Download Chapter 16: Endocrine System

Chapter 16: Endocrine System
Two types of glands:
•Exocrine
– secretions delivered to body surface or organ cavity via ducts
– delivers materials to locations contiguous to external environment
•Endocrine glands
– no ducts, release hormones into extracellular fluid or “blood”
– Hormones affect cell function
•Mixed glands
– Have both exocrine and endocrine function (pancreas)
Major endocrine glands:
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Hypothalamus
Pituitary
Thyroid
Parathyroid
Thymus
Adrenal cortex
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Adrenal medulla
Pineal
Pancreas
Ovaries
Testis
Other organs/tissues having endocrine function:
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Heart
Stomach
Duodenum
Placenta
– Kidneys
– Adipose tissue
– Liver
Two Major Classes of Hormones
1) Amino acid based (mostly hydrophilic)
– Catecholamines (epinephrine, norepinephrine & dopamine)
– Indolamines (serotonin, melatonin & histamine)
– Thyroid hormone (hydrophobic thyroxine T4 & triiodothyronine T3)
– peptide and protein hormones
2) Steroids – sex and adrenocortical hormones (cholesterol-based and hydrophobic)
Estrogens, progesterone, testosterone, DHEA, cortisol/corticosterone & aldosterone
Hormone Action
•Hormones alter target cell activity by one of two mechanisms
1) Second messengers:
ƒ Receptors embedded in cell membrane
ƒ Mechanism used by all hydrophilic (polar) hormones
nd
+2
ƒ Generate or release intracellular 2 messengers: cAMP, cGMP, Ca
2nd messengers diffuse throughout cell & “turn on the desired effect”
nd
ƒ 2 messengers are targeted for therapeutics (sildenafil increases
cGMP and promotes vasodilation in “certain” blood vessels)
ƒ
2) Direct gene activation:
ƒ Receptors in the nucleus
ƒ Used by hydrophobic hormones (Steroids & thyroid hormone)
ƒ Activate specific regions of the genetic code that encode for proteins
that are required for desired function.
Hypothalamus
• Regulates many endocrine functions from water balance to sex drive
• Most effects are mediated via pituitary gland
• Hypothalamic-pituitary-__________ axis.
Pituitary Gland (Hypophysis)
• “Master endocrine gland – regulates many other glands & processes”
• Location and size
– housed in sella turcica of sphenoid bone
• Has two lobes: anterior & posterior
Anterior Lobe secretes tropic hormones that regulate other endocrine glands:
• Thyroid stimulating hormone (TSH or thyrotropin) – regulates thyroid
• Follicle stimulating hormone (FSH) – regulates gonads
• Luteinzing hormone (LH) – regulates gonads
• Adrenocorticotropic hormone (ACTH or corticotrophin) – regulates adrenal cortex
• Prolactin (PRL) – regulates mammary glands
• GH, somatotropin or growth hormone – stimulates liver to make growth factors
Negative feedback & hypothalamic – pituitary – (thyroid/cortical/gonadal) axes
Function of the Posterior Pituitary
•Stores and releases oxytocin (OT) and anti-diuretic hormone (ADH)
•Both produced in hypothalamus & transported down
Posterior Lobe Hormones & Regulation
•ADH
– Hyperosmolarity (dehydration) causes hypothalamus to signal release
– ADH Æ kidneys ↑ water retention, Æ concentrated low volume urine
– Release is inhibited by alcohol
– Functions also as neurotransmitter – implicated in male monogamy.
•Oxytocin
– labor contractions, lactation, sexual arousal & female affection.
– neuroendocrine reflexes trigger release
1) Suckling infant→ stimulates nerve endings →
hypothalamus → posterior lobe → oxytocin → milk
ejection.
2) Uterine contraction→ stimulates nerve endings →
hypothalamus → posterior lobe → oxytocin → uterine
contraction.
– higher brain centers
•milk ejection reflex can be triggered by a baby's cry.
•ADH & oxytocin are both 7 amino acid peptides and share considerable structural
similarity – thus it is important for pregnant women to avoid being ____________ !
Adrenal Cortex
•Corticosteroids
– Mineralocorticoid: aldosterone
+
+
•Promotes Na & H2O retention + K secretion. controls BP via renin:
angiotensinogen
low BP Æ kidneys Æ
renin
angiotensin I
lungs Æ angiotensin converting enzyme (ACE)
angiotensin II (potent vasoconstrictor)
aldosterone (Na+ & H2O retention)
– Glucocorticoids: cortisol & corticosterone
•Alters metabolism: stimulates release of glucose into blood
•Antiinflammatory effect becomes immune suppression with long-term
– sex steroids
•Adrenal androgens (including DHEA which other tissues convert to
testosterone) and estrogen (important after menopause)
Ovary
•Regulated by hypothalamic-pituitary-gonadal axis – Chapter 27
Testes
•Regulated by hypothalamic-pituitary-gonadal axis
•Interstitial cells (between seminiferous tubules)
– LH Æ produce testosterone
•Seminiferous tubules
– FSH Æ promotes sperm production
Other Organs with Endocrine Function:
•Heart
– atrial natriuretic peptide released with an increase in BP
↓ blood volume + ↓ BP by ↑ Na+ and H2O loss by kidneys
•Stomach and small intestines (produce 10 enteric hormones)
– coordinate digestive motility and secretion
– Ghrelin – produced by stomach stimulates appetite
– Gastrin – produced by stomach increases gastric (stomach acid secretion)
– Secretin – produced by duodenum stimulates pancreas to release
bicarbonate & neutralize stomach acid.
– Cholecystokinin – produced by duodenum stimulates gallbladder to
contract and stimulates release of digestive enzymes from the pancreas
•Placenta
– secretes HCG, estrogen, progesterone: regulate pregnancy (Chapt 28)