Integumentary System and Thermoregulation (3B) - MCAT Biological and Biochemical Foundations of Living Systems

Epidermis, dermis, hypodermis (subcutaneous tissue). These layers form the skin's structure, with the epidermis as the protective outer barrier, dermis providing support and elasticity, and hypodermis offering insulation and anchorage.

Keratinized stratified squamous epithelium. This epithelial type provides a tough, waterproof barrier due to multiple layers of flattened cells filled with keratin, ideal for protection against environmental stressors.

Limits water loss and blocks pathogen/chemical entry. As the outermost layer of dead, keratinized cells, it forms an impermeable shield that prevents dehydration and invasion by external threats.

Stratum basale (germinativum). This deepest layer houses mitotic stem cells that continuously generate new keratinocytes to replace those shed from the surface.

Stratum lucidum. Present in areas of high friction, this translucent layer of dead cells adds extra protection and thickness to withstand mechanical stress.

Keratin. This fibrous protein forms intermediate filaments in keratinocytes, conferring mechanical strength and water resistance to the epidermis.

Produce melanin to protect against ultraviolet radiation. These cells synthesize pigment that absorbs harmful UV rays, shielding DNA in keratinocytes from mutations and damage.

Langerhans cells (dendritic cells). Derived from bone marrow, these immune cells capture skin antigens and migrate to lymph nodes to activate T-cell responses.

Merkel cells. Associated with sensory nerve endings, these cells transduce mechanical stimuli into neural signals for tactile discrimination.

Dense irregular connective tissue. With collagen and elastin fibers oriented randomly, this tissue resists multidirectional stresses while providing flexibility and strength.

Reticular dermis. As the deeper, denser layer, it supports skin appendages and vasculature crucial for nutrient delivery and sensory functions.

Adipose and areolar CT; insulation, cushioning, energy storage. These loose connective tissues store fat for thermal regulation, absorb shocks, and serve as an energy reserve.

Sebaceous gland; lubricates and provides antimicrobial barrier. Associated with hair follicles, it secretes oily sebum that moisturizes skin and inhibits microbial growth via fatty acids.

Eccrine sweat gland. Widely distributed, these glands produce watery sweat that evaporates to dissipate heat and maintain core temperature.

Apocrine sweat gland. Located in axillary and anogenital regions, these glands secrete viscous fluid influenced by hormones, contributing to scent signaling.

Sympathetic nervous system (cholinergic fibers). These fibers release acetylcholine to stimulate sweat secretion, enabling rapid cooling responses to heat or stress.

Hair standing; contraction of arrector pili muscle. Sympathetic activation causes this smooth muscle to contract, erecting hairs to trap insulating air in cold conditions.

Anterior hypothalamus (preoptic area). This region senses elevated core temperature and initiates sweating and vasodilation to promote heat loss.

Posterior hypothalamus. This area detects hypothermia and activates shivering and vasoconstriction to generate and conserve body heat.

Evaporation of sweat. As sweat vaporizes, it absorbs latent heat from the skin, providing efficient cooling when other mechanisms are insufficient.

Decreases heat loss by reducing skin blood flow. By narrowing vessels, it limits warm blood exposure to cold skin, minimizing heat dissipation to maintain core warmth.

Convection. This process involves heat transfer from the body to surrounding fluid in motion, enhanced by wind or water flow.

Conduction. This direct transfer occurs when the skin touches a cooler object, allowing heat to flow down the temperature gradient.

$BMI = \frac{m}{h^2}$. Using mass in kilograms and height in meters squared, it quantifies adiposity for assessing health risks.