The idea that one bleeds more easily at high altitudes is a common misconception. Altitude profoundly affects the blood, but the resulting changes often increase the risk of clotting rather than excessive bleeding. Exposure to the unique high-altitude environment triggers a cascade of physiological adjustments intended to improve oxygen delivery throughout the body. These adjustments involve changes to blood volume, blood component concentration, and the mechanisms that control clotting and flow.
The Initial Physiological Response to Altitude
The primary environmental stressor at high altitude is the reduced partial pressure of oxygen, leading to hypobaric hypoxia. The lower atmospheric pressure means fewer oxygen molecules enter the lungs with each breath, even though the air still contains 21% oxygen. The body’s immediate, short-term reaction is to increase ventilation and elevate the heart rate to compensate for the lower oxygen availability.
This initial response also includes a rapid fluid shift, where plasma volume decreases as fluid moves out of the blood vessels. This fluid loss is partially due to increased breathing, which leads to greater water vapor loss, and initial fluid excretion by the kidneys. The resulting hemoconcentration is a swift mechanism to increase the relative concentration of oxygen-carrying red blood cells before new cells are produced.
Altitude’s Impact on Blood Coagulation
Hypoxic conditions directly influence the body’s hemostatic system, leading to a hypercoagulable state—a tendency for blood to clot. Low oxygen levels can activate platelets, the small cell fragments that initiate clotting. Platelet aggregation and activity often increase upon initial ascent, promoting clot formation.
The coagulation cascade is also affected by the hypoxic environment. Certain clotting factors, such as fibrinogen (the precursor to the fibrin mesh that stabilizes a clot), increase with prolonged high-altitude exposure. Furthermore, the body’s system for breaking down clots, called fibrinolysis, may be suppressed or altered. This combination of increased clot formation and reduced clot breakdown significantly shifts the balance toward thrombosis.
Changes in coagulation parameters, such as an increase in D-dimer (a marker of fibrin breakdown), suggest continuous activation of the clotting system. While the overall environment favors clotting, some acute exposure studies show a temporary increase in bleeding time. However, the predominant and long-term risk associated with altitude exposure is the heightened potential for blood clots, such as deep vein thrombosis (DVT).
Hemoconcentration and Blood Viscosity
The body’s medium-to-long-term adaptation involves increasing the number of red blood cells (RBCs) in a process called erythropoiesis. The kidneys detect low oxygen and release the hormone erythropoietin, which stimulates the bone marrow to produce more RBCs. This compensatory increase in RBCs elevates the hematocrit, the percentage of blood volume occupied by red blood cells.
This increase in oxygen-carrying cells, combined with the initial plasma volume reduction, results in hemoconcentration and a substantial rise in blood viscosity. At high altitudes, blood viscosity—the thickness and stickiness of the blood—can increase significantly.
Higher blood viscosity impedes blood flow, making the blood move sluggishly through the circulatory system. This slowed flow, known as stasis, is a critical factor in Virchow’s triad, the well-known mechanism for the formation of blood clots. This physical property of thicker blood reinforces the chemical and cellular changes that promote a pro-clotting environment, increasing the risk of thrombotic events like stroke or DVT.
Clinical Considerations for High Altitude Travel
The hypercoagulable state and increased blood viscosity at high altitude have important practical implications for travelers and residents. Individuals with pre-existing cardiovascular conditions or a history of blood clots are at greater risk when traveling to high elevations. The risk of venous thromboembolism, which includes DVT and pulmonary embolism, is a significant concern for lowlanders ascending to high altitudes.
For those undergoing surgery or requiring blood thinners, the altered coagulation system presents a complex challenge. The body’s shift toward clotting may necessitate adjustments to anticoagulant therapy, such as warfarin, to maintain proper clotting balance. Maintaining adequate hydration is one of the most effective ways to mitigate risk, as it counteracts hemoconcentration, reduces blood viscosity, and improves circulation.