Physiology of the skin.
The skin is the largest organ in the body, accounting for about 16% of total body weight. It serves as an interface between the internal environment and the external world, carrying out a diverse range of physiological functions essential for survival.
Functions of the Skin:
The skin's main physiological functions can be categorized as:
- Protection: The skin acts as a barrier, safeguarding underlying tissues from mechanical injury, pathogens, radiation, and excessive water loss.
- Sensory: The skin houses a vast network of sensory receptors, providing crucial information about touch, temperature, pressure, pain, and vibration to the nervous system.
- Thermoregulation: The skin plays a vital role in maintaining body temperature through mechanisms like sweating, vasoconstriction, and vasodilation of cutaneous blood vessels.
- Endocrine and Metabolic: The skin participates in endocrine and metabolic functions, including the synthesis of vitamin D, sex steroids, and the production of leptin, a hormone involved in energy balance and appetite regulation.
Structure of the Skin:
The skin is composed of two main layers: the epidermis and the dermis.
- 1. Epidermis:
- The epidermis, the outermost layer, consists of stratified squamous epithelium. The predominant cell type in the epidermis is keratinocytes, which produce keratin, a fibrous protein providing strength and resilience to the skin. Keratinocytes also produce cytokines in response to injury.
- The epidermis is further divided into distinct layers:
- Stratum basale: The innermost layer, adhering to the dermis, serves as the site of keratinocyte proliferation.
- Stratum spinosum: Cells from the basal layer migrate to this layer, forming connections via desmosomes.
- Stratum granulosum: Keratinocytes in this layer contain granules and Odland bodies, which release lipids into the intercellular space, enhancing cohesion between cells.
- Stratum corneum: The outermost layer, consisting of corneocytes (keratinocytes that have lost their nuclei and organelles), forms the protective barrier of the skin.
The epidermis serves as a physical barrier, regulating the passage of substances into and out of the body.
While it offers protection against unregulated loss of water and electrolytes and invasion by microorganisms, it is not entirely impermeable. Water, ions, and certain substances can be absorbed, albeit at varying rates depending on their chemical properties and the body region. Alcohols and non-polar substances tend to be absorbed more readily than water and ions. The face and back of the hand are examples of areas with higher permeability.