Skin: Color & Functions

• Skin is the largest organ, its thickness varies from 1 to 2 mm on average.
• It is thickest on the sole of the foot, palm of the hand and in the interscapular region (about 5 mm).
• It is thinnest over eyelids and penis (about 0.5 mm).

Color of Skin

• Skin color depends on pigmentation and hemoglobin in the blood.
• Melanin, a brown pigment, is responsible for skin color and is synthesized by melanocytes in the epidermis.
• Melanin is derived from tyrosine through the action of tyrosinase, forming DOPA and then melanin.
• Melanin deficiency leads to albinism.
• Hemoglobin in cutaneous blood vessels affects skin coloration.
  • Skin becomes pale when hemoglobin decreases.
  • Skin becomes pink when blood rushes due to vasodilation (blushing).
  • Skin becomes bluish during cyanosis (excess reduced hemoglobin).

Functions of Skin

• The skin has protective, sensory, storage, synthetic, regulatory, excretory, and absorptive functions
• These include: protecting against bacteria and toxins via lysozyme, cytokines, and antimicrobial peptides.
• Protection from mechanical blows.
• Protection from ultraviolet rays.
• Melanin and the epidermis absorb ultraviolet rays.
• It is the largest sense organ in the body.
• It has many nerve endings forming cutaneous receptors.
• It stores fat, water, chloride, sugar, and blood.
• It synthesizes vitamin D3 with sunlight on cholesterol.
• It regulates body temperature through radiation, conduction, convection, evaporation, and sweat secretion.
• Lipid content of sebum prevents heat loss in cold environments.
• Regulation of water and electrolyte balance occurs by excreting water and salts through sweat.
• Excretion of waste materials like urea, salts, and fatty substances takes place through the skin.
• Absorption of fat-soluble substances and ointments occurs through the skin.

Secretory Function

• The skin secretes sweat through sweat glands and sebum through sebaceous glands.
• Sweat regulates body temperature and water balance and sebum keeps skin smooth.

Glands of Skin

• The skin contains sebaceous and sweat glands.
• Sebaceous glands secrete sebum, containing free fatty acids, triglycerides, sterols, waxes, and paraffin.
• Sebum's free fatty acids have antibacterial and antifungal effects, preventing skin infections.
• The lipid nature of sebum keeps skin smooth and oily, protecting it from desquamation and dryness.
• It also prevents heat loss from the body, especially useful in cold climates.
• Sebaceous glands activate at puberty due to sex hormones, potentially leading to acne.
• Eccrine and apocrine glands are the two types of sweat glands.

Eccrine vs Apocrine Sweat Glands

• Sweat is practically odorless and odor from sweat is produced by bacteria.
• Apocrine glands are scent glands that enlarge at sexual maturity and change with the menstrual cycle.
• Apocrine sweat is considered attractive in some cultures.
• Clothing traps stale sweat, which leads to bacterial degradation, and creates body odor(bromhidrosis) which may indicate metabolic disorder or poor hygiene.
• Apoeccrine glands have characteristics of both apocrine and eccrine glands.
• They are larger than eccrine glands but smaller than apocrine and have ducts opening onto the skin surface.
• They are sensitive to cholinergic activity and secrete more sweat than eccrine and apocrine glands.
• Pheromones are secreted by apocrine glands that cause behavioral or physiological changes in others.
• Pheromones influence the behavior and reproductive cycle in mammals.

Pheromones

• Phermones are in urine and vaginal fluid.
• Pheromones influence behavioral and reproductive cycles.
• They stimulate receptors in the vomeronasal organ, transmitting impulses to the hypothalamus and influence the menstrual cycle via pituitary hormonal axis.
• Presence of women stimulates men's beards to grow faster.
• Presence of men influences female ovulation.
• Vaginal secretions contain copulines, which raise men's testosterone levels.

Body Temperature

• Core temperature is constant ± 0.6°C.
• The internal body temperature is regulated in the head and trunk organs.
• Surface temperature fluctuates with the surrounding temperature.
• Oral temperature ranges from 36.6 to 37.2°C.
• Core temperature is about 37.8°C (100°F).
• Febrile temperature is above 37.2°C, hyperpyrexia is above 41.6°C.
• Subnormal temperature is below 36.6°C and hypothermia is below 35°C.
• Rectal temperature estimates core temperature, usually 37.2° to 37.6°C approximately 0.6°C higher than oral temperature.

Physiological Variations in Body Temperature

• In infants, body temperature varies with the environment, due to improper regulation during infancy.
• Children have slightly higher (0.5°C) temperatures than adults due to physical activity.
• Elderly people have slightly lower temperatures due to less heat production.
• Females have lower body temperatures due to low basal metabolic rate, decreasing slightly further during menstruation.
• Diurnal variation: temperature is lowest in early morning and highest in the afternoon.
• After meals: body temperature rises slightly (0.5°C) after meals.
• During exercise: temperature rises due to heat production in muscles.
• During sleep: body temperature decreases by 0.5°C.
• During emotional events: body temperature increases.
• Menstrual cycle: temperature rises (0.5°C to 1°C) after ovulation and decreases (0.5°C) during menstruation.
• Female sex hormone reduction causes hot flashes in postmenopausal women.
• Skin, subcutaneous tissues, and fat insulate the body, with better insulation in women.

Sources of Body Heat

• Heat balance occurs when heat production equals heat loss.
• Imbalance in heat production and loss affects body heat and temperature.
• Metabolic activities: major portion of heat produced in the body relates to food metabolism and is know as heat of metabolism.
• Fat metabolism produces more heat (9 calories/L O2) than carbohydrate (4.7 calories/L O2) or protein (4.5 calories/L O2) metabolism.
• Liver is the organ where maximum amount of amount of heat is produced due to metabolic activities.
• Muscular activity: Heat is produced in muscle during rest (muscle tone) and activities (heat of activity), about 80% by skeletal muscles.
• Hormones: Thyroxin, sympathetic stimulation, and adrenaline increase heat production.
• Hot food and drinks and radiation from the Environment: Body gains heat through radiation occur when the surrounding temperature is higher.
• Shivering refers to shaking of the body caused by rapid involuntary contraction or twitching of the muscles as during exposure to cold.
• Brown fat tissue produces enormous body heat.

Mechanisms of Heat Loss

• Heat travels away as heat waves or rays (radiation).
• Heat transfer is promoted by movement of a cooler medium (convection).
• Heat is used to change a liquid (such as sweat) to a vapor (evaporation).
• Heat is transferred to a cooler object (conduction).
• Increasing fan speed and environmental humidity impact heat loss.

Methods of Heat Loss

• Body heat flows beneath the skin through venous plexus supplied by skin capillaries and arteriovenous anastomoses, prominent in nose, lips, ears, toes, and fingers.
• Blood flow rate into the venous plexus varies greatly depending on the constriction of arterioles.

Thermoregulation

• Vasoconstriction is controlled by the sympathetic adrenergic nervous system.
• Vasodilation is achieved by the inhibition of the sympathetic adrenergic nervous system, sympathetic cholinergic nerve and local vasodilators.
• Blushing is controlled by sympathetic discharge of sympathetic cholinergic fibers to sweat glands.
• Eccrine glands are innervated by cholinergic sympathetic fibers.
• Sweating and cutaneous blood flow are inhibited by atropine and autonomic nervous system diseases.
• Tissue temperature affects cell protein denaturation and biological functions.
• Sweating which visible wetness of the skin occurs, is called diaphoresis
• Human skin has 2-3 million sweat glands, most concentrated in the forehead, scalp, armpits, palms, and soles.
• The preoptic area in the anterior hypothalamus stimulates sweating.
• Impulses are transmitted through sympathetic outflow to the skin.
• Epinephrine or norepinephrine can stimulate glands, too.
• Sweat glands on the palms and soles are activated by emotional stimuli, while axillary sweating is stimulated by both thermoregulatory changes as well as emotional stimuli.

Sweat Secretion

• Eccrine glands are innervated by cholinergic sympathetic fibers, sweating and the associated increase in cutaneous blood flow are inhibited by atropine and in diseases of the autonomic nervous system.
• Stimulation of the preoptic area in the anterior hypothalamus causes sweating.
• The impulses are transmitted in the autonomic pathways to the cord and thence through the sympathetic outflow to the skin everywhere in the body and innervate sweat gland by muscarinic cholinergic nerve fibers.
• The secretory portion of the sweat gland secretes primary secretion or precursor secretion, made of a protein plasma filtrate, controlled by cholinergic sympathetic nerve fibers.
• Fluid travels through the duct portion of the gland, where sodium and chloride are reabsorbed.
• Decreased reabsorption depends on sweating rate, lowering osmotic pressure and concentrating urea, lactic acid, and potassium ions.
• Heat acclimatization comes in two stages; profuse sweating upon exposure to the climate, but decreases as exposure continues, and decreased concentration chloride in the sweat
• Aldosterone occurs due to chloride depletion.

Brain Centers in Thermoregulation

• Temperature regulation occurs with nervous feedback mechanisms which are located in the hypothalamus (hypothalamic thermostat).
• Peripheral thermoreceptors (skin) and central thermoreceptors (hypothalamus, spinal cord, etc.) detect changes in body temperature.
• Skin contains both cold and warmth receptors.
• Deep body temperature receptors respond to core temperature.

Temperature-Decreasing Mechanisms

• Vasodilation and decreased peripheral resistance increases the rate of heat transfer by eightfold.
• Sweating causes an increase in evaporative heat loss when the body core temperature rises above 37°C.
• Reduction of heat production is achieved by inhibiting mechanisms like shivering.

Temperature-Increasing Mechanisms

• Skin vasoconstriction diminishes heat transfer.
• Piloerection (hairs standing on end) entraps an insulating layer of air.
• Increased heat production via shivering, epinephrine/norepinephrine release, and thyroxin secretion (chemical thermogenesis)
• Inhibition of sweating.
• Brown fat thermogenesis occurs via mitochondrial uncoupling.

Fever

• Fever resets the hypothalamic thermostat.
• Endogenous pyrogen (EP) is released from monocytes and macrophages and alters thermoreceptor firing.
• Prostaglandins, interleukins, interferons, and tumor necrosis factor mediate fever.
• Aspirin reduces fever by restoring normal thermoreceptor activity and inhibits prostaglandin synthesis.

Hyperthermia

• It elevates the body temperature when there is an imbalance between heat production and heat loss.
• Sustained exercise is the most common cause of hyperthermia in normal people.
• Exposure to excessive heat causes heat cramps, heat exhaustion, and heatstroke.
• Heat cramps are painful muscle spasms from excessive electrolyte loss.
• Heat exhaustion leads to hypotension, dizziness, vomiting, and fainting.
• Heatstroke exhibits higher core body temperatures and skin is flushed.
• Body becomes flushed and dry.
• Malignant hyperthermia is a fatal disorder associated with certain anesthetics and/or succinylcholine, leading to accelerated metabolism in skeletal muscle.
• Drugs cause a drastic and uncontrolled increase in skeletal muscle oxidative metabolism, eventually leading to collapse and death.

Hypothermia

• Hypothermia comes from exposure to cold weather or immersion.
• Positive feedback loops put the body at risk.
• A drop below 33°C (91°F) metabolic rate will fall so low that heat cannot keep up.
• Cardiac fibrillation may occur below 32°C (90°F).
• Alcohol accelerates heat loss by dilating cutaneous blood vessels.