Endocrine System Overview

Endocrine System Overview

• The endocrine system is the body's second major control system, working alongside the nervous system to regulate bodily functions.
• It achieves this by releasing chemical messengers called hormones, which affect the metabolic activities of cells.
• Endocrine glands produce hormones; exocrine glands produce non-hormonal substances like saliva and sweat, which are carried via ducts to the surface.

Hormones

• Hormones are chemical messengers, similar to neurotransmitters, that act on specific target tissues.
• Target tissues possess receptors that are activated by specific hormones.
• Most hormonal activity is regulated by negative feedback mechanisms.

Hormone Action

• Hormones can influence cell function in multiple ways, including altering membrane permeability, stimulating protein synthesis, activating or deactivating enzymes, and affecting cell division.
• Some hormones (water-soluble) rely on second messenger systems to trigger cellular responses, while others (lipid-soluble) can directly enter cells and impact gene expression.

Types of Hormones

• Water-soluble hormones (most amino acid-based hormones): Cannot cross cell membranes directly and require surface receptors to initiate a response.
• Lipid-soluble hormones (steroid and thyroid hormones): Can cross cell membranes and directly affect gene expression inside the cell.
• Hormones are categorized by their chemical structure: amino acid-based or steroid hormones (derived from cholesterol).

Second Messenger System

• The hormone binds to a surface receptor, stimulating G-proteins.
• G-proteins activate cyclic AMP (cAMP), a second messenger, which activates protein kinases.
• Protein kinases induce changes within the cell.

Direct Gene Activation

• Lipid-soluble hormones directly enter cells and bind to receptors within the nucleus.
• This hormone-receptor complex binds to specific DNA regions, influencing gene expression and protein synthesis.

Hormone Regulation

• Negative feedback systems control blood hormone levels.
• Humoral stimuli: Blood levels of specific chemicals trigger hormone release (e.g., calcium levels).
• Neural stimuli: Nervous system signals hormone release (e.g., "fight-or-flight" response).
• Hormonal stimuli: Other hormones trigger hormone release (e.g., hypothalamus releasing hormones that stimulate the anterior pituitary).
• The nervous system can modulate hormone release, altering levels in response to stress.

Hormone Removal

• Hormones are removed from the blood by:
  • Degrading enzymes.
  • Kidneys (excretion in urine).
  • Liver (breakdown).
• Hormone half-life varies, indicating how quickly hormones are removed.

Speed and Duration of Hormone Action

• Hormone effects can be immediate or require activation within target cells.
• Hormone effects may persist for seconds to hours, depending on the specific hormone and its removal rate.

Hypothalamus and Pituitary Gland

• The hypothalamus connects to the pituitary gland (hypophysis) via the infundibulum.
• The pituitary gland is often called the "master gland" because it controls other endocrine glands.

Anterior Pituitary Hormones

• Growth Hormone (GH): Stimulates somatic growth; hyposecretion causes dwarfism, hypersecretion gigantism.
• Thyroid-stimulating Hormone (TSH): Stimulates thyroid hormone production.
• Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex hormone production, crucial for long-term stress response.
• Follicle-stimulating Hormone (FSH): Stimulates gamete production (sperm and eggs).
• Luteinizing Hormone (LH): Stimulates hormone production in gonads (estrogen, progesterone, testosterone).
• Prolactin (PRL): Stimulates milk production.

Posterior Pituitary Hormones

• Oxytocin: Stimulates uterine contractions and milk release; promotes bonding.
• Antidiuretic Hormone (ADH): Regulates water balance by decreasing urine production, increasing blood volume, and pressure. Hyposecretion leads to diabetes insipidus.

Thyroid Gland

• Releases hormones that regulate metabolism.
• Calcitonin: Lowers blood calcium levels.
• Thyroxine (T4) and Triiodothyronine (T3): Increase metabolic activity throughout the body; hyposecretion causes hypothyroidism, hypersecretion hyperthyroidism.

Parathyroid Gland

• Parathyroid Hormone (PTH): Increases blood calcium levels; vital for skeletal, kidneys, and intestinal function. Hyposecretion causes hypocalcemia, hypersecretion hypercalcemia.

Adrenal Cortex

• Aldosterone: Regulates sodium and potassium balance; hypersecretion can cause hypertension and edema. Hyposecretion leads to excessive water loss and dehydration.
• Glucocorticoids (cortisol): Regulates long-term stress response via protein breakdown and glucose formation. Hypersecretion leads to Cushing's syndrome, while hyposecretion is Addison's disease

Adrenal Medulla

• Releases catecholamines (epinephrine, norepinephrine) in response to the stress response or fight-or-flight response.

Pancreas

• Insulin: Lowers blood glucose levels.
• Glucagon: Raises blood glucose levels.
• Disorders like Type 1 and Type 2 diabetes impact insulin function and blood glucose regulation.

Other Endocrine Glands

• Pineal gland: Produces melatonin, regulating circadian rhythms.
• Gonads (ovaries/testes): Produce sex hormones (estrogen, progesterone, testosterone), crucial for sexual development and reproduction.
• Thymus: Produces thymosin, impacting the immune system.
• Placenta: Releases hormones vital for pregnancy.
• Heart: Releases ANP, lowering blood pressure.
• Kidney: Produces erythropoietin, stimulating red blood cell production, and renin, regulating blood pressure.
• GI Tract, Skin, and Adipose Tissue: Produce hormones for digestion, vitamin D regulation, and appetite control, respectively.

Effects of Environmental Pollutants

• Exposure to certain pollutants can disrupt endocrine function, causing problems with various hormones.

Endocrine System Changes with Age

• Endocrine function generally declines with age, affecting hormone levels (growth hormone, thyroid hormone, estrogen) and impacting metabolic processes.

Explore the intricacies of the endocrine system, which plays a vital role in regulating bodily functions alongside the nervous system. Learn about the production and action of hormones, including how they influence various target tissues through complex feedback mechanisms.