Skin

"The skin is the largest organ of the body, with a total area of about 20 square feet. The skin protects us from microbes and the elements, helps regulate body temperature, and permits the sensations of touch, heat, and cold.
Skin has three layers: The epidermis, the outermost layer of skin, provides a waterproof barrier and creates our skin tone.
The dermis, beneath the epidermis, contains tough connective tissue, hair follicles, and sweat glands.
The deeper subcutaneous tissue (hypodermis) is made of fat and connective tissue.
The skin’s color is created by special cells called melanocytes, which produce the pigment melanin. Melanocytes are located in the epidermis."

Bruise

A bruise, also called a contusion, forms because the soft tissues of your body have been bumped. Some people bruise easily, whereas others may have tougher skin tissue. When these soft tissues are injured, small veins and capillaries (the tiniest blood vessels) under the skin sometimes break. Red blood cells leak out of these blood vessels. These red blood cells that collect under your skin cause that bluish, purplish, reddish, or blackish mark.