Acid-Base Balance

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Last updated: June 19, 2026Bookmark

Table Of Contents

Overview
Metabolic Acidosis
Metabolic Alkalosis
Respiratory Acidosis
Respiratory Alkalosis

Overview

The normal body pH is 7.35-7.45 (average 7.40).

This slightly alkaline pH optimises oxygen delivery to tissues, enzyme activity, protein function, and biochemical reactions.

Carbon dioxide (CO2) – a major byproduct of aerobic respiration – plays a major role in acid-base regulation.

Definition of terms

pH	Interpretation
< 7.35	Acidemia
> 7.45	Alkalemia
7.35–7.45	Normal

Major acid-base disorders: There are four primary acid-base disorders

Disorder	Primary Abnormality
Metabolic Acidosis	Low pH and Low HCO3-
Metabolic Alkalosis	High pH and Elevated HCO3-
Respiratory Acidosis	Low pH and Elevated PaCO2
Respiratory Alkalosis	High pH and Low PaCO2

Compensation: This is when the body attempts to restore pH by activating the opposite system. It rarely returns pH completely to 7.40.

Primary Disorder	Compensation
Metabolic acidosis	Respiratory alkalosis (hyperventilation)
Metabolic alkalosis	Respiratory acidosis (hypoventilation)
Respiratory acidosis	Metabolic alkalosis (↑ HCO₃⁻ retention)
Respiratory alkalosis	Metabolic acidosis (↓ HCO₃⁻ retention)

Buffer Systems

Buffer system	Description
Bicarbonate buffer system	This is the most important extracellular buffer system. It is catalyzed by carbonic anhydrase, and prevents large changes in pH. Equation: CO2 + H20 ↔ H2CO3 ↔ HCO3- + H+
Phosphate buffer system	Important in regulating pH in urine
Protein buffers	Regulates intracellular pH
Haemoglobin buffer	Binds hydrogen ions and CO2 to regulate pH in blood

Related effects

Effect	Description
Bohr effect	Increased H+ and CO2 reduces haemoglobin affinity for oxygen. This enhances oxygen delivery to tissues
Haldane effect	Oxygenated haemoglobin releases CO2 more readily

Organ systems regulating Acid-Base balance

System	Description	Respone time
Respiratory system	This controls carbon dioxide by increasing or reducing ventilation.	Minutes to hours
Renal system	This controls hydrogen ion (H+) excretion and bicarbonate (HCO3-) reabsorption.	Days

Metabolic Acidosis

The anion gap can be used to determine the cause of metabolic acidosis.

Anion gap = (Sodium + Potassium) – (Chloride + Bicarbonate)
A normal value is 10 – 18 mmol/L

The urine anion gap can be used further to determine the cause of normal anion gap metabolic acidosis

Urine anion gap = (Sodium + Potassium) – Chloride
+20 to +90 = renal cause (reduced NH4+ excretion)
- 20 to -50 = gastrointestinal bicarbonate loss

Types of metabolic acidosis

Type	Description	Causes
High anion gap metabolic acidosis	This occurs when bicarbonate buffers excess acids	GOLMARK: Glycols (ethylene glycol, propylene glycol), oxoproline, lactate, methanol, aspirin (salicylates), renal failure, and ketones (DKA, starvation, and alcohol)
Normal anion gap metabolic acidosis	This occurs when bicarbonate is lost but chloride is retained. It is also known as hyperchloremic metabolic acidosis.	Severe diarrhoea, renal tubular acidosis, carbonic anhydrase inhibitors, and gastrointestinal losses

Winter’s Formula is used to assess respiratory compensation in metabolic acidosis

Expected PaCO2 = (1.5 x HCO30) + 8 +/- 2

Interpretation of Winter’s formula

Actual PaCO₂	Interpretation
Within range of excpected PaCO2	Appropriate compensation
Higher than expected PaCO2	Concurrent respiratory acidosis
Lower than expected PaCO2	Concurrent respiratory alkalosis

Metabolic Alkalosis

Metabolic alkalosis can be classified based on whether it is likely to improve by administering normal saline (chloride replacement).

Types of metabolic alkalosis

Type	Description	Causes
Chloride-responsive metabolic alkalosis	The body has lost both hydrogen ions (H+) and chloride ions (Cl-). When chloride is depleted, the kidneys try to conserve chloride by reabsorbing sodium together with bicarbonate. This perpetuates the alkalosis. Urine chloride is < 20 mEq/L since the body is chloride depleted.	Vomiting, nasogastric suction, hypovolemia, and diuretics
Non-chloride-responsive metabolic alkalosis	Alkalosis is maintained by other ongoing factors. Urine chloride is > 20 mEq/L	Hyperaldosteronism, severe potassium repletion, and mineralocorticoid excess

Respiratory Acidosis

Hypoventilation → CO2 retention → increased Carbonic acid → low pH

Causes
- Type 2 respiratory failure
- COPD
- Opiate overdose
- Severe obesity (obesity hypoventilation syndrome)
- CNS depression
- Brain injury

Respiratory Alkalosis

Hyperventilation → CO2 retention → increased Carbonic acid → low pH

Causes
- Panic attacks
- Anxiety
- Pulmonary embolism
- Pneumonia
- Salicylate intoxication
- Pregnancy
- Stroke
- Subarachnoid haemorrhage
- Meningitis

Reference Intervals ›

Biochemistry

ACTH	P: <80 ng/L
ALT	P: 5–35 U/L
Albumin	P: 35–50 g/L
Aldosterone	P: 100–500 pmol/L
Alk. phosphatase	P: 30–130 U/L
α-Amylase	P: 0–180 IU/dL
α-Fetoprotein	S: <10 kU/L
Angiotensin II	P: 5–35 pmol/L
ADH	P: 0.9–4.6 pmol/L
AST	P: 5–35 U/L
Bicarbonate	P: 24–30 mmol/L
Bilirubin	P: 3–17 μmol/L
BNP	P: <50 ng/L
CRP	P: <10 mg/L
Calcitonin	P: <0.1 mcg/L
Calcium (ionized)	P: 1.0–1.25 mmol/L
Calcium (total)	P: 2.12–2.60 mmol/L
Chloride	P: 95–105 mmol/L
Cholesterol	P: <5.0 mmol/L
VLDL	P: 0.128–0.645 mmol/L
LDL	P: <2.0 mmol/L
HDL	P: 0.9–1.93 mmol/L
Cortisol AM	P: 450–700 nmol/L
Cortisol Midnight	P: 80–280 nmol/L
CK ♂	P: 25–195 U/L
CK ♀	P: 25–170 U/L
Creatinine	P: 70–100 μmol/L
Ferritin	P: 12–200 mcg/L
Folate	S: 2.1 mcg/L
FSH	P: 2–8 U/L ♂; >25 menopause
GGT ♂	P: 11–51 U/L
GGT ♀	P: 7–33 U/L
Glucose (fasting)	P: 3.5–5.5 mmol/L
Growth hormone	P: <20 mu/L
HbA1C (DCCT)	B: 4–6%
HbA1C (IFCC)	B: 20–42 mmol/mol
Iron ♂	S: 14–31 μmol/L
Iron ♀	S: 11–30 μmol/L
Lactate (venous)	P: 0.6–2.4 mmol/L
Lactate (arterial)	P: 0.6–1.8 mmol/L
LDH	P: 70–250 U/L
LH	P: 3–16 U/L
Magnesium	P: 0.75–1.05 mmol/L
Osmolality	P: 278–305 mosmol/kg
PTH	P: 0.8–8.5 pmol/L
Potassium	P: 3.5–5.3 mmol/L
Prolactin ♂	P: <450 U/L
Prolactin ♀	P: <600 U/L
PSA	P: 0–4 mcg/mL
Protein (total)	P: 60–80 g/L
Red cell folate	B: 0.36–1.44 μmol/L
Renin (erect)	P: 2.8–4.5 pmol/mL/h
Renin (recumbent)	P: 1.1–2.7 pmol/mL/h
Sodium	P: 135–145 mmol/L
TBG	P: 7–17 mg/L
TSH	P: 0.5–4.2 mU/L
T4	P: 70–140 nmol/L
Free T4	P: 9–22 pmol/L
TIBC	S: 54–75 μmol/L
Triglycerides	P: 0.50–2.3 mmol/L
T3	P: 1.2–3.0 nmol/L
Troponin T	P: <0.1 mcg/L
Urate ♂	P: 210–480 μmol/L
Urate ♀	P: 150–390 μmol/L
Urea	P: 2.5–6.7 mmol/L
Vitamin B12	S: 0.13–0.68 nmol/L
Vitamin D	S: 50 nmol/L

Arterial Blood Gases

pH	7.35–7.45
PaCO₂	4.7–6.0 kPa
PaO₂	>10.6 kPa
Base excess	±2 mmol/L

Urine

Cortisol (free)	<280 nmol/24h
Hydroxyindole acetic acid	16–73 μmol/24h
Hydroxymethylmandelic acid	16–48 μmol/24h
Metanephrines	0.03–0.69 μmol/mmol cr.
Osmolality	350–1000 mosmol/kg
17-Oxogenic steroids ♂	28–30 μmol/24h
17-Oxogenic steroids ♀	21–66 μmol/24h
17-Oxosteroids ♂	17–76 μmol/24h
17-Oxosteroids ♀	14–59 μmol/24h
Phosphate (inorganic)	15–50 mmol/24h
Potassium	14–120 mmol/24h
Protein	<150 mg/24h
Protein/creatinine ratio	<3 mg/mmol
Sodium	100–250 mmol/24h

Haematology

WCC	4.0–11.0 ×10⁹/L
RBC ♂	4.5–6.5 ×10¹²/L
RBC ♀	3.9–5.6 ×10¹²/L
Hb ♂	130–180 g/L
Hb ♀	115–160 g/L
PCV ♂	0.4–0.54 L/L
PCV ♀	0.37–0.47 L/L
MCV	76–96 fL
MCH	27–32 pg
MCHC	300–360 g/L
RDW	11.6–14.6%
Neutrophils	2.0–7.5 ×10⁹/L (40–75%)
Lymphocytes	1.0–4.5 ×10⁹/L (20–45%)
Eosinophils	0.04–0.44 ×10⁹/L (1–6%)
Basophils	0–0.10 ×10⁹/L (0–1%)
Monocytes	0.2–0.8 ×10⁹/L (2–10%)
Platelets	150–400 ×10⁹/L
Reticulocytes	0.8–2.0% / 25–100 ×10⁹/L
Prothrombin time	10–14 s
APTT	35–45 s

Paediatric

Pulse Rate (bpm)
Neonate	140–160
Infant <1yr	120–140
1–5 years	110–130
5–12 years	80–120
>12 years	70–100
Respiratory Rate (tachypnoea)
0–2 months	≥60/min
2–12 months	≥50/min
1–5 years	≥40/min
>5 years	≥30/min
Blood Pressure (mmHg)
Term	65/45
1 year	75/50
4 years	85/60
8 years	95/65
10 years	100/70
Weight Formulas
3–12 months	(a + 9)/2 kg
1–6 years	2a + 8 kg
>6 years	(7a − 5)/2 kg
Haemoglobin (g/dL)
Term newborn	13–20
1 month	11–18
2 months	10–15
1–2 years	10–13
>2 years	11–14
MUAC (6 months–5 years)
Obese	>17.5 cm
Normal	13.5–17.4 cm
At risk	12.5–13.4 cm
Moderate malnutrition	11.5–12.4 cm
Severe malnutrition	<11.5 cm
Developmental Milestones
Social smile	1.5 months
Head control	4 months
Sits unsupported	7 months
Crawls	10 months
Stands unsupported	10–12 months
Walks	12–13 months
Talks	18 months
CSF WBC (/mm³)
Term newborn	0–25
>2 weeks	0–5

Calculator ›

Overview

Metabolic Acidosis

Metabolic Alkalosis

Respiratory Acidosis

Respiratory Alkalosis

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