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Endocrine Physiology Flashcards

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1
Q

What is endocrinology?

A

Endocrinology is the study of hormones, the glands and tissues that produce them, and their effects on the body’s functions.

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2
Q

What are the major endocrine glands in the human body?

A

the pituitary gland, hypothalamus, thyroid, parathyroid, adrenal glands, pancreas, ovaries (in females), and testes (in males).

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3
Q

What are hormones?

A

Hormones are chemical messengers secreted by endocrine glands directly into the blood to regulate various physiological processes like growth, metabolism, and reproduction.

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4
Q

What are the types of hormone signaling?

A

• Endocrine: Hormones travel through the bloodstream to distant target organs.
• Paracrine: Hormones act on nearby cells.
• Autocrine: Hormones act on the same cell that produced them.
• Juxtacrine: Hormones remain attached to the producing cell and affect adjacent cells.

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5
Q

How are hormones classified based on chemical structure?

A

• Peptide hormones: e.g., insulin
• Steroid hormones: e.g., cortisol
• Amino acid derivatives: e.g., epinephrine
• Fatty acid derivatives: e.g., prostaglandins

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6
Q

What is the role of the hypothalamus in endocrine function?

A

The hypothalamus links the nervous system to the endocrine system via the pituitary gland and controls many endocrine functions through releasing and inhibiting hormones.

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7
Q

What are the three main parts of the endocrine system?

A

Endocrine cells, hormones, and target cells

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8
Q

What are the main chemical classes of hormones?

A

• Proteins and polypeptides: e.g., insulin, pituitary hormones
• Steroids: e.g., cortisol, sex hormones
• Tyrosine derivatives: e.g., thyroid hormones, epinephrine, norepinephrine

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9
Q

What are telecrines, autocrines, paracrines, and neurocrines?

A

• Telecrines: Hormones released into the blood to act on distant target cells.
• Autocrines: Act on the same cell that secretes them.
• Paracrines: Affect nearby but different types of cells.
• Neurocrines: Neurotransmitters released into the blood by neurons, affecting distant target cells.

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10
Q

How are peptide and protein hormones synthesized?

A

Synthesized as preprohormones in the rough ER → cleaved to prohormones → processed in Golgi → stored in vesicles → released via exocytosis upon stimulation.

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11
Q

How are steroid hormones synthesized and secreted?

A

Synthesized from cholesterol and are lipid-soluble, allowing them to diffuse across cell membranes to act on intracellular receptors.

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12
Q

How are peptide and protein hormones synthesized?

A

Synthesized as preprohormones in the rough ER → cleaved to prohormones → processed in Golgi → stored in vesicles → released via exocytosis upon stimulation.

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13
Q

How are amine hormones produced?

A

Derived from tyrosine. Thyroid hormones are stored in thyroglobulin in the thyroid follicles; catecholamines (E & NE) are produced by the adrenal medulla.

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14
Q

What are typical blood concentrations and secretion rates of hormones?

A

Extremely small: concentrations range from 1 pg/ml to a few μg/ml; secretion rates in μg/day to mg/day.

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15
Q

How are amine hormones produced?

A

Derived from tyrosine. Thyroid hormones are stored in thyroglobulin in the thyroid follicles; catecholamines (E & NE) are produced by the adrenal medulla.

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16
Q

How are hormones transported in the blood?

A

• Water-soluble hormones (peptides, catecholamines): dissolve in plasma.
• Lipid-soluble hormones (steroids, thyroid hormones): bound to plasma proteins and inactive until dissociated.

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17
Q

How are hormones cleared from circulation?

A

• Metabolic destruction by tissues
• Binding to tissues
• Excretion in bile (liver)
• Excretion in urine (kidneys)

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18
Q

What are the three key characteristics of hormone action?

A

• Specificity: Acts only on cells with specific receptors
• Potency: Small amounts have large effects
• Interactions: Synergistic, antagonistic, or permissive actions between hormones

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19
Q

Where can hormone receptors be located?

A

• On the cell membrane (peptide hormones, catecholamines)
• In the cytoplasm or nucleus (steroid and thyroid hormones)

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20
Q

How do G protein–linked hormone receptors work?

A

Hormone binds receptor → activates G protein (α subunit swaps GDP for GTP) → dissociates → activates membrane enzyme → produces second messenger (e.g., cAMP, IP3) → triggers cellular response.

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21
Q

What are the types of membrane receptors for hormones?

A
  1. Ion channel–linked receptors
    1. G protein–linked receptors
    2. Enzyme-linked receptors
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22
Q

Q: What is the cAMP second messenger system?

A

Activated Gs protein stimulates adenylyl cyclase → converts ATP to cAMP → activates protein kinase A → phosphorylates proteins → cellular response.

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23
Q

What is the phospholipid second messenger system?

A

Hormone activates phospholipase C → splits PIP2 into IP3 and DAG → IP3 mobilizes Ca²⁺ from ER → DAG activates protein kinase C → cellular response.

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24
Q

What hormones use the cAMP second messenger system?

A

Epinephrine, norepinephrine, glucagon, LH, FSH, TSH, calcitonin, PTH, ADH

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25
What hormones use the phosphoinositide/Ca²⁺ system?
Epinephrine, norepinephrine, angiotensin II, GnRH, TRH, ADH
26
What is the mechanism of intracellular (nuclear/cytoplasmic) hormone receptors?
Hormone diffuses into the cell → binds receptor in cytoplasm or nucleus → hormone-receptor complex binds DNA hormone response element → gene transcription and mRNA translation → protein synthesis → functional change
27
What are the two parts of the pituitary gland?
• Adenohypophysis (anterior pituitary) • Neurohypophysis (posterior pituitary)
28
What are the major hypothalamic releasing/inhibiting hormones?
• TRH (Thyrotropin-releasing hormone) • GnRH (Gonadotropin-releasing hormone) • CRH (Corticotropin-releasing hormone) • GHRH (Growth hormone-releasing hormone) • PRF (Prolactin-releasing factor) • GHIH (Somatostatin) • PIH (Prolactin-inhibiting hormone) • MRF (Melanophore-stimulating hormone releasing factor) • MIF (Melanophore-stimulating hormone release-inhibiting factor)
29
How is the anterior pituitary regulated by the hypothalamus?
Via hypothalamic releasing/inhibitory hormones transported through the hypothalamic-hypophysial portal vessels. These hormones act on glandular cells to control secretion.
30
How does the hypothalamus control the posterior pituitary?
Hormones are produced by magnocellular neurons in the supraoptic and paraventricular nuclei and transported via axons to the posterior pituitary for release.
31
Which hormones are secreted by the anterior pituitary (adenohypophysis)?
• Growth hormone (GH) • Prolactin (PRL) • Thyroid-stimulating hormone (TSH) • Adrenocorticotropic hormone (ACTH) • Follicle-stimulating hormone (FSH) • Luteinizing hormone (LH) • Melanocyte-stimulating hormone (MSH)
32
Which hormones are secreted by the posterior pituitary (neurohypophysis)?
• Antidiuretic hormone (ADH / vasopressin) • Oxytocin (OXT)
33
What is the structure and source of human growth hormone (hGH)?
hGH is secreted by the anterior pituitary, has 191 amino acids, and a molecular weight of 22,005 Da.
34
What are the main growth-promoting effects of GH?
• Increases cell size, mitosis, and differentiation • Stimulates bone and muscle development • Mediated largely by IGF-1 (somatomedin C) produced by the liver
35
What are the metabolic effects of GH on protein metabolism?
• Increases protein synthesis • Enhances amino acid transport into cells • Increases RNA translation and DNA transcription • Decreases protein catabolism
36
How does GH affect fat metabolism?
• Increases lipolysis and free fatty acid levels in blood • Promotes fatty acid oxidation for energy, sparing protein and carbs
37
How does GH affect carbohydrate metabolism?
• Decreases glucose uptake in tissues • Increases hepatic glucose output • Causes hyperinsulinemia and insulin resistance
38
What regulates GH secretion?
Stimulated by: • Starvation, hypoglycemia, low fatty acids • Exercise, excitement, trauma • Ghrelin (from stomach), arginine Controlled by: • GHRH (stimulates) • GHIH/somatostatin (inhibits) from hypothalamus
39
When is GH secretion highest?
During the first 2 hours of deep sleep; secreted in pulses; levels rise in starvation.
40
What are the effects of GH deficiency and excess?
• Deficiency in childhood: Dwarfism • Excess in childhood: Gigantism • Excess in adults: Acromegaly (bone thickening, soft tissue growth)
41
What are the two main functions of ADH (vasopressin)?
1. Increases water reabsorption in kidney collecting ducts 2. Causes vasoconstriction, raising blood pressure
42
What are pituicytes, and what is their role?
Glial-like support cells in the posterior pituitary that do not secrete hormones but support the nerve terminals that do.
43
What stimulates ADH secretion?
• ↑ Extracellular fluid osmolarity • ↓ Blood volume • ↓ Blood pressure
44
Where is the thyroid gland located and what does it secrete?
Located below the larynx, anterior to the trachea. It secretes two major metabolic hormones: • Thyroxine (T4) • Triiodothyronine (T3)
45
Which is more potent: T3 or T4?
T3 is ~5 times more potent but present in smaller quantities and has a shorter duration. T4 is more abundant (93%) and is converted to T3 in tissues.
46
What is thyroglobulin and its role in hormone synthesis?
A large glycoprotein stored in thyroid follicles; contains tyrosine residues that combine with iodine to form T3 and T4.
47
What is iodide trapping and how does it occur?
Active transport of iodide into thyroid cells via Na⁺/I⁻ symporter on the basal membrane. Driven by Na⁺/K⁺ ATPase.
48
What enzyme catalyzes oxidation of iodide?
Thyroid peroxidase converts iodide (I⁻) into active iodine species (I₀ or I₃⁻), allowing binding to tyrosine.
49
What are MIT and DIT?
• MIT (monoiodotyrosine): 1 iodine bound • DIT (diiodotyrosine): 2 iodines bound These combine to form T3 and T4.
50
How are thyroid hormones stored and released?
Stored as part of thyroglobulin in colloid. Upon TSH stimulation, T3 and T4 are cleaved and released into blood.
51
How are T3 and T4 formed?
• T4: DIT + DIT • T3: MIT + DIT Both remain part of thyroglobulin until release.
52
How are thyroid hormones transported in the blood?
99% bind to plasma proteins (mostly thyroxine-binding globulin). Only free hormones are active.
53
How do thyroid hormones affect growth and the brain?
• Essential for normal growth in children • Critical for brain development in fetus and neonates • Deficiency can cause cretinism
54
How do thyroid hormones affect carbohydrate metabolism?
• ↑ Glucose uptake • ↑ Glycolysis, gluconeogenesis, and insulin secretion
55
How do thyroid hormones affect fat metabolism?
• ↑ Lipolysis and fatty acid oxidation • ↓ Fat stores • ↑ Plasma free fatty acids
56
What is the effect of thyroid hormone on basal metabolic rate (BMR)?
Can increase BMR by 60–100%, enhancing overall energy use and heat production.
57
What are the effects of thyroid hormones on protein metabolism?
• ↑ Protein synthesis • Also ↑ protein catabolism, especially in hyperthyroidism
58
What are the cardiovascular effects of thyroid hormones?
• ↑ Heart rate, cardiac output, blood flow, heart strength • No major change in mean arterial pressure
59
How do thyroid hormones affect the CNS and muscles?
• CNS: ↑ excitability; hyperthyroidism = tremors, insomnia • Muscles: Mild excess = vigor; excess = weakness; deficiency = sluggishness
60
How does thyroid hormone affect sleep and GI function?
• Hyperthyroid: insomnia due to excitability • GI: ↑ motility; hyperthyroid may cause diarrhea
61
Describe the hypothalamic-pituitary-thyroid (HPT) axis.
1. Hypothalamus releases TRH 2. TRH stimulates anterior pituitary to release TSH 3. TSH stimulates thyroid to produce T3 and T4
62
How does thyroid hormone affect sleep and GI function?
• Hyperthyroid: insomnia due to excitability • GI: ↑ motility; hyperthyroid may cause diarrhea
63
How does TSH affect the thyroid?
• ↑ Proteolysis of thyroglobulin • ↑ Iodide trapping • ↑ Iodination of tyrosine • ↑ Size and number of thyroid cells • Acts via cAMP second messenger system
64
Q: What is the role of TRH in thyroid regulation?
TRH is a tripeptide secreted by the paraventricular nucleus (PVN) of hypothalamus, stimulates TSH release. Cold exposure ↑ TRH.
65
What is the feedback effect of thyroid hormone?
Negative feedback: High T3/T4 inhibits TRH (hypothalamus) and TSH (anterior pituitary).
66
How do nerves regulate thyroid hormone secretion? A:
• Sympathetic stimulation ↑ secretion • Parasympathetic stimulation ↓ secretion
67
What is the Wolff-Chaikoff effect?
High iodide levels inhibit thyroid hormone synthesis via enzyme suppression (autoregulatory effect).
68
What are the two main parts of the adrenal gland?
1. Adrenal medulla (10–20%): secretes epinephrine & norepinephrine 2. Adrenal cortex (80–90%): secretes corticosteroids (glucocorticoids, mineralocorticoids, sex hormones)
69
What are the three zones of the adrenal cortex and what do they secrete?
• Zona glomerulosa (outer): aldosterone (mineralocorticoid) • Zona fasciculata (middle): cortisol, corticosterone (glucocorticoids), some androgens • Zona reticularis (inner): androgens, estrogens, small amount of cortisol
70
What is the main glucocorticoid and what is it synthesized from?
Cortisol (hydrocortisone) — synthesized from cholesterol in the adrenal cortex.
71
How does cortisol affect carbohydrate metabolism?
• ↑ Gluconeogenesis (6–10x) • ↓ Glucose uptake by cells • ↑ Blood glucose levels (can cause adrenal diabetes) • Antagonizes insulin action
72
What are cortisol’s effects on protein metabolism?
• ↓ Protein synthesis and ↑ protein breakdown (except in liver) • ↑ Amino acid mobilization • ↑ Liver protein and plasma proteins
73
What are cortisol’s effects on fat metabolism?
• ↑ Lipolysis and plasma free fatty acids • ↑ Fatty acid oxidation for energy • In excess, causes fat redistribution (e.g., moon face, buffalo hump)
74
How does cortisol help in stress response?
Stress (trauma, surgery, infection) → ↑ ACTH → ↑ cortisol Cortisol mobilizes energy (fat & amino acids), supports damaged tissue recovery, and helps maintain blood glucose.
75
What are cortisol’s effects on blood and vascular system?
• ↑ RBCs, neutrophils, monocytes, platelets • ↓ Eosinophils and lymphocytes • Permissive action: makes blood vessels more responsive to catecholamines for constriction
76
What are cortisol’s effects on CNS and digestive system?
• ↑ CNS excitability • ↑ Digestive secretions
77
What are the anti-inflammatory effects of cortisol?
1. Blocks early stages of inflammation 2. Accelerates resolution & healing Suppresses immune response in large doses
78
Describe the hypothalamic-pituitary-adrenal (HPA) axis.
1. Hypothalamus releases CRH 2. CRH stimulates anterior pituitary to release ACTH 3. ACTH stimulates adrenal cortex to release cortisol
79
How does ACTH stimulate cortisol secretion?
• Binds to receptors on adrenal cortex • Activates adenylyl cyclase → cAMP → protein kinase → hormone synthesis • Also causes hypertrophy of adrenal cells
80
How does cortisol exert negative feedback?
• Inhibits CRH in the hypothalamus • Inhibits ACTH in the anterior pituitary
81
What is the circadian rhythm of cortisol secretion?
• High in the morning, low at night • Cortisol, ACTH, and CRH levels follow a daily rhythm
82
What are the two major tissue types in the pancreas?
1. Acinar cells – secrete digestive enzymes into the duodenum 2. Islets of Langerhans – secrete hormones into the blood
83
What are the major cell types in the Islets of Langerhans and their hormones?
• Beta cells (60%): Insulin, Amylin • Alpha cells (25%): Glucagon • Delta cells (10%): Somatostatin • PP cells: Pancreatic polypeptide
84
Describe the structure and synthesis of insulin.
•Two amino acid chains (A & B) linked by disulfide bonds • Synthesized as preproinsulin → proinsulin → insulin + C-peptide in beta cells
85
How does insulin exert its effects?
• Binds to enzyme-linked receptors • Causes autophosphorylation of the receptor • Activates tyrosine kinase → phosphorylation cascade → metabolic effects
86
What are insulin’s effects on carbohydrate metabolism?
• ↑ Glucose uptake, use, and storage (as glycogen in liver & muscle) • ↑ Conversion of excess glucose into fatty acids • ↓ Gluconeogenesis in liver
87
What are insulin’s effects on fat metabolism?
• ↑ Fat synthesis and storage • ↓ Fat breakdown • Deficiency leads to increased free fatty acids, ketone bodies, and risk of ketoacidosis
88
What are insulin’s effects on protein metabolism and growth?
• ↑ Amino acid uptake • ↑ Protein synthesis • ↓ Protein breakdown • Works synergistically with GH to promote growth
89
What are the main stimuli for insulin secretion?
• ↑ Blood glucose (primary stimulus) • Certain amino acids and fatty acids • GI hormones: gastrin, secretin, CCK, GIP • Hormones: glucagon, GH, cortisol
90
How does the autonomic nervous system regulate insulin secretion?
• Parasympathetic stimulation (vagus): ↑ insulin • Sympathetic stimulation (α-adrenergic): ↓ insulin • Sympathetic (β-adrenergic): can ↑ insulin slightly

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