I. The Endocrine System

A. Functions to maintain stability of internal environment.

1. Performed by chemical messengers sent to specific cells.

2. Hormones sent to specific target cells by blood stream.

B. Endocrine glands secrete products directly into blood.

II. Classification of Hormones

A. General function

1. Tropic hormones target other endocrine glands.

2. Sex hormones target reproductive tissues.

3. Anabolic hormones stimulate anabolism in target cells.

B. Chemical structure

1. Steroid hormones: manufactured from cholesterol.

2. Nonsteroid hormones: synthesized from amino acids.

3. Protein hormones

4. Glycoprotein hormones

5. Peptide hormones

6. Amino-acid derivative hormones

C. Prostaglandins: tissue hormones. Function to integrate activities of neighboring cells.

III. How hormones work

A. Mechanisms of steroid hormone action

1. receptors normally found in cytoplasm of target cell

2. hormone diffuses into target cell

3. binds to receptor molecule: hormone-receptor complex.

4. HRC migrates into nucleus.

5. activates genetic sequence to transcribe mRNA.

6. mRNA moves out to ribosome.

7. protein synthesis begins.

8. more hormone receptor complexes formed= more protein molecules.

9. responses often slow

B. Mechanisms of non-steroid hormone action

1. second messenger mechanism

2. nonsteroid hormone acts as first messenger.

3. message passed into cell; second messenger triggers change.

4. differs from steroid hormone effects

5. disproportionately great effect.

6. operates much more quickly.

C. Regulation of Hormone Secretion

1. Negative feedback loop

2. Endocrine cell is sensitive to physiological changes produced by its target cells.

a. Example: PTH produces responses in its target cells that increase calcium concentration in blood.

b. Calcium concentration exceeds set point value

c. Parathyroid cells sense it, reduce output of PTH.

3. Secretion by many glands is regulated by hormones from a gland.

a. anterior pituitary produces TSH to stimulate thyroid gland.

b. anterior pituitary responds to changes in blood concentration.

c. hypothalamus secretes releasing and inhibiting hormones.

4. Hypothalamus is responsive to changes in controlled variable.

5. Input from nervous system

IV. Endocrine Glands

A. Pituitary Gland

1. Structure

a. infundibulum: stalk.

b. 2 glands

1. adenohypophysis: anterior pituitary made of endocrine tissue.

2. neurohypophysis: posterior pituitary made of neurosecretory tissue.

2. Location: ventral surface of brain in pituitary fossa of sella turcica.

B. Anterior Pituitary

1. Structure

a. 2 parts. pars anterior, pars intermedia.

b. tissue in irregular clumps of secretory cells.

2. Hormones Produced

a. Growth Hormone (GH) also called somatotropin- STH.

1. promotes protein anabolism for growth and tissue repair.

2. indirectly inhibits glucose metabolism.

3. indirectly increases blood glucose levels.

b. Prolactin

1. initiates milk secretion

2. supportive role with LH in maintaining corpus luteum.

c. Tropic hormones: have stimulating effect on endocrine glands.

1. Thyroid-stimulating hormone (TSH): promotes and maintains growth and development of thyroid.

2. Adrenocorticotropic hormone (ACTH): promotes, maintains normal growth and development of the cortex of the adrenal gland.

3. Gonadotropins: stimulate growth and maintenance of gonads.

a. Follicle stimulating hormone (FSH): in females, stimulates primary follicles in ovaries to grow toward maturity; stimulates follicle cells to secrete estrogens. in males, stimulates development of seminiferous tubules of testes, maintains spermatogenesis.

b. Luteinizing hormone (LH): in females, stimulates formation and activity of corpus luteum. in males, stimulates interstitial cells in testes to develop and secrete testosterone.

4. Control of secretion: releasing hormones from hypothalamus.

C. Hypothalamus hormones

1. Growth hormone releasing hormone

2. Growth hormone inhibiting hormone

3. Corticotropin-releasing hormone

4. Thyrotropin-releasing hormone

5. Gonadotropin releasing hormone

6. Prolactin releasing hormone

7. Prolactin inhibiting hormone

D. Posterior Pituitary

1. storage and release site: antidiuretic hormone, oxytocin.

2. release controlled by nervous stimulation.

3. Hormones Released

a. Antidiuretic hormone: prevents formation of large volume of urine.

b. Oxytocin

1. causes milk ejection from breasts of lactating women.

2. stimulates uterus to strengthen strong, muscular labor contractions.

E. Pineal Gland

1. Structure: tiny pine cone shaped structure.

2. Function: regulates hunger, sleeping, reproductive cycles, behavior.

3. Melatonin appears to inhibit LH secretion.

F. Thyroid gland

1. Structure: two large lateral lobes with a narrow connecting isthmus.

composed of follicles. interior filled with thyroid colloid.

2. Location: in neck on anterior and lateral surfaces of trachea.

3. Hormones

a. Thyroid hormone (TH) made of tetraiodothyronine (T₄) and triiodothyronine (T₃).

b. Calcitonin influences processing of Ca by bone cells.

G. Parathyroid glands

1. Location: posterior surface of thyroid’s lateral lobes.

2. Parathyroid hormone (PTH): acts on bone, kidneys, and intestinal cells by increasing their input of Ca into blood.

H. Adrenal Glands

1. Location: fit like cap over kidneys

2. Structure

a. Adrenal cortex: outer portion made of regular endocrine tissue.

1. Aldosterone most important of the mineralocorticoids. primary function seems to be maintenance of sodium homeostasis.

2. Glucocorticoids: cortisol, cortisone, corticosterone.

3. Gonadocorticoids: sex hormones released by cortex.

b. Adrenal medulla: inner portion made of neurosecretory tissue. epinephrine and norepinephrine prolong and enhance effects of sympathetic stimulation by ANS.

I. Pancreatic Islets

1. Structure: Pancreas 12-15 cm long.

2. Hormones

a. Glucagon: increases blood glucose levels. produces hyperglycemic effect.

b. Insulin: promotes movement of glucose, amino acids, fatty acids out of blood into tissue cells.

c. Somatostatin: primary role is regulating other endocrine cells of pancreatic islands.

d. Pancreatic polypeptide: influences digestion, distribution of food molecules.

J. Gonads

1. Testes: interstitial cells scattered among seminiferous tubules produce androgens.

a. Testosterone, the principal androgen, responsible for growth and maintenance of male sex characteristics, sperm production.

b. regulated by gonadotropin levels in blood.

2. Ovaries: produces sex hormones

Estrogens: steroid hormones; promote development and maintenance of female sex characteristics.

Progesterone: secreted by corpus luteum, maintains lining of uterus. necessary for successful pregnancy.

K. Placenta

1. Human Chorionic Gonadotropin (hCG): stimulates development and secretion by ovarian tissues.

L. Thymus

1. Location: gland in mediastinum just beneath sternum.

M. Gastric and Intestinal Mucosa

1. Gastrin, Secretin, Cholecystokinin-pancreazymin (CKK) coordinate secretory and motor activities involved in the digestive process.

N. Heart

1. Atrial natriuretic hormone: increases secretion in response to increase in stretch of atrial wall caused by abnormally high blood pressure or blood volume.

2. Principal Effect: promote loss of sodium in urine, resulting in water loss.

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