Physiology · Blood, hemostasis and basic immunity

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 · Hip bone · Sternum · Liver

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.

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.

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See it in 3D

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

In Biochemistry

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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 (OpenStax (Rice University), 2022); 18.3 Erythrocytes (OpenStax (Rice University), 2022); Chronic Kidney Disease (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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