[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Renal, fluids and acid–base](https://durgical.com/en/physiology/#s-renal) › Respiratory acidosis and alkalosis

[Physiology](https://durgical.com/en/physiology/) · [Renal, fluids and acid–base](https://durgical.com/en/physiology/#s-renal)

# Respiratory acidosis and alkalosis

- Mechanism
- Exam priority: high

Breathing too little builds up CO₂ and acidifies the blood; breathing too much blows off CO₂ and makes it alkaline. The kidneys compensate over days.

Updated 2026-11-01

Also called: Hypercapnia, Hypocapnia

**See it in 3D:** [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/) · [Diaphragm](https://durgical.com/en/anatomy/diaphragm/) · [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Key points

- Respiratory acidosis = high PaCO₂ (hypoventilation); respiratory alkalosis = low PaCO₂ (hyperventilation).
- Acute acidosis: HCO₃⁻ rises ≈ 1 per 10 mmHg of PaCO₂; chronic: ≈ 3.5 (3–4) per 10.
- Acute alkalosis: HCO₃⁻ falls ≈ 2 per 10 mmHg; chronic: ≈ 4–5 per 10.
- Causes of acidosis: respiratory centre depression (opioids, sedatives), muscle weakness, severe COPD, obesity hypoventilation.
- Causes of alkalosis: anxiety, pain, fever, hypoxemia (altitude, pulmonary embolism), early sepsis, salicylates, pregnancy.
- At altitude (Bogotá), hypoxia keeps up a mild chronic respiratory alkalosis that the kidney compensates.

## What it is

Respiratory acid–base disorders start with a change in PaCO₂ from hypoventilation (acidosis) or hyperventilation (alkalosis). CO₂ is hydrated to carbonic acid, so moving PaCO₂ moves pH. Cell proteins and hemoglobin buffer first; then, over 2 to 5 days, the kidney adjusts HCO₃⁻.

## How it works, step by step

- Alveolar ventilation falls: CO₂ produced by tissues accumulates and PaCO₂ rises.
- CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻ (carbonic anhydrase): H⁺ rises and pH falls.
- Within minutes, hemoglobin and cell proteins buffer H⁺; HCO₃⁻ rises slightly.
- If it lasts days, the proximal tubule and collecting duct secrete more H⁺, excrete more NH₄⁺ and reclaim all HCO₃⁻, so HCO₃⁻ rises considerably.
- In respiratory alkalosis the reverse happens: the kidney excretes HCO₃⁻ and secretes less H⁺.
- CO₂ crosses the blood–brain barrier quickly, so PaCO₂ changes immediately alter CSF pH, cerebral blood flow and consciousness.

## Equations

**CO₂ hydration**CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

**carbonic anhydrase**
speeds the first step

Raising PaCO₂ pushes the reaction to the right.

**Renal compensation (rule of thumb)**ΔHCO₃⁻ ≈ k × ΔPaCO₂ / 10

**k (acidosis)**
≈ 1 acute; ≈ 3.5 chronic

**k (alkalosis)**
≈ 2 acute; ≈ 4–5 chronic

The rules in detail are on the Biochemistry page (acid–base compensation).

## Comparison

| Feature | Respiratory acidosis | Respiratory alkalosis |
| --- | --- | --- |
| Primary change | PaCO₂ ↑ (hypoventilation) | PaCO₂ ↓ (hyperventilation) |
| Renal compensation | Retains HCO₃⁻, excretes more H⁺ (days) | Excretes HCO₃⁻ (days) |
| Brain | Vasodilation, headache, drowsiness, CO₂ narcosis | Vasoconstriction, dizziness, paresthesias |
| Potassium and calcium | K⁺ slightly ↑ | Ionized Ca²⁺ ↓ (tetany), K⁺ ↓ |
| Typical causes | Opioids, COPD, neuromuscular disease | Anxiety, altitude, embolism, early sepsis, pregnancy |

## Clinical notes

- Opioid overdose: slow breathing, high PaCO₂ and low pH; naloxone reverses the respiratory depression.
- In COPD with chronic hypercapnia, HCO₃⁻ is high. Ventilating abruptly down to a 'normal' PaCO₂ leaves a post-hypercapnic metabolic alkalosis.
- Asthma attack: respiratory alkalosis at first; a PaCO₂ that 'normalises' or rises signals fatigue and is a red flag.
- On arrival at altitude, hyperventilation causes alkalosis; over the following days the kidney excretes HCO₃⁻ (acclimatisation).

## Memory aid

'Lungs move CO₂ in minutes, kidneys move bicarbonate in days': that is why HCO₃⁻ tells you how long the disorder has lasted.

## Test yourself

Think of the answer, then tap the question to see it.

## PaCO₂ 60 mmHg and HCO₃⁻ 26 mEq/L: acute or chronic?

Acute. With a 20 mmHg rise, acute change raises HCO₃⁻ by ≈ 2 (≈ 26) and chronic by ≈ 7 (≈ 31). Renal compensation has not happened yet.

## Why does anxiety-driven hyperventilation cause perioral tingling and carpopedal spasm?

Alkalosis increases calcium binding to albumin; ionized calcium falls and nerves become hyperexcitable.

## Why can a normal PaCO₂ be a bad sign in an asthma attack?

Early on, the patient hyperventilates and PaCO₂ is low. If it rises to 'normal', the muscles are tiring and ventilation is no longer enough: respiratory failure is near.

## Why does acute hypercapnia cause headache and drowsiness?

CO₂ crosses the blood–brain barrier, acidifies the CSF and dilates cerebral vessels: blood flow and intracranial pressure rise and the cortex is depressed.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/respiratory-acidosis-and-alkalosis/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/)
- [Diaphragm](https://durgical.com/en/anatomy/diaphragm/)
- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Related topics

- [Reading an arterial blood gas step by step](https://durgical.com/en/physiology/reading-an-arterial-blood-gas-step-by-step/)
- [Control of breathing, chemoreceptors and lung reflexes](https://durgical.com/en/physiology/control-of-breathing-chemoreceptors-and-lung-reflexes/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)
- [Hypoxemic and hypercapnic respiratory failure](https://durgical.com/en/physiology/hypoxemic-and-hypercapnic-respiratory-failure/)
- [CO₂ transport and the Haldane effect](https://durgical.com/en/physiology/co-transport-and-the-haldane-effect/)
- [Renal handling of H⁺ and bicarbonate](https://durgical.com/en/physiology/renal-handling-of-h-and-bicarbonate/)
- [Cerebral blood flow, brain metabolism and intracranial pressure](https://durgical.com/en/physiology/cerebral-blood-flow-brain-metabolism-and-intracranial-pressure/)

## Related conditions

- [Respiratory acidosis](https://durgical.com/en/conditions/respiratory-acidosis/)
- [Respiratory alkalosis](https://durgical.com/en/conditions/respiratory-alkalosis/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Asthma](https://durgical.com/en/conditions/asthma/)
- [Hypocalcaemia](https://durgical.com/en/conditions/hypocalcaemia/)

## In Biochemistry

- [Acid–base compensation rules](https://durgical.com/en/biochemistry/topics/acid-base-compensation-rules/)
- [Bicarbonate–CO₂ buffer system](https://durgical.com/en/biochemistry/topics/bicarbonate-co-buffer-system/)

## In Pharmacology

- [Opioids and antagonists](https://durgical.com/en/pharmacology/classes/opioids-and-antagonists/)
- [Naloxone](https://durgical.com/en/pharmacology/naloxone/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [26.5 Disorders of Acid-Base Balance](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-5-disorders-of-acid-base-balance) (OpenStax (Rice University), 2022); [Respiratory acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000092.htm) (MedlinePlus, US National Library of Medicine, 2026); [Respiratory alkalosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000111.htm) (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 31). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/physiology/respiratory-acidosis-and-alkalosis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/respiratory-acidosis-and-alkalosis/