Physiology · Blood, hemostasis and basic immunity

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 · Hip bone · Left ventricle

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 mmHgArterial 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 contentCaO₂ = 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 deliveryDO₂ = 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	<13 g/dL men, <12 g/dL non-pregnant women, <11 g/dL pregnancy	raised at altitude
MCV	≈80–100 fL
Reticulocytes	≈0.5–2.5 %

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

Sorting anemia by red cell size

MCV	Typical causes	Clue
Microcytic (<80 fL)	Iron deficiency, thalassemia, anemia of inflammation (late), sideroblastic	Ferritin low in iron deficiency; normal or high in thalassemia
Normocytic (80–100 fL)	Acute bleeding, hemolysis, chronic kidney disease, inflammation	Reticulocytes high in bleeding/hemolysis, low in kidney disease
Macrocytic (>100 fL)	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.

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.

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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: 18.3 Erythrocytes (OpenStax (Rice University), 2022); Complete Blood Count (CBC) (MedlinePlus (U.S. National Library of Medicine), 2024); Iron Deficiency and Microcytic Hypochromic Anemia (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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