Kidneys	Hours to days	Regulate bicarbonate and hydrogen ion excretion

Chemical Buffers

Buffers are substances that reduce sudden changes in pH. The most important extracellular buffer system is the bicarbonate buffer system. It links carbon dioxide, water, carbonic acid, hydrogen ions and bicarbonate.

The lungs influence the CO2 side of the equation. The kidneys influence the HCO3- and H+ side. This is why respiratory and metabolic processes are connected rather than separate silos.

Respiratory Regulation

The lungs regulate carbon dioxide. Carbon dioxide behaves as an acid because it combines with water to form carbonic acid, which can dissociate into hydrogen ions and bicarbonate.

If a patient hyperventilates, they blow off CO2 and pH rises. If a patient hypoventilates, they retain CO2 and pH falls. This is the basis of respiratory alkalosis and respiratory acidosis.

Renal Regulation

The kidneys regulate bicarbonate and hydrogen ions. They reabsorb filtered bicarbonate, generate new bicarbonate and excrete hydrogen ions in urine. This makes renal regulation slower than respiratory regulation, but essential for sustained correction.

For example, in chronic respiratory acidosis from long-standing CO2 retention, the kidneys gradually retain more bicarbonate to buffer the extra acid load. This renal compensation is why chronic respiratory acidosis may have a less dramatic pH fall than acute respiratory acidosis.

Acidosis vs Alkalosis

Acidosis is a process that tends to lower pH. Alkalosis is a process that tends to raise pH.

This is different from acidaemia and alkalaemia:

• Acidaemia means the measured blood pH is low: pH below 7.35.
• Alkalaemia means the measured blood pH is high: pH above 7.45.
• Acidosis is the process pushing the pH down.
• Alkalosis is the process pushing the pH up.

This distinction matters because patients can have mixed disorders. A patient may have metabolic acidosis and respiratory alkalosis at the same time. The final pH may be low, normal or high depending on which process is stronger.

Respiratory vs Metabolic Disorders

Acid-base disorders are grouped by the primary variable that is abnormal. Respiratory disorders primarily involve carbon dioxide. Metabolic disorders primarily involve bicarbonate or fixed acids.

Respiratory Disorders

In respiratory disorders, the primary abnormality is CO2. If CO2 rises, pH falls and respiratory acidosis develops. If CO2 falls, pH rises and respiratory alkalosis develops.

Metabolic Disorders

In metabolic disorders, the primary abnormality is HCO3- or non-volatile acid load. If bicarbonate falls or acid load rises, metabolic acidosis develops. If bicarbonate rises or hydrogen ions are lost, metabolic alkalosis develops.

Disorder group	Primary variable	Main organ system	Examples
Respiratory	CO2	Lungs and ventilation	COPD, opioid overdose, hyperventilation
Metabolic	HCO3- or fixed acid	Kidneys, metabolism, gastrointestinal tract	DKA, lactic acidosis, vomiting, diarrhoea

The Four Primary Acid-Base Disorders

Most beginner acid-base interpretation starts with the four primary disorders. Each disorder has a direction of pH change and a primary variable responsible for that change.

Primary disorder	Primary abnormality	Expected pH direction	Common causes
Metabolic acidosis	Low HCO3- or increased fixed acid	Low pH	DKA, lactic acidosis, renal failure, diarrhoea
Metabolic alkalosis	High HCO3- or hydrogen ion loss	High pH	Vomiting, diuretics, mineralocorticoid excess
Respiratory acidosis	High CO2	Low pH	COPD, drug overdose, neuromuscular weakness
Respiratory alkalosis	Low CO2	High pH	Anxiety, sepsis, pregnancy, pain, hypoxaemia

Metabolic Acidosis

Metabolic acidosis occurs when bicarbonate is reduced or fixed acid load is increased. The pH falls because the metabolic component of the buffer system is pushing toward acidity.

Common causes include diabetic ketoacidosis, lactic acidosis, renal failure and diarrhoea. The lungs compensate by increasing ventilation to lower CO2.

Metabolic Alkalosis

Metabolic alkalosis occurs when bicarbonate is increased or hydrogen ions are lost. Vomiting is a classic cause because gastric acid is lost from the body. Diuretics can also contribute by causing volume contraction, chloride loss and potassium abnormalities.

Respiratory Acidosis

Respiratory acidosis occurs when ventilation is inadequate and CO2 is retained. The pH falls because CO2 acts as an acid in the bicarbonate buffer system. Acute respiratory acidosis can be dangerous because the kidneys have not yet had time to compensate.

Respiratory Alkalosis

Respiratory alkalosis occurs when ventilation is excessive relative to CO2 production. CO2 falls, so pH rises. It may occur with anxiety, pain, sepsis, pregnancy, liver failure, salicylate poisoning or hypoxaemia-driven hyperventilation.

High-Yield Acid-Base Pattern Table

Disorder	pH	PaCO2	HCO3-	Primary problem
Metabolic acidosis	Down	Down	Down	Low bicarbonate or increased fixed acid
Metabolic alkalosis	Up	Up	Up	High bicarbonate or hydrogen ion loss
Respiratory acidosis	Down	Up	Up	Carbon dioxide retention
Respiratory alkalosis	Up	Down	Down	Excessive carbon dioxide loss

Physiological Compensation

Compensation is the body's attempt to minimise a change in pH. It does not fix the underlying disorder. It only reduces the severity of the pH disturbance while the primary problem continues.

A key principle is that compensation moves in the same direction as the primary abnormality:

• In metabolic acidosis, HCO3- is low. The lungs compensate by lowering CO2 through hyperventilation.
• In metabolic alkalosis, HCO3- is high. The lungs may compensate by retaining CO2 through reduced ventilation, although this is limited by the need to maintain oxygenation.
• In respiratory acidosis, CO2 is high. The kidneys compensate by retaining or generating more bicarbonate.
• In respiratory alkalosis, CO2 is low. The kidneys compensate by excreting more bicarbonate.

Primary disorder	Compensating system	Compensatory response
Metabolic acidosis	Lungs	Increase ventilation, lower CO2
Metabolic alkalosis	Lungs	Reduce ventilation, raise CO2
Respiratory acidosis	Kidneys	Retain/generate HCO3- and excrete H+
Respiratory alkalosis	Kidneys	Excrete HCO3-

Simple Approach To Acid-Base Interpretation

A beginner should approach acid-base interpretation in a consistent order. The goal is not to memorise every formula immediately, but to build a reliable mental map.

1. Check pH. Is there acidaemia, alkalaemia or a near-normal pH?
2. Check CO2. Is CO2 moving in a direction that explains the pH?
3. Check HCO3-. Is bicarbonate moving in a direction that explains the pH?
4. Identify the primary disorder. Decide whether the primary abnormality is respiratory or metabolic.
5. Assess compensation. Ask whether the other system is responding appropriately.

For example, if pH is low and HCO3- is low, the primary process is likely metabolic acidosis. If pH is low and CO2 is high, the primary process is likely respiratory acidosis. This simple pattern recognition is the foundation of ABG interpretation.

More detailed interpretation requires expected compensation formulas, anion gap assessment, delta ratio and clinical context. Those topics belong in a dedicated ABG interpretation article.

Clinical Examples

Clinical context is essential. The same pH abnormality may have different causes depending on the patient, history, examination and laboratory findings.

Example	Typical pattern	Primary disorder	Why it happens
Diabetic ketoacidosis	pH down, HCO3- down	Metabolic acidosis	Ketoacid accumulation consumes bicarbonate
COPD exacerbation	pH down, CO2 up	Respiratory acidosis	Inadequate ventilation causes CO2 retention
Vomiting	pH up, HCO3- up	Metabolic alkalosis	Loss of gastric acid and chloride promotes alkalosis
Panic attack	pH up, CO2 down	Respiratory alkalosis	Hyperventilation removes excessive CO2

These examples are deliberately simple. Real patients may have mixed disorders. A septic patient may have lactic acidosis and respiratory alkalosis at the same time. A patient with COPD may have chronic respiratory acidosis plus metabolic alkalosis from diuretics. Always interpret the blood gas with the clinical picture.

Frequently Asked Questions