Chapter 13 - Endocrine system

Endocrine System Overview

  • The endocrine system is essential for regulating body functions and maintaining homeostasis.
  • It coordinates communication within the body through hormones.

Major Endocrine Glands

  • Pituitary Gland
  • Thyroid Gland
  • Parathyroid Glands
  • Adrenal Glands
  • Pancreas
  • Pineal Gland
  • Thymus
  • Ovaries
  • Testes

Definitions and Types of Secretions

  • Endocrine Glands: Composed of cells, tissues, and organs that secrete hormones directly into the bloodstream.
  • Paracrine Secretion: Affects nearby cells.
  • Autocrine Secretion: Affects the cells that secrete them.
  • Nervous System: Releases neurotransmitters (e.g., acetylcholine) for quick responses.
  • Endocrine System: Releases hormones into the bloodstream; effects are longer lasting than nervous responses.

Hormones and Their Secretion

  • Each endocrine gland releases specific hormones:
    • Hypothalamus:
    • Gonadotropin Releasing Hormone (GnRH) / Luteinizing Hormone Releasing Hormone (LHRH)
    • Pituitary Gland (Anterior):
    • Follicle Stimulating Hormone (FSH)
    • Growth Hormone (GH)
    • Luteinizing Hormone (LH)
    • Thyroid Stimulating Hormone (TSH)
    • Pituitary Gland (Posterior):
    • Oxytocin (OT)
    • Thyroid Gland:
    • Calcitonin
    • Thyroxine (T4)
    • Triiodothyronine (T3)
    • Adrenal Gland:
    • Medulla: Epinephrine and Norepinephrine
    • Cortex: Aldosterone and Cortisol
    • Pancreas:
    • Glucagon, Insulin, Somatostatin

Types of Hormones

  1. Steroid Hormones:
    • Composed of lipids with complex rings of carbon and hydrogen atoms.
    • Examples: Testosterone, Estrogen, Cortisol, Aldosterone.
  2. Non-steroid Hormones:
    • Have shorter chains of amino acids; break down quicker in the body.
    • Considered "first messengers" for quicker responses, similar to instant messages.

Mechanism of Hormone Secretion

  • Hormone secretion is primarily controlled by negative feedback mechanisms.
  • Hormone effects can be short-lived (minutes) or long-lasting (days).
  • Hormones are broken down by enzymes, mainly from the liver, to cease effects.

Detailed Structure of the Pituitary Gland

  • Located at the base of the brain, controlled by the brain and connected to hypothalamus via the pituitary stalk.
  • Lobes:
    • Anterior Lobe:
    • Hormones released include Growth Hormone, Prolactin, Thyroid Stimulating Hormone, Adrenocorticotropic Hormone (ACTH), Follicle Stimulating Hormone (FSH), and Luteinizing Hormone (LH).
    • Posterior Lobe:
    • Regulated by nerve impulses from hypothalamus that trigger hormone release.

Disorders of the Pituitary Gland

  1. Dwarfism:
    • Caused by deficiency of growth hormone before puberty; individuals have short stature with normal body proportions and mental development. Treatment must start before ossification of bones completes.
  2. Giantism:
    • Caused by oversecretion of Growth Hormone during childhood; height may exceed eight feet and can have metabolic problems.
  3. Acromegaly:
    • Caused by oversecretion of Growth Hormone after puberty; results in thickening of bones and enlargement of body parts.

Posterior Pituitary Hormones

  • Antidiuretic Hormone (ADH):
    • Reduces urine production by decreasing H2O secretion from kidneys; causes vasoconstriction to increase blood pressure.
  • Oxytocin:
    • Stimulates milk ejection during lactation.

Thyroid Gland Functions

  • Capable of removing iodine from the blood; produces:
    • T4 (Thyroxine) - stimulates metabolism.
    • T3 (Triiodothyronine) - more active form.
    • Calcitonin - involved in calcium metabolism.

Disorders of the Thyroid Gland

  1. Hypothyroidism:
    • Low metabolic rate leading to cold sensitivity, fatigue, weight gain.
  2. Hyperthyroidism:
    • High metabolic rate, restlessness, weight loss; often treated with medications for excess T4.

Adrenal Gland Overview

  • Located on top of each kidney, consists of:
    • Adrenal Cortex: Secretes steroid hormones (Aldosterone, Cortisol, sex hormones).
    • Adrenal Medulla: Secretes non-steroid hormones (Epinephrine and Norepinephrine).

Effects of Adrenal Medulla Hormones

  • Increase in heart rate, blood pressure, airflow to lungs, and blood glucose levels, while decreasing digestive activities.

Disorders of the Adrenal Cortex

  1. Cushing’s Disease:
    • Hypersecretion of cortisol, causing muscle wasting, elevated blood glucose, high blood pressure, and “moon face” appearance.
  2. Addison’s Disease:
    • Involves electrolyte imbalances, low blood pressure, fatigue, and increased skin pigmentation; can be fatal.

Pancreatic Functions

  • The pancreas has both endocrine and exocrine functions.
  • Endocrine Function: Hormone secretion into body fluids.
  • Exocrine Function: Secretion of digestive enzymes via pancreatic ducts.
  • Major hormones include:
    • Glucagon (from alpha cells): Increases blood glucose levels.
    • Insulin (from beta cells): Decreases blood glucose levels.
    • Somatostatin (from delta cells): Regulates both insulin and glucagon secretion.

Disorders of the Pancreas

  1. Hypoglycemia: High blood glucose levels.
  2. Diabetes Mellitus:
    • Type 1 (Juvenile/Insulin-Dependent): Autoimmune destruction of beta cells.
    • Type 2 (Non-Insulin Dependent): Body cells fail to recognize insulin; more common and milder than Type 1.
    • Treatments vary for both types involving insulin administration and lifestyle changes.

Other Endocrine Glands

  • Pineal Gland:
    • Located in the brain, regulates circadian rhythm through melatonin secretion.
  • Thymus Gland:
    • Produces thymosins that support T lymphocyte development, crucial for immune response.
  • Ovaries: Produce estrogen and progesterone.
  • Testes: Produce testosterone.

Effects of Stress on the Endocrine System

  • Types of stress include:
    • Psychological Stress: Threat, loss, fear, anger.
    • Physical Stress: Temperature extremes, infections, injuries, oxygen deficiency.
  • Response Stages to Stress:
    1. Alarm Stage: Fight or flight response; immediate but short-lived.
    2. Resistance Stage: Prolonged response leading to increased cortisol release.
    3. Exhaustion Stage: Occurs months later; depletion of nutrients, imbalanced electrolytes, immune suppression, potential fatality due to prolonged cortisol exposure.