2. Endocrine System part 5A

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Pineal Gland
Hangs from the roof of the third ventricle of the brain
Secretes melatonin
Believed to trigger the body’s sleep/wake cycle
Believed to coordinate the hormones of fertility in humans and to inhibit the reproductive system until maturity occurs
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Thyroid Gland
Found at the base of the throat, inferior to the Adam’s apple
Consists of two lobes and a connecting isthmus
Follicles are hollow structures that store colloidal material
Produces two hormones
Thyroid hormone
Calcitonin
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5. Figure 9.6a The thyroid gland.
Thyroid cartilage
Epiglottis
Common carotid
artery
Isthmus of
thyroid gland
Trachea
Left subclavian artery
Brachiocephalic
artery
Left lobe of
thyroid gland
Aorta
(a) Gross anatomy of the thyroid gland, anterior view

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Thyroid Gland
Thyroid hormone
Major metabolic hormone
Controls rate of oxidation of glucose to supply body heat and chemical energy
Needed for tissue growth and development
Composed of two active iodine-containing hormones
Thyroxine (T4)—secreted by thyroid follicles
Triiodothyronine (T3)—conversion of T4 at target tissues
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7. Figure 9.6b The thyroid gland.
Colloid-filled
follicles
Figure 9.6b The thyroid gland.
Follicle cells
Parafollicular cells
(b) Photomicrograph of thyroid gland
follicles (380x)

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Thyroid Gland
Calcitonin
Decreases blood calcium levels by causing calcium deposition on bone
Antagonistic to parathyroid hormone
Produced by parafollicular cells found between the follicles
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9. Figure 9.6b The thyroid gland.
Colloid-filled
follicles
Figure 9.6b The thyroid gland.
Follicle cells
Parafollicular cells
(b) Photomicrograph of thyroid gland
follicles (380x)

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Parathyroid Glands
Tiny masses on the posterior of the thyroid
Secrete parathyroid hormone (PTH)
Most important regulator of calcium ion (Ca2+) homeostasis of the blood
Stimulates osteoclasts to remove calcium from bone
Hypercalcemic hormone (increases blood calcium levels)
Stimulates the kidneys and intestine to absorb more calcium
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11. Figure 9.7 Hormonal control of the level of calcium ions in the blood.
Calcitonin
Calcitonin
stimulates
calcium salt
deposit in bone.
Thyroid gland
releases calcitonin.
Stimulus
Rising blood
Ca2+ levels
IMBALANCE
Calcium homeostasis of blood:
9–11 mg/100 ml
BALANCE
BALANCE
Stimulus
Falling blood
Ca2+ levels
IMBALANCE
Thyroid
gland
Osteoclasts
degrade bone
matrix and release
Ca2+ into blood.
Parathyroid
glands
Parathyroid
glands release
parathyroid
hormone (PTH).
PTH

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Thymus
Located in the upper thorax, posterior to the sternum
Largest in infants and children
Decreases in size throughout adulthood
Produces a hormone called thymosin
Matures some types of white blood cells
Important in developing the immune system
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Adrenal Glands
Sit on top of the kidneys
Two regions
Adrenal cortex—outer glandular region has three layers that produce corticosteroids
Mineralocorticoids are secreted by outermost layer
Glucocorticoids are secreted by middle layer
Sex hormones are secreted by innermost layer
Adrenal medulla—inner neural tissue region
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14. Figure 9.8 Microscopic structure of the adrenal gland.
Capsule
Mineralocorticoid- secreting area
Kidney
Glucocorticoid- secreting area
Cortex
Adrenal gland
Adrenal cortex
Medulla
Cortex
Sex hormone- secreting area
Kidney
Medulla
Adrenal
medulla

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Adrenal Glands
Hormones of the adrenal cortex
Mineralocorticoids (mainly aldosterone)
Produced in outer adrenal cortex
Regulate mineral (salt) content in blood, particularly sodium and potassium ions
Regulate water and electrolyte balance
Target organ is the kidney
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Adrenal Glands
Hormones of the adrenal cortex (continued)
Release of aldosterone is stimulated by:
Humoral factors (fewer sodium ions or too many potassium ions in the blood)
Hormonal stimulation (ACTH)
Renin and angiotensin II in response to a drop in blood pressure
Aldosterone production is inhibited by atrial natriuretic peptide (ANP), a hormone produced by the heart when blood pressure is too high
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17. Figure 9.9 Major mechanisms controlling aldosterone release from the adrenal cortex.
Decreased Na+ or
increased K+ in
blood
Stress
Hypothalamus
Decreased
blood volume and/or blood pressure
Corticotropin- releasing hormone
Anterior pituitary
Increased
blood pressure
or blood volume
ACTH
Kidney
Renin
Heart
Indirect stimulating effect via angiotensin
Atrial natriuretic peptide (ANP)
Angiotensin II
Direct stimulating effect
Inhibitory effect
Mineralocorticoid- producing part of adrenal cortex
Enhanced secretion
of aldosterone targets
kidney tubules
Increased absorption
of Na+ and water;
increased K+ excretion
Increased blood volume and
blood pressure