1. The skin is an organ that contains thousands of smaller organs: each hair, nail, gland, and many of the nerve endings (those with accessory connective tissues, but not the bare dendrites) are separate organs.

2. There is considerable latitude for variation in your answer here. However, the functions of the apocrine sweat glands, axillary and pubic hair, beard, and eyebrows would not be clear if we think only of western industrialized society. They take on much more meaning if we compare human cultures and consider their functions in other primate species. The vestigial body hair and arrector pili muscles would make no sense at all if not for the fact that humans evolved from mammals in which these structures were functional.

3. The two layers of the dermis are functionally different, and this is reflected in their histological differences. The upper one-fifth, the papillary layer, plays two roles. (1) It as an arena of immune defense, which is best served by a loosely organized areolar tissue in which immune cells can freely migrate to a site of infection. (2) Its dermal papillae keep the epidermis from sliding across the dermis when stress is applied to the skin. The lower four-fifths, or reticular layer, gives the skin much of its toughness and resilience and binds the skin to underlying tissue. These roles are well-served by its thick tangle of collagen bundles. Such a thick mass of collagen would be somewhat incompatible with the immune function of the papillary layer.

4. In cold weather, dermal arteries constrict to reduce heat loss from the body, and blood flow through the skin is reduced. Skin tissue continues to extract oxygen from this blood, but fresh, oxygenated blood does not flow in to replace it as quickly. Therefore, blood in the the dermal capillaries becomes oxygen-poor and the skin appears cyanotic, as when you're so cold your lips are blue.

5. Excessive exposure to UV radiation damages DNA and can trigger cancer, so it is important to screen out UV. However, enough UV must reach the dermal blood capillaries to promote the synthesis of vitamin D. If the epidermis were too good a UV filter, a person could have a vitamin D deficiency.

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