The Structure and Vital Functions of Human Skin

Skin Vascularization: Blood Supply

The skin is nourished by blood vessels found in the dermis and hypodermis, which transport oxygen and other essential nutrients.

These vessels form two main networks (plexuses):

• Subpapillary (Superficial) Plexus: Located in the deep dermis.
• Subdermal Plexus: Located in the hypodermis.

Skin Innervation: Nerve Endings

The skin is one of the body's most richly innervated sensory organs. There are two main types of nerve endings in the skin:

1. Receiving (Sensory) Endings

   These perceive external environmental stimuli and transmit them to the brain. They allow us to appreciate the sensations of touch, cold, heat, and pain.

2. Effector Endings

   These innervate the blood vessels, perspiration glands, and arrector pili muscles (erector muscles).

Skin Appendages (Accessory Structures)

The skin includes several important accessory structures:

• Hair and nails
• Glands:
  • Sebaceous glands (produce sebum)
  • Sweat glands (Eccrine and Apocrine)

Essential Functions of the Skin

1. Protection and Barrier Functions

Preventing Water Loss (Dehydration)

The stratum corneum, due to its chemical composition of lipids and keratin, prevents the massive outflow of water, thereby protecting the body from dehydration.

Barrier Against Substance Penetration

The skin hinders the passage of substances into the body. However, this barrier is not absolute, allowing small amounts of cosmetics and drugs to penetrate the skin.

Protection from Physical Trauma

The skin confers great elasticity, flexibility, and endurance. It is able to absorb friction and tension without tearing.

Defense Against Solar Radiation

Melanin formation by melanocytes is stimulated by solar radiation. Melanocytes secrete this pigment, which is deposited on basal keratinocytes, forming a protective barrier that prevents radiation from passing into the dermis.

Furthermore, the action of sunlight stimulates basal keratinocytes to actively divide, leading to the thickening of the stratum corneum, which acts as an additional radiation filter.

Protection Against Pathogens

The surface of our skin maintains an acidic pH of approximately 5.5 (the acid mantle), which hinders the development of pathogenic microorganisms.

2. Thermoregulation and Homeostasis

The skin helps maintain a constant body temperature (homeostasis).

• When cold: Blood vessels in the dermis constrict (vasoconstriction), decreasing blood flow and reducing sweating.
• When warm: Blood vessels dilate (vasodilation), increasing blood flow, which facilitates increased eccrine sweating for cooling.

3. Secretory Functions

The substances produced by the skin are responsible for most of the protective mechanisms it develops. The skin produces four primary substances:

• Keratin
• Melanin
• Sweat
• Sebum