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# Types of hypoxia and what hypoxia does to the body

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Hypoxia means cells get too little oxygen. The fault can be in the lungs, the blood, the flow or the cells themselves.

Updated Nov 13, 2026

Also called: Hypoxemic, anemic, stagnant and histotoxic hypoxia, Tissue hypoxia

## Key points

- Hypoxemic hypoxia is the only type with a low PaO₂; anemic, stagnant and histotoxic hypoxia start with a normal PaO₂.
- Anemia and CO poisoning lower O₂ content (CaO₂); a pulse oximeter reads normal in anemia and falsely normal in CO poisoning.
- Stagnant (circulatory) hypoxia: normal arterial values, but tissues extract more, so mixed venous SvO₂ falls.
- Histotoxic hypoxia (cyanide): cells cannot use O₂, so venous blood stays red and SvO₂ is high, with lactic acidosis.
- Supplemental O₂ helps most in hypoventilation, V/Q mismatch and diffusion limits; it helps little in true shunt and in histotoxic hypoxia.
- Cyanosis needs enough deoxygenated hemoglobin: it may be absent in severe anemia and in CO poisoning despite real hypoxia.

## What it is

Hypoxia is an oxygen supply too small for what the tissues need. Oxygen has to be loaded in the lungs, carried by hemoglobin, delivered by blood flow and used by mitochondria, and each step can fail. Naming the failing step tells you which blood values will be abnormal and whether extra oxygen will help.

## How it works, step by step

- Low O₂ at the mitochondria slows oxidative phosphorylation, so ATP output falls.
- Cells switch to anaerobic glycolysis: lactate and H⁺ build up and blood pH falls.
- Ion pumps that need ATP slow down; Na⁺ and water enter cells, which swell.
- The brain fails first: judgment, memory and coordination worsen, then drowsiness, seizures and coma.
- The heart reacts with tachycardia and the carotid bodies drive faster, deeper breathing.
- If hypoxia persists, cell membranes and mitochondria are damaged and cells die.

### Oxygen–hemoglobin dissociation curve

**P50** 17.2 mmHg **Arterial blood** 59 % **Mixed venous blood** 54 % **O₂ given up to the tissues** 5 %

With 40% HbCO those sites no longer carry O₂ (the plateau falls) and the rest release it less well (left shift): that is why the harm is greater than in an anaemia of the same size. A standard pulse oximeter may still read normal.

Left shift (P50 17.2 mmHg): Hb holds O₂ more tightly and gives up less in the tissues.

- Current curve
- Normal curve (pH 7.40 · 37 °C)
- Arterial blood (PO₂ 100)
- Mixed venous blood (PO₂ 40)
- O₂ given up to the tissues

CO lowers the plateau and shifts the curve left: content falls although PaO₂ is normal. Compare with «Normal».

## Reference values (healthy adult at sea level, breathing air)

| Parameter | Value | Note |
| --- | --- | --- |
| PaO₂ | ≈ 80–100 mmHg | falls with age and altitude |
| SaO₂ | ≈ 95–99% | |
| CaO₂ | ≈ 20 mL O₂/dL | with Hb 15 g/dL |
| Mixed venous SvO₂ | ≈ 70–75% | PvO₂ ≈ 40 mmHg |
| Lactate |

| Type (examples) | PaO₂ | SaO₂ | CaO₂ | SvO₂ | Response to O₂ |
| --- | --- | --- | --- | --- | --- |
| [Hypoxemic (altitude, hypoventilation, V/Q mismatch, shunt)](https://durgical.com/en/conditions/high-altitude-illness/) | Low | Low | Low | Low | Good (poor in true shunt) |
| [Anemic (anemia, CO poisoning, methemoglobinemia)](https://durgical.com/en/conditions/iron-deficiency-anaemia/) | Normal | Normal in anemia; low in CO and MetHb | Low | Low | Small, except faster CO removal |
| [Stagnant or circulatory (shock, heart failure, arterial occlusion)](https://durgical.com/en/conditions/hypovolaemic-shock/) | Normal | Normal | Normal | Low | Small: flow is the problem |
| [Histotoxic (cyanide blocking cytochrome c oxidase)](https://durgical.com/en/conditions/cyanide-poisoning/) | Normal | Normal | Normal | High | Little: cells cannot use O₂ |

## Clinical notes

- A smoke-inhalation victim with headache and confusion but SpO₂ 99% may have CO and cyanide poisoning; co-oximetry measures carboxyhemoglobin directly.
- Severe anemia can starve tissues with a perfect oximeter reading, because the oximeter sees the percentage, not the amount of hemoglobin.
- At high altitude the brain shows the first effects: headache, poor sleep and slowed thinking, as in acute mountain sickness.

## Memory aid

Lungs, Load, Lorry, Labour: low PaO₂ (lungs), too little loaded Hb (blood), slow lorry (flow), cells refuse the labour (histotoxic).

## Test yourself

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

## A patient has PaO₂ 95 mmHg, normal SaO₂ and CaO₂, but a mixed venous SvO₂ of 45%. Which type of hypoxia fits best?

Stagnant (circulatory) hypoxia: arterial blood is normal, but low flow makes tissues extract more O₂, so venous saturation falls, as in shock or heart failure.

## Why is mixed venous O₂ saturation high in cyanide poisoning?

Cyanide blocks cytochrome c oxidase, so mitochondria cannot use the O₂ that is delivered; less is extracted and venous blood returns nearly as saturated as arterial blood.

## Which type of hypoxia shows a low PaO₂?

Only hypoxemic hypoxia (low inspired PO₂, hypoventilation, V/Q mismatch, diffusion limit or shunt). The other three start with a normal PaO₂.

## Why may a patient with hemoglobin 6 g/dL and tissue hypoxia not look cyanotic?

Cyanosis needs a certain amount of deoxygenated hemoglobin in capillary blood; with so little hemoglobin that amount is rarely reached.

## Why does breathing 100% O₂ correct hypoxemia from V/Q mismatch but not from a large right-to-left shunt?

Poorly ventilated units still receive some O₂ and their PAO₂ rises; shunted blood never meets alveolar gas, so it stays desaturated and mixes into arterial blood.

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## Connections

## Related topics 5

- [The causes of hypoxemia](https://durgical.com/en/physiology/the-causes-of-hypoxemia/)
- [Oxygen content, delivery and extraction](https://durgical.com/en/physiology/oxygen-content-delivery-and-extraction/)
- [Carbon monoxide, methemoglobin and cyanide](https://durgical.com/en/physiology/carbon-monoxide-methemoglobin-and-cyanide/)
- [Oxygenation: SpO₂, PaO₂, cyanosis and oxygen therapy](https://durgical.com/en/physiology/oxygenation-spo-pao-cyanosis-and-oxygen-therapy/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)

## Related conditions 6

- [Carbon monoxide poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/)
- [Cyanide poisoning](https://durgical.com/en/conditions/cyanide-poisoning/)
- [Methemoglobinemia](https://durgical.com/en/conditions/methemoglobinemia/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [High-altitude illness (AMS, HACE, HAPE)](https://durgical.com/en/conditions/high-altitude-illness/)

## In Biochemistry 1

- [Cytochrome c oxidase (complex IV)](https://durgical.com/en/biochemistry/enzymes/cytochrome-c-oxidase/)

## In Pharmacology 1

- [Oxygen](https://durgical.com/en/pharmacology/oxygen/)

[← 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: [Carbon monoxide poisoning: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002804.htm) (MedlinePlus, US National Library of Medicine, 2025); [Methemoglobinemia: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000562.htm) (MedlinePlus, US National Library of Medicine, 2025); [Iron Deficiency and Microcytic Hypochromic Anemia](https://www.ncbi.nlm.nih.gov/books/NBK560876/) (StatPearls (NCBI Bookshelf), 2025); [Hypovolemia and Hypovolemic Shock](https://www.ncbi.nlm.nih.gov/books/NBK513297/) (StatPearls (NCBI Bookshelf), 2025).

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