Physiology of the Integumentary System

Exam Tip: Remember that the epidermis is avascular, meaning it has no blood vessels. All nutrients and oxygen must reach the surface cells via diffusion from the underlying dermal blood vessels.

Thermoregulation and the Blood Reservoir

The integumentary system is essential for maintaining a stable internal body temperature through the manipulation of blood flow and sweat production.

Heat Dissipation and Conservation

When the body needs to cool down, the dermal blood vessels undergo vasodilation. This increases blood flow to the surface, allowing heat to escape into the environment. Simultaneously, eccrine sweat glands release sweat; as this moisture evaporates from the skin, it carries heat away from the body.

Conversely, in cold environments, the body undergoes vasoconstriction. Dermal vessels narrow to divert blood toward the body's core, protecting vital organs. However, extreme exposure to cold can lead to necrosis (pathological cell death), such as in the case of frostbite.

The Blood Reservoir

The dermis is highly vascularized and serves as a blood reservoir, holding approximately 5% of the body's total blood supply at any given time.

Absorption and Excretion

The skin's unique chemical properties allow it to participate in the movement of substances into and out of the body.

• Absorption: Because the skin's barrier is hydrophobic (lipid-based), it repels water-based substances. However, it can absorb lipids and lipid-soluble substances, such as steroid hormones.
• Excretion: The skin assists the renal system by eliminating nitrogen-containing wastes, including ammonia, urea, and uric acid, through perspiration.

Perspiration occurs in two forms: insensible perspiration (unconscious water loss) and sensible perspiration (visible sweating). While sweating aids cooling, it also results in some salt loss.

Pigmentation and Vitamin Synthesis

The visual appearance of the skin is a result of various chromophores and metabolic processes.

Skin Color

Skin color is determined by the interaction of three main pigments:

1. Melanin: Provides brown and black tones.
2. Carotene: Provides orange tones.
3. Haemoglobin: Provides pink and red tones via the blood in the dermis.

Vitamin D Synthesis

The skin plays a vital role in endocrine health through vitamin D synthesis. When the skin is exposed to sunlight, a precursor called 7-dehydrocholesterol is converted into Vitamin D. This vitamin is crucial for the homeostatic maintenance of calcium and phosphates in the blood, which are necessary for bone health.

Accessory Structures: Glands and Hair

The integumentary system includes several accessory structures that reside primarily in the dermis.

Sweat Glands

There are two primary types of sweat glands:

1. Eccrine Glands: Distributed all over the body, these are primarily responsible for thermoregulation and secreting dermcidin.
2. Apocrine Glands: Found in the armpits and genital regions, these become active at puberty. They are thought to be involved in pheromone secretion, which may influence sexual attraction.

Hair and Sebaceous Glands

Hair consists of dead, keratinized cells. It provides protection (e.g., eyelashes protecting eyes) and aids in thermoregulation by trapping air via the erector pili muscle. Associated with hair follicles are sebaceous glands, which secrete sebum (a mix of lipids). Sebum lubricates the hair and skin and has bactericidal properties to inhibit bacterial growth.