Ch 16B - Endocrine System
Chapter 16B1 Endocrine System
Overview of the endocrine system and its role in regulating various bodily functions through hormones.
Key glands involved: pituitary, thyroid, adrenal, and pancreas.
Focus on the endocrine system, its glands, and function.
Thyroid Gland Structure
Location: Anterior neck, butterfly-shaped on trachea.
Consists of:
Isthmus: Connects the two lobes.
Follicles: Produce glycoprotein thyroglobulin.
Colloid: Contains thyroglobulin and iodine, precursor to hormones.
Parafollicular Cells: Produce calcitonin.
Thyroid Hormone (TH)
Types: T4 (thyroxine) and T3 (triiodothyronine).
Functions:
Major metabolic hormone affecting almost every cell.
Increases basal metabolic rate (calorigenic effect).
Regulates growth and development, critical for nervous system and reproduction.
Maintains blood pressure by increasing adrenergic receptors.
Synthesis of Thyroid Hormone
Intricate process involving:
Thyroglobulin synthesis.
Iodide trapping.
Iodide oxidation and iodination of tyrosine.
Linking iodinated tyrosines to form T3/T4.
Endocytosis of colloid and lysosomal cleavage.
Secretion into the bloodstream.
Regulation of TH Secretion
Negative Feedback: Falling TH levels stimulate TSH release.
Influencing Factors: GHIH, dopamine, cortisol, TRH during pregnancy or cold exposure.
Calcitonin
Produced in response to high calcium levels; inhibits osteoclast activity.
Parathyroid Gland
Function: Secretes parathyroid hormone (PTH) in response to low calcium levels.
Effects:
Stimulates osteoclasts.
Enhances calcium reabsorption by kidneys.
Promotes vitamin D activation.
Adrenal Gland Structure
Located atop kidneys, contains:
Cortex: Produces corticosteroids.
Medulla: Produces catecholamines (epinephrine and norepinephrine).
Adrenal Cortex Hormones
Mineralocorticoids: Regulates electrolytes; e.g., aldosterone.
Glucocorticoids: e.g., cortisol, impacts metabolism and stress response.
Gonadocorticoids: Sex hormones, influence puberty and secondary characteristics.
Clinical Imbalances
Thyroid: Myxedema (hyposecretion), Graves’ disease (hypersecretion).
Adrenal: Addison’s disease (hyposecretion), Cushing’s syndrome (hypersecretion).
Pancreas
Contains exocrine (digestive) and endocrine (glucose regulation) cells:
Insulin: Lowers blood glucose.
Glucagon: Raises blood glucose.
Hormonal Regulation by Other Organs
Various hormones released by heart, kidneys, gastrointestinal tract, and adipose tissue.
Developmental Aspects
Endocrine function changes with age; hormonal exposure to environmental pollutants affect function.
Chapter 16B2 Endocrine System
Focus on the endocrine system, its glands, and their functions.
Thyroid Gland Structure
Location: Found in the anterior neck, it has a butterfly shape and is located on the trachea, just below the larynx.
Consists of:
Isthmus: The bridge of tissue that connects the two lobes of the thyroid gland, usually situated over the second and third rings of the trachea.
Follicles: Spherical structures that produce glycoprotein thyroglobulin which is essential for hormone production.
Colloid: A gel-like substance within the follicles that contains thyroglobulin and iodine; it acts as a precursor to thyroid hormones.
Parafollicular Cells (C cells): These cells are responsible for the production of calcitonin, a hormone that helps in calcium metabolism.
Thyroid Hormone (TH)
Types:
T4 (thyroxine): Contains four iodine atoms, this hormone plays a major role in controlling the body's metabolic rate.
T3 (triiodothyronine): Contains three iodine atoms and is considered the more active form of thyroid hormone, significantly affecting energy level and metabolic activities.
Functions:
Major metabolic hormone affecting almost every cell in the body, influencing metabolic processes.
Increases the basal metabolic rate (greater calorigenic effect), thereby enhancing calorie consumption and heat production.
Regulatory role in growth and development; crucial for brain maturation and reproduction.
Maintains blood pressure by increasing the number and sensitivity of adrenergic receptors.
Synthesis of Thyroid Hormone
An intricate process involving several key steps:
Thyroglobulin synthesis: Occurs in the follicular cells and is secreted into the follicle.
Iodide trapping: Active transport of iodide from the bloodstream into the follicular cell.
Iodide oxidation and iodination of tyrosine: Converts iodide into an active form that can bind to tyrosine residues on thyroglobulin.
Linking iodinated tyrosines: Forms T3 and T4 by the coupling of iodinated tyrosines.
Endocytosis of colloid: Thyroglobulin enters the follicular cells through endocytosis and is processed by lysosomes, releasing T3 and T4.
Secretion into the bloodstream: Both hormones are secreted into circulation, where they bind to transport proteins.
Regulation of TH Secretion
Negative Feedback: Falling TH levels stimulate the secretion of Thyroid Stimulating Hormone (TSH) from the anterior pituitary, which in turn stimulates TH production.
Influencing Factors:
Growth hormone inhibitory hormone (GHIH), dopamine, cortisol, and Thyrotropin-Releasing Hormone (TRH) during specific conditions such as pregnancy or cold exposure.
Calcitonin
Produced in response to high levels of calcium in the blood; it functions to inhibit osteoclast activity, thereby reducing bone resorption and lowering calcium levels in the blood.
Parathyroid Gland
Function: Secretes parathyroid hormone (PTH) in response to low calcium levels in the blood.
Effects of PTH:
Activates osteoclasts to break down bone and release calcium into the bloodstream.
Enhances calcium reabsorption from the kidneys, reducing urinary losses.
Promotes the activation of vitamin D in the kidneys, which increases intestinal absorption of calcium.
Adrenal Gland Structure
Located atop the kidneys, consists of two distinct parts:
Cortex: The outer portion that produces corticosteroids essential for various bodily functions, including metabolism and the immune response.
Medulla: The inner portion responsible for producing catecholamines (epinephrine and norepinephrine), which are key in stress responses.
Adrenal Cortex Hormones
Mineralocorticoids: Example is aldosterone, regulates electrolytes with a focus on sodium and potassium levels to maintain blood pressure.
Glucocorticoids: Included are hormones such as cortisol that influence metabolism and assist in managing stress responses.
Gonadocorticoids: Released sex hormones, such as androgens and estrogens, that influence sexual development and secondary sexual characteristics during puberty.
Clinical Imbalances
Thyroid Disorders:
Myxedema: Result of hyposecretion, leading to symptoms such as fatigue, weight gain, and sensitivity to cold.
Graves’ disease: Characterized by hypersecretion of thyroid hormones, resulting in symptoms like weight loss, increased heart rate, and heat intolerance.
Adrenal Disorders:
Addison’s disease: Result of adrenal hyposecretion leading to fatigue, low blood pressure, and darker skin pigmentation due to insufficient cortisol.
Cushing’s syndrome: Caused by hypersecretion affecting cortisol, leading to obesity, hypertension, and skin changes.
Pancreas
The pancreas contains both endocrine (insulin and glucagon production) and exocrine (digestive enzyme production) cells:
Insulin: A hormone that lowers blood glucose levels by promoting glucose uptake and storage in cells.
Glucagon: Raises blood glucose levels by promoting glycogen breakdown and glucose release from the liver.
Hormonal Regulation by Other Organs
Various hormones are secreted by other organs including but not limited to:
Heart: Produces atrial natriuretic peptide (ANP) which regulates blood pressure.
Kidneys: Release erythropoietin and renin, which influence blood pressure and red blood cell production.
Gastrointestinal Tract: Produces hormones like gastrin and secretin that regulate digestive processes.
Adipose Tissue: Releases leptin, which is involved in energy balance and appetite regulation.
Developmental Aspects
Endocrine function can change with age; hormonal imbalances can occur with aging, and hormonal exposure to environmental pollutants may disrupt normal endocrine function.