What causes blood to be dark and thick?

What Causes Blood to Be Dark and Thick

Blood becomes dark and thick primarily due to increased red blood cell mass (polycythemia/erythrocytosis), which elevates blood viscosity and reduces oxygen saturation, making the blood appear darker. 1

Primary Mechanisms

Increased Red Blood Cell Production (Erythrocytosis)

The most common cause of thick blood is increased red cell mass, which directly increases blood viscosity. 2, 3 This occurs through two main pathways:

• Compensatory erythrocytosis: In cyanotic conditions (like congenital heart disease with right-to-left shunting), chronic hypoxemia triggers the kidneys to release erythropoietin, stimulating bone marrow to produce more red blood cells in an attempt to improve tissue oxygenation 1
• Primary polycythemia vera: A clonal myeloproliferative disorder where bone marrow overproduces red blood cells independent of oxygen needs 1, 3

Why Blood Appears Dark

Blood appears darker when it contains more deoxygenated hemoglobin. 1 Specifically:

• Arterial oxygen saturations below 75% result in visible cyanosis and darker blood color 1
• Increased quantities of reduced (deoxygenated) hemoglobin greater than 5 g/100 mL blood cause central cyanosis 1
• Right-to-left shunting in cyanotic heart disease results in low systemic arterial oxygen saturation, producing darker blood 1

Secondary Factors That Worsen Blood Thickness

Iron Deficiency Paradox

Iron deficiency actually worsens blood viscosity despite causing anemia. 1, 4 This occurs because:

• Iron-deficient red cells become rigid, microcytic microspheres (8 μm diameter) that are less deformable in capillaries (4-6 μm diameter) 1
• These rigid cells increase resistance to flow in the microcirculation, paradoxically worsening hyperviscosity symptoms 1, 4
• Iron deficiency is the strongest independent predictor of cerebrovascular events—not the hematocrit level itself 4, 5

Plasma Composition Changes

Blood viscosity increases through elevated acute phase reactants and abnormal proteins: 6

• Increased fibrinogen and other coagulation proteins 1
• Hypergammaglobulinemia in inflammatory states 6
• Decreased fibrinolytic activity and increased plasminogen activator inhibitor 1

Clinical Conditions Causing Dark, Thick Blood

Cyanotic Congenital Heart Disease

This represents the classic scenario of both dark AND thick blood: 1

• Right-to-left shunting causes arterial hypoxemia (dark blood) 1
• Compensatory erythrocytosis increases hematocrit to 65% or higher (thick blood) 1
• Decompensated erythrocytosis occurs when aortic oxygen saturations fall below 75% 1

Polycythemia Vera

A primary bone marrow disorder causing excessive red cell production: 1, 3

• Hemoglobin exceeds 18.5 g/dL in men or 16.5 g/dL in women 7
• JAK2 mutation present in up to 97% of cases 7
• Blood viscosity can reach twice normal values, causing vascular occlusions 8

Secondary Polycythemia

Chronic hypoxic conditions stimulate erythropoietin production: 7

• Chronic obstructive pulmonary disease (COPD) 7
• Obstructive sleep apnea producing nocturnal hypoxemia 7
• Smoker's polycythemia from chronic carbon monoxide exposure 7
• High altitude residence (physiologic adaptation) 7

Dangerous Consequences of Thick, Dark Blood

Thrombotic Complications

Hyperviscosity is the major factor in thrombogenesis: 2, 8

• Cerebrovascular accidents (stroke) 5, 2, 8
• Myocardial infarction 5, 2, 8
• Deep vein thrombosis and pulmonary embolism 2
• Laminated thrombi in pulmonary arteries (up to 30% of cyanotic patients) 1, 5

Paradoxical Bleeding

Despite thick blood, patients face hemorrhagic complications: 1, 5

• Platelet dysfunction and thrombocytopenia 1
• Acquired von Willebrand disease (in over one-third of polycythemia vera patients) 1
• Hemoptysis occurs in up to 100% of Eisenmenger patients 1, 5

Hyperviscosity Symptoms

Clinical manifestations include: 1, 5

• Headache, faintness, dizziness, fatigue 1, 5
• Tinnitus, blurred vision 1, 5
• Paresthesia of fingers, toes, and lips 1, 5
• Muscle pain and weakness 1, 5

Critical Management Pitfall

Avoid routine phlebotomy—it causes more harm than benefit. 4, 5 Therapeutic phlebotomy should ONLY be performed when: 4

• Hemoglobin exceeds 20 g/dL AND hematocrit exceeds 65% 4
• Patient has moderate-to-severe hyperviscosity symptoms 1, 4
• Dehydration and iron deficiency have been excluded 1, 4
• Equal volume replacement with normal saline is provided 4

Repeated phlebotomies without meeting these criteria cause iron depletion, which paradoxically increases stroke risk by creating rigid, poorly deformable red cells with reduced oxygen-carrying capacity. 4, 5