Integumentary System
& Thermoregulation
Master how the skin protects, senses, and regulates temperature: the structures, feedback loops, and MCAT-tested mechanisms of the body's largest organ.
MCAT Biology
What Is the Integumentary System?
The integumentary system is the organ system that forms the external covering of the body. It includes the skin (the largest organ by surface area), hair, nails, and associated glands (sweat and sebaceous).
On the MCAT, this system is tested within Foundational Concept 3B: structure and integrative functions of the main organ systems. Expect questions on skin layers, gland function, and thermoregulatory feedback loops.
Three Layers of Skin
Key Cell Types in the Epidermis
Keratinocytes
Produce keratin, the tough structural protein. Make up ~90% of epidermal cells. Form waterproof barrier.
Melanocytes
Synthesize melanin pigment. Transferred to keratinocytes via melanosomes. Protect against UV radiation.
Langerhans Cells
Dendritic cells of the immune system. Antigen-presenting cells that detect and phagocytose foreign pathogens.
Merkel Cells
Associated with sensory nerve endings. Detect light touch and pressure (mechanoreceptors).
Core Functions of the Skin
1
Protection: Physical barrier against pathogens, chemicals, and UV radiation. Acid mantle (pH ~5.5) inhibits bacterial growth.
2
Thermoregulation: Sweat glands cool via evaporation; dermal blood vessels vasodilate or vasoconstrict to modulate heat loss.
3
Sensation: Thermoreceptors, nociceptors, and mechanoreceptors detect temperature, pain, pressure, and touch.
4
Vitamin D Synthesis: UV-B converts 7-dehydrocholesterol in the skin to cholecalciferol (\( \text{vitamin D}_3 \)).
5
Excretion: Sweat glands eliminate small amounts of urea, salts, and water.
Thermoregulation: Negative Feedback
Four Mechanisms of Heat Loss
Radiation
Emission of infrared heat waves from the body to cooler surroundings. Accounts for ~60% of heat loss at rest.
Conduction
Direct transfer of heat through physical contact with a cooler surface (e.g., sitting on a cold bench).
Convection
Movement of air or water currents across the skin that carry heat away. Wind chill amplifies this effect.
Evaporation
Sweat vaporizes from the skin surface, using body heat as the energy source for the phase change (\( \Delta H_{\text{vap}} \)).
Vasodilation vs. Vasoconstriction
| Vasodilation | Vasoconstriction | |
|---|---|---|
| Trigger | Body temp above set point | Body temp below set point |
| Blood Vessels | Arterioles widen | Arterioles narrow |
| Blood Flow to Skin | Increased | Decreased |
| Skin Appearance | Flushed / red | Pale / bluish |
| Net Effect | Heat radiates out | Heat retained in core |
| Accompanies | Sweating | Shivering, piloerection |
Eccrine vs. Apocrine Sweat Glands
Eccrine Glands
Found over most body surfaces (especially palms, soles, forehead). Produce dilute, watery sweat (mostly NaCl + urea). Open directly onto skin surface. Primary role: thermoregulation.
Apocrine Glands
Located in axillae and groin. Produce thicker, lipid-rich secretion into hair follicles. Activated at puberty. Odorless until bacteria metabolize the secretion. Stimulated by stress and hormones.
How to Solve Thermoregulation Questions
1
Identify the stimulus: Is body temperature rising or falling relative to the 37 °C set point?
2
Name the receptor: Peripheral or central thermoreceptors detect the deviation.
3
Identify the control center: The hypothalamus compares input to the set point and initiates a response.
4
State the effectors: Sweat glands, dermal blood vessels (smooth muscle), skeletal muscle (shivering), or arrector pili muscles.
5
Confirm negative feedback: Does the response oppose the original change? If yes, this restores homeostasis.
Example 1: Exercise in Hot Weather
Scenario: A runner exercises in 35 °C weather. Core temperature rises to 39 °C. Trace the thermoregulatory response.
1
Stimulus: Core temperature rises above the 37 °C set point due to metabolic heat from muscle contraction.
2
Receptor: Central and peripheral thermoreceptors detect the increase.
3
Control center: Hypothalamus activates cooling mechanisms.
4
Effectors: Vasodilation of dermal arterioles + eccrine sweat glands secrete sweat.
5
Result: Evaporative and radiative heat loss → core temperature decreases toward 37 °C (negative feedback).
Example 2: Cold Exposure
Scenario: A hiker is stranded in 0 °C conditions. Core temperature drops to 35 °C. What responses occur?
Vasoconstriction
Dermal arterioles constrict, shunting blood to the core and reducing heat loss from the skin surface.
Shivering
Involuntary skeletal muscle contractions generate heat via ATP hydrolysis without performing useful work.
Piloerection
Arrector pili muscles contract, raising body hair. In humans this is vestigial but traps air in furry mammals.
Example 3: Fever and the Set Point
Scenario: During bacterial infection, pyrogens are released. How does the body's thermostat change?
Skin as an Immune Barrier
Physical Barriers
Keratinized stratified squamous epithelium resists abrasion. Tight junctions between cells prevent pathogen entry. Constant desquamation sheds attached microbes.
Chemical Barriers
Acid mantle (pH ~5.5) from sebum and sweat. Defensins (antimicrobial peptides) kill bacteria. Sebum contains fatty acids toxic to many pathogens.
Common Mistakes
✕ Confusing Fever with Hyperthermia
Fever involves a raised set point (pyrogen-mediated). Hyperthermia is uncontrolled temperature rise with a normal set point (e.g., heat stroke).
✕ Eccrine = Adrenergic
Eccrine glands are innervated by sympathetic cholinergic fibers (ACh), not norepinephrine. This is a classic MCAT trap.
✕ Epidermis Has Blood Vessels
The epidermis is avascular. All blood supply is in the dermis. Nutrients reach the epidermis by diffusion.
Summary Table
| Feature | Epidermis | Dermis | Hypodermis |
|---|---|---|---|
| Tissue Type | Stratified squamous epithelium | Dense connective tissue | Adipose + loose connective |
| Blood Supply | None (avascular) | Rich vascular plexus | Present |
| Key Cells | Keratinocytes, melanocytes | Fibroblasts, macrophages | Adipocytes |
| Role in Thermoreg. | Barrier, sweat pore exit | Blood vessels, sweat glands | Insulation (fat layer) |
| Key Proteins | Keratin, melanin | Collagen, elastin | Lipids |
Question 1 of 5
Which layer of the skin is avascular and relies entirely on diffusion for nutrient delivery?
The epidermis contains no blood vessels. All oxygen and nutrients reach it by diffusing from the dermal capillary plexus below.
Question 2 of 5
During a fever, the hypothalamic set point is elevated. What happens initially when the set point rises from 37 °C to 39 °C?
When the set point rises, current body temperature (37 °C) is now below the target. The body responds as if it is cold: vasoconstriction + shivering to generate heat until the new set point is reached.
Question 3 of 5
Eccrine sweat glands are innervated by the sympathetic nervous system but release which neurotransmitter at the neuroeffector junction?
This is a key exception: sympathetic postganglionic fibers to eccrine glands are cholinergic, releasing ACh instead of the typical norepinephrine. This is frequently tested on the MCAT.
Question 4 of 5
When ambient temperature exceeds body temperature, which is the only effective mechanism for heat loss?
Radiation, conduction, and convection all require a temperature gradient from body to environment. When the environment is hotter, heat flows into the body via those routes. Only evaporation can still remove heat because it depends on the phase change of water, not a temperature gradient.
Question 5 of 5
A patient presents with uncontrolled body temperature rise, dry skin (no sweating), and a normal hypothalamic set point. This condition is best described as:
Hyperthermia occurs when heat gain exceeds the body's ability to dissipate it, but the hypothalamic set point remains at 37 °C. In fever, pyrogens raise the set point itself. The key distinction: fever = elevated set point; hyperthermia = overwhelmed cooling with a normal set point.