Energy balance and body temperature

Sweat glands receive sympathetic fibers that release acetylcholine onto muscarinic receptor proteins: the sympathetic cholinergic exception.

• Parasympathetic fibers releasing acetylcholine:
• Correct: Sympathetic cholinergic fibers releasing acetylcholine:
• Sympathetic fibers releasing norepinephrine:

Fat cells release leptin roughly in proportion to how much fat they hold, so leptin signals the size of energy stores.

• That the stomach is empty:
• That blood glucose is falling:
• Correct: How large the body's fat stores are:

Fat supplies about 9 kcal per gram, so 10 g supplies about 90 kcal.

• About 40 kcal:
• Correct: About 90 kcal:
• About 10 kcal:

Oxygen per day: 200 mL/min × 1,440 min/day = 288,000 mL = 288 L. Energy: 288 L × 4.8 kcal/L = 1,382 kcal/day.

• Correct: About 1,380 kcal/day: Correct. 288 L × 4.8 kcal/L ≈ 1,380 kcal/day.
• About 960 kcal/day: 960 is 200 × 4.8, the kcal for 200 liters, not for a day's oxygen use.
• About 1,730 kcal/day: 1,730 kcal/day is the answer for 250 mL/min, not 200.
• About 13,800 kcal/day: This is ten times too large: 288,000 mL is 288 L, not 2,880 L.

Across the nervous, integumentary and cardiovascular systems: the heat load raises skin blood flow and sweating, but humidity blocks evaporation, so core temperature climbs. Sweat losses and blood held in the skin cut venous return and stroke volume, and heart rate rises to compensate. This is the setting for heat exhaustion and heat stroke.

• Skin blood flow: up. The hypothalamus drives more active vasodilation through sympathetic cholinergic nerves as core and skin temperature rise.
• Sweat rate: up. The same nerves drive the sweat glands harder in response to the greater heat load.
• Share of her sweat that evaporates: down. Humid air is close to saturated, so evaporation slows and much of the sweat drips off.
• Core temperature: up. Air near skin temperature blocks radiation and convection, and humid air limits evaporation, so heat loss falls short of heat production.
• Stroke volume: down. Sweat losses shrink plasma volume and more blood sits in the widened skin vessels, so less returns to fill the heart.
• Heart rate: up. With a smaller stroke volume, heart rate rises to keep cardiac output up for both muscles and skin.

A core temperature above 40 °C with confusion is heat stroke. Damage depends on how long the core stays that hot, so the priority is rapid cooling, best by immersing him in cold water, before transport if that can be done safely.

• Give a fever-reducing drug by mouth: His set point is normal; heat gain has outrun heat loss. Fever-reducing drugs lower a raised set point, so they do not help, and he cannot swallow safely while confused.
• Wrap him in a blanket until the ambulance arrives: A blanket would trap heat and let his temperature keep rising.
• Give salty fluids and let him rest seated: Fluids and rest suit heat exhaustion, where the brain works normally. His confusion marks heat stroke, and he should not drink while confused.
• Correct: Cool him rapidly, ideally by cold-water immersion: Correct. Rapid cooling is the treatment for heat stroke.

With less fat, fat cells release less leptin, so the hypothalamus drives hunger and saves energy. Ghrelin from the stomach also tends to rise after weight loss. Both push toward regaining the lost weight.

• Correct: Lower leptin and higher ghrelin: Correct. Falling leptin and rising ghrelin both raise hunger.
• Higher leptin and lower ghrelin: High leptin and low ghrelin would both reduce hunger.
• Higher insulin and higher leptin: Higher insulin and leptin would signal plenty and reduce hunger.
• Lower ghrelin and lower insulin: Lower ghrelin would reduce hunger, not raise it.

In the cold, sympathetic nerves release norepinephrine onto skin arterioles, narrowing them. Less warm blood reaches the surface, so the shell insulates the core.

• Cold thermoreceptors in the skin increase their firing: This step is right. Skin sensors report the cold first, before the core cools.
• Sensory signals reach the preoptic area of the hypothalamus: This step is right. The preoptic area is the temperature control center.
• Correct: Sympathetic nerves widen the arterioles of the skin: This is the error. Skin arterioles narrow in the cold; they widen in the heat.
• Skeletal muscles begin to shiver: This step is right. Shivering raises heat production.
• Heat loss falls and heat production rises: This step is right. Both effects oppose the fall in temperature: negative feedback.

At a core temperature near or below 30 °C, the heart muscle is cold and electrically unstable. Jostling can trigger ventricular fibrillation. Rescuers keep him horizontal, handle him gently and rewarm him.

• Moving him would restart shivering and exhaust him: Shivering stops in moderate hypothermia because heat production fails, and it would help rather than harm.
• Movement makes his skin vessels constrict further: Skin vessel tone is not the danger here.
• Correct: His cold heart can fibrillate if handled roughly: Correct. A cold heart is prone to fatal rhythms.
• His bones are brittle from the cold: Cold does not make bones brittle at these temperatures.

Thyroid hormones raise the number of sodium–potassium pumps and mitochondria in cells, so they use more ATP and oxygen. Her BMR rises, she spends more energy than she eats, and the extra energy is released as heat.

• Correct: Her raised BMR makes her spend more energy, released as heat: Correct. A raised BMR explains both weight loss and feeling hot.
• Her appetite hormones have stopped working properly: Her hunger is rising in response to greater spending, which shows appetite regulation is working.
• She is absorbing much less of her food from the gut: The weight loss comes from greater spending, not poor absorption.
• Her brown fat has been replaced by ordinary white fat: Hyperthyroidism raises heat production; losing brown fat would lower it.