This article was last reviewed on
This article waslast modified on
5 January 2018.

What are they?

Acidosis and alkalosis are terms used to describe abnormal conditions when a patient’s blood pH may not fall within the healthy range. Measuring the hydrogen ion concentration, and calculating the pH, of blood is a way of finding out how acidic or alkaline the blood is. Normal blood pH must be within a narrow range of 7.35-7.45 so that the body’s metabolic processes can work properly and can deliver the right amount of oxygen to tissues. Many diseases and other conditions can cause a patient’s blood pH to fall outside of these healthy limits.

In the human body, normal metabolism generates large quantities of acids (effectively compounds that produce a free hydrogen ion) that must be removed to keep a normal pH balance. Disruption of this balance can be caused by a build-up of acid or alkali (base) or by an increased loss of acid or alkali (see the diagram of ‘taps and drains’ below). Alkalis, or bases, are compounds that remove a free hydrogen ion. Acidosis occurs when blood pH falls below 7.35, indicating an increase in hydrogen ion concentration. Alkalosis occurs when blood pH rises above 7.45, indicating a reduction in hydrogen ion concentration. Both of these conditions act as an alarm to the body; they trigger actions intended to restore the balance and to 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 carbon dioxide (CO2), which forms an acid when in solution (dissolved in the blood). Within physical limits, the body can raise and lower the rate of breathing to alter the amount of carbon dioxide that is breathed out. This can affect blood pH within seconds to minutes. The kidneys remove some acids in the urine and the kidneys also produce and regulate the retention of bicarbonate (HCO3-), a base (alkali) that increases the blood’s pH. Changes in bicarbonate concentration occur more slowly than changes in carbon dioxide, taking hours or days. Often, both of these processes proceed at the same time, continuing until the balance is restored or until the body’s ability to compensate is exhausted or overwhelmed. Diseases affecting either the lungs or the kidneys as well as other metabolic conditions can interfere with the regulation of blood pH.

Figure 1: Taps and Drains

  • The blood’s pH must remain between 7.35 and 7.45.
  • The body tries to have a constant balance between incoming/produced acids and bases (tap on) and removal of acids and bases (drain open).
  • Imbalances of the flow from the ‘taps’ and removal through the ‘drains’ lead to acidosis (acid sink overflow) or alkalosis (base sink overflow).
  • Balance can be restored by increasing removal (faster flow through the ‘drain’) and/or by decreasing flow in (slowing the flow through the ‘tap’).

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

Acid-base disorders are divided into two broad categories, respiratory and metabolic. Those that affect breathing and cause changes in carbon dioxide 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 bicarbonate 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 that result in the impairment of normal metabolic pathways and so 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 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.

Accordion Title
About Acidosis and Alkalosis
  • Common causes of acid-bases disorders

    Respiratory acidosis
    Reduced carbon dioxide removal

    Decreased breathing (due to drugs or to central nervous system disorders)
    Slow breathing
    Lung disease
    Respiratory muscle/nerve disease (myasthaenia gravis, botulism, amyotrophic lateral sclerosis, Guillain-Barre syndrome)

    Respiratory alkalosis
    Increased carbon dioxide removal

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

    Metabolic acidosis
    Decreased bicarbonate, 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 bicarbonate, due to gain, or to loss of acid

    Prolonged vomiting
    Severe dehydration

  • Tests

    Testing is used to identify whether a patient has an acid-base disorder, to determine how severe the disorder is, and to help diagnose underlying diseases such as diabetic ketoacidosis or poisoning. Testing is also done to monitor critically ill patients, as well as patients with long term diseases of acid-base balance, such as chronic lung disease and kidney disease. The primary tests used to identify, evaluate, and monitor acid-base disorders are blood gases and electrolytes.

    Blood gases are a group of tests performed together on an arterial blood sample (blood obtained from an artery instead of a vein). They are a snapshot of the blood’s pH, the amount of oxygen dissolved in the blood (PO2), and the amount of carbon dioxide dissolved in the blood (PCO2). Using these results the amount of bicarbonate in the blood (HCO3-) can be calculated. The sample must be arterial to get an accurate measurement of oxygen and carbon dioxide as the concentration of these gases change as the blood moves through the body tissues. This means the sample has to be taken from an artery, usually at the wrist, which can be slightly more painful than normal venous blood sampling (venepuncture) as arteries have nerves and muscles in the vessel walls. Arteries can also spasm when irritated making the procedure slightly more likely to fail and strong pressure for a couple of minutes must be exerted afterwards to prevent bruising, this being much more important than in normal venepuncture.

    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 acid
    Respiratory alkalosis Greater than 7.45 Low Low Kidney decreases production of HCO3- and excretion of acid

    Electrolytes refers to a group of four tests: Na+ (sodium), K+ (potassium), Cl- (chloride) and bicarbonate (total carbon dioxide content).

    An anion gap can be calculated from the electrolytes and provides a clue to the cause of the acid/base imbalance.

    Depending on the suspected cause, a number of other tests may be requested: Renal profile, glucose, ketones in blood and ketones in urine, lactate, aspirin (salicylate), ethylene glycol, and methanol, to name a few.

  • Treatment

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