L35-2

Endocrine System Overview

• The endocrine system is a second messenger system using chemical messengers (hormones) released into the bloodstream.
• Hormones control many bodily processes, including: reproduction, growth and development, mobilization of body defenses, homeostasis, and metabolism.
• Hormones are produced by specialized cells and secreted into extracellular fluids, then transported to target cells via the blood.
• These hormones regulate the activity of other cells.

Hormone Chemistry

• Hormones are categorized into different chemical types: amino acid-based hormones (e.g., proteins, peptides, amines) and steroids, made from cholesterol, and prostaglandins, derived from fatty acids.

Mechanisms of Hormone Action

• Hormones act on specific tissues, or target cells/organs.
• Target cells must have specific protein receptors for the hormone to bind.
• Binding hormones influences the target cell's function.

Hormone Release Control

• Hormone levels are regulated by a negative feedback loop.
• A stimulus or low hormone levels trigger hormone release.
• Hormone release stops once the appropriate blood level is reached.

Hormonal Stimuli of Endocrine Glands

• Endocrine glands are activated by other hormones.
• Changes in blood levels of certain ions can trigger hormone release.

Pituitary Gland

• A small gland, about the size of a grape; suspended from the hypothalamus by a stalk.
• Protected by the sphenoid bone.
• Has two lobes: anterior (glandular tissue) and posterior (nervous tissue).

Hormones of the Anterior Pituitary

• Six hormones are produced in the anterior pituitary.
• Two hormones affect non-endocrine targets, and four others stimulate other endocrine glands (tropic hormones).
• All anterior pituitary hormones are proteins, or peptides, and operate via second-messenger systems.
• Mostly regulated by hormonal stimuli; primarily negative feedback.

Growth Hormone (GH)

• A general metabolic hormone affecting skeletal muscles and long bones.
• Promotes amino acid incorporation into proteins and fat breakdown for energy.

Functions of Other Anterior Pituitary Hormones

• Prolactin (PRL): Important for milk production in females.
• Adrenocorticotropic hormone (ACTH): Regulates adrenal cortex endocrine activity.
• Thyroid-stimulating hormone (TSH): Influences thyroid growth and activity.
• Gonadotropic hormones:
  • Follicle-stimulating hormone (FSH): Stimulates follicle development in ovaries, and sperm development in testes.
  • Luteinizing hormone (LH): Triggers ovulation, helps the ruptured follicle become the corpus luteum, and stimulates testosterone production in males (also known as interstitial cell-stimulating hormone (ICSH)).

Hormones of the Posterior Pituitary

• Oxytocin: Stimulates uterine contractions during labor and milk ejection.
• Antidiuretic hormone (ADH): Inhibits urine production and, in high amounts, causes vasoconstriction, increasing blood pressure (also known as vasopressin).

Thyroid Gland

• Consists of two lobes connected by an isthmus.
• Produces two hormones: thyroid hormone and calcitonin.

Thyroid Hormone

• A major metabolic hormone composed of two iodine-containing hormones:
  • Thyroxine (T4): Secreted by thyroid follicles.
  • Triiodothyronine (T3): Conversion of T4 at target tissues.

Calcitonin

• Lowers blood calcium levels by promoting calcium deposition into bone.
• Antagonistic to parathyroid hormone.
• Produced by parafollicular cells (C cells).

Parathyroid Glands

• Tiny masses on the posterior of the thyroid.
• Secrete parathyroid hormone (PTH).
• Stimulates osteoclasts to release calcium from bone, and stimulates the kidneys and intestine to absorb more calcium.
• Raises blood calcium levels.

Adrenal Glands

• Located on top of kidneys.
• Composed of two parts: cortex (outer, glandular) and medulla (inner, neural tissue).

Hormones of the Adrenal Cortex

• Mineralocorticoids (mainly aldosterone): Produced in the outer adrenal cortex. Regulates mineral content in blood, water, and electrolyte balance; targets the kidney. Stimulated by renin and aldosterone, inhibited by atrial natriuretic peptide.
• Glucocorticoids (e.g., cortisone, cortisol): Produced in the middle adrenal cortex layer and promote normal cell metabolism, aiding resistance to long-term stressors. Released in response to increased blood ACTH levels.
• Sex hormones (androgens and some estrogen): Produced in the inner adrenal cortex layer. Androgens are male sex hormones, and some estrogen is a female sex hormone.

Hormones of the Adrenal Medulla

• Produces similar hormones (catecholamines):
  • Epinephrine: Aids body in dealing with short-term stressors.
  • Norepinephrine: Aids body in dealing with short-term stressors.

Pancreatic Islets

• The pancreas is a mixed gland.
• The islets produce:
  • Insulin: Allows glucose to enter cells (released from beta cells).
  • Glucagon: Allows glucose to leave cells (released from alpha cells).
• These hormones work as antagonists to maintain blood sugar homeostasis.

Pineal Gland

• Located on the third ventricle of the brain.
• Secretes melatonin, which helps establish wake/sleep cycles.

Thymus

• Located posterior to the sternum.
• Largest in infants/children. Produces thymosin.
• Matures some types of white blood cells, important in developing the immune system.

Hormones of the Ovaries

• Estrogens: Produced by Graafian follicles or the placenta; stimulates development of secondary female characteristics, matures female reproductive organs, helps prepare the uterus, helps maintain pregnancy, and prepares the breasts.
• Progesterone: Produced by the corpus luteum and acts with estrogen to bring about the menstrual cycle, aiding implantation of an embryo.

Hormones of the Testes

• Interstitial cells in testes produce androgens.
• Testosterone is the most crucial androgen. Responsible for adult male sex characteristics, promotes growth/maturation of the male reproductive system, and is essential for sperm production.

Other Hormone-Producing Tissues and Organs

• Parts of the small intestine
• Parts of the stomach
• Kidneys
• Heart
• Scattered endocrine cells in various tissues

Endocrine Function of the Placenta

• Produces hormones to maintain pregnancy and delivery.
• Produces human chorionic gonadotropin (HCG) alongside other hormones like estrogen and progesterone.

Developmental Aspects of the Endocrine System

• Most endocrine organs work smoothly until old age.
• Menopause is caused by low ovarian efficiency.
• Reduced estrogen levels pose problems.
• Growth hormone production declines with age.
• Many endocrine glands decline in output as we age.