HA

• The skin contains three layers: the epidermis, the dermis, and the subcutaneous tissues.
• The most superficial layer, the epidermis, is thin, devoid of blood vessels, and itself divided into two layers: an outer horny layer of dead keratinized cells and an inner cellular layer where both melanin and keratin are formed.
• The epidermis depends on the underlying dermis for its nutrition.
• The dermis is well supplied with blood
• The color of normal skin depends primarily on four pigments: melanin, carotene, oxyhemoglobin, and deoxyhemoglobin.
• he amount of melanin, the brownish pigment of the skin, is genetically determined and is increased by exposure to sunlight. Carotene is a golden yellow pigment that exists in subcutaneous fat and in heavily keratinized areas such as the palms and soles.
• Another yellow color in the skin may be jaundice, due to deposition of bilirubin in the skin.
• Hemoglobin, which circulates in the red cells and carries most of the oxygenof the blood, exists in two forms. Oxyhemoglobin, a bright red pigment, pre-dominates in the arteries and capillaries.
• As blood passes through the capillary bed, 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.
• If the oxygen level in the arterial blood is low, cyanosis is central and indicates decreased oxygenation in the patient.
• If the oxygen level is normal, cyanosis is peripheral.
• Adults have two types of hair: vellus hair, which is short, fine, inconspicuous, and relatively unpigmented; and terminal hair, which is coarser, thicker, more conspicuous, and usually pigmented. Scalp hair and eyebrows examples of terminal hair.
• Nails protect the distal ends of the fingers and toes.
• Sebaceous glands produce sebum
• Sweat glands are of two types: eccrine and apocrine. The eccrine glands are widely distributed, open directly onto the skin surface, and by their sweat production help to control body temperature. In contrast, the apocrine glands are found chiefly in the axillary and genital regions, and are stimulated by emotional stress.