Correct Answer: B. Increased blood flow to skin
When the hypothalamic thermostat is reset from point A to point C (a higher set point), the body initially perceives itself as being "cold" relative to the new set point, even though core body temperature has not actually changed. This is the critical discriminator: the question asks what happens in stage A (the transition phase immediately after reset) compared to stage B (the plateau phase when the new set point is reached).
In stage A, the hypothalamus drives heat-generating mechanisms (shivering, chemical thermogenesis) and heat-conserving mechanisms (vasoconstriction). However, as core temperature rises toward the new set point C, stage B begins. Once the body temperature approaches the new set point, the thermostat no longer perceives a deficit. At this point, heat-dissipating mechanisms activate: cutaneous vasodilation increases blood flow to the skin to radiate excess heat and prevent overshooting the new set point.
The key physiological principle is that increased blood flow to skin is a heat-dissipating response that occurs when body temperature approaches or exceeds the set point. In stage A (early phase), vasoconstriction dominates; in stage B (later phase), vasodilation dominates. This is the transition from heat generation/conservation to heat dissipation, making increased skin blood flow the hallmark of stage B relative to stage A.
Why the other options are wrong
A. Inhibition of chemical thermogenesis — This is backwards. In stage A (immediately after reset to a higher set point), the hypothalamus perceives a temperature deficit and activates chemical thermogenesis (non-shivering thermogenesis via brown adipose tissue and metabolic rate increase), not inhibits it. Inhibition occurs in stage B when the set point is approached. This option reverses the temporal sequence of thermoregulatory responses. C. Shivering — Shivering is a heat-generating mechanism that occurs in stage A when the body perceives cold relative to the new set point. However, the question asks what happens in stage A compared to stage B. Shivering is prominent in stage A but diminishes in stage B as temperature rises. The question structure implies a response that is more active in stage B, not stage A, making this a trap for students who identify stage A responses without comparing them to stage B. D. Sweating — Sweating is a heat-dissipating mechanism that occurs when body temperature exceeds the set point. In stage A, the body is still below the new set point C, so sweating does not occur. Sweating becomes prominent only in stage B or later if temperature overshoots. This option confuses the timing of thermoregulatory responses and is a classic NBE trap for students who know sweating is a dissipation response but misidentify when it activates.
High-Yield Facts
- Hypothalamic set point reset causes the body to perceive itself as cold (stage A) even though absolute temperature is unchanged, triggering heat-generating responses.
- Stage A (heat generation phase): vasoconstriction, shivering, increased metabolic rate, and chemical thermogenesis dominate to raise body temperature toward the new set point.
- Stage B (plateau phase): once body temperature approaches the new set point, vasodilation and increased skin blood flow activate to prevent overshoot and dissipate excess heat.
- Cutaneous vasodilation and increased skin blood flow are heat-dissipating mechanisms that become prominent as body temperature approaches or exceeds the set point.
- The temporal sequence of thermoregulation is critical: heat conservation/generation first (stage A), then heat dissipation (stage B) as the set point is approached.
Mnemonics
SET POINT RESET SEQUENCE COLD → HEAT → COOL: When set point rises, body perceives COLD (stage A: shivering, vasoconstriction, thermogenesis to HEAT up), then as temperature rises toward new set point, COOL mechanisms activate (vasodilation, sweating). Stage B is the COOL phase. VASO-RESPONSE RULE Below set point = Vasoconstriction (conserve heat); At/above set point = Vasodilation (dissipate heat). Stage A = below new set point = constriction; Stage B = approaching new set point = dilation.
NBE Trap
NBE pairs "shivering" and "chemical thermogenesis" with stage A (correct) but asks "compared to stage B," trapping students who identify stage A responses without recognizing that the question demands a stage B-predominant response. Increased skin blood flow is the only option that is more active in stage B than stage A.
Clinical Pearl
In clinical practice, when a patient develops fever (hypothalamic set point reset due to pyrogens), the initial phase (stage A) presents with chills, shivering, and vasoconstriction—patients feel cold despite rising temperature. As fever plateaus (stage B), vasodilation occurs and patients feel flushed with warm, dry skin. Understanding this biphasic response prevents misinterpretation of fever physiology in Indian hospital settings.
_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 73 (Body Temperature Regulation); Harrison's Principles of Internal Medicine, Ch. 16 (Fever and Hyperthermia)_