Endocrine System & Hormones

Endocrine System Overview

• The endocrine system is a major regulating system for maintaining body functions and homeostasis.
• Endocrine glands are ductless, secreting hormones directly into the circulation.
• Hormones, including amines, proteins, and steroids, regulate various bodily functions. Specific examples of hormones include thyroxine, epinephrine, norepinephrine, insulin, growth hormone, calcitonin, antidiuretic hormone, oxytocin, cortisol, aldosterone, estrogen, progesterone, and testosterone.

Objectives

• Objectives for learning about the endocrine system include describing mechanisms of hormone action.
• Identifying the major glands and hormones of the body.
• Discussing the functional interactions of hormones.
• Describing feedback mechanisms and antagonistic actions of various hormones in the body.

Pituitary Gland

• The pituitary gland, also known as the hypophysis, plays a role in regulating several body functions.
• It is attached to the hypothalamus by a short stalk called the infundibulum.
• It has two parts: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis).

Posterior Pituitary Gland

• Hormones produced by the hypothalamus and stored in the posterior pituitary include antidiuretic hormone (ADH) and oxytocin.
• ADH increases water reabsorption in the kidneys, decreasing urination and sweating, and potentially raising blood pressure.
• Oxytocin stimulates uterine contractions during labor and milk release from mammary glands.

Anterior Pituitary Gland

• Hormones released by the anterior pituitary, controlled by releasing hormones from the hypothalamus, include growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), prolactin, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

Thyroid Gland

• The thyroid gland is located in the neck, and its structural units are thyroid follicles made of cuboidal epithelium.
• Follicles produce thyroxine (T4) and triiodothyronine (T3). Iodine is necessary for the synthesis of these hormones.
• Thyroid also produces calcitonin, which helps regulate calcium and phosphate levels.

Thyroid Hormones (T4 & T3)

• Thyroid hormones (T4 and T3) are stimulated by TSH from the anterior pituitary.
• They regulate energy production and protein synthesis, contributing to overall body function throughout life.
• Their activity is reflected in the functioning of the brain, muscles, heart, and other organs.
• Specific effects include increasing cell respiration and metabolism of carbohydrates, fats, and excess amino acids, and increasing the rate of protein synthesis within cells.

Thyroid Gland: Calcitonin

• Calcitonin plays a role in maintaining normal blood levels of calcium (Ca2+) and phosphate
  • it helps reduce the amount of Ca2+ reabsorbed into the blood from the bones
• The release of calcitonin is stimulated by high blood calcium (hypercalcemia)

Parathyroid Glands

• The parathyroid glands, which are located behind the thyroid gland, secrete parathyroid hormone.
• Parathyroid hormone (PTH) raises blood calcium levels and lowers blood phosphate levels.
• Its release is stimulated by low blood calcium levels (hypocalcemia).
• Its specific effects include increasing calcium and phosphate absorption from bone into the blood; vitamin D activation, and increasing calcium absorption from the intestines, while increasing phosphate excretion in the kidneys.

Pancreas

• The pancreas is located in the upper left quadrant of the abdominal cavity.
• The islets of Langerhans in the pancreas produce hormones, including glucagon (alpha cells) and insulin (beta cells).

Glucagon

• Glucagon is released when blood glucose is low (hypoglycemia).
• It stimulates the liver to convert glycogen to glucose (glycogenolysis).
• It also increases the use of fats and excess amino acids for gluconeogenesis to raise blood glucose levels.

Insulin

• Insulin is released when blood glucose is high (hyperglycemia), typically after eating.
• It increases cell permeability to glucose, facilitates glucose uptake into cells for energy production.
• Insulin promotes conversion of excess glucose to glycogen in the liver and skeletal muscles (glycogenesis) and increases amino acid and fatty acid transport into cells for synthesis.

Adrenal Glands

• Adrenal glands are located on top of each kidney.
• They have two parts: the adrenal medulla and the adrenal cortex.

Adrenal Medulla

• The adrenal medulla secretes epinephrine and norepinephrine (catecholamines).
  • These hormones are released during stressful situations.
  • Norepinephrine is released in smaller amounts. It primarily causes vasoconstriction in skin, viscera, and skeletal muscle raising blood pressure
  • Epinephrine is released in larger amounts. It increase heart rate, and force of cardiac contraction stimulating blood vessels to dilate in skeletal muscle and constrict in other areas

Adrenal Cortex

• The adrenal cortex secretes steroid hormones:
  • Mineralocorticoids (such as aldosterone) regulate salt and water balance, often stimulated by stress
  • Glucocorticoids (such as cortisol) regulate metabolism. They influence glucose, fat, and protein metabolism and reduce inflammation
  • Sex hormones (such as estrogen and androgens)

Aldosterone

• Its release is stimulated by low blood sodium levels, low blood volume, or low blood pressure.
• Aldosterone primarily increases sodium reabsorption in kidneys, decreasing sodium excretion, and increasing potassium excretion. This action helps maintain water and electrolyte balances.

Cortisol

• Cortisol release stimulated by ACTH (anterior pituitary) during stressful situations.
• It increases the use of fats and excess amino acids for energy (gluconeogenesis) and decreases the use of glucose for energy (glucose sparing).

Ovaries

• The ovaries produce estrogen and progesterone.
• Estrogen is primarily released from cells in the ovarian follicle and is stimulated by FSH (anterior pituitary).
• Estrogen supports the maturation of the ovum, stimulates the growth of blood vessels in the uterine lining (endometrium), and develops secondary sex characteristics.
• Progesterone completes uterine preparation for a possible pregnancy by supporting glycogen storage and further maturation of the uterine lining (endometrium). It is stimulated by LH (anterior pituitary).

Testes

• The testes produce testosterone.
  • LH stimulates testosterone secretion.
  • Testosterone promotes sperm maturation and the development of male secondary sex characteristics at puberty, and closes epiphyseal plates in the long bones.
• Inhibin decreases the anterior pituitary's FSH secretion in response to high testosterone levels. This interaction ensures a constant rate of spermatogenesis.

Hormone Action Mechanisms

• There are two main hormone receptor action mechanisms:
  • Two-messenger mechanism: protein hormones bind to receptors, initiating a signal transduction pathway that involves the production of a second messenger (e.g., cyclic AMP).
  • Steroid hormone mechanism: steroid hormones diffuse through the cell membrane, bind to receptors in the cytoplasm or nucleus, forming a complex that modifies gene expression.

Overview of the endocrine system and its role in maintaining body functions and homeostasis. Covers hormone types, major glands, and hormone interactions. Explores feedback mechanisms and hormone actions.