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Why can severe burns lead to a dangerous drop in blood pressure?

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Cross-section diagram of human skin showing burn depth by degree, with arrows indicating that first-degree burns affect only the epidermis, second-degree burns reach the dermis, third-degree burns extend into subcutaneous tissue, and fourth-degree burns penetrate to muscle. — burn This diagram shows how burn injuries are classified by severity. First-degree burns damage the outer skin, known as the epidermis. First-degree burns are the least severe form of burn injury. Second-degree burns damage the outer skin and some of the underlying skin, known as the dermis, which also includes some connective tissue. Third-degree burns damage all the layers of skin, including subcutaneous tissue. Fourth-degree burns, which are the most severe, destroy every layer of skin and also damage muscle, tendons, and bones.

Severe burns can lead to a dangerous drop in blood pressure—a condition known as secondary shock—primarily due to a significant reduction in total blood volume.12

When the skin is severely damaged, it loses its protective ability to contain body fluids. This results in the leakage of plasma and electrolytes from the circulatory system into the damaged tissue and out through the wound. If the volume of fluid lost exceeds approximately one-fifth of the total blood volume, the amount of blood returning to the heart becomes insufficient to maintain adequate cardiac output.13

Furthermore, the loss of salts, such as sodium and potassium, disrupts the osmotic balance of the blood and body fluids. As the circulating volume contracts, the body’s compensatory mechanisms eventually fail, causing arterial blood pressure to fall sharply. This process underscores the critical importance of replacing lost fluids and electrolytes in burn patients to prevent circulatory collapse.132