The skin plays critical roles in thermoregulation, protection, sensation, metabolism, and most importantly, thermoregulation. This article explores the core physiological functions of the skin, the structure and role of sweat glands, and the intricate neural and endocrine mechanisms involved in thermoregulation, including disorders like hyperthermia, hypothermia, and fever.
Functions of the Skin
The skin is multifunctional, contributing to both systemic and localized physiology.
1. Protection
- Acts as a physical barrier against mechanical injuries, pathogens, UV radiation, and toxic substances.
- The stratum corneum, composed of keratinized cells, provides water resistance and structural integrity.
2. Sensory Reception
- Richly innervated with mechanoreceptors, thermoreceptors, and nociceptors.
- Enables tactile sensation, temperature perception, and pain detection.
3. Thermoregulation
- Regulates heat through sweat production and vasodilation/vasoconstriction.
- Controlled by the hypothalamic thermoregulatory center.
4. Metabolism
- Participates in vitamin D synthesis upon exposure to UV-B light.
- Plays a role in immune surveillance through Langerhans cells.
5. Excretion
- Eliminates urea, ammonia, salts, and water through sweat.
6. Aesthetic and Social Role
- Skin color, texture, and condition significantly influence psychological and social interactions.
Sweat Glands: Types, Secretion Control, and Functions
Sweat glands are integral components of the skin’s thermoregulatory apparatus. There are two primary types:
1. Eccrine Glands
- Location: Found all over the body, especially palms, soles, and forehead.
- Structure: Simple coiled tubular glands located in the dermis.
- Secretion: Watery sweat composed of water, Na+, Cl⁻, urea, ammonia.
- Function: Key role in thermoregulation through evaporative cooling.
- Control: Regulated by the sympathetic nervous system using acetylcholine as the neurotransmitter.
2. Apocrine Glands
- Location: Axillae, genital region, perianal areas.
- Secretion: Milky, protein-rich, becomes odorous when degraded by bacteria.
- Function: Limited thermoregulation; more involved in scent release and emotional sweating.
- Control: Sympathetically innervated, primarily adrenergic.
Thermoregulation
Thermoregulation is the process by which the human body maintains its core temperature around 37°C despite environmental changes. It involves multiple systems: neural sensors, the hypothalamus, effectors (skin, blood vessels, sweat glands, muscles), and hormones.
1. Thermoregulatory Center – The Hypothalamic Thermostat
The preoptic area of the anterior hypothalamus serves as the body’s thermostat.
- Warm receptors in the skin and hypothalamus activate heat-loss mechanisms.
- Cold receptors stimulate heat-conservation or heat-production mechanisms.
Thermoreceptors send afferent signals to the hypothalamus, which compares actual body temperature with the set point and initiates corrective actions through autonomic and behavioral responses.
2. Heat Dissipation Mechanisms
When body temperature rises:
a. Vasodilation
- Cutaneous blood vessels dilate, increasing blood flow to the skin for heat loss via radiation and convection.
b. Sweating
- Evaporation of sweat from the skin surface dissipates heat. Controlled by sympathetic cholinergic fibers.
c. Behavioral Adjustments
- Seeking shade, reducing physical activity, and wearing light clothing.
3. Heat Conservation Mechanisms
When the body is exposed to cold:
a. Vasoconstriction
- Decreases blood flow to the skin, preserving core temperature.
b. Piloerection
- Contraction of arrector pili muscles traps insulating air near the skin (ineffective in humans).
c. Shivering Thermogenesis
- Involuntary rhythmic contractions of skeletal muscles generate heat.
d. Non-shivering Thermogenesis
- In neonates and infants, brown adipose tissue is metabolized to produce heat under sympathetic stimulation.
4. Effects of Hypothalamic Stimulation
- Anterior hypothalamus: Stimulates heat loss (sweating, vasodilation).
- Posterior hypothalamus: Stimulates heat conservation and production (shivering, vasoconstriction).
Physiological Responses to Environmental Temperature Extremes
Exposure to Heat
- Increased skin blood flow.
- Profuse sweating and electrolyte loss.
- Risk of dehydration, cramps, or heat stroke.
Exposure to Cold
- Peripheral vasoconstriction.
- Shivering and increased metabolic rate.
- Risk of frostbite or hypothermia in extreme cases.
Disorders of Thermoregulation
Thermoregulatory failure can result from disease, drugs, or environmental extremes.
1. Hypothermia
- Core temperature <35°C.
- Causes: Prolonged cold exposure, hypothyroidism, sepsis.
- Symptoms: Bradycardia, confusion, shivering, coma.
- Management: Passive and active rewarming.
2. Hyperthermia
- Core temperature >40°C without a change in hypothalamic set point.
- Causes: Heat stroke, certain drugs (e.g., neuroleptics, anesthetics), exertion.
- Symptoms: Tachycardia, delirium, seizures.
- Management: Rapid cooling, fluid resuscitation, addressing underlying cause.
3. Fever
- Pyrogen-induced rise in hypothalamic set point via prostaglandin E2.
- Phases:
- Chill: Vasoconstriction and shivering.
- Plateau: Temperature matches new set point.
- Defervescence: Sweating and vasodilation reduce temperature.
- Treatment: Antipyretics like paracetamol, NSAIDs.
Conclusion
The skin and its associated structures like sweat glands are pivotal in maintaining homeostasis, especially thermal balance. Thermoregulation is a sophisticated process managed by neural, endocrine, and behavioral mechanisms, coordinated primarily by the hypothalamus. Understanding these principles is essential in clinical contexts ranging from environmental exposure to thermoregulatory disorders such as hyperthermia, hypothermia, and fever. Through this, medical students gain a holistic view of how physiology underpins the body’s survival under diverse external conditions.