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The Endocrine System
Endocrine system
• Includes all cells and endocrine tissues that produce
hormones or paracrine factors
Endocrine vs Nervous System
• Nervous system performs short term crisis
management
• Endocrine system regulates long term ongoing
metabolic
• Endocrine communication is carried out by
endocrine cells releasing hormones
– Alter metabolic activities of tissues and organs
– Target cells
• Paracrine communication involves chemical
messengers between cells within one tissue
Control of Endocrine Activity
• Endocrine reflexes are the counterparts of
neural reflexes
• Hypothalamus regulates the activity of the
nervous and endocrine systems
– Secreting regulatory hormones that control the
anterior pituitary gland
– Releasing hormones at the posterior pituitary
gland
– Exerts direct neural control over the endocrine
cells of the adrenal medullae
Three Methods of Hypothalamic Control
over the Endocrine System
Figure 18.5
Hypothalamic Control of Adenohypophysis
• Hypothalamus regulates secretion of hormones
• Secretes releasing factors to release hormones
• Secretes inhibiting hormones to turn off secretion of
hormones
The Hypothalamus
• The hypothalamus sends a chemical stimulus to the
anterior pituitary to releasing hormones stimulate the
synthesis and release of hormones
– TRH - Thyrotropin releasing hormone (TRH) >> release of TSH
– CRH - Corticotropin releasing hormone (CRH) >> release of ACTH
– GnRH - Gonadotropin releasing hormone (GNRH) >> release of
FSH and LH
– GHRH – Growth hormone releasing hormone >> release of GH
Hypothalamic Stimulation–from CNS
The Pituitary Gland - Hypophysis
• Attached to the hypothalamus by the infundibulum
• Two basic divisions of the pituitary gland
– Anterior pituitary or Adenohypophysis
– Posterior pituitary or Neurohypophysis
Figure 25.3a–c
The Pituitary Gland - Hypophysis
• Releases nine important peptide hormones
• All nine bind to membrane receptors and use cyclic AMP as a
second messenger
Anterior Pituitary
• Pars distalis - largest division of the
adenohypophysis
– Contains five different types of endocrine cells
• Somatotropic cells -secrete growth hormone (GH)
• Mammotropic cells - secrete prolactin (PRL)
• Thyrotropic cells - secrete thyroid-stimulating hormone
(TSH)
• Corticotropic cells - secrete adrenocorticotropic
hormone (ACTH) and melanocyte-stimulating hormone
(MSH)
• Gonadotropic cells - secrete follicle-stimulating
hormone (FSH) and luteinizing hormone (LH)
– Tropic hormones
• TSH, ACTH, FSH, and LH
• Regulate the secretion of hormones by other endocrine
glands
Hormones of the Adenohypophysis
• Thyrotropin releasing hormone promotes the release of
TSH
• Thyroid stimulating hormone (TSH) triggers the release of
thyroid hormones
• Corticotropin releasing hormone causes the secretion of
ACTH
• Adrenocorticotropic hormone (ACTH) stimulates the
release of glucocorticoids by the adrenal gland
Hormones of the Adenohypophysis
• Follicle stimulating hormone (FSH) - stimulates
follicle development and estrogen secretion in
females and sperm production in males
• Luteinizing hormone (LH) causes ovulation and
progestin production in females and androgen
production in males
• Gonadotropin releasing hormone (GNRH)
promotes the secretion of FSH and LH
Hormones of the Adenohypophysis
• Prolactin (PH) –stimulates the development of
mammary glands and milk production
• Growth hormone (GH or somatotropin) - stimulates
cell growth and replication
• Melanocyte stimulating hormone (MSH) -stimulates
melanocytes to produce melanin
Posterior Pituitary
• Structurally part of the
brain
• Contains axons of
hypothalamic nerves
• Manufactures antidiuretic
hormone (ADH)
– Decreases the amount
of water lost at the
kidneys
– Elevates blood pressure
• Manufactures oxytocin
– Stimulates contractile
cells in mammary glands
– Stimulates smooth
muscle cells in uterus
Figure 25.5
Table 25.1
Feedback Control of Endocrine Secretion
Figure 18.8a
Negative Feedback Controls:
Long & Short Loop Reflexes
Figure 7-14: Negative
feedback loops in the
hypothalamicanterior
pituitary pathway
Negative Feedback Controls:
Long & Short Loop Reflexes
Figure 7-15: Control pathway for cortisol secretion
Thyroid
• Lies near the thyroid cartilage of the larynx
Thyroid Follicles and Thyroid Hormones
• Thyroid gland contains numerous follicles
– Release several hormones such as thyroxine (T4) and
triiodothyronine (T3) that regulate metabolism
• increases protein synthesis
• promotes glycolysis, gluconeogenesis, glucose uptake
– C cells produce calcitonin - helps regulate calcium concentration in
body fluids
The Thyroid Follicles
Figure 18.12b
Thyroid hormones
• Held in storage
• Bound to mitochondria, thereby increasing
ATP production
• Bound to receptors activating genes that
control energy utilization
• Exert a calorigenic effect
• C cells produce calcitonin - helps regulate
calcium concentration in body fluids
Four parathyroid glands
• Embedded in the posterior surface of the thyroid gland
• Chief cells produce parathyroid hormone (PTH) in
response to lower than normal calcium concentrations
• Parathyroid hormones are regulators of calcium levels in
healthy adults
Homeostatic Regulation of Calcium Ion
Concentrations
Figure 18.15
Adrenal Cortex
• Synthesizes and releases steroid hormones called corticosteroids
• Different corticosteroids are produced in each of the three layers
– Zona glomerulosa – mineralocorticoids (chiefly aldosterone)
– Zona fasciculata – glucocorticoids (chiefly cortisol)
– Zona reticularis – gonadocorticoids (chiefly androgens)
Adrenal Cortex
• Secretes over 30 different steroid hormones
(corticosteroids)
– Mineralocorticoids
• Aldosterone: maintains electrolyte balance
– Glucocorticoids
• Cortisol:
– Stimulates gluconeogenesis
– Mobilization of free fatty acids
– Glucose sparing
– Anti-inflammatory agent
– Gonadocorticoids
• testosterone, estrogen, progesterone
Mineralocorticoids
• Regulate the electrolyte concentrations of extracellular
fluids
• Aldosterone – most important mineralocorticoid
– Maintains Na+ balance by reducing excretion of sodium from the
body
– Stimulates reabsorption of Na+ by the kidneys
• Aldosterone secretion is stimulated by:
– Rising blood levels of K+
– Low blood Na+
– Decreasing blood volume or pressure
Glucocorticoids (Cortisol)
• Help the body resist stress by:
– Keeping blood sugar levels relatively constant
– Maintaining blood volume and preventing water shift
into tissue
• Cortisol provokes:
– Gluconeogenesis (formation of glucose from noncarbohydrates)
– Rises in blood glucose, fatty acids, and amino acids
Gonadocorticoids (Sex Hormones)
• Most gonadocorticoids secreted are androgens
(male sex hormones), and the most important
one is testosterone
• Androgens contribute to:
– The onset of puberty
– The appearance of secondary sex characteristics
– Sex drive in females
• Androgens can be converted into estrogens
after menopause
Pancreatic Islets
• Clusters of endocrine cells within the pancreas
called Islets of Langerhans or pancreatic islets
– Alpha cells secrete glucagons
– Beta cells secrete insulin
– Delta cells secrete GH-IH
Insulin and glucagon
• Insulin lowers blood glucose by
increasing the rate of glucose uptake and
utilization
• Glucagon raises blood glucose by
increasing the rates of glycogen
breakdown and glucose manufacture by
the liver
Regulation of Blood Glucose Concentrations
Figure 18.19