Metabolic processes regulated by thyroid hormones Flashcards
What is calcitonin and where is it produced?
Calcitonin is a polypeptide hormone produced by parafollicular (C) cells of the thyroid gland. It helps lower blood calcium levels.
How does calcitonin work to lower blood calcium levels?
Calcitonin inhibits bone resorption and stimulates calcium uptake and incorporation into bone.
When is calcitonin released and what inhibits its release?
Calcitonin is secreted in response to high blood calcium levels and inhibited when blood calcium levels decline.
How is metabolic rate defined?
Metabolic rate is the rate of energy expenditure per hour, measured by the total heat produced by all chemical reactions and mechanical work of the body.
What is the role of calcitonin in adults and children?
Calcitonin is most important in childhood for skeleton development, and has minimal role in adults.
What are the effects of hyperthyroidism and hypothyroidism on metabolic rate?
Hyperthyroidism results in a high metabolic rate, weight loss, and poor heat tolerance, while hypothyroidism leads to a low metabolic rate, obesity, and poor cold tolerance.
What is Basal Metabolic Rate (BMR)?
BMR is the rate at which energy is expended by the body under controlled conditions (12 hours after a meal, at rest) to maintain essential activities.
How is BMR measured?
BMR can be measured by oxygen consumption (indirectly) or by using a calorimeter to measure heat production.
What factors influence BMR?
Body surface area, age, gender, stress, and hormones are key factors. The body surface area is the most important determinant of BMR.
How does stress affect metabolic rate?
Stress, whether physical or emotional, increases metabolic rate by releasing norepinephrine and epinephrine, which increase fat catabolism and heat production.
How does age and gender affect BMR?
BMR is inversely proportional to age and higher in males than females due to more muscle and less fat in males.
What role does thyroid hormone play in regulating metabolic rate?
Thyroid hormone (TH) increases oxygen consumption and accelerates metabolic rate by increasing ATP usage for the Na/K pump, generating heat as a byproduct.
What is food-induced thermogenesis?
Food-induced thermogenesis refers to the increased metabolic rate following food ingestion, especially after protein-rich meals, primarily due to the liver’s increased metabolic activity.
How does exercise affect TMR?
Exercise can increase metabolic rate by 15-20 times, and this elevated rate can remain for hours after the activity.
What is Total Metabolic Rate (TMR)?
TMR is the total rate of kcal consumption to fuel all activities, including BMR and voluntary activities like exercise.
What are the four mechanisms of heat exchange?
- Radiation: Loss of heat through infrared waves.
- Conduction: Heat transfer through direct contact.
- Convection: Movement of warm air rising and being replaced by cooler air.
- Evaporation: Heat loss due to water vaporization from the body.
How does fasting affect metabolic rate?
Fasting or low caloric intake decreases metabolic rate, slowing the breakdown of food reserves.
What is insensible water loss?
Insensible water loss is continuous evaporation of water from the skin, lungs, and mouth, accounting for about 10% of basal heat loss, not regulated by body temperature.
What is the role of the hypothalamus in thermoregulation?
The hypothalamus integrates thermoregulatory information and initiates heat-loss or heat-promoting mechanisms, similar to a thermostat.
What are heat-promoting mechanisms?
- Vasoconstriction: Restricts blood flow to the skin to conserve body heat.
- Shivering: Involuntary muscle contractions that generate heat.
What is “chemical or nonshivering thermogenesis”?
It’s the increase in metabolic rate and heat production in response to cold, triggered by the release of epinephrine and norepinephrine from the adrenal medulla. It’s well-documented in infants but controversial in adults.
How does thyroxine release contribute to heat conservation?
Lowering external temperature stimulates the hypothalamus to release TRH, leading to increased thyroid hormone (T3 & T4) secretion, which boosts metabolic rate and heat production, helping to maintain body temperature.
What are some behavioral modifications for heat loss?
Reducing activity, seeking cooler environments (shade), increasing convection (e.g., using a fan), and wearing light, loose clothing to reflect radiant energy.
What are some behavioral modifications that conserve heat?
Wearing warmer clothing, ingesting warm fluids, reducing exposed body surface area, and increasing physical activity help conserve heat.
How does vasodilation help in heat loss?
Vasodilation increases blood flow to the skin, promoting heat loss through radiation, conduction, and convection.
What triggers heat-loss mechanisms?
When core body temperature increases, the hypothalamus activates the heat-loss center and inhibits the heat-promoting center.
What role does sweating play in heat loss?
Sweating is activated when the body is overheated. It cools the body through evaporation, which is more effective in dry environments.
What is hyperthermia, and how does it affect the body?
Hyperthermia occurs when heat loss mechanisms fail, leading to elevated body temperature and a vicious cycle of increased metabolic rate, which further raises body temperature. This can result in heat stroke, which is potentially fatal.
What is fever?
Fever is a controlled hyperthermic response, typically caused by infection. Pyrogens reset the hypothalamus, raising body temperature to fight infection.
What contributes to energy output?
Energy output consists of heat loss (~60%) and work (ATP production).
What is the energy intake equation?
Energy intake equals the energy provided by food oxidation, which is the energy released when food molecules are broken down.
What happens when energy intake equals energy output?
When energy intake equals energy output, body weight remains stable.
What is the difference between hunger and satiety?
Hunger is the physiological need to eat, while satiety is the feeling of satisfaction after eating.
What is the role of appetite?
Appetite is the desire for specific foods, influenced by memory and associations, and doesn’t always reflect the body’s caloric needs.
What are key centers in appetite regulation?
The arcuate nucleus in the hypothalamus, the lateral hypothalamic area (LHA), and the ventromedial nucleus (VMH) are central to appetite regulation.
How do neural signals and nutrients affect appetite?
How do neural signals and nutrients affect appetite?
A: Stretch receptors in the GI tract inhibit appetite. Elevated plasma glucose and amino acids also suppress appetite, while low levels stimulate it.
What hormones regulate appetite in the short term?
Insulin, cholecystokinin, and glucagon play roles in short-term appetite regulation, while ghrelin stimulates hunger.
What is the role of leptin in appetite regulation?
Leptin, secreted by adipose tissue, inhibits appetite by binding to receptors in the arcuate nucleus and increasing CART expression, signaling that fat stores are sufficient.
