emoglobin, which circulates in the red cells and carries most of the oxygen of the blood, exists in two forms. Oxyhemoglobin, a bright red pigment, predominates in the arteries and capillaries. An increase in blood flow through the arteries to the capillaries of the skin causes a reddening of the skin, while e opposite change usually produces pallor. The skin of light-colored perns is normally redder on the palms, soles, face, neck, and upper chest.
blood passes through the capillary bed, some of the oxyhemoglobin loses its oxygen to the tissues and changes to deoxyhemoglobin—a darker and somewhat bluer pigment. An increased concentration of deoxyhemoglobin in cutaneous blood vessels gives the skin a bluish cast known as cyanosis.
Cyanosis is of two kinds, depending on the oxygen level in the arterial blood. If this level is low, cyanosis is central. If it is normal, cyanosis is peripheral. Peripheral cyanosis occurs when cutaneous blood flow decreases and slows, and tissues extract more oxygen than usual from the blood. Peripheral cyanosis may be a normal response to anxiety or a cold environment.
Skin color is affected not only by pigments but also by the scattering of light as it is reflected back through the turbid superficial layers of the skin or vessel walls. This scattering makes the color look more blue and less red. The bluish color of a subcutaneous vein is a result of this effect; it is much bluer than the venous blood obtained on venipuncture.
As people age their skin wrinkles, becomes lax, and loses turgor. The vascu-larity of the dermis decreases and the skin of light-skinned persons tends to look paler and more opaque. Comedones (blackheads) often appear on the cheeks or around the eyes. Where skin has been exposed to the sun it looks weatherbeaten: thickened, yellowed, and deeply furrowed. Skin on the backs of the hands and forearms appears thin, fragile, loose, and transparent, and may show whitish, depigmented patches known as pseudoscars. Well-demarcated, vividly purple macules or patches, termed actinic purpura, may also appear in the same areas, fading after several weeks. These purpuric spots come from blood that has leaked through poorly supported capillaries and has spread within the dermis. Dry skin (asteatosis)—a common problem—is flaky, rough, and often itchy. It is frequently shiny, especially on the legs, where a network of shallow fissures often creates a mosaic of small polygons.
Some common benign lesions often accompany aging: cherry angiomas
(p._), which often appear early in adulthood, seborrheic keratoses (p._), and, in sun-exposed areas, actinic lentigines or "liver spots" (p.__) and actinic keratoses (p._). Elderly people may also develop two common skin cancers: basal cell carcinoma and squamous cell carcinoma (p._).
Jails lose some of their luster with age and may yellow and thicken, especially on the toes.
Hair on the scalp loses its pigment, producing the well-known graying. As early as 20, a man's hairline may start to recede at the temples; hair loss at th e vertex follows. Many women show a less severe loss of hair in a similar pattern. Hair loss in this distribution is genetically determined.
In both sexes, the number of scalp hairs decreases in a generalized pattern, and the diameter of each hair diminishes.
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