Chap013_-endocrine_system

Major Components of the Endocrine System

Hypothalamus

• Regulates hunger, thirst, sleep, wakefulness, body temperature, and involuntary mechanisms.

Thyroid Glands

• Regulate energy levels and metabolism.

Pancreas

• Aids in digestion of carbohydrates, proteins, fats.

• Produces insulin to control blood sugar levels.

Ovaries and Testes

• Influence circulation, mental vigor, and sex drive.

Pituitary Gland

• Controls other endocrine glands, influences growth, metabolism, and regeneration.

Parathyroid

• Secretes hormones necessary for calcium absorption.

Thymus

• Builds resistance to diseases.

Adrenal Glands

• Secrete compounds like cortisone and adrenaline for emergency reactions.

Mechanisms of Hormone Action

Hormonal Signaling Process

• Endocrine Cells: Release hormones into circulation.

• Receptors: Hormones bind to target cell receptors, instigating effects.

G-Protein-Linked Receptors

• Utilize second messengers (e.g. cAMP).

• Amplifies cellular responses through enzyme activation (e.g. protein kinase A).

Amplification of Hormone Activity

• Each hormonal signal escalates through multiple reactions, activating many target enzymes, leading to pronounced systemic effects.

Steroid Hormones

• Easily diffuse through cell membranes and bind to receptors inside cells.

  • Steroid hormones diffuse easily through lipid bilayer of cell membranes

    • carriers transport thyroid hormones

• Gene expression is changed through binding of the hormone receptor complex to specific DNA binding sites

• Slow onset compared to water-soluble hormones, no significant amplification cascade.

• Upregulation: Increased receptor number improving responsiveness to hormones. This process allows cells to become more sensitive to hormones by increasing the availability of receptors on the cell surface.

• Downregulation: Decreased receptor number reducing responsiveness to hormones. This process occurs when there is a high level of hormone presence over time, leading to a decrease in receptor availability and, consequently, reduced sensitivity of the target cells to those hormones.

Hypothalamus and Pituitary Interaction

Hypophyseal Portal System

• Hypothalamic neurons secrete hormones that regulate anterior pituitary hormone release utilizing the portal system.

• Hormones released: TSH, FSH, LH, ACTH, GH, PRL.

Feedback Mechanisms

• Hormone levels regulated by negative feedback loops between hypothalamus, pituitary, and target glands.

Hormone Examples and Effects

Thyroid Hormone

• TRH from hypothalamus stimulates TSH release, influences metabolic rate, growth, and development.

• Negative feedback: High levels of T3 decrease TRH and TSH secretion. Thyroid Hormone - TRH from the hypothalamus stimulates TSH release, which has several key effects: - **Metabolic Rate**: Increases basal metabolic rate (BMR), leading to higher energy expenditure. - **Calorigenic Effect**: Enhances thermogenesis, contributing to heat production in the body. - **Sympathomimetic Activity**: Increases responsiveness to catecholamines (e.g., adrenaline), enhancing the body's reaction to stress. - **Cardiovascular Effects**: Improves cardiac output and helps maintain heart rate, promoting overall cardiovascular health. - **Growth**: Stimulates growth and development in children, influencing physical and cognitive maturation. - **Nervous System**: Enhances neural activity, affecting mood, reflexes, and overall mental function. - Negative feedback: High levels of T3 decrease TRH and TSH secretion.

Growth Hormone (GH)

• Released from the anterior pituitary, promotes growth and metabolism.

• Excess can lead to gigantism or acromegaly.

Cortisol

• Primary glucocorticoid from adrenal cortex; regulates stress response, increases plasma glucose.

  • inceases catabolism

  • muscle breakdown

  • free fatty acids

  • blood glucose (from non carbohydrates)

  • SNS response

• Suppresses: Immune and inflammatory systems

Insulin

• Secreted by pancreatic beta cells; lowers blood glucose.

• Insulin action: enhances glucose uptake, promotes glycogen formation and fat storage.

Diabetes Mellitus

• Type 1: Insulin-dependent, autoimmune destruction of β-cells, results in lack of insulin.

• Type 2: Insulin resistance, more common, complications can affect various body systems.

Pathophysiological Conditions

Hypothyroidism Symptoms

• Dry hair, weight gain, depression, cold intolerance, among others.

Hyperthyroidism Symptoms

• Hair loss, weight loss, rapid heartbeat, bulging eyes.

Diabetes Insipidus

• Inadequate ADH leads to excessive urination, risk of severe fluid imbalance.

Hormonal Feedback Mechanisms

• PTH: Secreted in response to low calcium levels, regulates calcium homeostasis in bones and kidneys.

• Melatonin: Regulates circadian rhythms, influenced by light exposure.

Regulation of Blood Glucose

• Insulin: Promotes cellular uptake of glucose; aids in energy storage.

• Glucagon: Raises blood glucose by promoting glycogen breakdown.

Summary of Hormonal Targets and Effects

• Tissues affected by hormones include liver (gluconeogenesis), muscle (glucose uptake), and adipose tissue (lipogenesis).

Key Concept Definitions

• Upregulation: Increased receptor number improving responsiveness.

• Downregulation: Decreased receptor number reducing responsiveness.

Important Terms and Their Functions

• Glycogen: Storage form of glucose, essential for energy regulation in mammals.

• Adrenaline: Prepares body for fight-or-flight response, increasing heart rate and energy availability.

• Calcitonin: Produced by thyroid, lowers blood calcium levels.