The Integumentary System: Functions and Aging

The Integumentary System

Overview

The integumentary system, primarily composed of the skin, serves as the body's first line of defense and plays a vital role in protection, sensation, temperature regulation, and synthesis of essential molecules. This study guide covers the major functions of the skin, factors influencing skin color, and the effects of aging on skin structure and function.

Skin Color

Factors Contributing to Skin Color

Skin color is determined by several pigments and physiological factors. These include:

• Melanin: A pigment produced by melanocytes, responsible for yellow, tan, reddish brown, brown, and black hues. Melanin accumulates in areas such as freckles and moles and provides protection against ultraviolet (UV) radiation. Genetic differences affect the type and amount of melanin produced, not the number of melanocytes.

• Carotene: An orange pigment that accumulates in the stratum corneum, dermis, and hypodermis. Carotene is used to synthesize Vitamin A, which is essential for vision.

• Hemoglobin: The oxygen-carrying protein in red blood cells (RBCs). Hemoglobin imparts a reddish color to the skin, which varies with blood flow and oxygenation. Increased blood flow (e.g., during exercise) makes the skin appear more red.

• Bilirubin: A yellow pigment resulting from the breakdown of hemoglobin during RBC death. Elevated bilirubin levels, often due to liver disease (e.g., jaundice), cause yellowing of the skin.

Clinical Significance: Changes in skin color can indicate underlying health conditions, such as cyanosis (bluish skin due to low oxygen), pallor (paleness due to reduced blood flow), or jaundice (yellowing due to liver dysfunction).

Functions of the Skin

Chemical and Mechanical Protection

The skin provides both physical and chemical barriers to protect the body:

• Skin Continuity: Closely-packed cells, tight junctions, and desmosomes form a multi-layered barrier that prevents pathogen entry.

• Hardness: The protein keratin gives the skin its toughness and resistance to abrasion.

• Prevention of Water Loss and Entry: Glycolipids in the skin limit water loss and block external water entry.

• Melanin: Protects against UV radiation damage.

• Secretions: Sweat and sebum create an acidic environment (the acid mantle), which is bactericidal and inhibits microbial growth.

Excretion

• Sweat: Removes water, salts, and nitrogenous wastes from the body.

Immunity

• Dendritic cells (epidermis) and macrophages/leukocytes (dermis) provide immune surveillance and defense against pathogens.

Temperature Regulation

The skin regulates body temperature through blood flow and sweat production:

• Insensible perspiration: Continuous, unnoticed water loss (~0.5 L/day).

• Sensible perspiration: Noticeable sweating, which can reach several liters per day.

• Blood flow adjustments:

  • Vasodilation: Blood vessels widen, increasing heat loss across the skin (cooling effect).

  • Vasoconstriction: Blood vessels narrow, decreasing heat loss (warming effect).

• Sweating adjustments: Increased sweat gland activity promotes evaporative cooling.

These processes are regulated by the hypothalamus in the brain.

Blood Vessel Response Table

Response	Blood Vessel Action	Effect on Heat Loss
Cooling	Vasodilation	Increases heat loss
Warming	Vasoconstriction	Decreases heat loss

Vitamin D Synthesis

The skin is essential for the synthesis of Vitamin D:

• Precursor: 7-dehydrocholesterol in the skin is converted to cholecalciferol (Vitamin D3) upon UV exposure.

• Activation: Cholecalciferol is further modified by the liver and kidneys to form calcitriol, the active form of Vitamin D.

• Function: Calcitriol is crucial for calcium absorption in the digestive tract and for maintaining stable blood calcium levels.

Equation:

$\text{7-dehydrocholesterol} \xrightarrow{UV} \text{cholecalciferol (Vit. D_3)} \xrightarrow{liver, kidneys} \text{calcitriol}$

Tactile Sensations

The skin contains numerous sensory receptors that detect various stimuli:

• Tactile (Merkel) discs: Detect light touch.

• Free nerve endings: Sense pain, temperature, and crude touch.

• Tactile (Meissner) corpuscles: Detect fine touch and texture.

• Lamellated (Pacinian) corpuscles: Sense deep pressure and vibration.

• Bulbous (Ruffini) corpuscles: Detect skin stretch and sustained pressure.

• End bulbs: Respond to light touch.

• Hair follicle receptors (hair root plexus): Detect hair movement.

These receptors allow the skin to sense touch, pain, hot, cold, pressure, and vibration.

Aging and the Skin

Major Changes with Age

Aging leads to several structural and functional changes in the skin:

• Fibroblast activity decreases: Reduced production of collagen and elastic fibers, leading to a thinner dermis and increased wrinkling.

• Gland atrophy: Sweat and sebaceous glands become less active, causing dry, itchy skin and impaired temperature regulation.

• Melanocyte activity decreases: Skin becomes paler and less protected from UV radiation; hair may turn gray or white due to reduced pigment.

• Vitamin D production decreases: May result in lower bone density and muscle weakness.

• Blood flow to skin decreases: Skin feels colder and heals more slowly.

• Reduced immune cells: Fewer melanocytes and dendritic cells increase the risk of damage, infection, and cancer.

• Subcutaneous fat decreases: Less insulation, leading to increased sensitivity to cold.

• Hair changes: Hair becomes thinner, more brittle, and less pigmented; hair follicles become less active.

Clinical Implications: Aging skin is more prone to injury, infection, and slower healing.

Review Questions

• Why is skin referred to as the acid mantle?

• Why is sunlight important to bone health?

• Which alteration in skin color may indicate a liver disorder?

• What effect would decreased oxygenation have on skin color? Why does your skin appear more red when you exercise?