Blood Viscosity Increases as Temperature Drops
One of the most immediate and impactful effects of hypothermia is the increase in blood viscosity, or thickness. As the blood's temperature decreases, its resistance to flow increases significantly. This makes it harder for the heart to pump blood through the body, especially to the extremities, and contributes to the body's overall struggle to maintain circulation and heat.
The Impact of Hemoconcentration and Fluid Shifts
Alongside the direct effect of temperature on viscosity, other changes contribute to the thickening of the blood. The body responds to cold stress by constricting peripheral blood vessels (peripheral vasoconstriction) to preserve core body heat. This shunts blood away from the limbs and skin towards the vital organs, leading to a phenomenon known as hemoconcentration—an increase in the concentration of red blood cells relative to the plasma volume. This causes blood to become even more viscous. Furthermore, cold-induced diuresis, an increased urination response to cold, further reduces the plasma volume, exacerbating hemoconcentration.
The Paradox of Coagulation Disorders
Hypothermia disrupts the body's delicate hemostatic balance, causing a complex coagulation disorder known as hypothermic coagulopathy. While it might seem that slower blood flow would lead to easier clotting, the reality is more complicated and depends heavily on the severity of the cold. The effects on coagulation are paradoxical and dangerous.
Impaired Clotting Factor Activity
Normal blood clotting relies on a series of enzymatic reactions involving various coagulation factors. These enzymes are temperature-sensitive and become significantly less active as the body's core temperature drops below 34°C (93.2°F). This impairs the initiation and propagation of the clotting cascade, increasing the risk of uncontrolled bleeding, particularly in cases of trauma. Standard laboratory tests, which warm blood samples to 37°C before analysis, often fail to detect this impairment, making accurate diagnosis challenging.
Altered Platelet Function
Platelets, the blood cells responsible for forming clots, are also profoundly affected. Their activation and aggregation are impaired by low temperatures, directly contributing to the body's reduced ability to form stable blood clots. However, in mild to moderate hypothermia (above 33°C), studies have shown that platelets might become more responsive to activation stimuli, a kind of protective mechanism, especially in the peripheries. A significant risk, particularly during the rewarming process, is the rebound effect. Rewarming can trigger a surge of activated platelets into the bloodstream, increasing the risk of dangerous micro-thrombi (tiny clots) forming and causing thrombotic disorders.
Fluid and Electrolyte Disturbances
The kidney's function is also impacted by hypothermia, leading to fluid and electrolyte imbalances. Cold diuresis, mentioned earlier, results in increased fluid loss, while impaired renal function can affect the body's ability to regulate electrolyte levels. This can cause unpredictable and wide fluctuations in essential minerals like potassium, which can significantly affect heart rhythm and function.
Comparison of Normothermic and Hypothermic Blood
| Characteristic | Normothermic Blood (37°C) | Hypothermic Blood (<35°C) |
|---|---|---|
| Viscosity | Normal, optimal for flow | Increased; becomes thick and sluggish |
| Coagulation | Normal, efficient clotting | Impaired, especially below 34°C |
| Platelet Function | Normal aggregation | Inhibited aggregation, yet can be primed to activate |
| Hemoconcentration | Absent | Present due to fluid shifts |
| Blood Flow | Efficient and widespread | Reduced, especially in extremities |
| pH | Stable (approx. 7.4) | Increases as a result of temperature effects |
The Rewarming Phase: A Critical Period
The rewarming phase is not without its own risks. As the body warms, blood vessels in the periphery begin to dilate, allowing cold, acidic, and potassium-rich blood from the limbs to rush back into the core circulation. This can cause a sudden and dramatic drop in core temperature (afterdrop) and trigger serious arrhythmias or cardiovascular collapse. Additionally, the release of activated platelets during rewarming can lead to a hypercoagulable state, increasing the risk of thrombosis. Careful, monitored rewarming is essential to manage these complex and potentially fatal blood changes.
Conclusion
In summary, hypothermia triggers a dangerous domino effect in the bloodstream. The initial cold exposure causes peripheral vasoconstriction and hemoconcentration, which thickens the blood and restricts flow. As core temperature drops further, the entire hemostatic system is compromised: enzymatic activity in the coagulation cascade slows, while platelet function is impaired. This complex combination of increased blood viscosity and inhibited clotting creates a high-risk state for both bleeding and unwanted clot formation, especially during the critical rewarming process. Understanding what happens to your blood when you have hypothermia is key to appreciating the seriousness of this medical emergency and highlights why prompt, expert medical attention is essential for recovery. For more detailed information on hypothermia, consult the National Institutes of Health.
Visit the National Institutes of Health for more information on hypothermia.
