Endocrine

Learning Objectives

• By the end of this lecture, you should be able to:

  • Describe the functions of the four tissues within various organs of the endocrine system.

  • Compare and contrast the function of organs within the endocrine system.

  • Discuss the endocrine system’s function as a messenger pathway of the body.

  • Compare and contrast the responses of the endocrine system when the 'fight or flight' response is triggered.

  • Describe the impact of the endocrine system on fluid, glucose, and electrolyte balance.

Overview of the Endocrine System

• Coordination & Control:

  • Sensory System

  • Endocrine System

  • Nervous System

Communication through Hormones

• The endocrine system communicates through chemical signals called hormones.

  • Hormones are released into the blood and travel to target organs via blood.

Functions of Hormones

• Effects of the hormones secreted by the endocrine system include:

  • Growth

  • Reproduction

  • Metabolism

  • Regulation of water and electrolyte balance

  • Digestion

Endocrine Glands

• Definition: Ductless glands that secrete hormones into the blood.

• Hormones: Chemical substances that perform specific functions.

• Classification of Hormones:

  • Steroids

  • Proteins and protein-related substances

Hormone Targeting and Receptors

• Each hormone binds to a specific tissue through hormone receptors:

  • Types of receptors:

  • Membrane receptors

  • Intracellular receptors

  • Mechanism: Hormones operate via a lock-and-key mechanism with receptors, potentially initiating a second messenger response.

Control of Hormone Secretion

• Hormone secretion is regulated through:

  • Feedback Loops:

  • Negative Feedback: Inhibits further secretion when hormone levels are sufficient.

  • Positive Feedback: Enhances secretion when more is needed.

  • Biorhythms: Rhythmic alteration in hormone secretion rates.

  • Central Nervous System Control: Activation of the hypothalamus stimulates the sympathetic nervous system.

Hormonal Transport

• Blood Composition: Blood is 92% water.

• Protein Hormones: Hydrophilic, easily transported by blood to target organs.

• Steroid Hormones: Hydrophobic, require special carrier chemicals for transport in blood.

Mechanism of Hormone Action

• For Protein Hormones:

  • Cannot diffuse through the cell membrane; they bind to receptors on the cell membrane, stimulating the production of a second messenger (e.g., cAMP) to activate specific enzymes immediately.

• For Steroid Hormones:

  • Diffuse through the cell membrane and bind to intracellular receptors, altering cellular functions.

  • Slower acting than protein hormones but results in longer-lasting effects.

Feedback Mechanisms of Hormone Regulation

• Negative Feedback:

  • A self-regulating system that maintains hormone levels within a normal range. If the target tissue becomes too active, secretion decreases.

• Positive Feedback:

  • A bodily change stimulates further change in the same direction (e.g., oxytocin during labor).

Biorhythms

• Control blood levels of hormones through rhythmic alterations:

  • Circadian Rhythms: 24-hour cycles (e.g., sleep-wake cycle).

  • Monthly Rhythms: E.g., menstrual cycles.

Hypothalamus

• The hypothalamus controls many internal body processes and maintains homeostasis by receiving information from various receptors regarding:

  • Body temperature

  • Water balance

  • Blood pressure

  • Nutrients

  • Hormones

Pituitary Gland

• Known as the Master Gland, the pituitary gland controls the activity of other hormone-secreting glands and is divided into:

  • Anterior Pituitary

  • Posterior Pituitary

Hormones of the Pituitary Gland

• Anterior Pituitary Hormones:

  • Thyroid-Stimulating Hormone (TSH)

  • Adrenocorticotropic Hormone (ACTH)

  • Growth Hormone (GH)

  • Prolactin (PRL)

  • Follicle Stimulating Hormone (FSH)

  • Luteinizing Hormone (LH)

• Posterior Pituitary Hormones:

  • Oxytocin

  • Antidiuretic Hormone (ADH)

Control of Anterior Pituitary

• Regulated by hormones from the hypothalamus:

  • Inhibiting Hormones (turn off secretion)

  • Releasing Hormones (turn on secretion)

Anterior Pituitary Hormones - Classes

• Non-Tropic Hormones:

  • Act directly on target organs (e.g., Growth Hormone stimulates growth in tissues).

• Tropic Hormones:

  • Act on other endocrine glands to stimulate the secretion of a second hormone (e.g., TSH stimulates the thyroid).

Growth Hormone (GH)

• Promotes growth in bones, muscles, and cells.

Prolactin (PRL)

• Stimulates milk production in mammary glands.

Anterior Pituitary - Tropic Hormones

• Major tropic hormones include:

  • TSH: Stimulates thyroid gland to release T3 and T4.

  • ACTH: Stimulates adrenal glands.

  • Gonadotropins (FSH & LH): Stimulate gonads (ovaries & testes).

The Sex Glands

• Produce hormones necessary for the development of sexual characteristics and maintenance of reproductive organs:

  • Female: Ovaries (Estrogens, Progesterone)

  • Male: Testes (Androgens)

Thyroid Gland

• Located at the front of the neck, attached to the voice box and upper trachea.

• Contains two lobes connected by an isthmus.

• Secretes two types of hormones:

  • Triiodothyronine (T3)

  • Tetraiodothyronine (T4) (Thyroxine)

Functions of Thyroid Hormones

• Regulate metabolism and energy expenditure.

• Essential for the proper maturation of the nervous system and normal growth and development.

• T3 is more potent than T4.

Regulation of Thyroid Hormone Secretion

• Regulated by the Hypothalamus via Thyrotropin-Releasing Hormone (TRH) and TSH from the anterior pituitary.

Need for Iodine

• Iodine is essential for the synthesis of thyroid hormones; deficiency leads to decreased T3 and T4 production.

• Calcitonin: Regulates blood calcium and phosphate levels.

Parathyroid Glands

• Located behind the thyroid gland and not controlled by the pituitary.

• Secrete Parathyroid Hormone (PTH) which raises blood-calcium levels in response to low calcium.

Blood-Calcium Regulation

• Calcitonin Functions:

  • Stimulates osteoblasts to form bone, suppressing osteoclast activity (decreasing blood calcium).

• Parathyroid Hormone Functions:

  • Stimulates osteoclasts, allowing calcium release into the blood, increases intestinal absorption, and promotes renal reabsorption of calcium from urine.

Adrenal Glands

• Located above the kidneys; divided into:

  • Medulla: Reacts to stress (sympathetic nervous system), secreting catecholamines (epinephrine and norepinephrine).

  • Cortex: Regulates metabolism, blood pressure, and sex hormones, secreting glucocorticoids (like cortisol) and mineralocorticoids (like aldosterone).

Fight or Flight Response

• Epinephrine and norepinephrine actions include:

  • Converting glycogen to glucose.

  • Bronchodilation to enhance respiratory efficiency.

  • Increasing heart rate and blood pressure to deliver oxygen and glucose.

  • Diverting blood to skeletal muscles from less critical organs.

Hormones of the Adrenal Cortex

• Glucocorticoids (Cortisol): Increase blood glucose levels.

• Mineralocorticoids (Aldosterone): promote sodium retention and potassium release, increasing blood pressure.

• Sex Hormones (Androgens): Stimulate puberty and reproductive activities.

Control of the Adrenal Cortex

• Regulated by ACTH from the anterior pituitary through negative feedback mechanisms.

Posterior Pituitary Gland

• Stores hormones produced in the hypothalamus, releasing them in response to nervous stimulation:

  • Oxytocin: Stimulates contractions in uterus and milk ejection from mammary glands.

  • Antidiuretic Hormone (ADH): Promotes water reabsorption in kidneys and is inhibited by alcohol.

Cells of the Pancreas

• Contains Islets of Langerhans with:

  • Alpha Cells: Secrete glucagon to raise blood-glucose levels.

  • Beta Cells: Secrete insulin to lower blood-glucose levels.

Functions of Insulin

• Transport glucose into cells, regulates carbohydrate, protein, and fat metabolism.

• Mechanism of action requires binding to insulin receptors on target cell membranes.

Functions of Glucagon

• Raises blood glucose by stimulating both glycogenolysis and gluconeogenesis in the liver.