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[Physiology](https://durgical.com/en/physiology/) · [Blood, hemostasis and basic immunity](https://durgical.com/en/physiology/#s-blood)

# Anemia physiology and compensation

- Mechanism
- Exam priority: high

Anemia means too little hemoglobin to carry oxygen. The body copes by pumping faster and letting go of oxygen more easily.

Updated 2026-11-01

Also called: Anemia, Low hemoglobin

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/) · [Hip bone](https://durgical.com/en/anatomy/hip-bone/) · [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)

## Key points

- Anemia lowers CaO₂ but leaves PaO₂ and SaO₂ normal: pulse oximetry looks fine.
- 2,3-BPG rises within hours to days and shifts the O₂ curve to the right, so more O₂ is unloaded per gram of Hb.
- Cardiac output rises (lower viscosity, lower afterload, sympathetic drive): tachycardia, bounding pulse, flow murmur; severe chronic anemia can cause high-output heart failure.
- Reticulocyte count separates marrow failure (low) from blood loss or hemolysis (high).
- Mean corpuscular volume (MCV) sorts the cause: microcytic, normocytic or macrocytic.
- Cyanosis needs ≈5 g/dL of deoxygenated Hb, so a severely anemic patient may be hypoxic yet not blue.

## What it is

Anemia is a fall in hemoglobin concentration below the reference for age, sex, pregnancy and altitude. It lowers arterial O₂ content while PaO₂ and SaO₂ stay normal, so the problem is the cargo, not the lungs. The body compensates in the heart, the red cell itself, the circulation and the marrow; symptoms appear when these limits are reached.

## How it works, step by step

- Hemoglobin falls → arterial O₂ content (CaO₂) falls, while PaO₂ and saturation stay normal.
- Less O₂ delivered → tissues extract a larger fraction → venous PO₂ and saturation drop.
- Red cells make more 2,3-BPG → the dissociation curve shifts right → more O₂ is released at tissue PO₂.
- Blood is thinner (low viscosity) → systemic resistance falls → venous return and stroke volume rise.
- Sympathetic activation raises heart rate and contractility → cardiac output rises.
- Flow is redirected from skin and kidney to heart and brain → pallor, cool skin.
- Renal cortex senses hypoxia (HIF) → erythropoietin rises → marrow makes more cells if iron, B12 and folate are available.
- When reserve runs out: exertional dyspnea, fatigue, angina in a stenotic coronary, and finally high-output failure.

### Oxygen–hemoglobin dissociation curve

**P50** 26.8 mmHg **Arterial blood** 97 % **Mixed venous blood** 75 % **O₂ given up to the tissues** 23 %

Normal curve. Tap a point (arterial, venous or P50) or move a control.

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

Raise 2,3-BPG and watch the curve move right: more O₂ is unloaded at tissue PO₂, as in chronic anemia.

## Equations

**Arterial O₂ content**CaO2 = 1.34 × Hb × SaO2 + 0.003 × PaO2

**Hb**
hemoglobin, g/dL

**SaO₂**
arterial saturation, fraction

**PaO₂**
arterial PO₂, mmHg

Normal ≈20 mL O₂/dL. Halving Hb halves CaO₂; the dissolved part is tiny.

**Oxygen delivery**DO2 = CO × CaO2 × 10

**CO**
cardiac output, L/min

**10**
converts dL to L

Raising CO is the main short-term defence of DO₂ in anemia.

## Reference values (adults, sea level; labs vary)

| Parameter | Value | Note |
| --- | --- | --- |
| Hemoglobin, men | ≈13.5–17.5 g/dL | |
| Hemoglobin, women | ≈12.0–15.5 g/dL | |
| WHO anemia threshold |

| MCV | Typical causes | Clue |
| --- | --- | --- |
| [Microcytic (100 fL)](https://durgical.com/en/conditions/megaloblastic-anaemia/) | B12 or folate deficiency, alcohol, liver disease, drugs | Hypersegmented neutrophils point to B12/folate |

## Clinical notes

- Colombia: hemoglobin cut-offs are adjusted upward with altitude (by roughly 1 g/dL around Bogotá, ~2600 m; the exact figure depends on the WHO table used), so a sea-level 'normal' value can be anemic in Bogotá.
- Acute hemorrhage: hemoglobin is normal at first, because cells and plasma are lost together; it falls only as fluid shifts in or saline is given.
- Hemolysis: high reticulocytes, high unconjugated bilirubin, high LDH, low haptoglobin.

## Memory aid

Anemia is an empty truck on a busy road: the road (lungs) is fine, so the body sends more trucks per minute (cardiac output) and unloads each one fully (2,3-BPG).

## Test yourself

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

## Why is pulse oximetry usually normal in a patient with hemoglobin 7 g/dL?

Saturation is the fraction of available Hb carrying O₂. The lungs work, so the little Hb present is fully saturated; it is the total O₂ content that is low.

## How does 2,3-BPG help in chronic anemia?

It binds deoxyhemoglobin, lowers its O₂ affinity and shifts the curve right, so tissues receive more O₂ from each gram of Hb at the same capillary PO₂.

## Why does cardiac output rise in anemia even without a change in blood volume?

Low viscosity lowers vascular resistance and raises venous return, and tissue hypoxia triggers vasodilation and sympathetic drive; together they raise stroke volume and heart rate.

## A patient has anemia with a low reticulocyte count. What does that tell you?

The marrow is not responding: lack of iron, B12, folate or erythropoietin (kidney disease), inflammation or marrow failure, rather than bleeding or hemolysis.

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

## See it in 3D

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

- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Hip bone](https://durgical.com/en/anatomy/hip-bone/)
- [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)

## Related topics

- [Erythropoiesis and erythropoietin](https://durgical.com/en/physiology/erythropoiesis-and-erythropoietin/)
- [The red cell life cycle and the spleen](https://durgical.com/en/physiology/the-red-cell-life-cycle-and-the-spleen/)
- [Body iron balance](https://durgical.com/en/physiology/body-iron-balance/)
- [Oxygen content, delivery and extraction](https://durgical.com/en/physiology/oxygen-content-delivery-and-extraction/)
- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)

## Related conditions

- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Megaloblastic anaemia (B12 or folate deficiency)](https://durgical.com/en/conditions/megaloblastic-anaemia/)
- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)
- [Pernicious anaemia](https://durgical.com/en/conditions/pernicious-anaemia/)
- [Glucose-6-phosphate dehydrogenase (G6PD) deficiency](https://durgical.com/en/conditions/glucose-6-phosphate-dehydrogenase-deficiency/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)

## In Biochemistry

- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)
- [Iron metabolism](https://durgical.com/en/biochemistry/topics/iron-metabolism/)
- [Cobalamin (vitamin B12)](https://durgical.com/en/biochemistry/topics/cobalamin/)

[← 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: [18.3 Erythrocytes](https://openstax.org/books/anatomy-and-physiology-2e/pages/18-3-erythrocytes) (OpenStax (Rice University), 2022); [Complete Blood Count (CBC)](https://medlineplus.gov/lab-tests/complete-blood-count-cbc/) (MedlinePlus (U.S. National Library of Medicine), 2024); [Iron Deficiency and Microcytic Hypochromic Anemia](https://www.ncbi.nlm.nih.gov/books/NBK560876/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 33). 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/anemia-physiology-and-compensation/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/anemia-physiology-and-compensation/