Ch. 16 Endocrine System

Functions of the endocrine system in maintaining homeostasis.

• release hormones into the bloodstream to influence metabolic activities and regulate metabolism

• when stressed, hormones release to respond to stressors

General mechanisms by which hormones work.

• hormones are long-distance chemical signals that travel in blood/lymph thruout the body

• its solubility in water affects how a hormone is transported in the blood, how long it lasts b4 it is degraded, and what receptors it can act upon

• a hormone can alter plasma membrane permeability/membrane potential by opening/closing ion channels, stimulates synthesis of enzymes & other proteins within the cell, activates/deactivates enzymes, induce secretory activity, and stimulates mitosis

Differentiate the major chemical classes of hormones.

• amino acid based:

  • most hormones are in this class and are usually water soluble and cannot cross plasma membrane

• steroids:

  • synthesized from cholesterol (only gonadal and adrenocortical) so they’re lipid soluble (can cross plasma membrane)

• eicosanoids:

  • include leukotrienes (signal for inflammation) and prostaglandins

Compare and contrast the receptors of each class; and the mechanism of response each class elicits.

• water-soluble hormones (all amino acid-based ones except thyroid) act on receptors in plasma membrane and are usually coupled by regulatory molecules called G proteins to 1+ intracellular second messengers

• lipid soluble (steroid & thyroid) act on receptors inside cell, which activates genes

• amino acid-based hormones work thru intracellular second messengers (cyclic AMP)

  1. Hormone binds receptor (first messenger) in plasma membrane

  2. Receptor activates an inactive G protein by changing shape, which allows GDP(off) to be displaced by GTP (on)

  3. G protein activates adenylate cyclase. activated g protein binds to adenylate cyclase

  4. adenylate cyclase converts ATP to cyclic AMP

  5. cyclic AMP activates protein kinases (enzymes that phosphorylate others) and may affect some processes

Discuss the control of endocrine organs.

• Humoral stimuli (bodily fluids)

  • hormones respond to changing blood levels (parathyroid hormone & insulin)

• neural stimuli

  • nerve fibers stimulate hormone release, such as stress (norepinephrine & epinephrine)

• hormonal stimuli

  • respond to other hormones produced by other endocrine glands

Describe the role of the hypothalamus in endocrine control.

• the hypothalamus controls release of hormones from the pituitary gland in 2 ways:

  1. posterior pituitary (neurohypophysis) release neurohormones received from hypothalamus. since this lobe is more for storage, it’s not considered a true endocrine gland.

  2. anterior pituitary (adenohypophysis) is a true glandular structure & releases hormones thru the bloodstream

• the posterior pituitary release these hormones

  • Oxytocin: uterine contractions & nursing (uterus & breasts) & comes from neurons in the paraventricular

  • Antidiuretic hormone: prevents urine formation (kidneys) & comes from the supraoptic nuclei

• the anterior pituitary releases these hormones

  • Growth hormone: growth (liver, muscle, bone)

  • thyroid-stimulating hormone: stimulates thyroid gland to release thyroid hormone (thyroid)

  • adrenocorticotropic hormone: promotes release of glucocorticoids & androgens (adrenal cortex)

  • follicle-stimulating hormone: stimulates ovarian follicle maturation & production of estrogen & stimulates sperm production (ovaries & testes)

  • luteinizing hormone: triggers ovulation & estrogen release & testosterone production (ovaries & testes)

  • prolactin: promotes lactation (breasts)

Describe the location, hormones, and functions of the following endocrine glands: pituitary, thyroid, parathyroid, adrenal, pancreas, ovaries, testes, pineal, and thymus.

Pituitary:

• seated in the sella turcica of the sphenoid bone

• releases oxytocin, antidiuretic hormone, growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, & prolactin

• produces several hormones to regulate bodily functions

Thyroid:

• located in anterior neck, on the trachea (largest pure endocrine structure)

• releases thyroid hormone (thyroxine & triiodothyronine) & calcitonin

• increases basal metabolic rate, body heat production, regulates tissue growth & development, and maintains BP

Parathyroid:

• found in the posterior of the thyroid gland, as small, yellow-brown structures

• releases parathyroid hormone

• controls Ca balance in the blood

Adrenal:

• found atop of the kidneys cushioned in fat

• releases mineralocorticoids (aldosterone), glucocorticoids (cortisol), gonadocorticoids (androgens), & catecholamines (epinephrine & norepinephrine)

• mostly cope with stress

Pancreas:

• located partially behind stomach

• α cells release glucagon (less numerous & hyperglycemic) & β cells release insulin (more numerous & hypoglycemic)

• regulate blood glucose levels

Ovaries:

• located in the female’s abdominopelvic cavity

• produces estrogen & progesterone

• maturation of reproductive organs, & puberty (such as breast development & menstruation)

Testes:

• located in scrotum

• produces testosterone

• maturation of reproductive organs, enhanced sex drive, & sperm production

Pineal:

• hangs from roof of diencephalon

• secretes melatonin (which is derived from serotonin)

• regulation of circadian rhythms

Thymus:

• located deep to the sternum

• secrete hormones that stimulate the development of T-cell maturation

Discuss the roles of the major stress-response hormones.

• for short-term stress, the adrenal medulla releases epinephrine & some norepinephrine (acute)

• for long-term stress, the adrenal cortex releases glucocorticoids (chronic)