Chapter 13: Hormonal Regulation of Homeostasis

Chapter 13: Hormonal Regulation of Homeostasis

• The endocrine system works in cooperation with the nervous system to control body functions.
• There are many connections between the two systems. Examples include epinephrine which functions as both a neurotransmitter and a hormone.
• The goal of the endocrine system is to maintain homeostasis, a stable state within the body despite changes in the external environment.

Hormones

• Hormones are "chemical messengers" that regulate the activities of organs and tissues in the endocrine system.

• Hormones' mechanism of action:

  • Endocrine cells release hormones into the bloodstream.
  • The hormone travels through the body to a target tissue.
  • The hormone binds to receptor sites on the target tissue, triggering an effect.

• The endocrine system is organized into several key glands:

  • Hypothalamus
  • Pituitary gland
  • Thyroid gland
  • Thymus glands
  • Adrenal gland
  • Pancreas
  • Testes (in males)
  • Ovaries (in females)

Glands

• Endocrine glands release hormones directly into the bloodstream or lymph.
  • Exocrine glands release fluids (secretions) through ducts into a body cavity outside the body. Examples include sweat glands, mucous glands and digestive glands.

Feedback Loops

• Many hormones are regulated by negative feedback mechanisms.
  • When a specific blood concentration of a hormone is reached, or when the targeted cells have responded to it, the endocrine gland is then inhibited (it stops releasing the hormone).
• Positive feedback is a system that creates a self-amplifying effect. More hormone is produced when the concentration of the hormone reaches a certain threshold. Oxytocin is an example.

Hormones of the Anterior Pituitary

• Prolactin (PRL): stimulates milk production in mammary glands.
• Human Growth Hormone (hGH): promotes growth in nearly all cells, connective tissue, bones, and other tissues.
• It stimulates protein synthesis, cell division, growth, and fat breakdown. It's most active during puberty.
• Disorders related to hGH include pituitary dwarfism (hyposecretion) in children; giantism in children; and acromegaly (hypersecretion) in adults.
• Symptoms of Acromegaly:
  • Headache
  • Enlarged heart, liver, and kidneys
  • Coarse facial features, forehead, nose, tongue and lips
  • Widely spaced teeth
  • Other: fatigue, excessive perspiration, visual disturbances, breathing difficulties, missed menstrual periods, and impotence.

Other Pituitary Hormones

• Thyroid Stimulating Hormone (TSH): stimulates the thyroid gland to release thyroxine (T4).
• Adrenocorticotropic Hormone (ACTH): stimulates the adrenal cortex to release cortisol.
• Follicle-Stimulating Hormone (FSH) & Luteinizing Hormone (LH): stimulate the gonads to release sex hormones.

Hormones of the Posterior Pituitary

• Anti-diuretic Hormone (ADH, also called vasopressin): regulates blood pressure and water balance. It is stimulated by low blood pressure and inhibited by negative feedback exerted by increased blood pressure
• Oxytocin: is involved in labor processes and breastfeeding by stimulating uterine contractions and milk release.

Thyroid Gland

• The thyroid gland regulates metabolic rate by releasing thyroxine (T4).
• It also lowers blood calcium levels by releasing calcitonin.

Parathyroid Gland

• The parathyroid gland releases parathyroid hormone (PTH), which breaks down bone, releases calcium into the blood, and tells kidneys to reabsorb calcium from urine.
• PTH increases blood calcium levels.
• PTH is stimulated by low blood calcium levels. It is inhibited by high blood calcium levels or calcitonin.

Adrenal Glands

• The adrenal gland has two parts:

  • Adrenal medulla: responsible for the short-term stress response releases adrenaline/noradrenaline which prepares the body for flight-or-fight. It is controlled by the sympathetic nervous system.
  • Adrenal cortex: responsible for the long-term stress response releases corticoids (such as glucocorticoids like cortisol, mineralocorticoids like aldosterone, and gonadocorticoids).
    • Cortisol increases blood glucose. It is stimulated by ACTH, from the anterior pituitary and inhibited by negative feedback from cortisol.
    • Aldosterone increases blood pressure. It is stimulated by hormones, not nerves.
    • Gonadocorticoids (e.g. testosterone): normally insignificant in adults; can cause female masculinization if hypersecreted.

• Addison's disease is a disorder caused by hyposecretion damage to the adrenal cortex. Symptoms include low blood sugar, sodium/potassium imbalances, low blood pressure, weight loss, fatigue, and weakness.

Pancreas

• The pancreas is both an endocrine and exocrine gland.
  • Exocrine: secretes juices through ducts to the small intestine for digestion.
  • Endocrine: secretes hormones, such as glucagon and insulin, to regulate blood sugar levels.
  • Islets of Langerhans: contain specialized cells (alpha and beta cells) that secrete glucagon and insulin, respectively.
    • Alpha cells produce glucagon: when blood glucose levels are low, glucagon increases blood glucose levels by stimulating glycogen breakdown in the liver and fat breakdown in adipose tissue.
    • Beta cells produce insulin: when blood glucose levels are high, insulin lowers blood glucose levels, by stimulating cells to take up glucose; stimulating glycogen storage in the liver; and stimulating fat storage.
• Diabetes mellitus results from hyposecretion by Beta cells.

Diabetes Mellitus

• Type I: Characterized by a lack of insulin production, usually develops in youth but may appear at any age. Symptoms are sudden onset; risk factors are mostly genetic and treatment includes daily insulin injections.
• Type II: Characterized by body cells becoming resistant to insulin, usually develops in adulthood. Risk factors include being overweight, high body fat content, high blood pressure and high cholesterol. Treatment may include changes in diet, exercise, and oral medication

Gestational diabetes

• Temporary type II diabetes triggered by high levels of pregnancy hormones.