1. The Endocrine System

2. Endocrine Glands Ductless glands Include
Release their hormone into the surrounding tissue fluid
Typically have rich vascular and lymphatic drainage to receive the hormone
Include
Neuroendocrine organ
Hypothalamus
Endocrine glands
Pituitary, Thyroid, Parathyroid, Adrenal, Pineal, and Thymus
Areas of endocrine tissue in the organ
Pancreas and gonads (ovaries and testes)

3. The Chemistry of Hormones
Hormones are chemical substances, secreted by cells into the extracellular fluids, tat regulate the metabolic function of other cells in the body
Amino acid based
Water soluble
Cannot cross the plasma membrane
Steroid based
Fat soluble
Can cross the plasma membrane

4. Mechanism of Hormone Action
Target cells
The tissue cells the hormone is influencing
Alter cell activity by increasing or decreasing normal cellular processes
Hormonal stimulus typically produces one or more of the following changes
Alters the plasma membrane permeability or membrane potential, or both, by opening or closing ion channels
Stimulates synthesis of proteins or regulatory molecules such as enzymes within the cell
Activates or deactivates enzymes
Induces secretory activity
Stimulates mitosis

5. Mechanism of Action for Amino Acid Based Hormones
Step 1 – the hormone, acting as the first messenger, binds to its receptor
Step 2 – hormone binding causes the receptor to change shape, allowing it to bind a G protein and activate it (turn it on)
Step 3 – activated G protein binds to the effector enzyme adenylate cyclase and can either stimulate or inhibit its action
Step 4 – if stimulated, adenylate cyclase generates the second messenger cAMP
Step 5 – cAMP triggers a cascade of chemical reactions beginning with the activation of protein kinase A
Step 6 – protein kinase A triggers responses of target cell (activates enzymes, stimulates cellular secretion, opens ion channels, etc.) A variety of reactions may occur in the same target cell at the same time.

6. Mechanism of Action for Steroid Based Hormones
Step 1 – hormone passes through the plasma membrane and into the nucleus
Step 2 – hormone binds to and activates its receptor
Step 3 – hormone-receptor complex then binds to hormone response elements on the DNA strand
Step 4 – gene is activated and a protein is made

7. Mechanism of Hormone Action cont.
In order for a target cell to respond to a hormone, the cell must have specific protein receptors on its plasma membrane or in its interior to which that hormone can bind
A hormone receptor responds to hormone biding by prompting the cell to perform, or turn on, some gene-determined “preprogrammed” function
Target cell activation depends on
Blood levels of the hormone
Numbers of receptors on the target cell
Affinity of the bond between the hormone and receptor

8. Mechanism of Hormone Action cont.
Up-regulation
Response of target cell where it forms more receptors in response to rising blood levels of its specific hormone
Down-regulation
Response of target cell where it removes receptors and responds less vigorously in response to prolonged exposure of its specific hormone
Permissiveness
One hormone cannot exert its full effects without another hormone being present
Synergism
Two hormones amplify the effects of each other so that the response is more than if each acted alone
Antagonism
Two hormones act to cancel each other out

9. Mechanism of Hormone Action cont.
Most hormones are regulated by a negative feedback mechanism
Humoral Stimuli
Secrete hormones in direct response to changing blood levels of certain critical ions and nutrients
Neural Stimuli
Nerve fibers stimulate hormone release
Hormonal Stimuli
Release hormones in response to hormones produced by other endocrine glands

10. Major Endocrine Organs

11. The Pituitary Gland
Attached to the hypothalamus by a stalk called the infundibulum
Posterior lobe
Stores and releases hormones made by the hypothalamus
Releases oxytocin and antidiuretic hormone (ADH) (also called vasopressin)
Anterior lobe
Manufactures and secretes growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL)
Also releases the prohormone pro-opiomelanocortin (POMC) which is used to make adrenocorticotropic hormone, two natural opiates (enkephalin and beta endorphin), and melanocyte-stimulating hormone (MSH)
Secretion is controlled by the hypothalamus

14. Growth Hormone (GH) Target organs Effects Stimulated by Inhibited by
All cells but mainly liver, skeletal muscle, bone, and cartilage
Effects
Stimulates somatic growth
Mobilizes fats
Has a daily rhythm with the highest levels occurring during evening sleep
Secretion peaks during adolescence and then declines with age
Stimulated by
Low blood levels of GH, hypoglycemia, increase blood levels of amino acids, low levels of fatty acids, exercise and other types of stressors, and estrogens
Inhibited by
Feedback inhibition, hyperglycemia, hyperlipidemia, obesity, and emotional deprivation

15. Thyroid-Stimulating Hormone (TSH)
Target organs
Thyroid gland
Effects
Stimulates normal development and secretory activity of the thyroid gland
Stimulated by
Hormones sent from the hypothalamus
Indirectly by pregnancy and, in infants, cold temperatures
Inhibited by
Negative feedback

16. Adrenocorticotropic Hormone (ACTH)
Target organs
Adrenal cortex
Effects
Promotes release of glucocorticoids and androgens
Help the body resist stressors
Has a daily rhythm with levels peaking in the morning shortly before awakening
Stimulated by
Fever, hypoglycemia, and other stressors
Inhibited by
Negative feedback

17. Follicle-Stimulating Hormone (FSH)
Target organ
Ovaries and testes
Effects
In females it stimulates ovarian follicle maturation and estrogen production
In males it stimulates sperm production
Stimulated by
Hormones sent by the hypothalamus
Inhibited by
Negative feedback

18. Luteinizing Hormone (LH)
Target organ
Ovaries and testes
Effects
In females it triggers ovulation and stimulates ovarian production of estrogen and progesterone
In males it promotes testosterone production
Stimulated by
Hormones sent by the hypothalamus
Inhibited by
Negative feedback

19. Prolactin (PRL) Target organ Effects Stimulated by Inhibited by
Breast secretory tissue
Effects
Promotes lactation
Stimulated by
Hormones released by the hypothalamus
Release enhanced by estrogen, birth control pills, breast-feeding, and dopamine blocking drugs
Inhibited by
Dopamine

20. Oxytocin Target organ Effects Stimulated by Inhibited by
Uterus and breasts
Effects
Stimulates uterine contractions, initiates labor, initiates mild ejection
Acts as a feel good hormone that creates bonding behaviors
Plays a role in sexual arousal and orgasm
Plays a role in nurturing and affectionate behavior
Stimulated by
Stretching of the cervix and uterus and suckling of an infant during breastfeeding
Positive interactions between people
Positive feedback
Inhibited by
Lack of appropriate neural stimuli

21. Antidiuretic Hormone (ADH)
Also known as vasopressin
Target organs
kidneys
Effects
Stimulates kidney tubule cells to reabsorb water
Prevents wide swings in water balance helping prevent dehydration and water intoxication
Incases of severe blood loss it causes vasoconstriction that increases blood pressure
Has a daily rhythm with highest levels occurring during evening sleep
Prevents frequent need to wake up to urinate
Children who wet the bed and shift workers often have disruption in the rhythm
Stimulated by
Increased osmolality of blood, decreased blood volume, pain, some drugs (nicotine, morphine, and barbiturates), low blood pressure
Inhibited by
Adequate hydration and alcohol
Drugs used to treat high blood pressure and congestive heart failure

22. The Thyroid Gland
Located in the anterior neck, on the trachea just inferior to the larynx
Manufactures and secretes thyroid hormone (TH)
Often referred to as the body’s major metabolic hormone
Made up of two iodine containing hormones, thyroxine (T4) and triiodothyronine (T3)
Synthesis of TH is stimulated by TSH from the pituitary gland
Manufactures and secretes calcitonin

25. Thyroid Hormone Target organ Effects Stimulated by Inhibited by
Except for the adult brain, spleen, testes, uterus, and the thyroid gland itself it effects every cell in the body
Effects
Stimulates enzymes concerned with glucose oxidation it increases basal metabolic rate and body heat production
Helps maintain blood pressure
Regulates tissue growth and development and is especially critical for normal skeletal and nervous system development and maturation and for reproductive capabilities
Stimulated by
Falling TH blood levels trigger release of TSH and ultimately more TH
Inhibited by
Negative feedback
Rising levels of glucocorticoids
Excessively high blood iodine concentration

26. Calcitonin Target organ Effects Stimulated by Inhibited by Skeleton
Lower blood calcium levels
Direct antagonist of parathyroid hormone
Regulation is short lived but rapid
Appears to be only important in childhood
Stimulated by
Excessive blood calcium levels
Inhibited by
Declining blood calcium levels

27. The Parathyroid Gland
Tiny, yellow-brown glands are located in the posterior aspect of the thyroid gland

30. Parathyroid Hormone (PH)
Target organs
Bone, intestines, kidney
Effect
Calcium balance of the blood
Calcium and phosphorus released from the bone into the blood
Increased calcium absorption from food
Promotes activation of vitamin D
Increases calcium reabsorption in kidney
Stimulated by
Falling blood calcium
Inhibited by
Excessive blood calcium levels

31. The Adrenal Glands
Paired adrenal glands are pyramid shaped organs sitting atop the kidneys where they are enclosed in a fibrous capsule and a cushion of fat
Hormones help us cope with stressful situations
Inner adrenal medulla
Synthesize catecholamines
epinephrine and norepinephrine
Outer adrenal cortex
Secretes over two dozen steroid hormones collectively called corticosteroids
Mineralocorticoids, glucocorticoids, gonadocorticoids

34. Mineralocorticoids Mainly aldosterone Target organ Effect
Distal part of the kidney tubules
Effect
Regulation of the electrolyte concentrations in extracellular fluids
Stimulates reabsorption of Na+ from the forming urine, perspiration, saliva, and gastric juice
Also causes water retention and K+ excretion
Stimulated by
Rising blood levels of K+, low levels of Na+, and decreasing blood volume
Inhibited by
Decreasing blood levels of K+, high levels of blood Na+, and increasing blood volume

35. Glucocorticoids Mainly cortisol Target organ Effect Stimulated by
Body cells
Effect
Keep blood glucose levels fairly constant and maintain blood pressure and resist stressors
Promote gluconeogenesis and hyperglycemia, mobilize fat for energy, stimulate protein breakdown, depress inflammatory and immune responses
Levels peak right before waking and wane just after we fall asleep
Cycles are interrupted by acute stress
Stimulated by
ACTH
Inhibited by
Negative feedback

36. Gonadocorticoids Mainly androgens Target organ Effect Stimulated by
These are later converted to testosterone and estrogen
Target organ
Sex organs
Effect
Onset of puberty and development of secondary sex characteristics
Female libido
Estrogen after menopause
Stimulated by
ACTH
Inhibited by
Not known but does not seem to be negative feedback

37. Catecholamines Epinephrine and Norepinephrine Target organ Effect
Those affected by the sympathetic nervous system
Effect
Increase heart and metabolic rate, increase blood pressure by vasoconstriction
Flight or fight response
Stimulated by
sympathetic nervous system
Inhibited by
Lack of neuronal stimulus

38. The Pancreas Located behind the stomach in the abdomen
Produce an enzyme rich juice that is ducted into the small intestine during food digestion
Alpha cells produce glucagon
Beta cells produce insulin

41. Glucagon Target organ Effect Stimulated by Inhibited by Liver
Hyperglycemic agent
Breakdown glycogen to glucose
Synthesis of glucose from lactic acid and noncarbohydrate molecules
Release of glucose to the blood by liver cells
Fall in amino acid blood concentration
Stimulated by
Falling blood glucose levels, sympathetic nervous system stimulation, rising amino acid levels
Inhibited by
Rising blood glucose, insulin, somatostatin

42. Insulin Target organ Effect Stimulated by Inhibited by Body cells
Lower blood glucose
Enhancing membrane transport of glucose into body cells (especially muscle and fat)
Inhibits the breakdown of glycogen to glucose and conversion of amino acids or fats to glucose
So would also cause the production of glycogen, amino acids, and fats
Stimulated by
Elevated blood glucose levels, rising levels of amino acids and fats in the blood, release of acetylcholine by parasympathetic nerve fibers
Inhibited by
Falling blood glucose levels

43. The Gonads
Produce steroid sex hormones identical to those produced by adrenal cortical cells just in larger amounts
Ovaries
Small, oval organs located in the female’s abdominopelvic cavity
Produce estrogen and progesterone
Testes
Located in the extra-abdominal skin pouch called the scrotum
Produce testosterone
Regulated by gonadotropin release

46. Estrogen
Responsible for maturation of the reproductive organs and appearance of secondary sex characteristics
Acting with progesterone, promote breast development and cyclic changes in the uterine mucosa

47. Progesterone
Acting with estrogen, promote breast development and cyclic changes in the uterine mucosa

48. Testosterone
Initiates the maturation of the male reproductive organs, appearance of secondary sex characteristics, and sex drive
Necessary for sperm production

49. The Pineal Gland
Tiny, pine cone shaped gland that hangs from the roof of the third ventricle in the diencephalon

52. Melatonin
Powerful antioxidant and amine hormone derived from serotonin
Concentrations rise and fall in a daily cycle
Peak levels occur during the night to make us drowsy
Lowest levels occur around noon
Indirectly receives input from the visual pathways concerning intensity and duration of sunlight
In some animals mating behavior and gonadal size are controlled this way
May affect body temperature, sleep, and appetite cycles

53. Other Hormones Gastrin, Secretin, and Cholecystokinin (CCK)
Erythropoietin (EPO)
Renin
Cholecalciferol
Atrial natriuretic peptide
Leptin
Resistin