Thermoregulation in the Human Body

Thermoregulation

• Thermoregulation is a homeostatic process that maintains the body's core temperature at a constant 37°C (98°F) despite changes in external temperatures.
• Core temperature remains constant even when exposed to extreme temperatures, ranging from 55°F to 130°F.
• Skin temperature refers to the skin's ability to lose heat to the surroundings.

Normal Core Temperature

• Normal core temperature ranges from less than 36°C to greater than 37.5°C.
• The average normal core temperature is between 98°F and 98.6°F orally, and 1°F higher when measured rectally.
• Core temperature can increase during exercise and vary depending on surrounding temperatures.
• It can rise to 101°F to 104°F when extremely active, and drop to 96°F when cold.

Heat Production

• Heat is a by-product of metabolism, influenced by the metabolic rate of the body.
• Factors that affect heat production include:
  • Basal metabolic rate
  • Muscle activity (shivering)
  • Thyroxine
  • Epinephrine and norepinephrine
  • Sympathetic stimulation
  • Chemical activity
  • Digestion and absorption

Heat Loss

• Heat loss occurs through the transfer of heat from organs and tissues to the skin, and then to the air.
• The rate of heat loss is influenced by the speed of heat transfer to the skin and the surrounding air.
• The insulator system of the body, comprising subcutaneous tissues, acts as a heat insulator, conducting heat ⅓ as readily as other tissues.
• Blood flow to the skin from the body core provides an effective means of heat transfer.

Control of Heat Conduction to the Skin

• The sympathetic nervous system controls heat conduction to the skin by regulating blood flow through arterioles and arteriovenous anastomoses.
• Vasoconstriction and vasodilation play a crucial role in this process.

Basic Physics of Heat Loss

• Heat loss from the skin surface occurs through three mechanisms:
  • Radiation: when the body temperature is higher than surrounding objects, heat is radiated towards the surroundings.
  • Conduction: direct contact between the skin and an object results in heat loss (about 3% of total heat loss).
  • Evaporation: 0.58 calories of heat are lost for each gram of water evaporated.

Cooling Effect of Air and Water

• Air movement increases heat loss, with low velocity air resulting in a cooling effect proportional to the square root of wind velocity.
• Conduction and convection of heat from a person suspended in water are significantly more effective than in air, resulting in faster cooling.

Conductive Heat Loss

• Conductive heat loss occurs through direct contact with an object, accounting for approximately 3% of body heat loss.
• Molecules of skin undergo vibratory motion, facilitating heat transfer.

Convective Heat Loss

• Convective heat loss occurs when there is equal heat in the air and body, resulting in air convection.
• When a person is sitting, 15% of body heat loss is due to convective heat loss, assuming no gross air movement.
• Air movement enhances convective heat loss, with increased cooling effects at higher wind velocities.

Cooling Effect of Wind

• Wind replaces air constantly, increasing heat loss, with the cooling effect proportional to the square root of wind velocity.
• Faster wind currents result in more efficient cooling.

Conduction and Convection in Water

• Heat conduction and convection in water are approximately 1,000 times more efficient than in air.
• When in water, the body cools more efficiently than in air.

Evaporation

• Evaporation results in heat loss of approximately 0.58 calories per gram of water evaporated.
• Even without sweating, evaporation occurs through breathing and skin, resulting in a loss of around 600-700ml of water per day, equivalent to 16-19 calories per hour.
• Evaporation is a necessary cooling mechanism at high air temperatures, as body heat increases when surroundings are hot.
• People without sweat glands are at risk of heat stroke due to their inability to cool themselves through evaporation.

Clothing and Heat Loss

• Clothing reduces conductive and convective heat loss, depressing the rate of heat loss to about 1/6th.
• Wet clothing makes it difficult to radiate heat out, further reducing heat loss.

Temperature-Decreasing Mechanisms When the Body Is Too Hot

• Vasodilation of skin blood vessels increases the rate of heat transfer to the skin, allowing the body to cool down
• Sweating is an effective way to remove heat, as a 1°C increase in body core temperature causes sweat to remove 10 times the basal rate of body heat production
• Decrease in heat production is achieved through the inhibition of shivering and chemical thermogenesis

Temperature-Increasing Mechanisms When the Body Is Too Cold

• Skin vasoconstriction occurs throughout the body, stimulated by the posterior hypothalamic sympathetic centers
• Piloerection (hair standing upright) helps to create a thick layer to protect the skin from cold surroundings
• Increase in thermogenesis (heat production) is achieved through:
  • Shivering, stimulated by the primary motor center in the posterior hypothalamus
  • Chemical thermogenesis (non-shivering thermogenesis), which increases the rate of cellular metabolism through sympathetic circulation, norepinephrine, and epinephrine
  • Thyroxine secretion, which is stimulated by the cooling of the anterior hypothalamic-preoptic area and the release of thyrotropin-releasing hormone from the hypothalamus

Body Temperature Control

• The set point for temperature control is 37.1°C, which the body continually attempts to maintain.
• The body temperature control mechanisms work to bring the body temperature back to this set point level.

Feedback Gain for Body Temperature Control

• Feedback gain is the ratio of the change in environmental temperature to the change in body core temperature minus 1.0.
• For every 25°- to 30°C- change in environmental temperature, the body core temperature changes about 1°C.

Skin Temperature and Body Temperature Control

• Skin temperature can slightly alter the set point for core temperature control.

Behavioral Control of Body Temperature

• The temperature controlling area in the brain gives a psychic sensation of being overheated.
• When the body is cold, it receives signals from skin and deep body receptors, resulting in a feeling of being cold.
• Behavioral control of body temperature is most effective in cold environments.

Fever

• Fever is an elevation of body temperature (> 99°F) caused by the "resetting of the thermostat" due to toxic substances or abnormalities in the brain's temperature-regulating centers
• Causes of fever include bacterial/viral infections, trauma, lesions of CNS, exposure to high temperatures, and drug-induced factors
• Fever is characterized by increased heat production (shivering, increased metabolism) and diminished heat loss (vasoconstriction, warm and flushed skin)
• Fever subsides through sweating

Thermoregulation

• Thermoreceptors detect changes in the balance between heat loss and production due to metabolic rate (exercise)
• There are two types of thermoreceptors: peripheral (on skin) and central (in hypothalamus, spinal cord, and abdominal organs)
• Output from the hypothalamus is sent to effectors via sympathetic nerves to sweat glands, skin arterioles, and adrenal medulla, and to motor neurons to skeletal muscles
• Core temperature is maintained relatively constantly
• Peripheral thermoreceptors help identify heat and cold

Sweating and Its Regulation

• Stimulation of the anterior hypothalamus-preoptic area in the brain causes sweating through the autonomic pathways to the skin
• Sweat glands are innervated by cholinergic nerve fibers (acetylcholine-secreting) that run in sympathetic nerves
• Sweat glands can also be stimulated by epinephrine or norepinephrine in the blood, particularly during exercise when these hormones are secreted by the adrenal medullae to help the body lose excessive heat

Sweat Gland Structure

• A sweat gland is a tubular structure consisting of two main parts: a deep subdermal coiled portion and a duct portion.
• The coiled portion is responsible for secreting sweat and is located deep in the subdermal layer.
• The duct portion passes outward through the dermis and epidermis of the skin.

Sweat Secretion Process

• The secretory portion of the sweat gland produces a fluid called primary secretion or precursor secretion.
• The primary secretion undergoes modification as it flows through the duct, resulting in changes to the concentrations of its constituents.

Regulation of Body Temperature

• The precise regulation of body temperature depends on various factors, including wind movement, air moisture, and surroundings.
• A nude person in dry air between 55°F and 130°F can maintain a normal body core temperature between 97°F and 100°F.

Role of the Hypothalamus

• The hypothalamus plays a crucial role in regulating body temperature, with almost all temperature-regulating mechanisms operating through it.
• The hypothalamus contains temperature-regulating centers that detect changes in body temperature.

Temperature Regulation Mechanisms

• When the hypothalamus detects abnormal body temperature, it institutes procedures to decrease or increase body temperature.
• These procedures are activated through neuronal effector mechanisms.

Neuronal Effector Mechanisms for Thermoregulation

• Hypothalamic temperature centers play a crucial role in detecting deviations in body temperature from the normal range.
• When the body temperature is too high or too low, the hypothalamic temperature centers trigger mechanisms to decrease or increase the body temperature.
• These mechanisms are activated to maintain a stable body temperature, which is essential for proper bodily functions.