Endocrine System: Pineal Gland

Study Notes

Attached to the roof of the third ventricle, beneath the posterior end of the corpus callosum
Undergoes involution (shrinkage) after age 7, down 75% by the end of puberty
Synchronizes physiological functions with 24-hour circadian rhythms of daylight and darkness
Synthesizes melatonin from serotonin during the night, with fluctuations seasonally with changes in day length

Plays a role in three systems: endocrine, lymphatic, and immune
Located in the mediastinum superior to the heart
Undergoes involution after puberty
Site of maturation of T cells, important in immune defense
Secretes hormones (thymopoietin, thymosin, and thymulin) that stimulate development of other lymphatic organs and activity of T lymphocytes

Largest endocrine gland, composed of two lobes and an isthmus below the larynx
Dark reddish brown color due to rich blood supply
Normally not palpable and not visible
If enlarged in size, called Goiter
Secretes thyroxine (T4) and triiodothyronine (T3)
T4 is converted to T3, increasing metabolic rate, O2 consumption, heat production, appetite, growth hormone secretion, and ale-ness
Parafollicular (C or clear) cells secrete calcitonin, stimulating osteoblast activity and bone formation

a-Largest endocrine gland: True
b-Two lobes and an isthmus below the larynx: True
c-Secretes thyroxine (T4) and triiodothyronine (T3): True
d-Thyroid hormones decrease metabolic rate, O2 consumption, heat production, appetite: False
e-Parafollicular cells secrete calcitonin: True

Usually four glands, partially embedded in the posterior surface of the thyroid gland
Connective tissue capsule

Small gland that sits on top of each kidney, retroperitoneal like the kidney
Adrenal cortex and medulla formed by the merger of two fetal glands with different origins and functions
Adrenal medulla: inner core, 10% to 20% of the gland, acts as an endocrine gland and sympathetic ganglion of the sympathetic nervous system
Adrenal cortex: surrounds the adrenal medulla, produces more than 25 steroid hormones called corticosteroids or corticoids

Inervated by sympathetic fibers
When stimulated, releases catecholamines (epinephrine and norepinephrine) and a trace of dopamine directly into the bloodstream
Effect is longer lasting than neurotransmitters
Increases ale-ness and prepares the body for physical activity
Increases blood pressure, heart rate, blood flow to muscles, pulmonary air flow, and metabolic rate
Decreases digestion and urine production

Secretes three main types of steroid hormones:
- Mineralocorticoids ( zona glomerulosa): regulate the body's electrolyte balance, e.g., aldosterone stimulates Na+ retention and K+ excretion
- Glucocorticoids (zona fasciculata): especially cortisol, stimulates fat and protein catabolism, gluconeogenesis, and anti-inflammatory effect
- Sex steroids (zona reticularis): androgens and estrogens, set libido throughout life

Exocrine digestive gland and endocrine cell clusters (pancreatic islets) found retroperitoneal, inferior, and posterior to the stomach
Beta cells: secrete insulin, lowering blood glucose levels
Alpha cells: secrete glucagon, released between meals when blood glucose concentration is falling
Delta cells: secrete somatostatin, partially suppresses secretion of glucagon and insulin
G cells: secrete gastrin, stimulates stomach acid secretion, motility, and emptying

Ovarian hormones: estradiol, progesterone, and inhibin
Functions of estradiol and progesterone:
- Development of female reproductive system and physique, including adolescent bone growth
- Regulation of menstrual cycle, sustaining pregnancy, and preparing mammary glands for lactation
Inhibin: suppresses FSH secretion from the anterior pituitary
Testicular hormones: testosterone, androgens, estrogen, and inhibin
Functions of testosterone:
- Stimulates development of male reproductive system in the fetus and adolescent, and sex drive
- Sustains sperm production

Skin: keratinocytes convert a cholesterol-like steroid into cholecalciferol using UV from the sun
Liver: calcidiol, angiotensinogen, and erythropoietin
Kidneys: convert calcidiol to calcitriol, the active form of vitamin D, and secrete renin and erythropoietin
Heart: cardiac muscle secretes natriuretic peptides, decreasing blood volume and blood pressure
Stomach and small intestine: secrete at least 10 enteric hormones, coordinating digestive motility and glandular secretion