Endocrine System Overview Quiz

Endocrine System Overview

• The endocrine system is one of the body's two main control systems, the other being the nervous system.
• It controls and integrates many bodily processes.
• Key processes include: reproduction, growth and development, electrolyte, water, and nutrient balance in the blood, cellular metabolism and energy balance, and mobilization of body defenses.

Endocrine Glands and Hormones

• Endocrine glands secrete hormones into the systemic circulation.
• Hormones are transported in the blood to tissues where they have a specific action.
• They are distinct from exocrine glands such as meibomian, lacrimal, and sweat glands.

What is a Hormone?

• Classically, a hormone is a chemical substance secreted by specialized endocrine cells directly into the blood to affect distant target cells.
• Target cells can sometimes be local, in which case the substances are referred to as paracrines or autocrines.

Hormone Composition

• Amino acid based:
  • Polypeptides: cannot pass through plasma membranes by diffusion but are water-soluble; these are the most prevalent type (e.g., TRH, TSH, GnRH, FSH, LH, insulin, glucagon)
  • Modified amino acids: small, cannot pass through plasma membranes by diffusion but are water-soluble (e.g., thyroxine)
• Steroids: synthesized from cholesterol, small, fat-soluble, and can pass through plasma membranes (e.g., cortisol, estrogen, testosterone)

Stimuli for Hormone Release

• Humoral stimuli: Hormone release is triggered by altered levels of certain ions or nutrients in the blood (e.g., PTH release in response to low blood calcium).
• Neural stimuli: Hormone release is triggered by neural input (e.g., epinephrine and norepinephrine release from the adrenal medulla in response to stress).
• Hormonal stimuli: Hormone release is triggered by another hormone (e.g., hormones from the hypothalamus stimulating other endocrine glands).

Hormone Release Regulation

• Negative feedback: Hormone B inhibits secretion of gland A. Hormone A stimulates secretion of gland B. Hormone B has an action on target cells. This is a common mechanism to regulate hormone release.

Major Endocrine Glands

• Pineal: sleep-wake cycle
• Pituitary: growth, lactation, thyroid function
• Thyroid: metabolism
• Thymus: T cell maturation
• Adrenal: stress response
• Pancreas: glucose metabolism
• Ovaries: ovulation
• Testes: spermatogenesis

Pineal Gland

• Secretes melatonin, which peaks at night.
• Causes drowsiness and lowers body temperature.
• Regulated by the suprachiasmatic nucleus (SCN) in the hypothalamus.

Hypothalamus

• Receives input from higher brain centers (limbic system - emotion) and visceral/somatic senses via the brain stem.
• Regulates hormone release from the pituitary gland.
• It is part of the CNS and links the CNS to the endocrine system.

Pituitary Gland

• Lies close to the optic nerves.
• Has two regions:
  • Anterior lobe: secretes multiple hormones from various cell types
  • Posterior lobe: secretes oxytocin and vasopressin

Anterior Pituitary Cells

• Secretes various hormones:
  • Somatotrophs: growth hormone
  • Lactotrophs: prolactin
  • Corticotrophs: ACTH
  • Thyrotrophs: TSH
  • Gonadotrophs: FSH and LH

Anterior Pituitary Regulation

• Hypothalamic nuclei release neuroendocrine signals (e.g., TRH, GnRH, PIF) which travel down the portal vein.
• Signals act on specific cells in the anterior pituitary, stimulating the release of hormones into the hypophyseal vein.

Pituitary Hormones and their Relationship to Hypothalamus, Anterior Pituitary

• Neuroendocrine signals (e.g. TRH) from the hypothalamus stimulate release of specific hormones (e.g. TSH) from the anterior pituitary.
• Other signals have inhibitory effects.
• There is close coordination and regulation between the hypothalamus and the anterior pituitary.

Posterior Pituitary

• Secretes oxytocin and vasopressin (ADH).
• Hormones stored in the supraoptic and paraventricular nuclei before release.
• Release is regulated by neural signals from the hypothalamus.

FSH and LH in Females

• GnRH from the hypothalamus stimulates cyclical release of LH and FSH from the pituitary, regulating the menstrual cycle.
• LH triggers ovulation.
• FSH promotes ovarian follicle development and estrogen/progesterone secretion.

FSH and LH in Males

• GnRH, LH, and FSH are involved in similar ways to regulate male reproductive function as in females.
• LH stimulates testosterone production in the testes.
• FSH helps maintain spermatogenesis in the testes.

Growth Hormone

• Released by the pituitary gland.
• Stimulates growth through the liver and various other tissues.
• Important for skeletal muscle protein synthesis, bone growth and chondrocyte activity.
• Excessive secretion can lead to acromegaly.

Thyroid Gland

• Secretes thyroid hormones (thyroxine (T4) and triiodothyronine (T3)) into the bloodstream.
• Thyroid hormone release is regulated by negative feedback involving the hypothalamus and pituitary.
• Hormones affect growth, metabolism, body temperature regulation, and mood.
• Iodine is necessary for thyroid hormone production.

Parathyroid Glands

• Contain cells that secrete parathyroid hormone (PTH).
• PTH regulates blood calcium levels through effects on the kidneys ( increasing calcium reabsorption.
• PTH also has an impact on bone and the GI tract to regulate Calcium homeostasis levels.

Adrenal Glands

• Sit atop the kidneys.
• Have three regions (cortex and medulla):
  • Cortex: secretes corticosteroids (aldosterone, cortisol, androgens) that are involved in electrolyte balance, blood pressure, metabolism, immune function and stress response. ACTH from the anterior pituitary regulates corticosteroid secretion.
  • Medulla: secretes adrenaline and noradrenaline to help with emotional and physical stress response. Medulla hormones are controlled by sympathetic innervation.

Pancreas

• An endocrine and exocrine gland.
• Secretes insulin and glucagon for blood glucose regulation.
• Insulin lowers blood glucose.
• Glucagon raises blood glucose.
• Contains pancreatic islets with alpha (glucagon) and beta (insulin) cells.

Beta Cells and Diabetes

• Beta cells produce insulin.
• Type 1 diabetes is an autoimmune disease that damages beta cells preventing insulin secretion. This results in increased levels of blood glucose. Type 2 diabetes is characterized by decreased responsiveness to insulin.
• The pancreas also releases certain hormones like SOMATOSTATIN and GASTRIN as well as others.