40.1 Introduction to the Skin and Its Function

Skin Structure and Function

The skin is the body's largest organ, and understanding its structure is essential for pharmacology because how the skin is built directly affects how drugs are absorbed, distributed, and applied topically. This section covers the three main layers of the skin, their functions, and why all of this matters for nursing practice.

Epidermis and Dermis

The epidermis is the outermost layer of skin, composed of stratified squamous epithelium. It serves as the body's primary protective barrier against UV radiation, microorganisms, and chemicals. The epidermis consists of five distinct layers (covered in detail below) and undergoes continuous renewal through keratinization, the process by which cells differentiate, mature, and eventually become the tough outer surface of your skin.

The dermis sits directly beneath the epidermis and is composed of connective tissue, primarily collagen and elastin fibers. It provides mechanical support and elasticity. The dermis is densely packed with structures that serve critical functions:

• Blood vessels supply nutrients and oxygen to the skin and regulate temperature
• Lymph vessels support immune function and fluid balance
• Nerve endings enable sensory perception (touch, pressure, temperature, pain)
• Hair follicles produce hair
• Sweat glands help regulate body temperature

The dermis plays a major role in thermoregulation through blood vessel dilation (to release heat) and constriction (to retain heat), and in sensory perception through specialized receptors distributed throughout its layers.

Layers of the Epidermis

Moving from the surface inward, the five layers of the epidermis are:

1. Stratum corneum (horny layer): The outermost layer, made up of dead, flattened keratinocytes filled with keratin, a tough fibrous protein. This layer provides a waterproof barrier and is the main physical defense against external threats. For pharmacology, this is the layer that topical drugs must penetrate to reach deeper tissues.

2. Stratum lucidum (clear layer): Present only in thick skin, such as the palms and soles. It consists of dead, flattened keratinocytes and helps reduce friction and shear forces in areas subject to heavy mechanical stress.

3. Stratum granulosum (granular layer): Contains keratinocytes with keratohyalin granules, which are precursors to keratin. This layer produces lipids (ceramides, cholesterol, fatty acids) that contribute to the skin's barrier function and plays a key role in forming the stratum corneum above it.

4. Stratum spinosum (spinous layer): Several layers of keratinocytes connected by desmosomes (strong cell-to-cell junctions). This layer provides mechanical strength and is involved in synthesizing keratin and structural proteins like involucrin and loricrin.

5. Stratum basale (basal layer): The deepest epidermal layer, anchored to the dermis by hemidesmosomes. It contains stem cells that continuously divide to replenish the epidermis. Melanocytes in this layer produce melanin, the pigment that protects against UV radiation.

Dermis and Hypodermis

Composition and Functions of the Dermis

The dermis has two sublayers:

1. Papillary dermis: The upper layer, made of loose connective tissue with capillary loops that supply nutrients to the epidermis above.
2. Reticular dermis: The deeper layer, composed of dense irregular connective tissue (collagen and elastin fibers) that gives skin its strength and elasticity.

Key functions of the dermis:

• Provides structural support and elasticity
• Nourishes the epidermis and regulates body temperature via blood vessels (vasodilation to lose heat, vasoconstriction to retain heat)
• Supports immune function and fluid balance through lymph vessels
• Enables sensory perception through nerve endings
• Houses hair follicles, sweat glands (for thermoregulation), and sebaceous glands (which produce sebum to lubricate the skin)

Temperature regulation works through two mechanisms: blood vessels dilate to increase heat loss or constrict to conserve heat, and sweat glands produce sweat that evaporates from the skin surface to cool the body.

Sensory receptors in the dermis include:

• Meissner's corpuscles (papillary dermis): detect light touch and texture
• Pacinian corpuscles (reticular dermis): respond to deep pressure and vibration
• Ruffini endings (reticular dermis): sense sustained pressure and skin stretching
• Free nerve endings (throughout dermis): detect temperature, pain, and itch

Hypodermis and Dermatologic Disorders

The hypodermis (also called the subcutaneous layer) is the deepest layer of the skin. It's composed of loose connective tissue and adipose tissue (fat cells) and serves three main purposes: insulation (retaining body heat), energy storage (as triglycerides), and cushioning for underlying muscles and bones.

Disorders affecting the hypodermis can change skin appearance and function:

• Cellulite: Dimpling of the skin caused by uneven fat distribution in the hypodermis, more common in women.
• Lipodystrophy: Abnormal distribution or loss of body fat. This can result from certain medications (such as antiretroviral drugs) or medical conditions (insulin resistance, autoimmune disorders). Nurses should monitor injection sites for signs of lipodystrophy in patients on long-term subcutaneous injections.

Impact on drug absorption and distribution is where this anatomy becomes directly relevant to pharmacology:

1. The hypodermis acts as a reservoir for lipophilic drugs (drugs that dissolve in fat), which can affect how quickly they reach systemic circulation.

2. Drugs given via subcutaneous injection are absorbed through blood vessels in the hypodermis. This route provides slower absorption compared to intramuscular or intravenous routes.

3. Several factors influence subcutaneous drug absorption:

   • Blood flow (affected by exercise and temperature)
   • Body fat composition (varies between individuals)
   • Injection site (abdomen, thigh, and upper arm all have different absorption rates)

4. Certain medications are specifically designed for subcutaneous administration to take advantage of sustained release or localized action. Common examples include insulin, heparin, and growth hormone.