Endocrine System Overview

Endocrine System Characteristics

• The endocrine system is composed of endocrine glands and specialized endocrine cells located throughout the body.
• Endocrine glands and cells secrete small amounts of chemical messengers called hormones into the bloodstream.
• Hormones travel via general blood circulation to target tissues.
• Target tissues contain receptors for specific hormones.
• Hormones trigger specific responses in target tissues.

Classes of Chemical Messengers

• Autocrine: Released by cells and have a local effect on the same cell type.
• Paracrine: Released by cells that affect other cell types in close proximity.
• Neurotransmitters: Secreted by nerve cells.
• Hormones: Secreted into the blood and bind to receptors on target tissues.

Types of Hormones

• Water-soluble hormones: Include proteins, peptides, and amino acids. They are the most common type of hormone. Examples include growth hormone, antidiuretic hormone, and prolactin.
• Lipid-soluble hormones: Include steroids and eicosanoids. Examples include luteinizing hormone and androgens.

Hormone Receptors and Mechanisms of Action

• A hormone can only stimulate cells that have the corresponding receptor for that hormone.
• The portion of a receptor molecule where a hormone binds is called a receptor site.
• Receptor sites are specific, allowing only one hormone to bind to them.
• Some hormones, like epinephrine, can bind to a "family" of structurally similar receptors.
• This specificity is attributed to molecular shape and chemical characteristics.
• Lipid-soluble hormones bind to nuclear receptors due to their lipid solubility and small molecular size, allowing them to pass through the cell membrane easily.
• Water-soluble hormones bind to membrane-bound receptors because they are polar molecules and cannot pass through the cell membrane.

Regulation of Hormone Levels in the Blood

• Negative feedback: This system regulates hormone levels. When a hormone level rises, negative feedback inhibits further production of the hormone, thus maintaining homeostasis.
• Positive feedback: This system amplifies the initial stimulus. The response enhances the original stimulus, leading to a rapid change.

Glands and Hormones

• Gland: An organ that produces and releases hormones.
• Hormones: Chemical messengers that regulate the activity of cells and organs.

Pituitary Gland

• The pituitary gland is a small gland located in the brain.
• It is controlled by the hypothalamus.
• The pituitary gland is divided into two regions: anterior and posterior.

Hypothalamic Control of the Anterior Pituitary Gland

• The hypothalamus controls the production of anterior pituitary hormones.
• Hypothalamic neurons secrete releasing hormones that stimulate the production and secretion of specific hormones by the anterior pituitary.
• Inhibiting hormones from the hypothalamus decrease the secretion of specific anterior pituitary hormones.

Hypothalamic Control of the Posterior Pituitary

• The posterior pituitary gland stores and releases hormones produced by neuroendocrine cells in the hypothalamus.
• The two hormones released from the posterior pituitary are antidiuretic hormone (ADH) and oxytocin.

Anterior Pituitary Hormones

• Growth Hormone (GH):
  • Target tissues: Most tissues.
  • Functions: Stimulates the growth of bones, muscles, and organs.
  • Abnormalities:
    • Excess GH causes giantism.
    • Deficiency in GH causes pituitary dwarfism.
• Thyroid-Stimulating Hormone (TSH):
  • Target tissue: Thyroid gland.
  • Functions: Regulates thyroid gland secretions.
  • Abnormalities:
    • Excess TSH causes enlargement of the thyroid gland.
    • Deficiency in TSH causes shrinkage of the thyroid gland.
• Gonadotropins:
  • Luteinizing Hormone (LH):
    • Females: Target tissue: Ovaries. Function: Promotes ovulation and progesterone production.
    • Males: Target tissue: Testes. Function: Production of sperm and testosterone.
  • Follicle-Stimulating Hormone (FSH):
    • Females: Target tissue: Follicles in ovaries. Function: Follicle maturation and estrogen secretion.
    • Males: Target tissue: Seminiferous tubules (testes). Function: Sperm production.
• Prolactin:
  • Target tissues: Mammary glands and ovaries.
  • Functions: Milk production.
• Melanocyte-Stimulating Hormone (MSH):
  • Target tissues: Melanocytes in the skin.
  • Functions: Stimulates melanin production in melanocytes.
• Adrenocorticotrophic Hormone (ACTH):
  • Target tissues: Cells of the adrenal cortex.
  • Functions: Stimulates the production of certain corticosteroids.

Hormones of the Posterior Pituitary

• Antidiuretic Hormone (ADH):
  • Target tissues: Kidneys.
  • Functions: Conserves water.
  • Abnormalities: Diabetes insipidus
    • Low ADH causes the kidneys to produce large amounts of dilute (watery) urine.
    • Can lead to dehydration and thirst.
• Oxytocin:
  • Target tissues: Uterus.
  • Functions: Increases uterine contractions during labor.

Thyroid Gland

• One of the largest glands in the body.
• Requires iodine to function.
• Secretes thyroid hormone (T3 and T4) and calcitonin.
• Thyroid Hormones (T3 and T4):
  • Target tissues: Most tissues.
  • Functions: Regulate metabolic rates and are necessary for growth.
• Calcitonin:
  • Target tissues: Bone and some other tissues.
  • Function: Reduces blood calcium levels when they are high.

Thyroid Hormone Disorders

• Hypothyroidism:
  • Decreased metabolism.
  • Weight gain, reduced appetite, fatigue, low body temperature.
• Hyperthyroidism:
  • Increased metabolism.
  • Weight loss, increased appetite, nervousness and anxiety.

Pancreas Actions

• The pancreas is a mixed gland with both exocrine and endocrine portions.
• The exocrine portion of the pancreas secretes digestive enzymes.
• The endocrine part of the pancreas consists of pancreatic islets (Islets of Langerhans), dispersed throughout the exocrine portion.

Pancreas Hormones

• Insulin:
  • Target tissues: Liver, skeletal muscle, adipose tissue.
  • Functions: Regulates blood glucose levels.
    • After a meal, when glucose levels are high, insulin is secreted.
    • Excess glucose is stored as glycogen.
• Glucagon:
  • Target tissues: Liver.
  • Function: Regulates blood glucose levels.
    • Between meals, when glucose levels drop, glucagon is secreted.
    • Glucagon stimulates the breakdown of glycogen into glucose.

Testes Hormone

• Testosterone:
  • Target tissues: Most tissues.
  • Functions: Aids in sperm and reproductive organ development and function.

Ovarian Hormones

• Estrogen and Progesterone:
  • Target tissues: Most tissues.
  • Functions: Involved in uterine and mammary gland development and the menstrual cycle.