A blister is a small pocket of fluid in the upper layers of the skin. Most blisters are filled with a clear fluid called serum. Serum is the part of the blood that remains after red blood cells and clotting agents have been removed.
A blister usually forms because the outer layer of the skin has been damaged. Fluid collects under the damaged layer of skin, cushioning the tissue underneath. This protects the tissue from further damage and allows it to heal.
Blisters are sometimes filled with blood (called blood blisters) or pus (if they become infected).
A blood blister usually forms when a small blood vessel close to the surface of the skin ruptures (breaks) and blood leaks between the layers of skin. This can happen if the skin is crushed, pinched or tightly squeezed.
Blisters are common and can be caused by:
- friction to the skin, for example from shoes that rub,
- contact with chemicals, such as detergent,
- heat, for example from sunburn or a scald, and
- medical conditions, such as chickenpox and impetigo (see Useful links).
Most blisters heal naturally and do not require medical attention.
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Red blood cells
Red blood cells transport oxygen around the body and remove carbon dioxide.
Tissue
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.
Blood vessel
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessel are veins, arteries and capillaries.
Rupture
A rupture is a break or tear in an organ or tissue.
