Page 474 - Atlas of Histology with Functional Correlations

conserved by constriction of superficial blood vessels, decreased blood flow to

the skin, and maintaining more heat in the body core.

Sensory Perception

The skin is a large sensory organ for the external environment. Numerous

encapsulated and free sensory nerve endings within the skin respond to stimuli
for temperature (heat and cold), touch, pain, and pressure.

Excretion

Through the production of sweat by the sweat glands, water, sodium salts, urea,
and nitrogenous wastes are excreted through the surface of the skin.

Formation of Vitamin D

The skin is a major source of vitamin D. The keratinocytes are not only the
primary source of vitamin D for the body but also possess the enzymes to
metabolize vitamin D to its active metabolite. Vitamin D is formed from
precursor molecules synthesized in the epidermis during exposure of the skin to

ultraviolet rays from the sun. Vitamin D becomes essential for calcium
absorption from the intestinal mucosa and for proper mineral metabolism.

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