Chapter 1 - An Introduction to the Human Body - Figure 1.1 - Page 3: 1

The level of organization at which two or more tissues combines to perform specific functions is the organ level. The heart, lungs, stomach and skin are examples of the organ level of structural organization The skin which is the largest organ of the integumentary system, is a good example of organ level structure and function. The skin is made up of two or three anatomic divisions-- some experts do not consider the subcutaneous layer a proper part of the skin. The two main sections are the epidermis on the outside, and the dermis beneath the epidermis, but above the subcutaneous layer. The keratinized cells of the dermis protect the body from physical trauma, injury by chemicals, and the invasion of parasites and microbes. Melanin produced by melanocytes of the epidermis also protects the nuclei of skin cells from ultra violet radiation damage. The epidermis is composed largely of keratinized epithelial cell,s, but the dermis is largely a layer of dense irregular connective tissue. Both the matrix and cells of the dermis enable the skin to carry out some important functions that benefit the organism. The dense irregular fibers of collagen, and elastin give the skin great tensile strength, flexibility, and elasticity. Therefore, the skin does not tear easily, and it recoils when stretched, or deformed. The rich vascular network of the dermis supports the avascular dermis with nutrients, and it removes waste products of epidermal cell metabolism. Furthermore, the dermal blood supply plays a role in helping the body maintain temperature homeostasis. The muscles in the walls of dermal blood vessels can control the volume of blood going to the skin. On a hot day, or after vigorous exercise, more blood goes to the dermal blood vessels; this increases heat loss from the surface of the ski, and serves to keep the body from heating up. The adipose layer of the dermis also functions in temperature regulation. It forms an insulating layer that helps to dampen temperature fluctuations in the body .

Sweat glands, hair, nails, and sensory receptors are also important parts of the skin: they play vital roles in facilitating certain skin functions. Sweat glands function in temperature regulation, and they also play a minor excretory role. Hair is less important in humans than in some other mammals, but a full head of hair can contribute to warmth in winter, because in a cold environment, most of the body heat lost is lost through the head. Sensory receptors like Merkel cells ,Meissner's corpuscles, Pacinian corpuscles, and nerve endings make the skin an important sense organs. The skin can perceive and transmit sensations of warmth, coolness, pain, itch, tickle and pressure. Although the subcutaneous layer is excluded from this discussion, it does help the skin to perform its special functions.