Polycythaemia and Blood Viscosity
Polycythaemia refers to an abnormally high concentration of red blood cells, haemoglobin, or haematocrit. While anaemia reduces the blood’s oxygen-carrying capacity, polycythaemia increases it, but at a cost. Excessive red cells make the blood more viscous, slowing flow and increasing the risk of thrombosis. Understanding the physiology of polycythaemia is crucial in recognising symptoms, interpreting laboratory results, and managing complications such as clotting, stroke, and tissue hypoxia.
What You Need to Know
Polycythaemia refers to an increase in red blood cell concentration in the blood, resulting in elevated haemoglobin and haematocrit. This may occur either because the total red cell mass is truly increased or because plasma volume has fallen, making the blood more concentrated. While a higher red cell count improves oxygen-carrying capacity, it also increases blood viscosity, meaning the blood becomes thicker and more resistant to flow. This has important consequences for cardiovascular workload, tissue perfusion, and clotting risk.
Polycythaemia is broadly classified into two major types:
relative polycythaemia, where plasma volume is reduced (for example, due to dehydration or diuretic use) while red cell mass is normal
absolute polycythaemia, where total red cell mass is increased
Absolute polycythaemia can be further divided into primary and secondary forms, depending on the underlying mechanism driving red cell production.
In primary polycythaemia, the bone marrow produces red blood cells independently of normal regulatory signals. This occurs most commonly in polycythaemia vera, a myeloproliferative disorder usually caused by a JAK2 mutation that leads to uncontrolled erythropoiesis. In secondary polycythaemia, red cell production increases appropriately in response to elevated erythropoietin levels triggered by chronic hypoxia or, less commonly, ectopic erythropoietin production by renal or hepatic tumours.
Although elevated red cell mass increases oxygen delivery, it also thickens the blood and slows flow through small vessels. This raises vascular resistance, increases afterload on the heart, and predisposes to thrombosis, stroke, and myocardial infarction. Polycythaemia therefore represents a balance between improved oxygen transport and impaired circulation, with clinical risk rising sharply as haematocrit increases.
Beyond the Basics
Normal Regulation of Red Blood Cell Mass
Red blood cell production is normally governed by a finely tuned oxygen-sensing system designed to balance oxygen delivery against blood flow efficiency. The kidneys continuously monitor how much oxygen is being delivered to their tissues. When oxygen levels fall, they respond by releasing erythropoietin, a hormone that stimulates the bone marrow to increase red blood cell production. As oxygen delivery improves, erythropoietin secretion falls and erythropoiesis slows. This negative feedback loop keeps red cell mass matched to metabolic demand rather than allowing it to drift upward unchecked.
This system ensures that blood remains fluid enough to flow easily through capillaries while still carrying sufficient oxygen. Polycythaemia arises when this balance is disrupted — either because the kidneys are persistently signalling hypoxia or because the bone marrow is no longer responding appropriately to regulatory signals.
Increased Red Blood Cell Production and Oxygen Sensing
In many cases of polycythaemia, the body genuinely believes that oxygen delivery is inadequate. Chronic lung disease, sleep-disordered breathing, and high-altitude exposure all reduce the amount of oxygen entering the blood, triggering sustained erythropoietin release. In the short term, this is adaptive, as increasing red cell mass improves oxygen transport. However, this compensation carries a cost. As red cell numbers rise, blood becomes progressively more viscous. Thickened blood flows more slowly through small vessels, limiting oxygen diffusion into tissues. Over time, a mechanism that initially improves oxygen delivery can begin to undermine it by impairing microcirculatory flow.
Primary Polycythaemia: Autonomous Bone Marrow Activity
Primary polycythaemia represents a fundamentally different physiological failure and considered a type of blood cancer. In this setting, red blood cell production becomes disconnected from oxygen-sensing altogether. In polycythaemia vera, gene mutations in growth-regulating pathways within bone marrow stem cells cause uncontrolled expansion of erythroid precursors. The marrow continues producing red cells even when oxygen levels are normal or high.
Erythropoietin levels are typically suppressed, but this suppression has little effect because red cell production is no longer dependent on hormonal signalling. The result is a steady rise in haematocrit and blood viscosity without any underlying need for increased oxygen delivery.
Secondary Polycythaemia: Sustained Erythropoietin Drive
In secondary polycythaemia, excessive red blood cell production is driven by persistently elevated erythropoietin. This usually results from chronic hypoxia (such as from smoking, lung disease, or high altitude) but it may also occur when tumours produce erythropoietin inappropriately.
Although the kidneys are responding appropriately to perceived oxygen deficiency, the prolonged increase in red cell mass eventually becomes maladaptive. Rising viscosity increases resistance to flow, reducing tissue perfusion and placing strain on the cardiovascular system. The paradox of secondary polycythaemia is that oxygen content rises while oxygen delivery falls.
Relative Polycythaemia and Plasma Volume Loss
Not all cases of elevated haemoglobin result from true increases in red blood cell mass. In relative polycythaemia, plasma volume falls while the number of red cells remains unchanged. Dehydration, diuretics, burns, or severe physiological stress can all produce this effect by removing fluid from the circulation. Although red cell mass is normal, haemoconcentration increases blood thickness and reduces flow through small vessels. From a haemodynamic perspective, the circulation behaves as though red cell mass were increased, exposing the patient to similar risks of impaired perfusion and thrombosis.
Blood Viscosity and the Circulation
As haematocrit rises, blood viscosity increases steeply. Thickened blood flows more slowly, particularly in capillaries, where resistance is highest. The heart must generate higher pressures to maintain flow, increasing cardiac workload and afterload. Sluggish flow also promotes platelet activation and interaction with the vessel wall, increasing the risk of clot formation. These haemodynamic changes explain why polycythaemia is associated with both arterial and venous thrombosis, as well as symptoms related to poor tissue perfusion.
Oxygen Delivery and Tissue Hypoxia
Oxygen delivery depends not only on how much oxygen is carried in the blood, but also on how efficiently blood flows through the microcirculation. In polycythaemia, rising viscosity slows capillary transit and limits diffusion of oxygen into tissues. Even though each millilitre of blood contains more oxygen, less oxygen reaches cells where it is needed. This is why patients with polycythaemia may develop neurological symptoms such as headache, dizziness, visual disturbance, and fatigue despite having elevated haemoglobin levels. The problem lies in impaired delivery rather than inadequate supply.
Conceptual Contrast with Anaemia
Polycythaemia and anaemia depict opposite failures of the same physiological system. Anaemia limits oxygen transport by reducing the number of oxygen-carrying cells. Polycythaemia limits oxygen delivery by making blood too viscous to flow efficiently. In both cases, tissues experience relative hypoxia through different mechanisms. This highlights a central principle of physiology: effective oxygen delivery depends on balance between red cell mass and blood flow, not simply on maximising haemoglobin levels.
Clinical Connections
Symptoms
Polycythaemia produces a characteristic cluster of symptoms where both increased blood viscosity and altered tissue perfusion are present. Patients may present with a ruddy or plethoric complexion, headaches, visual blurring, dizziness, fatigue, and a sensation of pressure or fullness in the head. Neurological symptoms, such as tingling, burning pain in the hands and feet, or transient visual loss, occur because sluggish microcirculatory flow reduces oxygen delivery to sensitive tissues. Intense pruritus (itching), particularly after hot showers, is a classic feature of polycythaemia vera and due to abnormal histamine release from increased basophils and mast cells.
The most serious clinical risk associated with polycythaemia is thrombosis. Thickened blood and increased platelet activation predispose to clot formation in both arteries and veins, leading to conditions such as:
stroke or transient ischaemic attack
myocardial infarction
deep vein thrombosis or pulmonary embolism
portal or hepatic vein thrombosis
These complications may occur even in patients with relatively mild symptoms, making early recognition and management essential.
Management
Management depends on whether polycythaemia is primary, secondary, or relative. In polycythaemia vera, treatment aims to reduce haematocrit and lower thrombotic risk. Regular venesection (therapeutic blood removal) decreases red cell mass and viscosity, while low-dose aspirin reduces platelet activation. In higher-risk patients, cytoreductive therapy may be required to suppress bone marrow activity and limit red cell production.
In secondary polycythaemia, treatment focuses on correcting the underlying cause rather than removing blood. Improving oxygenation through smoking cessation, treatment of chronic lung disease, management of sleep apnoea, or optimisation of cardiac function reduces erythropoietin drive and gradually normalises red cell mass. Inappropriate venesection in this setting can worsen tissue hypoxia by removing needed oxygen-carrying capacity.
Relative polycythaemia is treated by restoring plasma volume through adequate hydration and correction of contributing factors such as diuretics or fluid losses. Once plasma volume normalises, haemoglobin and haematocrit fall without any need for direct red cell removal.
Because polycythaemia can mimic dehydration or stress responses on blood tests, clinical context is crucial. Careful assessment of oxygen status, hydration, erythropoietin levels, and underlying disease allows clinicians to distinguish between benign haemoconcentration and pathological red cell overproduction, a distinction that directly affects both treatment and prognosis.
Concept Check
How does polycythaemia affect blood viscosity?
What distinguishes primary from secondary polycythaemia?
Why might a patient with polycythaemia experience itching after a hot shower?
Why does polycythaemia increase the risk of thrombosis?
How is secondary polycythaemia managed?