Blood circulates throughout the human body, transporting oxygen, nutrients, and waste products. Its fluid nature often leads to questions about its behavior in cold temperatures. Understanding how blood responds to frigid conditions involves examining its composition and the physical changes it undergoes.
How Blood Responds to Freezing Temperatures
Blood is composed mainly of water and dissolved substances like salts, proteins, and sugars. These solutes lower its freezing point below that of pure water, a phenomenon called freezing point depression. Pure water freezes at 0°C (32°F), but blood, due to its dissolved components, typically begins to freeze at a slightly lower temperature, often between -0.53°C and -0.56°C (30.9°F to 31.0°F). Some studies suggest a broader range, around -2°C to -3°C (28.4°F to 26.6°F).
When blood freezes, ice crystals begin to form, initially in the extracellular fluid. As these crystals grow, they draw water out of the cells, causing them to dehydrate. This process can lead to significant damage to red blood cells, potentially causing their membranes to rupture, a process known as lysis. Such cellular destruction impairs blood’s ability to carry oxygen and perform other functions.
What Happens When Blood Freezes in the Body
When a living organism is exposed to severe cold, the body’s tissues can freeze, leading to conditions like frostbite. Frostbite involves the freezing of water within cells and in the spaces between them, not primarily within major blood vessels. Ice crystals developing in these tissues can physically disrupt cell structures and impede blood flow by blocking capillaries.
The body counteracts cold exposure through vasoconstriction, narrowing blood vessels to redirect warmer blood away from extremities and towards the core. Prolonged cold exposure and reduced blood flow can lead to a lack of oxygen and nutrients in affected tissues, resulting in cellular damage and tissue death.
How Blood is Stored and Protected
Medical practices consider blood’s freezing point to maintain its viability for transfusions. In blood banks, whole blood and red blood cells are stored under refrigeration, between 2°C and 6°C (35.6°F and 42.8°F). This range prevents freezing, minimizes bacterial growth, and preserves cellular integrity for up to 35 to 42 days, depending on the solutions used.
For longer-term preservation of specific blood components like red blood cells (for rare types), plasma, or stem cells, cryopreservation is used. This involves freezing components at very low temperatures, often below -80°C or -196°C in liquid nitrogen. To prevent damaging ice crystal formation, cryoprotectants like glycerol are added. These protect cells and allow for storage extending for years.