Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 45.3 Endocrine Glands Respond to Diverse Stimuli in Regulating Animal Physiology
Hormones regulate a wide range of physiological processes, including calcium balance, stress response, reproductive development, and daily rhythms. These functions are carried out by a complex network of endocrine glands and signaling pathways that respond to internal and environmental stimuli.
1. Calcium Homeostasis
- Parathyroid hormone (PTH) raises blood calcium when levels drop below ~10 mg/100 mL
- PTH stimulates bone breakdown, kidney reabsorption of Ca²⁺, and vitamin D activation
- Active vitamin D increases Ca²⁺ absorption in the intestines
- Calcitonin, from the thyroid, lowers blood calcium by inhibiting bone breakdown and enhancing Ca²⁺ excretion in the kidneys
- In humans, calcitonin is mainly important during childhood
2. Adrenal Medulla and Fight-or-Flight
- Triggered by nerve impulses from the hypothalamus
- Epinephrine and norepinephrine are secreted from the adrenal medulla
- These hormones increase blood glucose, heart rate, breathing rate, and redirect blood flow to vital organs and muscles
- Effects vary by tissue: e.g., vasodilation in skeletal muscle, vasoconstriction in intestines
- Rapid, systemic responses to stress
3. Adrenal Cortex and Long-Term Stress
- Activated by ACTH from the anterior pituitary
- Releases corticosteroids:
- Glucocorticoids (e.g. cortisol): increase glucose production from proteins, suppress immune function
- Mineralocorticoids (e.g. aldosterone): promote Na⁺ and water retention, increasing blood pressure
- Chronic use of glucocorticoids (like cortisone) can impair metabolism and immunity
4. Sex Hormones
- Androgens, estrogens, and progesterone are made primarily in the gonads
- Testosterone drives male reproductive development and secondary sexual traits (voice, muscles)
- Estradiol maintains the female reproductive system and secondary traits
- Progesterone supports pregnancy by maintaining uterine tissues
- These hormones are regulated by FSH and LH from the anterior pituitary, controlled by GnRH from the hypothalamus
5. Melatonin and Biological Rhythms
- Secreted by the pineal gland, especially at night
- Regulated by the SCN (suprachiasmatic nucleus) in the hypothalamus
- Involved in sleep-wake cycles and seasonal reproduction
- Melatonin release tracks night length, adjusting physiological timing
6. Evolution of Hormone Function
- Hormone roles have diverged across species
- Thyroid hormone regulates metabolism in mammals, but in frogs it also triggers tail resorption during metamorphosis
- Prolactin influences milk production in mammals, but also regulates osmoregulation, reproduction, and metabolism in other vertebrates
- MSH (melanocyte-stimulating hormone) affects skin color in fish and amphibians; in mammals, it also regulates hunger and metabolism
7. Endocrine Disruptors
- DES, a synthetic estrogen, disrupted reproductive development in humans
- Chemicals like bisphenol A (BPA) may interfere with hormonal pathways
- Endocrine disruptors can mimic or block normal hormone function, with lasting developmental effects
In a Nutshell
Hormones are essential for maintaining homeostasis, coordinating stress responses, guiding development, and controlling behavior. From calcium levels to fight-or-flight reactions to reproductive cycles, endocrine pathways are diverse yet tightly regulated. These systems evolve and adapt, but are also vulnerable to disruption by environmental chemicals.