Chapter 9

The Endocrine System

• Along with Nervous System, the Endocrine System coordinates and directs the activities of body cells

• Nervous System uses impulses to make muscles and glands to respond quickly to changes inside and outside the body

• Endocrine System uses chemical messengers released into the blood to respond slowly to changes

Hormones

• Chemical messengers are called hormones

• Hormones control several major processes– Reproduction– Growth and development– Mobilization of body defenses– Maintenance of much of homeostasis– Regulation of metabolism

• Hormones are produced by specialized cells

• Cells secrete hormones into extracellular fluids

• Blood transfers hormones to target sites

• These hormones regulate the metabolic activity of other cells

• Hormones regulate processes that go on for long periods

• Hormones can be classified as– Amino acid-based molecules – proteins,

peptides, amines– Steroids – made from cholesterol– Prostaglandins – made from highly active

lipids

Mechanisms of Hormone Action

• Hormones work by binding to target cell membrane

• Hormones affect only certain tissues or organs (target cells or organs)

• Target cells must have specific protein receptors

• Hormone binding influences the working of the cells

Effects Caused by Hormones

• Changes that follow hormone binding depend on the specific hormone and the target cell type

• Typical changes include:– Changes in plasma membrane permeability or

electrical state– Synthesis of proteins, such as enzymes– Activation or inactivation of enzymes– Stimulation of mitosis

Mechanisms of Hormone Action

• Two hormone mechanisms trigger changes in the cell

• Figure 9.1

• Direct gene activation – the steroid hormone mechanism

• A second messenger system – nonsteroid (amino acid-based) mechanism

Steroid Hormone Action

• Diffuse through the plasma membrane of target cells – lipid-soluble

• Enter the nucleus

• Bind to a specific protein within the nucleus

• Bind to specific sites on the cell’s DNA

• Activate genes that result in synthesis of new proteins

Steroid Hormone Action

STEROID HORMONE ANIMATIONFigure 9.1a

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Nonsteroid Hormone Action

• Hormone binds to a membrane receptor• Hormone does not enter the cell• Sets off a series of reactions that activates

an enzyme• Catalyzes a reaction that produces a

second messenger molecule – cAMP or various other possiblities

• Oversees additional intracellular changes to promote a specific response

Nonsteroid Hormone Action

NONSTEROID HORMONE ANIMATIONFigure 9.1b

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Prostaglandins

• Group of biochemicals that also regulate cells like hormones

• Produced by many types of cells

• Act more locally than hormones

• Effect organs where they are produced

• Synthesized just before release, not stored in cells

Control of Hormone release

• Negative Feedback mechanisms – Regulate blood levels of hormones– Secretion is triggered (internal or external

stimulus), rising hormone levels inhibit further hormone release

– Blood levels of hormones vary within a narrow range

• 3 major categories of stimuli to activate endocrine organs– Hormonal stimulation – organs stimulated by

other hormones– Humoral stimulation – changing blood levels

stimulate release of hormones– Neural stimulation – nerve fibers stimulate

release of hormones

Hormonal Stimuli of Endocrine Glands

• Endocrine glands are activated by other hormones

• Hypothalamus hormones stimulate pituitary gland secretion

• Pituitary hormones stimulate other endocrine organs

Figure 9.2a

• Hormones secreted by thyroid, adrenal cortex, testis (final target glands) “feed back” to pituitary gland and inhibit more pituitary gland secretion

• When target gland secretion is too low, hypothalamus again stimulates the pituitary gland

Humoral Stimuli of Endocrine Glands

• Changing blood levels of certain ions stimulate hormone release

• Low blood Ca levels stimulate release of PTH

• When Ca levels rise, PTH release is inhibited

Figure 9.2b

Neural Stimuli of Endocrine Glands

• Nerve impulses stimulate hormone release

• Most are under control of the sympathetic nervous system

Figure 9.2c

• Sympathetic Nervous System stimulation of the adrenal gland releases norepinephrine and epinephrine during periods of stress.

Major Endocrine Organs

• Pituitary, thyroid, parathyroid, adrenal, pineal, thymus, pancreas, gonads (ovaries and testis)

• Hypothalamus is part of the CNS, but is also considered an endocrine organ since it produces several hormones

• Some glands are purely endocrine, others are mixed – endocrine and exocrine

• Exocrine glands – release substances onto body substances or into body cavities through ducts

• Endocrine glands – release substances into blood

Pituitary Gland

• Size of a grape

• Hangs by a stalk from the hypothalamus

• Protected by the sphenoid bone

• Has two functional lobes– Anterior pituitary – glandular tissue– Posterior pituitary – nervous tissue

Hormones of the Anterior Pituitary

• Six anterior pituitary hormones– Two affect non-endocrine targets

• Growth hormone • Prolactin

– Four stimulate other endocrine glands (tropic hormones)

• Thyrotropic hormone• Adrenocorticotropic hormone• 2 Gonadotropic hormones

Hormones of the Anterior Pituitary

Figure 9.4

• Characteristics of all anterior pituitary hormones– Proteins (or peptides)– Act through second-messenger systems– Regulated by hormonal stimuli, mostly

negative feedback

Hormones of the Anterior Pituitary

• Growth hormone - GH– Effects growth of skeletal muscle and long

bones– Builds up proteins– Cells grow in size and divide– Breaks down fats to produce energy

• Prolactin – PRL– Stimulates and maintains milk production in

mothers after birth– Function in males is not known

• Adrenocorticotropic hormone - ACTH – Regulates endocrine activity of adrenal cortex

• Thyroid-stimulating hormone– TSH– Also called thyrotropic hormone - TH – Influences growth and activity of thyroid gland

• Gonadotropic hormones• Follicle-stimulating hormone – FSH

– Matures follicles of ovaries, produces estrogen– In males, stimulates sperm development

• Luteinizing hormone – LH– Triggers ovulation of egg from the ovary– Produces progesterone and estrogen– In males, called interstitial cell-stimulating hormone,

stimulates testosterone production

Pituitary-Hypothalamus

• Anterior pituitary controls the activity of other glands : adrenals, thyroid, ovaries, testes

• Anterior pituitary is controlled by hypothalamus

• Hypothalamus produces 2 other hormones that are transported along neurosecretory axons to posterior pituitary – figure 9.5

Hormones of Posterior Pituitary

• Hormones are made by hypothalamus and transported to and stored by posterior pituitary

• Nerve impulse from hypothalamus triggers release of posterior hormones

• Oxytocin – Released during childbirth and nursing– Stimulates contraction of uterine muscles

• Antidiuretic hormone – ADH– Diuresis – urine production– ADH causes kidneys to reabsorb water from

urine, urine volume decreases, blood volume increases

– Alcoholic drinks inhibit ADH secretion resulting in increase in urine output, dehydration

– Diuretic drugs inhibit ADH

Thyroid gland

• Located at the base of the throat

• Consists of 2 lobes joined by a central isthmus

• Composed of hollow structures called follicles which are filled with a colloid which produces thyroid hormones

• Produces 2 iodine-containing hormones and calcitonin

Thyroid Hormones

• Thyroxine – T4

– Major hormone of the thyroid follicles– 4 iodine atoms– Controls metabolism

• Triiodothyronine - T3

– Formed by conversion of thyroxine at the target cell

– 3 iodine atoms

• Calcitonin – Decreases blood calcium levels by helping

calcium to be deposited in the bones– Made by C cells, parafollicular cells, in the

connective tissue around the follicles– Released into blood in response to high blood

levels of calcium

Parathyroid Glands

• Tiny masses of gland tissue found on posterior surface of thyroid

• Secretes parathyroid hormone which acts antagonistically to calcitonin

Parathyroid Hormone

• Parathyroid hormone – PTH– Increases calcium levels in the blood by

stimulating osteoclasts to break down bone tissue to increase calcium levels in blood

Adrenal Glands

• Bean-shaped gland on top of kidneys• Contains glandular cortex and neural tissue

medulla region• Adrenal cortex produces 3 groups of steroid

hormones called corticosteroids: mineralocorticoids, glucocorticoids, and sex hormones

• Adrenal medulla produces catecholamines: epinephrine and norepinephrine

Adrenal cortex Hormones

• Mineralcorticoids, mainly aldosterone, produced by outermost cortex cell layer– Regulates concentration of sodium and

potassium ions by causing kidney to absorb sodium ions and water, and excrete potassium ions

– Increases blood volume and blood pressure

• Glucocorticoids including cortisone and cortisol are produced by the middle cortical layer – Promote normal cell metabolism, especially

breakdown of fats and proteins, to help the body resist long-term stresses (Figure 9.12)

– Increase blood glucose levels– Control inflammation– Regulated by blood levels of ACTH

• Sex hormones including androgens and estrogens produced by innermost cortex layer– Both are produced regardless of gender

Adrenal Medulla Hormones

• Stimulated by sympathetic nervous system, releases catecholamines epinephrine and norepinephrine

• Epinephrine - also called adrenaline

• Coping with short-term stressful situations– Increase heart rate, blood pressure, blood

glucose level, dilate air passageways in lungs– Increase oxygen to the brain

Controlling stress

• Glucocorticoids ( ) vs. Catecholamines ( )

• Figure 9.12

• Cortisone & cortisol – adrenal cortex– Cope with prolonged or continuing stressors,

resistance stage

• Epinephrine – adrenal medulla– Prepare body for alarm stage of stress

Pancreas

• Located close to the stomach in abdominal cavity

• A mixed gland – Figure 9.13– Exocrine cells – acinar tissue – work with the

digestive system, produce enzymes– Endocrine gland cells – pancreatic islets –

scattered throughout acinar tissue, produce 2 hormones: insulin & glucagon

– Insulin produced in beta cells, glucagon produced in alpha cells

Pancreatic Hormones

• Insulin – beta cells– Increases cells ability to transport glucose

from the blood into cell through the cell membrane

– Speeds up cell’s ability to convert glucose into energy or glycogen to be stored in the liver

– Insulin secretion is stimulated by high glucose levels in blood. As glucose levels decrease, insulin release ends. Negative feedback

– Without insulin, no glucose gets into cells

• Glucagon – alpha cells– Antagonist to insulin– Stimulates liver to break down glycogen to

glucose and release glucose into blood– Stimulated by low blood glucose levels

• Figure 9.14 – antagonistic effects of insulin and glucagon

Pineal Gland

• Small, cone-shaped

• Roof of third ventricle of brain

• Only one hormone secreted, melatonin– “sleep trigger”– Establishes the body’s day-night cycle– Peak levels at night cause drowsiness, low

levels at daylight

Thymus Gland

• Upper thorax, posterior to sternum

• Large in infants and children

• Size decreases in adulthood

• Produces thymosin– Incubator to help white blood cells mature– Important role in immune response

Gonads

• Male and female gonads produce same sex hormones as those produced in adrenal cortex

• Female gonads – ovaries

• Male gonads - testes

Ovaries

• Almond-shaped, located in pelvic cavity

• Produce sex cells, eggs

• Produce 2 steroid hormones: estrogen and progesterone

• Do not function until gonadotropic hormones of anterior pituitary gland stimulate them at puberty

Ovarian Hormones

• Estrogen– stimulates maturation of reproductive organs– Prepares uterus to receive fertilized egg by causing

cyclic changes in uterine lining – menstrual cycle

• Progesterone – Acts with estrogen to bring about menstrual cycle– Assures that uterine muscle will not abort implanted

embryo– Produced by corpus luteum, glandular structure of

ovary

Testes

• Suspended in a sac outside pelvic cavity

• Produce sex cells, sperm

• Produce testosterone– Stimulates maturation of reproductive organs– Necessary for production of sperm– Stimulated by gonadotropic hormone of

anterior pituitary hormone at puberty

Development of Endocrine System

• Embryonic development varies– Pituitary – epithelium of oral cavity, neural tissue of

hypothalamus– Pineal – neural tissue– Thyroid, thymus, pancreas – mucosa of digestive tract

• Late middle age – estrogen production declines– menopause

• Most endocrine organs function well until old age