Final answer:
Secondary union healing is when a wound, typically one that is gaping and cannot be easily closed, heals from the bottom outward through the formation of granulation tissue.
Step-by-step explanation:
With secondary union healing, a wound heals by granulation from the bottom of the wound outward. This type of wound healing is required when there is a gaping wound that cannot be brought together easily, and it involves filling the wound with granulation tissue, contraction of the wound edges, and eventually the formation of a scar. Granulation tissue is a combination of new blood vessels, collagen fibers, and fibroblasts that fill the wound area. The process of wound healing involves multiple stages: stopping blood loss through clotting, combating infection through immune response, and repairing the wound with collagen deposition by fibroblasts. If the wound is deeper than one quarter of an inch, sutures may be applied to promote primary union and reduce scarring.