Detailed Notes – 2 -Chapter-19-Chemical Coordination And Integration

19.2.4 Thyroid Gland

1. Anatomy:

   • The thyroid gland consists of two lobes located on either side of the trachea (windpipe), connected by a thin flap of connective tissue called the isthmus.

2. Composition:

   • The thyroid gland is made up of follicles and stromal tissues.
   • Each thyroid follicle contains follicular cells surrounding a cavity.

3. Hormones Produced:

   • The follicular cells synthesize two main hormones:
     • Tetraiodothyronine or thyroxine (T4)
     • Triiodothyronine (T3)
   • Proper synthesis of these hormones requires iodine.

4. Effects of Thyroid Hormones:

   • Hypothyroidism: Insufficient levels of thyroid hormones due to iodine deficiency or other causes can lead to hypothyroidism. This can result in an enlarged thyroid gland (goiter) and various health issues, including stunted growth (cretinism), mental retardation, irregular menstrual cycles in women, and other symptoms.
   • Hyperthyroidism: Excessive production of thyroid hormones, often due to thyroid cancer or nodules, results in hyperthyroidism. This condition can cause symptoms such as enlarged thyroid gland, protruding eyeballs (exophthalmos), increased basal metabolic rate, weight loss, and is sometimes associated with Graves’ disease.
   • Regulation of Metabolism: Thyroid hormones play a crucial role in regulating the basal metabolic rate and metabolism of carbohydrates, proteins, and fats.
   • Red Blood Cell Formation: Thyroid hormones support the process of red blood cell formation (erythropoiesis).
   • Water and Electrolyte Balance: Thyroid hormones influence the maintenance of water and electrolyte balance in the body.

5. Thyrocalcitonin:

   • The thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT).
   • Thyrocalcitonin regulates blood calcium levels, helping to lower calcium levels by promoting its deposition in bones.

19.2.5 Parathyroid Gland 

1. Anatomy:

   • Humans typically have four parathyroid glands.
   • These glands are located on the posterior (back) side of the thyroid gland, with one pair situated within each of the two lobes of the thyroid gland.

2. Hormone Secretion:

   • The parathyroid glands secrete a peptide hormone called parathyroid hormone (PTH).

3. Regulation of PTH Secretion:

   • The secretion of PTH is regulated by the circulating levels of calcium ions in the blood.

4. Functions of Parathyroid Hormone (PTH):

   • Increase in Blood Calcium Levels: PTH acts to increase the levels of calcium ions (Ca2+) in the blood.
   • Bone Resorption: PTH stimulates the process of bone resorption (dissolution/demineralization), releasing calcium from bones into the bloodstream.
   • Renal Tubule Reabsorption: PTH stimulates the reabsorption of calcium ions by the renal tubules in the kidneys, preventing their loss in urine.
   • Calcium Absorption: PTH increases the absorption of calcium from digested food in the intestines.
   • Hypercalcemic Hormone: PTH is considered a hypercalcemic hormone because it raises blood calcium levels.

5. Role in Calcium Balance:

   • Along with thyrocalcitonin (TCT) from the thyroid gland, PTH plays a significant role in maintaining calcium balance in the body.
   • PTH acts to elevate blood calcium levels when they are too low, ensuring that calcium levels remain within a narrow range necessary for proper physiological functioning.

19.2.6 Thymus

1. Anatomy:

   • The thymus gland is a lobular structure located in the chest cavity between the lungs and behind the sternum (breastbone), on the ventral side of the aorta (a large blood vessel).

2. Role in Immune System Development:

   • The thymus gland plays a crucial role in the development of the immune system, particularly in the maturation of T-lymphocytes (T cells).
   • T cells are a type of white blood cell involved in cell-mediated immunity, which is the immune response involving the direct action of cells against pathogens.

3. Hormone Secretion:

   • The thymus gland secretes peptide hormones called thymosins.

4. Functions of Thymosins:

   • Differentiation of T-Lymphocytes: Thymosins play a major role in the differentiation of T-lymphocytes within the thymus gland.
   • Cell-Mediated Immunity: T-lymphocytes produced in the thymus are crucial for cell-mediated immunity, where these cells directly target and eliminate infected or abnormal cells.
   • Promotion of Antibody Production: Thymosins also promote the production of antibodies by other immune cells, contributing to humoral immunity, which involves the production of antibodies to neutralize pathogens.

5. Age-Related Changes:

   • Thymus Degeneration: The thymus gland tends to degenerate with age, particularly in older individuals.
   • Decreased Thymosin Production: Degeneration of the thymus leads to a decreased production of thymosins.
   • Weakened Immune Responses: As a result, the immune responses of older individuals may become weaker due to reduced T-cell maturation and function.

19.2.7 Adrenal Gland

1. Anatomy and Location:

   • The body has one pair of adrenal glands, located on top of each kidney.

2. Composition:

   • The adrenal gland is composed of two main tissues: the adrenal medulla and the adrenal cortex.
   • The adrenal medulla is centrally located, while the adrenal cortex lies outside the medulla.

3. Hormones Secreted by Adrenal Medulla:

   • Adrenal medulla secretes two hormones: adrenaline (epinephrine) and noradrenaline (norepinephrine), collectively known as catecholamines.
   • These hormones are rapidly released in response to stress or emergency situations, initiating the “fight or flight” response.
   • Effects of adrenaline and noradrenaline include increased alertness, pupil dilation, increased heart rate and strength of heart contraction, increased respiration rate, and mobilization of glucose from glycogen stores.

4. Layers of Adrenal Cortex:

   • The adrenal cortex can be divided into three layers:
     • Zona glomerulosa (outer layer)
     • Zona fasciculata (middle layer)
     • Zona reticularis (inner layer)

5. Hormones Secreted by Adrenal Cortex:

   • The adrenal cortex secretes several hormones collectively known as corticoids.
   • Glucocorticoids: These corticoids are involved in carbohydrate metabolism, with cortisol being the main glucocorticoid. They stimulate gluconeogenesis, lipolysis, and proteolysis, and inhibit cellular uptake and utilization of amino acids. Glucocorticoids also have anti-inflammatory and immune-suppressive effects.
   • Mineralocorticoids: These corticoids regulate water and electrolyte balance. Aldosterone is the main mineralocorticoid, stimulating sodium and water reabsorption and potassium and phosphate excretion in the kidneys, thereby maintaining electrolyte balance and blood pressure.
   • Androgenic Steroids: Small amounts of androgenic steroids are also secreted by the adrenal cortex, contributing to the development of axial, pubic, and facial hair during puberty.

6. Functions of Cortisol and Aldosterone:

   • Cortisol is involved in maintaining cardiovascular function, kidney function, and stimulating red blood cell production.
   • Aldosterone regulates electrolyte balance, body fluid volume, osmotic pressure, and blood pressure.

19.2.8 Pancreas

1. Composition:

   • The pancreas is a composite gland that acts as both an exocrine and an endocrine gland.

2. Endocrine Pancreas:

   • The endocrine pancreas consists of clusters of cells called Islets of Langerhans.
   • Islets of Langerhans represent only about 1 to 2 percent of the pancreatic tissue.
   • The two main types of cells in the Islets of Langerhans are α-cells and β-cells.

3. Hormones Secreted:

   • α-cells secrete glucagon.
   • β-cells secrete insulin.

4. Functions of Glucagon:

   • Glucagon is a peptide hormone that plays a role in maintaining normal blood glucose levels.
   • It acts mainly on liver cells (hepatocytes) and stimulates:
     • Glycogenolysis: Breakdown of glycogen into glucose, leading to increased blood sugar levels (hyperglycemia).
     • Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources, also contributing to hyperglycemia.
   • Glucagon reduces cellular glucose uptake and utilization, further contributing to hyperglycemia.

5. Functions of Insulin:

   • Insulin is a peptide hormone that regulates glucose homeostasis.
   • It acts mainly on hepatocytes and adipocytes (fat cells), enhancing cellular glucose uptake and utilization.
   • Insulin promotes the movement of glucose from the blood into hepatocytes and adipocytes, leading to decreased blood glucose levels (hypoglycemia).
   • It stimulates glycogenesis: conversion of glucose into glycogen for storage in liver and muscle cells.

6. Role in Glucose Homeostasis:

   • Glucose homeostasis in the blood is maintained jointly by insulin and glucagon.
   • Insulin and glucagon work in opposition to each other to regulate blood glucose levels, ensuring they remain within a narrow range.

7. Diabetes Mellitus:

   • Prolonged hyperglycemia can lead to a complex disorder called diabetes mellitus.
   • Diabetes mellitus is associated with symptoms such as loss of glucose through urine and the formation of harmful compounds known as ketone bodies.
   • Diabetic patients are often treated with insulin therapy to manage their blood glucose levels.