6.6 Hormones and Homeostasis

Question 1

Homeostasis

Answer 1

Homeostasis is the maintenance of a constant internal
environment within physiological tolerance limits
* A disease ensues if a factor deviates from its normal range
Physiological processes are regulated by negative feedback
* An effect is antagonistic (opposite) to the stimulus
* This means the detected change is reversed

Question 2

Endocrine system

Answer 2

The endocrine system releases chemical messengers called
hormones into the blood to act on distant target cells
* Hormones only act on the cells with a specific receptor
The endocrine system works in tandem with the nervous
system to maintain physiological balance (homeostasis)
* Hormones initiate slower responses (longer durations)

Question 3

Thermoregulation

Answer 3

Body temperature is regulated by the hormone thyroxin
* Thermoreceptors (skin) send signals to the hypothalamus
* Thyroxin is released from the thyroid gland when body
temperature is low and increases metabolism (generates heat)
Thyroxin production requires iodine and a deficiency will
result in goitre (enlargement of the thyroid gland)

Question 4

Circadian Rhythms

Answer 4

Circadian rhythms are regulated by the hormone melatonin
* Photoreceptors (eyes) send signals to the hypothalamus
* Melatonin is secreted by the pineal gland (in the brain)
* Melatonin release is inhibited by light (levels high at night)
* High levels of melatonin promote sleep in diurnal animals
Changing time zones can disrupt melatonin release (jet lag)
* Melatonin supplements can recalibrate sleep patterns

Question 5

Appetite Control

Answer 5

Appetite suppression is regulated by the hormone leptin
* Adipose cells secrete leptin to suppress appetite (⇩ hunger)
* Leptin binds to receptors located in the hypothalamus
Over-eating causes more fat cells to be produced (obesity)
* Over time, obese people become desensitized to leptin
and therefore are more likely to continue to over-eat
* Hence, leptin treatments for obese people are ineffective
(obesity linked to leptin unresponsiveness – not a deficiency)

Question 6

Blood Glucose Regulation

Answer 6

Blood sugar levels are regulated by insulin and glucagon
* These hormones are secreted by cells in the pancreas
Insulin is secreted by β-cells to lower blood sugar levels
* Stimulates glucose uptake by the liver and adipose cells
* Increases the rate of glucose metabolism (⇧ respiration)
Glucagon is secreted by α-cells to raise blood sugar levels
* Stimulates glycogen breakdown within the liver
* Decreases the rate of glucose metabolism (⇩ respiration)

Question 7

Diabetes

Answer 7

Diabetes is a disorder that prevents blood sugar regulation
* It can be either type I (IDDM) or type II (NIDDM)