Endocrine System: Pineal Gland

Endocrine System

• The endocrine system consists of glands that produce and secrete hormones that regulate various bodily functions.

Pineal Gland

• Located beneath the posterior end of the corpus callosum, attached to the roof of the third ventricle.
• Undergoes involution (shrinkage) after age 7, decreasing by 75% by the end of puberty.
• Synthesizes melatonin from serotonin during the night, which synchronizes physiological functions with 24-hour circadian rhythms.
• Fluctuates seasonally with changes in daylight.

Thymus

• Located in the mediastinum, superior to the heart.
• Plays a role in the endocrine, lymphatic, and immune systems.
• Goes through involution after puberty.
• Site of maturation of T cells, which are important in immune defense.
• Secrete hormones (thymopoietin, thymosin, and thymulin) that stimulate the development of other lymphatic organs and activity.

Thyroid Gland

• Largest endocrine gland, composed of two lobes and an isthmus below the larynx.
• Dark reddish-brown color due to the presence of iodine atoms.
• Secrete hormones thyroxine (T4) and triiodothyronine (T3), which:
  • Increase metabolic rate, oxygen consumption, heat production, appetite, and growth hormone secretion.
  • Stimulate osteoblast activity and bone formation.
• Parafollicular (C) cells secrete calcitonin, which:
  • Stimulates osteoblast activity and bone formation.
  • Decreases blood calcium levels.

Parathyroid Glands

• Usually four glands partially embedded in the posterior surface of the thyroid gland.
• Secrete parathyroid hormone (PTH), which:
  • Increases blood calcium levels.
  • Promotes synthesis of calcitriol.
  • Increases absorption of calcium.
  • Increases bone resorption.

Adrenal Glands

• Small glands located 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:
  • Acts as an endocrine gland and sympathetic ganglion of the sympathetic nervous system.
  • Innervated by sympathetic fibers.
  • When stimulated, releases catecholamines (epinephrine and norepinephrine) and a trace of dopamine directly into the bloodstream.
  • Increases alertness, prepares body for physical activity, and increases blood pressure, heart rate, blood flow to muscles, and metabolic rate.
• Adrenal cortex:
  • Produces more than 25 steroid hormones called corticosteroids or corticoids.
  • Secrete five major steroid hormones from three layers of glandular tissue:
    • Zona glomerulosa (thin, outer layer): secretes mineralocorticoid (aldosterone) that regulates the body's electrolyte balance.
    • Zona fasciculata (thick, middle layer): secretes glucocorticoids, especially cortisol, which stimulates fat and protein catabolism, gluconeogenesis, and has anti-inflammatory effects.
    • Zona reticularis (narrow, inner layer): secretes sex steroids, androgens, and estrogens.

Pancreatic Islets

• Exocrine digestive gland and endocrine cell clusters (pancreatic islets) found retroperitoneal, inferior, and posterior to the stomach.
• Pancreatic islets:
  • Beta cells: secrete insulin, which lowers blood glucose levels.
  • Alpha cells: secrete glucagon, which increases blood glucose levels.
  • Delta cells: secrete somatostatin, which partially suppresses secretion of glucagon and insulin.
  • G cells: secrete gastrin, which stimulates stomach acid secretion, motility, and emptying.

Gonads

• Ovarian hormones:
  • Estradiol, progesterone, and inhibin.
  • Functions: development of female reproductive system and physique, regulate menstrual cycle, sustain pregnancy, and prepare mamma glands for lactation.
• Testicular hormones:
  • Testosterone, androgens, estrogen, and inhibin.
  • Functions: development of male reproductive system, sustain sperm production, and regulate sex drive.

Endocrine Functions of Other Tissues and Organs

• Skin: keratinocytes convert a cholesterol-like steroid into cholecalciferol using UV from the sun.
• Liver: secretes 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, which decrease blood volume and blood pressure.
• Stomach and small intestine: secrete at least 10 enteric hormones that coordinate digestive motility and glandular secretion.

Endocrine Disorders

• No specific details mentioned in the text.