24.6 Energy and Heat Balance

Thermoregulation and Heat Balance

Your body constantly produces heat through metabolic reactions, and it needs to balance that heat production with heat loss to keep your core temperature stable. This balance is critical because enzymes and cellular processes only function properly within a narrow temperature range. The hypothalamus orchestrates this entire system, acting as the body's thermostat.

Body Temperature Maintenance

The hypothalamus controls thermoregulation through two key regions:

• The preoptic area of the anterior hypothalamus contains temperature-sensitive neurons that detect changes in blood temperature.
• The posterior hypothalamus integrates temperature information from various body regions and initiates the appropriate response.

Together, these regions maintain a stable core temperature around 37°C (98.6°F) through three categories of response:

Heat production (thermogenesis) raises body temperature:

• Shivering thermogenesis uses involuntary, rapid skeletal muscle contractions to generate heat. This is why your teeth chatter and your muscles tense up in the cold.
• Non-shivering thermogenesis occurs primarily in brown adipose tissue (BAT), which metabolizes fatty acids specifically to produce heat rather than ATP. This is especially important in infants, who have more BAT than adults, and in cold-adapted individuals.

Heat loss mechanisms lower body temperature:

• Vasodilation of cutaneous blood vessels increases blood flow to the skin surface, allowing more heat to radiate and convect away from the body. This is why your skin looks flushed when you're overheated.
• Sweating promotes heat loss through evaporation. As sweat changes from liquid to vapor on your skin, it absorbs a large amount of thermal energy.

Heat conservation mechanisms reduce heat loss:

• Vasoconstriction of cutaneous blood vessels decreases blood flow to the skin, minimizing heat loss to the environment. This is why your skin turns pale and your fingers feel cold in low temperatures.
• Behavioral responses include seeking warm environments, adding clothing layers, and curling up to reduce exposed surface area.

Heat Exchange Mechanisms

The body exchanges heat with the environment through four physical processes:

• Radiation transfers heat via electromagnetic (infrared) waves without direct contact. Your body radiates heat to cooler surrounding objects like walls and furniture, and absorbs radiant heat from warmer objects like the sun. The rate of transfer is proportional to the temperature difference between surfaces.
• Conduction transfers heat directly between objects in physical contact. Heat flows from the warmer object to the cooler one. Walking barefoot on a cold tile floor or holding an ice cube are both examples of conductive heat loss.
• Convection transfers heat through the movement of fluids or gases across the body surface. A breeze carries warm air away from your skin and replaces it with cooler air, which is why wind chill makes cold temperatures feel even colder. Swimming in cool water produces the same effect, but faster, because water conducts heat about 25 times more efficiently than air.
• Evaporation converts liquid sweat to water vapor, absorbing heat from the body in the process. This is the most effective heat loss mechanism during exercise or in hot environments, and it's the only mechanism that can cool you when the ambient temperature is higher than your body temperature.

Thermoregulation and Homeostasis

Thermoregulation is a classic example of a negative feedback loop maintaining homeostasis. Because mammals are endotherms, they generate internal heat through metabolism and maintain a relatively constant core temperature regardless of the external environment.

When thermoregulation fails, two dangerous conditions can result:

• Heat stress (hyperthermia) occurs when the body cannot dissipate excess heat fast enough. Core temperature rises above normal, and in severe cases (heat stroke), thermoregulatory mechanisms can fail entirely.
• Hypothermia results from excessive heat loss or inadequate heat production, dropping core temperature below 35°C. Metabolic reactions slow, and organ function becomes impaired.

Acclimatization refers to gradual physiological adaptations to new environmental conditions. For example, someone who moves to a hot climate will, over days to weeks, begin sweating earlier and more efficiently, and their sweat will contain less sodium. Similar adaptations occur with altitude and cold exposure.

Basal Metabolic Rate Factors

Basal metabolic rate (BMR) is the minimum energy expenditure needed to maintain vital functions (breathing, circulation, cell maintenance) while the body is at complete rest. BMR is measured under standardized conditions: after a 12-hour fast, at complete physical rest, in a thermoneutral environment (20–25°C / 68–77°F).

Several factors influence BMR:

• Body size and composition: Larger individuals generally have a higher total BMR because they have more tissue to maintain. More specifically, lean body mass (muscle) is far more metabolically active than adipose tissue, so a muscular person will have a higher BMR than someone of the same weight with more body fat.
• Age: BMR decreases with age, primarily because of declining muscle mass and shifts in hormonal levels (lower testosterone, lower estrogen after menopause). BMR drops roughly 1–2% per decade after early adulthood.
• Sex: Males generally have a 5–10% higher BMR than females of comparable size, largely due to greater muscle mass and the metabolic effects of testosterone.
• Thyroid hormones: Thyroxine ($T_4$) and triiodothyronine ($T_3$) are the primary hormonal regulators of BMR. They stimulate oxygen consumption and cellular metabolism throughout the body. Hyperthyroidism elevates BMR, while hypothyroidism lowers it.

Harris-Benedict Equations estimate BMR based on sex, weight, height, and age:

• Males: $BMR = 88.362 + (13.397 \times weight_{kg}) + (4.799 \times height_{cm}) - (5.677 \times age_{years})$
• Females: $BMR = 447.593 + (9.247 \times weight_{kg}) + (3.098 \times height_{cm}) - (4.330 \times age_{years})$

The result is in kilocalories per day (kcal/day). You can compare an individual's calculated BMR to population norms to assess whether their metabolic rate is high, low, or average. Significant deviations from expected BMR may point to underlying conditions like thyroid disorders or malnutrition, or they may reflect metabolic adaptations seen in trained athletes or chronic dieters.