Anatomy

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The Endocrine System

Objectives • Compare the major chemical classes and general mechanisms of hormones. • Describe the location and structure of the pituitary gland, and explain its relationship

with the hypothalamus. • Describe the location and structure of each of the endocrine glands. • Identify the hormones produced by each of the endocrine glands and specify the

functions of those hormones. • Describe the functions of the hormones produced by the kidneys, heart, thymus, testes,

ovaries, and adipose tissue. • Explain how hormones interact to produce coordinated physiological responses.

Endocrine versus 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 by acting at target cells • Paracrine communication involves chemical messengers between cells within one

tissue

Endocrine system • Includes all cells and endocrine tissues that produce hormones or paracrine factors

Hormone structure • Amino acid derivatives

• Structurally similar to amino acids • Peptide hormones

• Chains of amino acids • Lipid derivatives

• Steroid hormones and eicosanoids

Hormones can be • Freely circulating

• Rapidly removed from bloodstream • Bound to transport proteins

Mechanisms of hormone action • Receptors for catecholamines, peptide hormones, eicosanoids are in the cell

membranes of target cells • Thyroid and steroid hormones cross the membrane and bind to receptors in the

cytoplasm or nucleus

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The Pituitary Gland or Hypophysis • Releases nine important peptide hormones • All nine bind to membrane receptors and use cyclic AMP as a second messenger

The anterior lobe (adenohypophysis) • Thyroid stimulating hormone (TSH)

• Triggers the release of thyroid hormones from thyroid gland • Thyrotropin releasing hormone promotes the release of TSH

• Adrenocorticotropic hormone (ACTH) • Stimulates the release of glucocorticoids by the adrenal gland • Corticotrophin releasing hormone causes the secretion of ACTH

• Follicle stimulating hormone (FSH) • Stimulates follicle development/estrogen secretion in females; sperm in

males • Leutinizing hormone (LH)

• Causes ovulation/progestin production in females; androgen production in males

• Gonadotropin releasing hormone (GNRH) • Promotes the secretion of FSH and LH

• 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

The posterior lobe of the pituitary gland (neurohypophysis) • Contains axons of hypothalamic nerves • Antidiuretic hormone (ADH)

• Decreases the amount of water lost at the kidneys • Elevates blood pressure

• Oxytocin (OT) • Stimulates contractile cells in mammary glands • Stimulates smooth muscle cells in uterus

The Thyroid Gland • Lies near the thyroid cartilage of the larynx • Two lobes connected by an isthmus • Release several hormones such as thyroxine (T4) and triiodothyronine (T3) • Hormones are bound to mitochondria, thereby increasing ATP production, control

energy utilization, Exert a calorigenic effect

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Cells of the thyroid gland • C cells produce calcitonin (CT)

• Helps regulate calcium concentration in body fluids • Works in a negative feedback mechanism with PTH

The Parathyroid Glands • Four parathyroid glands • Embedded in the posterior surface of the thyroid gland • Chief cells produce parathyroid hormone (PTH) when Ca++ is low in fluids

The Adrenal Glands

Adrenal cortex • Manufactures steroid hormones (corticosteroids) • Cortex produces

• Mineralocorticoids – increase renal reabsorption of Na+ and H2O • Glucocorticoids – glucose metabolism • Androgens – dominate sex hormone in females

Adrenal medulla • Produces epinephrine (~75-80 percent) • Produces norepinephrine (~25-30 percent)

The Pineal gland • Synthesize melatonin • Suggested functions include inhibiting reproductive function, protecting against

damage by free radicals, setting circadian rhythms

The Pancreas

The pancreatic islets • Clusters of endocrine cells within the pancreas called islets of Langerhans or

pancreatic islets • Alpha cells secrete glucagons • Beta cells secrete insulin

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

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Diabetes mellitus is high blood glucose

high urine output (full of extra glucose)

excessive thirst (loss of fluid)

excessive hunger (glucose not entering cells)

Type I –Insulin-dependent – genetic (juvenile)

Type II – Non-insulin dependent (age onset) decrease in insulin cell receptors

>90% all cases. Controlled by diet, exercise & drugs

Can also be gestational, or from reactions to drugs

The Endocrine Tissues of Other Systems

The Intestines • Produce hormones important to the coordination of digestive activities

The Kidneys • Produce calcitriol and erythropoietin (EPO) and the enzyme rennin

• Calcitriol = stimulates Ca++ and phosphate ion absorption along the digestive tract • EPO stimulates red blood cell production by bone marrow • Renin converts angiotensinogen to angiotensin I

Angiotensin I converted to angiotensin II in the lungs • Stimulates adrenal production of aldosterone • Stimulates pituitary gland release of ADH • Promotes thirst • Elevates blood pressure

The Heart • Specialized muscle cells produce natriuretic peptides when BP becomes excessive

• Generally oppose actions of angiotensin II

The Thymus • Produces thymosins

• Help develop and maintain normal immune defenses

The Gonads • Interstitial cells of the testes produce testosterone

• Most important sex hormone in males • In females, oocytes develop in follicles

• Follicle cells produce estrogens

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• After ovulation, the follicle cells form a corpus luteum that releases a mixture of estrogens and progesterone

Adipose tissues secrete • Leptin, a feedback control for appetite • Resistin, which reduces insulin sensitivity

Hormones often interact, producing • Antagonistic (opposing) effects • Synergistic (additive) effects • Permissive effects (one hormone is required for the other to produce its effect) • Integrative effects (hormones produce different but complimentary results)

Hormones and growth • Normal growth requires the interaction of several endocrine organs • Six hormones are important

• GH • Thyroid hormones • Insulin • PTH • Calcitriol • Reproductive hormones

Hormones and stress • Stress = any condition that threatens homeostasis • GAS (General Adaptation Syndrome) is our bodies response to stress-causing factors • Three phases to GAS

• Alarm phase (immediate, fight or flight, directed by the sympathetic nervous system)

• Resistance phase (dominated by glucocorticoids) • Exhaustion phase (breakdown of homeostatic regulation and failure of one or more

organ systems)

Hormones and behavior • Many hormones affect the CNS • Changes in the normal mixture of hormones significantly alters intellectual

capabilities, memory, learning, and emotional states

Endocrine system • Few functional changes with age

• Chief change is a decline in concentration of reproductive hormones