Endocrine System and Pituitary Gland

The Endocrine System

• The endocrine system maintains homeostasis (the body's internal balance).
• Endocrine glands secrete hormones that regulate metabolic processes.
• The major endocrine glands include the pituitary, thyroid, parathyroid, adrenal glands, pancreas, ovaries, testes, and the thymus and pineal glands.

Pituitary Gland and Hypothalamus

• The pituitary gland is located in the hypophyseal fossa of the sphenoid bone, below the hypothalamus.
• The pituitary gland and hypothalamus work together using negative feedback regulation.
• The hypothalamus secretes hormones that stimulate or inhibit the release of hormones from the anterior pituitary.

Hormones Secreted by the Anterior Pituitary

• Growth Hormone (GH) promotes growth in children and maintains and repairs tissues in adults.
• Prolactin is secreted after childbirth and stimulates lactation.
• Thyroid Stimulating Hormone (TSH) stimulates growth and activity of the thyroid gland.
• Adrenocorticotrophic Hormone (ACTH) stimulates activity of the adrenal cortex, increasing the production of steroid hormones, predominantly cortisol.
• Gonadotrophins include Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH), which stimulate the production of gametes and sex hormones.

Hormones Secreted by the Posterior Pituitary

• Oxytocin promotes contractions of the uterus and works with prolactin to produce breast milk.
• Antidiuretic Hormone (Vasopressin) influences water resorption in the kidney tubules, concentrating urine, and raising blood pressure.

Thyroid Gland

• The thyroid gland is positioned in the neck, anterior to the larynx and trachea.
• It secretes thyroxine (T4), triiodothyronine (T3), and calcitonin.
• Iodine is essential for the production of thyroid hormones.

Thyroid Hormones

• Thyroxine (T4) and triiodothyronine (T3) regulate:
  • Basal metabolic rate
  • Carbohydrate, protein, and lipid metabolism
  • Nervous and cardiovascular systems
  • Female reproductive cycle and lactation
  • Motility of the GI tract
• Calcitonin reduces blood calcium levels by stimulating calcium storage in bones and excretion in the kidneys.

Parathyroid Glands

• Four parathyroid glands are embedded in the thyroid gland, two per lobe.
• These glands produce parathyroid hormone (PTH), which is important for maintaining plasma calcium levels.

Parathyroid Hormone

• PTH raises blood calcium levels by influencing:
  • Calcium uptake by the small intestine
  • Calcium resorption by the kidneys
  • Calcium release from the skeletal system
• PTH works with calcitonin to regulate blood calcium levels.

Adrenal Glands

• The adrenal glands are located atop each kidney.
• Each adrenal gland consists of two parts:
  • The cortex: Outer part, essential to life
  • The medulla: Inner part

Adrenal Cortex

• The adrenal cortex secretes:
  • Glucocorticoids: Concerned with glucose metabolism and stress response.
    • Cortisol is a major glucocorticoid.
  • Mineralocorticoids: Concerned with electrolyte balance.
    • Aldosterone is a mineralocorticoid regulating blood volume and sodium and potassium levels.

Glucocorticoids

• Glucocorticoids combat stress by: - Increasing blood sugar by increasing new sugar formation. - Converting amino acids to glucose in the liver. - Promoting lipolysis, converting fatty acids and glycerol into glucose. - Breaking down proteins into amino acids. - Promoting sodium and water reabsorption in the renal tubules.

Aldosterone

• Aldosterone stimulates sodium and water reabsorption and potassium excretion in the kidneys.
• It is secreted when blood sodium levels are low or potassium levels are high.

Adrenal Medulla

• The adrenal medulla produces:
  • Adrenaline (epinephrine)
  • Noradrenaline (norepinephrine)

Adrenaline

• Adrenaline is involved in the fight-or-flight response.
• Its effects include:
  • Increased metabolic rate
  • Increased heart rate and blood pressure
  • Dilation of blood vessels in muscles, heart, and brain
  • Dilation of pupils
  • Dilation of bronchioles
  • Conversion of glycogen to glucose
  • Constriction of blood vessels in the skin.

Noradrenaline

• Noradrenaline maintains blood pressure by vasoconstriction, except in coronary arteries.

Pancreatic Islets

• The pancreatic islets are clusters of cells within the pancreas that produce hormones.

Hormones Secreted by Pancreatic Islets

• Insulin and glucagon regulate blood glucose levels.
• Somatostatin inhibits insulin and glucagon secretion.

Insulin

• Insulin reduces blood glucose levels by:
  • Stimulating glucose uptake and use by muscle and connective tissue cells.
  • Promoting glucose storage by increasing the conversion of glucose to glycogen, especially in the liver and skeletal muscles.

Glucagon

• Glucagon increases blood glucose levels by:
  • Converting glycogen to glucose.
  • Stimulating gluconeogenesis, the formation of new sugars.
• Glucagon secretion increases in response to low blood glucose levels.

Ovaries

• The ovaries produce:
  • Oestrogen: Regulates the menstrual cycle and brings about female sexual maturity.
  • Progesterone: Involved in the menstrual cycle and pregnancy.

Testes

• The testes produce:
  • Spermatozoa
  • Testosterone: Responsible for male sexual maturity and secondary sexual characteristics.

Thymus Gland

• The thymus gland is located behind the sternum in the chest.
• It secretes thymosin.

Thymosin

• Thymosin is crucial for the development of T-lymphocytes for cell-mediated immunity.
• It also promotes the growth and maturation of the thymus and other lymph tissue.
• Thymosin production declines with age, leading to a decline in the effectiveness of T-lymphocytes.

Pineal Gland

• The pineal gland is approximately 10 mm long and secretes melatonin.
• Melatonin secretion is suppressed by daylight.

Melatonin

• Melatonin coordinates circadian rhythms (24-hour cycles), potentially influencing the hypothalamus.