Acidosis and alkalosis



Last Review Date: April 5, 2017


What are they?

Acidosis and alkalosis are terms used to describe the abnormal conditions when a patient’s blood pH does not fall within the healthy range. Measuring the pH of blood is a way of determining how acidic or basic (alkaline) the blood is. Normal blood pH must be maintained within a narrow range of 7.35 - 7.45 to ensure that metabolic processes function properly and the right amount of blood is delivered to the tissues. Many diseases or situations can cause a patient’s blood pH to fall outside of these limits.

In the human body, normal metabolism generates large quantities of acids that must be eliminated to maintain a normal pH balance. Most of the acid is carbonic acid which is produced when carbon dioxide (CO2) combines with water in the body. Lesser quantities of lactic acid, ketoacids and other organic acids are also produced. This balance can be disrupted by a build-up of an acid or a base (alkali) or by an increased loss of an acid or a base (see Figure 1, below).

Acidosis occurs when blood pH falls below 7.35

Alkalosis occurs when blood pH rises above 7.45

Both of these conditions act as an alarm to the body; they trigger actions intended to restore the pH balance and return the blood pH to its normal range.

The major organs involved in regulating blood pH are the lungs and the kidneys. The lungs flush acid out of the body by exhaling CO2 (carbon dioxide). Within physical limits, the body can raise and lower the rate of breathing to alter the amount of CO2 that is breathed out. This can affect blood pH within seconds or minutes. The kidneys excrete some acids in the urine, and they produce and regulate the retention of HCO3- (bicarbonate), a base that increases the blood’s pH or alkalinity. Changes in HCO3- concentration occur more slowly than changes in CO2, taking hours or days. Often, both of these processes proceed at the same time, and they continue until the balance is restored or the body’s ability to compensate is exhausted or overwhelmed. Diseases that affect either the lungs or the kidneys as well as other metabolic conditions, can interfere with the regulation of blood pH.

Illstration of Acid-Base Balance

Figure 1: Taps and drains

  • The blood’s pH must remain between 7.35 and 7.45.
  • The body’s goal is to maitain a constant balance between incoming/produced acids and bases (tap 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 the dripping tap).

Acidosis or alkalosis can be an acute condition or it may be a chronic condition. Acidosis may not cause any symptoms or it may be associated with nonspecific symptoms such as fatigue, nausea and vomiting. Acute acidosis may also cause an increased rate and depth of breathing, confusion and headaches, and it can lead to seizures, coma, and in some cases death. Symptoms of alkalosis are often due to associated potassium (K+) loss and may include irritability, weakness and cramping.

Acid-base disorders are divided into two broad categories. Those that affect respiration and cause changes in CO2 concentration are called respiratory acidosis (low pH) or respiratory alkalosis (high pH). Respiratory acid-base disorders are commonly due to lung diseases or conditions that affect normal breathing. Disorders that affect HCO3- concentration are called metabolic acidosis (low pH) and metabolic alkalosis (high pH). Metabolic acid-base disorders may be due to kidney disease and a variety of other conditions. There are also known genetic abnormalities prevent normal metabolic pathways from functioning perfectly and can cause acid-base imbalance, usually acidosis. These are called inborn errors of metabolism (or genetic-metabolic disorders), and the acid-base effect is due to deficiencies or build-ups of different compounds, many of which are acidic in nature. Other disorders that can cause metabolic (non-respiratory) acid-base disorders include diabetes (diabetic ketoacidosis), severe vomiting and severe diarrhoea.


Common causes of acid-base disorders

Respiratory acidosis
Reduced CO2 elimination

  • Decreased respiratory drive (due to drugs or central nervous system disorders)
  • Hypoventilation
  • Lung disease
  • Respiratory muscle/nerve disease (myasthenia gravis, botulism, amyotrophic lateral sclerosis, Guillain-BarrĂ© syndrome)

Respiratory alkalosis
Increased CO2 elimination

  • Hyperventilation (due to anxiety, pain, shock)
  • Severe infection or fever
  • Liver failure
  • Pneumonia, pulmonary congestion or embolism

Metabolic acidosis
Decreased HCO3-, due to loss or to increased acid

  • Alcoholic ketoacidosis
  • Diabetic ketoacidosis
  • Kidney failure
  • Lactic acidosis
  • Toxins (overdose of salicylates [aspirin], methanol, ethylene glycol, toluene)
  • Gastrointestinal bicarbonate loss, such as from prolonged diarrhoea

Metabolic alkalosis
Increased HCO3-, due to gain, or to loss of acid

  • Diuretics
  • Prolonged vomiting
  • Severe dehydration

Tests

Tests are performed to determine whether a patient has an acid-base disorder and how severe the disorder is, and to help diagnose underlying diseases or conditions (such as diabetic ketoacidosis or the ingestion of a toxin) that have caused the acid-base disorder. Testing is also done to monitor critically ill patients, as well as patients with conditions known to affect acid-base balance, such as chronic lung disease and kidney disease. The primary tests used to identify, evaluate and monitor acid-base disorders are those that measure blood gases and electrolytes.

"Blood gases" refers to a group of tests performed together on an arterial blood sample (blood obtained from an artery instead of a vein). They provide a snapshot of the blood’s pH, PO2 (the amount of oxygen dissolved in the blood) and PCO2 (the amount of carbon dioxide dissolved in the blood). From these results, HCO3- (the amount of bicarbonate) can be calculated.

Results seen:

Acid-Base Disorder pH HCO3 PCO2 Body Compensation
Metabolic acidosis Less than 7.35 Low Low Increased breathing rate (hyperventilation) to increase CO2 elimination
Metabolic alkalosis Greater than 7.45 High High Slowed breathing (hypoventilation) to decrease CO2 elimination
Respiratory acidosis Less than 7.35 High High Kidney increases production of HCO3- and excretion of H+ (acid)
Respiratory alkalosis Greater than 7.45 Low Low Decreased production of HCO3- and excretion of H+

"Electrolytes" refers to a group of four tests that measure Na+ (sodium), K+ (potassium), Cl- (chloride) and bicarbonate (total CO2 content).

An anion gap (see electrolytes /Common questions, for an explanation) can be calculated from the electrolytes; this provides a clue to the cause of the acid/base imbalance.

Depending on the suspected cause, a number of other tests may be ordered, including an E/LFT, a test for ketones in blood and ketones in urine, and tests for lactate, salicylates, ethylene glycol and methanol, to name but a few.

Treatment
Most acid-base disorders do not require treatment of the abnormal pH. The goal is to identify what has caused the acid-base disorder and, when necessary, treat this underlying cause. However, doctors may give intravenous HCO3- to patients with dangerously low blood pH levels.


Related pages

On this site
Tests: Blood gases, electrolytes, glucose, lactate
Conditions: Alcoholism, kidney disease, diabetes

Elsewhere on the web
Merck Manual: Acid-Base Metabolism
myDr Ketoacidosis: a complication of diabetes
MedlinePlus: Acidosis, Alkalosis