How and why do blisters form on the feet?
A blister may form when the skin has been damaged by friction or rubbing, heat, cold or chemical exposure.
Fluid collects between the epidermis—the upper layer of the skin—and the layers below. This fluid cushions the tissue underneath, protecting it from further damage and allowing it to heal.
Intense rubbing can cause a blister, as can any friction on the skin if continued long enough. This kind of blister is most common after walking long distances or by wearing old or poorly fitting shoes.
Blisters are most common on the hands and feet, as these extremities are susceptible while walking, running, or performing repetitive motions, such as joystick manipulation whilst playing certain video games, digging with a shovel, playing guitar, etc. Blisters form more easily on moist skin than on dry or soaked skin, and are more common in warm conditions.
Less-aggressive rubbing over long periods of time may cause calluses to form rather than a blister. Both blisters and calluses can lead to more serious complications, such as foot ulceration and infection, particularly when sensation or circulation is impaired, as in the case of diabetes, neuropathy or peripheral artery disease (PAD).
Image Source: By AndryFrench - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=8492989