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

# Erythropoiesis and erythropoietin

- Regulation
- Exam priority: medium

The kidney measures oxygen. When it is low, it releases erythropoietin, which tells the bone marrow to make more red cells.

Updated 2026-11-01

Also called: Red cell production, EPO

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/) · [Hip bone](https://durgical.com/en/anatomy/hip-bone/) · [Sternum](https://durgical.com/en/anatomy/sternum/) · [Liver](https://durgical.com/en/anatomy/liver/)

## Key points

- About 90 % of EPO comes from peritubular interstitial cells of the renal cortex; the liver makes the rest (and most in the fetus).
- Hypoxia stabilizes HIF (hypoxia-inducible factor), which switches on the EPO gene; with enough O₂, HIF is hydroxylated and degraded.
- The kidney senses O₂ delivery, not PaO₂ alone: anemia, altitude, lung disease and high-affinity hemoglobin all raise EPO.
- Reticulocytes rise about 3–5 days after an EPO rise; it takes weeks to restore hemoglobin.
- Chronic kidney disease causes a normocytic, hypoproliferative anemia mainly from EPO deficiency.
- Iron, vitamin B12 and folate are the raw materials; lacking them blocks the response even with high EPO.

## What it is

Erythropoiesis is the production of red cells in the bone marrow, about 2 million every second in an adult. Its main regulator is erythropoietin (EPO), a hormone made mostly by interstitial cells of the renal cortex when they sense low oxygen. The response is a negative feedback loop: more red cells restore O₂ delivery and switch EPO off.

## How it works, step by step

- O₂ delivery to the renal cortex falls (anemia, altitude, hypoxemia).
- Prolyl hydroxylases, which need O₂, stop marking HIF-α for destruction → HIF accumulates.
- HIF enters the nucleus → EPO transcription rises within hours.
- EPO binds receptors on CFU-E and proerythroblasts → prevents their apoptosis and speeds division.
- Erythroblasts fill with hemoglobin, eject the nucleus and leave the marrow as reticulocytes.
- Reticulocytes lose their RNA in ≈1 day in the blood → mature red cells for ≈120 days.
- Higher hemoglobin restores renal O₂ delivery → HIF is degraded → EPO falls (negative feedback).

## Reference values

| Parameter | Value | Note |
| --- | --- | --- |
| Red cell production | ≈2 × 10⁶ cells per second | |
| Reticulocytes | ≈0.5–2.5 % | higher after bleeding |
| Red cell life span | ≈120 days | |
| Time from EPO rise to reticulocytosis | ≈3–5 days | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Regulation

- *+* **Tissue hypoxia (anemia, altitude, lung or heart disease)**: Stabilizes HIF and raises EPO
- *+* **Androgens**: Raise EPO and marrow sensitivity; men have higher hemoglobin
- *+* **Iron, B12, folate**: Allow the marrow to respond
- *−* **Chronic kidney disease**: Loss of EPO-producing tissue
- *−* **Inflammation (IL-6, TNF, hepcidin)**: Blunts EPO action and traps iron
- *−* **Hyperoxia, polycythemia**: Suppress EPO

## Clinical notes

- Living in Bogotá (~2600 m) leads to a higher steady-state hemoglobin than at the coast, because of chronic EPO stimulation.
- Polycythemia with high EPO points to hypoxia or an EPO-secreting tumor; with low EPO, to polycythemia vera.
- Erythropoiesis-stimulating agents in kidney disease need adequate iron stores to work.

## Memory aid

The kidney is the oxygen thermostat: when it 'feels cold' (low O₂), it turns on the EPO heater and the marrow warms the blood with new cells.

## Test yourself

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

## Why does the kidney make EPO when hemoglobin falls even though PaO₂ is normal?

Renal cells respond to the O₂ they actually receive. With low hemoglobin, O₂ content and delivery fall, cellular PO₂ drops, HIF is stabilized and EPO rises.

## Why is the anemia of chronic kidney disease usually normocytic with low reticulocytes?

Cells are made normally but too few, because EPO is lacking: the marrow is under-stimulated rather than short of iron or B12.

## A patient with severe iron deficiency has very high EPO. Why is the reticulocyte count still low?

EPO can only push precursors to divide; without iron they cannot make hemoglobin, so few mature cells are released.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/erythropoiesis-and-erythropoietin/)

## 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/)
- [Sternum](https://durgical.com/en/anatomy/sternum/)
- [Liver](https://durgical.com/en/anatomy/liver/)

## Related topics

- [Hematopoiesis](https://durgical.com/en/physiology/hematopoiesis/)
- [Anemia physiology and compensation](https://durgical.com/en/physiology/anemia-physiology-and-compensation/)
- [Polycythemia: physiological causes](https://durgical.com/en/physiology/polycythemia-physiological-causes/)
- [Body iron balance](https://durgical.com/en/physiology/body-iron-balance/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)

## Related conditions

- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)
- [Megaloblastic anaemia (B12 or folate deficiency)](https://durgical.com/en/conditions/megaloblastic-anaemia/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)

## In Biochemistry

- [Iron metabolism](https://durgical.com/en/biochemistry/topics/iron-metabolism/)
- [Cobalamin (vitamin B12)](https://durgical.com/en/biochemistry/topics/cobalamin/)
- [Folate (vitamin B9)](https://durgical.com/en/biochemistry/molecules/folate/)

[← 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: [18.2 Production of the Formed Elements](https://openstax.org/books/anatomy-and-physiology-2e/pages/18-2-production-of-the-formed-elements) (OpenStax (Rice University), 2022); [18.3 Erythrocytes](https://openstax.org/books/anatomy-and-physiology-2e/pages/18-3-erythrocytes) (OpenStax (Rice University), 2022); [Chronic Kidney Disease](https://www.ncbi.nlm.nih.gov/books/NBK535404/) (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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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/erythropoiesis-and-erythropoietin/