Human Anatomy & Physiology (9th Edition)

Published by Pearson
ISBN 10: 0321743261
ISBN 13: 978-0-32174-326-8

Chapter 19 - The Cardiovascular System: Blood Vessels - Review Questions - Page 749: 21


Blood flow that supplies nutrients and oxygen to the skin is autoregulated by mechanisms in the skin itself.The extensive network of capillary beds , plexuses and aterio-venous anastomoses respond primarily to changes in oxygen levels[O2], but also to changes in hydrogen ion concentration[H+] associated with metabolic activity. On the contrary, thermoregulation by changes in cutaneous blood flow involves both peripheral mechanisms and central temperature control centers in the hypothalamus.

Work Step by Step

The temperature receptors of the skin are naked sympathetic nerve endings ; some are slow unmyelinated fibers, and some are fast myelinated fibers. When there are significant temperature changes in the blood these receptors alert the temperature regulating centers in the hypothalamus. The center in the anterior hypothalamus is the temperature thermostat and the center in the posterior hypothalamus integrates and responds to impulses from peripheral and central temperature receptors. If body temperature falls below normal (37C), the hypotahlamic heat promotion center is activated. It ten sends impulses to cutaneous capillaries to constrict, and to arterio-venous anastmoses (shunts) to dilate . As a cosequence most of the blood bypasses the skin capillary beds and is redirected through shunts to the more vital organ s of the cor--brain, and organs of the thoracic and abdominal cavities. The skin cools and passive heat loss is prevented If body temperature rises above the normal range ( 32F--37F) sympathetic messages are sent from both peripheral (cutaneous) and central receptors to the hypothalamic regulating centers. . In response the posterior hypothalamus .sens out sympathetic impulses to inhibit or decrease the stimulation of the cutaneous vessels. As a result the precapillary sphincters open, the blood vessels dilate and warm blood flows int the capillary beds. . arm blood at the surface loses heat by the processes of radiation, convection and a little conduction. When it gets very warm the swaet gland incresetheir secretion of sweat. . Sweat pours out on the surface of the epidermis and evaporates as water vapor. Sweat is 99% water and the latent heat f vaporization is hih so evaporation of sweat is a very effective cooling process. Sweat produces an enzype that generates synthesis of bradykinin whch stimulates the production of nitric oxide (NO) from endothelial cells . Nitric oxide is a potent vaso dilator so it enhances blood flow int the skin. . Ordinarily the skin loses about 500 mls of water by evaporation each day. When the skin needs to lose heat by sweating the water los by evaporation rise to about 2L per day.
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