Metabolism and Thermoregulation

What occurs in the absorptive state of metabolism?

• 0-4 hrs after a meal, “fed” state

• Nutrients are either used for energy production or stored

• Insulin (secreted from β-cells) promotes anabolism

  • Glucose: used directly in skeletal muscles for ATP production or stored as glycogen in skeletal muscles or liver —> GLYCOGENESIS

  • Lipids: stored as triglycerides in adipose tissue —> LIPOGENESIS

  • Amino acids: used instantly for protein synthesis

What occurs in the post-absorptive state of metabolism?

• At least 4 hrs after a meal, “fasting” state

• Stored nutrients is broken down to maintain normal bodily functions

• Glucagon (secreted by α-cells) promotes catabolism

  • Glucose: broken down from glycogen in the liver and released into blood —> GLYCOGENOLYSIS

  • Glucose: synthesised from glycerol in triglycerides from adipose tissues —> GLUCONEOGENESIS

  • Lipids: triglycerides in adipose tissue is broken down into fatty acids —> LIPOLYSIS

What does Basal Metabolic Rate mean?

BMR is the energy expenditure per unit of time of an individual at rest in thermoneutral conditions, after at least 12 hrs of fasting. Encompasses energy used by the heart, brain, kidney, liver, and muscles.

How can BMR be measured?

• Can be measured directly through calorimetry, which detects the heat dissipated from the body

• Can be measured indirectly by monitoring O₂ consumption and CO₂ production through a mask

How is food intake regulated?

It is regulated by two hormones acting on Neuropeptide Y. NPY is an amino acid neurotransmitter found in the hypothalamus that induces hunger signals:

• Ghrelin - the hunger hormone

  • Secreted by enteroendocrine cells in the stomach when it’s empty

  • Promotes hunger by signalling the release of neuropeptide Y in the hypothalamus

  • Also triggered by sympathetic activation, linking hunger to stress and energy depletion

• Leptin - the satiety hormone

  • Secreted by adipocytes when fat stores are high

  • Signals the hypothalamus to suppress appetite by inhibiting the release of neuropeptide Y

  • Acts long-term: negative feedback loop to prevent excessive fat accumulation

What is conduction?

Conduction is the direct transfer of heat between adjacent molecules in physical contact.

• Lying on a cold floor, sitting in a hot bath

How does the sympathetic nervous system control skin blood vessels in response to temperature changes?

• In cold weather, vasoconstriction is triggered to reduce blood flow and heat loss, from the skin to conserve body heat

• In warm weather, vasodilation is triggered to increase blood flow to the skin and release heat to cool the body

How is heat loss controlled through evaporation?

Through thermoregulation, where the body is cooled down in hot external conditions or during exercise. Central core temperature is kept constant, but the skin and subcutaneous tissue regulate heat loss.

Heat is lost via phase change of water droplets on skin (sweat) from liquid to gas —> EVAPORATION

• This is a requires a high amount of energy

• If the environment is too humid, it limits this evaporation and impairs thermoregulation

What are two ways heat production is controlled?

Generally, all muscle activity generates heat as a by-product.

1. Shivering Thermogenesis: heat production via rapid involuntary muscle contractions. Produces no external work, only heat. Triggered by cold exposure and controlled by the hypothalamus.

2. Non-shivering Thermogenesis: involves brown adipose tissue, which is rich in mitochondria. Uncoupling proteins in these mitochondria divert energy from ATP production to heat production by uncoupling the electron transport chain.

   • Common in infants, rare in adults.

What happens during a fever?

A fever is an increase in core body temperature due to a reset in the hypothalamic “thermostat” to a higher point. Fevers are caused by infection, tissue damage, or physical trauma. Mechanism:

1. Pyrogens are released by macrophages (interleukin 1-β, TNF-α)

2. Pyrogens stimulate prostaglandin synthesis (PGE2), which acts on the hypothalamus to raise the temperature set point

What are the differences between hyperthermia, heat exhaustion, and heatstroke?

Hyperthermia: Rise in body temperature without a change in the hypothalamic set point (so not a fever). Common during exercise, and the body activates heat loss mechanisms (sweating & vasodilation).

Heat exhaustion: Severe hyperthermia caused by loss of fluid and decreased blood pressure as a result of vasodilation and decreased cardiac output. Can lead to weakness, dizziness, and possible fainting.

Heatstroke: Life-threatening progression of heat exhaustion where thermoregulatory mechanism fail completely, leading to collapse, seizure, unconsciousness. Higher risk in elderly and in extremely hot conditions without access to water.

What are the events leading up to a heatstroke?

1. Increased body temperature leads to sweating

2. Volume of bodily fluids decreases, which reduces blood pressure

3. Drop in blood pressure leads to vasoconstriction

4. Vasoconstriction leads to decreased heat loss and decreased sweating

5. Breakdown of thermoregulatory mechanisms lead to rapid increase in body temperature