Blood is mostly water, and like water, it can freeze when exposed to cold temperatures. However, blood does not solidify at the same temperature as pure water, which freezes at 0°C (32°F). The numerous components dissolved within the bloodstream act as a natural antifreeze, lowering the point at which ice crystals can begin to form. Understanding this difference involves exploring the physical chemistry of solutions and the specific makeup of human blood.
Why Blood Resists Freezing
The reason blood has a lower freezing point than pure water is a phenomenon known as freezing point depression. This is a colligative property of solutions, meaning the change in freezing point depends on the total number of dissolved particles, or solutes, present in the liquid. The greater the concentration of these particles, the more the freezing point is lowered.
Blood plasma, the liquid matrix of blood, is rich with various solutes that interfere with the formation of a solid water lattice. These dissolved substances include simple electrolytes, such as sodium and chloride ions, glucose, and various proteins like albumin. These particles physically obstruct water molecules from aligning into the ordered, crystalline structure of ice, allowing the blood to remain liquid at temperatures below 0°C.
The Actual Freezing Temperature
The specific temperature at which human blood begins to freeze is remarkably consistent and very close to the freezing point of water. Physiologically normal whole blood typically starts to solidify at approximately -0.5°C to -0.6°C (about 31.0°F). This precise temperature point is often used in medicine as a measure of a patient’s plasma osmolality, which is the concentration of dissolved particles in the blood.
A slight shift in this freezing temperature indicates a change in the concentration of solutes, such as in cases of severe dehydration. If the concentration of salts and proteins increases due to water loss, the freezing point will be depressed even further. Only extreme variations in plasma concentration, like severe illness or exposure to cryoprotectant agents, will significantly alter this typical freezing threshold.
Effects of Freezing on Blood Cells
When blood freezes, the damage to the cellular components is not solely caused by the cold, but primarily by the formation of ice crystals. As water turns to ice, the solutes are excluded, causing the remaining unfrozen liquid to become progressively more concentrated. This process creates a highly hypertonic environment outside the cells.
The concentrated external solution draws water out of the red blood cells, causing them to shrink and become dehydrated. Simultaneously, ice crystals forming inside or outside the cells can physically puncture the delicate cell membranes. This dual assault of physical damage and osmotic stress leads to cell rupture, a process known as hemolysis.
Hemolysis releases hemoglobin into the plasma, rendering the blood useless for oxygen transport. This destruction of the cellular structure is why frozen blood, outside of carefully controlled medical cryopreservation techniques, cannot be used for standard transfusions.