Integumentary System: Skin Layers

• The skin, along with its accessory structures such as hair, nails, sensory receptors, and glands, constitutes the integumentary system.
• It serves as a strong, flexible barrier between the body and the outside world.

Skin Layers

• Consists of two distinct layers: the epidermis (outer layer) and the dermis (inner layer).
• The outer layer, known as the epidermis, is composed of stratified squamous epithelium, with its thickness varying across different body regions.
• Blood vessels are absent in the epidermis due to its composition of stratified squamous epithelium; nourished by dermal blood vessels near its deepest layer, the stratum basale.
• Basal cells divide and grow, pushing older epidermal cells (keratinocytes) towards the skin surface, where they receive less nutrient supply and eventually die.
• Keratinocytes undergo keratinization, where their cytoplasm fills with tough, fibrous, waterproof keratin proteins.
• The inner layer, or dermis, is thicker than the epidermis.
• It comprises connective tissue with collagen and elastic fibers, as well as smooth muscle, nervous tissue, and blood.
• The epidermis is anchored to the dermis by a basement membrane, separating these two layers.
• Below the dermis lies the subcutaneous layer (hypodermis), composed of areolar and adipose tissue that binds the skin to underlying organs.
• The adipose tissue within the subcutaneous layer provides insulation, aiding in body heat conservation.
• Major blood vessels supplying the skin and underlying adipose tissue are located within the subcutaneous layer.

Skin Functions

• The skin performs various functions, including protective covering to prevent harmful substances/microorganisms from entering the body and slowing water loss by diffusion.
• Houses sensory receptors and excretes small amounts of waste.
• Melanocytes in the epidermis produce melanin, a pigment responsible for skin color, within organelles called melanosomes.
• Higher melanin production results in darker skin.
• Melanin also protects skin cell DNA by absorbing UV radiation to prevent mutations.
• Skin condition aids in the diagnosis of health issues.
• Skin color is influenced by both environmental/physiological factors and genetics.
• Bright red blood pigment hemoglobin indicates oxygen-rich blood, leading to a pinkish skin tone in fair-skinned individuals.
• Low blood oxygen results in dark red hemoglobin and a bluish skin appearance (cyanosis).
• Diet and diseases can also alter skin color, such as a diet high in yellow vegetables leading to an orange-yellow skin tone due to carotene accumulation in the stratum corneum and adipose tissue.
• Jaundice, a yellowish skin tone, may indicate liver malfunction.
• The skin synthesizes vitamin D, essential for bone and tooth development, by converting dehydrocholesterol into cholecalciferol upon sunlight exposure. The liver and kidneys then convert it to calcitriol (active vitamin D).
• The skin regulates body temperature.
• Blood vessels dilate and sweat glands secrete sweat when body temperature rises above normal.
• Blood vessels constrict and sweat glands become inactive when body temperature drops below normal.
• Skeletal muscles rhythmically contract if body temperature continues to drop.