Endocrine System Lecture Notes Flashcards

Overview of the Endocrine System

• System Function: The human body utilizes two systems to regulate the functions of other systems: the nervous system and the endocrine system.

• Definition of Endocrine Glands: These are ductless glands that secrete chemical messengers, known as hormones, directly into the bloodstream.

• Mechanism of Action: Hormones travel through the circulatory system to reach specific target organs where they exert their biological actions.

• Major Endocrine Glands:

  • Thyroid Gland

  • Thymus

  • Adrenal Gland

  • Testicles

  • Pituitary Gland

  • Pancreas

  • Ovary

Anatomy of the Pituitary Gland

• General Characteristics: Known as the "Master Gland," it is small in size, weighing approximately $0.5-1\,gm$.

• Location: Situated at the base of the skull within a small bony cavity called the sella turcica (also known as the hypophysial fossa).

• Attachment: It is connected to the floor of the 3rd ventricle of the brain by a stalk referred to as the pituitary stalk or infundibulum.

• Lobar Structure:

  • Adenohypophysis (Anterior Pituitary Gland): A large anterior lobe tissue.

  • Intermediate Lobe: A small lobe that is rudimentary in humans.

  • Neurohypophysis (Posterior Pituitary Gland): A large posterior lobe tissue.

Hypothalamic Connections with the Pituitary Gland

The hypothalamus and the pituitary gland form a critical link between the nervous system and the endocrine system via two distinct types of connections:

• Vascular Connection (Anterior Lobe): This involves the hypothalamo-hypophysial portal circulation.

  • Arterial twigs from the carotid arteries form a primary capillary plexus in the basal hypothalamus (median eminence).

  • Blood drains via portal hypophysial vessels down the pituitary stalk to a secondary capillary plexus in the anterior lobe.

  • Blood finally drains into the hypophysial vein.

• Nervous Connection (Posterior Lobe): This is the hypothalamo-hypophysial tract, consisting of neuronal bundles originating in the hypothalamus and terminating in the posterior pituitary.

The Hypothalamo-Hypophyseal Portal Circulation

• Definition of Portal Circulation: The presence of two sets of capillaries between two organs.

  • Examples include the connection between the hypothalamus and anterior pituitary, and the connection between the liver and intestine.

• Process and Pathways:

  • Specific nuclei in the hypothalamus (ventromedial, arcuate, pre-optic, and para-ventricular) secrete Releasing or Inhibiting hormones into the first set of the portal circulation.

  • These hormones travel to the second set of capillaries in the adenohypophysis to control the secretion of GH, Prolactin, TSH, ACTH, LH, and FSH.

Hypothalamic Releasing and Inhibiting Hormones

• Chemical Nature: These are polypeptides (proteins).

• Specific Types:

  • Growth hormone releasing hormone (GHRH): Stimulates GH release.

  • Growth hormone inhibiting hormone (GHIH / Somatostatin): Inhibits GH release.

  • Prolactin inhibiting hormone (PIH / Dopamine): Inhibits Prolactin release.

  • Thyrotropin releasing hormone (TRH): Stimulates TSH release.

  • Corticotropin releasing hormone (CRH): Stimulates ACTH release.

  • Gonadotropin releasing hormone (GnRH): Stimulates FSH and LH release.

Classification of Pituitary Hormones

The pituitary gland secretes 7 protein hormones, categorized by their chemical structure and the cells that produce them:

• By Chemical Structure:

  1. Polypeptide Hormones:

     • Adrenocorticotropic hormone (ACTH or corticotropin).

     • Prolactin (PRL, Mammotropin, or Lactogenic hormone).

     • Growth hormone (GH, Somatotropic hormone, or Somatotropin).

     • Beta lipotropin (contains amino acid sequences of endorphins and enkephalins).

  2. Glycoprotein Hormones:

     • Thyroid stimulating hormone (TSH or thyrotropin).

     • Luteinizing hormone (LH or interstitial-cell stimulating hormone).

     • Follicle stimulating hormone (FSH).

     • Note: FSH and LH together are called gonadotropic hormones because they control the gonads.

• By Cell Type (Anterior Pituitary):

  • Acidophils: Secretors of GH (from somatotropes) and Prolactin (from mammotropes).

  • Basophils: Secretors of TSH (thyrotropes), ACTH (corticotropes), and Gonadotropins (FSH and LH from gonadotropes).

• Posterior Pituitary Hormones:

  1. Antidiuretic hormone (ADH or vasopressin).

  2. Oxytocin.

Growth Hormone (GH / Somatotropin)

• Nature: Polypeptide hormone.

• Metabolism: GH has no carrier protein. It has a short half-life of $20\,minutes$ and is rapidly metabolized in the liver.

• Effects on Growth: Increases both the number and size of cells.

  • Viscera: Increases the size of all organs (e.g., heart, lung, stomach).

  • Skeleton (Before/During Puberty): Occurs before the ossification and union of the epiphysis with the shaft.

    • Promotes differentiation of chondrocytes.

    • Stimulates liver production of somatomedin-C (Insulin-like Growth Factor-1 or IGF-I).

    • Results in thickening of epiphyseal cartilage, linear growth (height), and increased thickness of long and flat bones.

  • Skeleton (After Puberty): Occurs after the union of the epiphysis and shaft.

    • GH increases the thickness of all bones (long and flat) but produces no linear growth.

Metabolic Actions of Growth Hormone

• Protein and Electrolyte Metabolism (Anabolic):

  • Stimulates soluble collagen synthesis for soft tissue growth.

  • Produces positive ($+\text{ve}$) nitrogen balance by reducing blood urea nitrogen and amino acid levels through protein synthesis utilization.

  • Increases $Ca^{2+}$ absorption from the gastrointestinal tract (GIT).

  • Reduces $Na^{+}$ and $K^{+}$ excretion in urine as they are diverted to growing tissues.

• Carbohydrate Metabolism (Anti-insulin / Diabetogenic):

  • Increases glucose output from the liver.

  • Decreases the utilization of glucose for energy.

  • Decreases the number of insulin receptors on cells, reducing cellular glucose uptake.

  • Decreases glucose phosphorylation inside cells.

• Fat Metabolism (Lipolytic / Ketogenic):

  • Stimulates the "hormone sensitive lipase" enzyme, leading to lipolysis and mobilization of free fatty acids (FFA).

  • Promotes the use of FFA for energy instead of glucose.

  • Excess GH can cause ketosis (conversion of FFA into ketone bodies), particularly during hypoglycemia, fasting, or stress.

Regulation of Growth Hormone Secretion

• Hypothalamic Control:

  • GHRH: Stimulates GH secretion from somatotropes.

  • GHIH (Somatostatin): Inhibits GH secretion from somatotropes.

• Feedback Mechanism: Long loop negative feedback where IGF-I inhibits GH secretion from the anterior pituitary and GHRH secretion from the hypothalamus.

• Factors Increasing Secretion ($\uparrow\uparrow$):

  • Insufficient calorie intake: Acute (hypoglycemia, low FFA) or Chronic (fasting, starvation).

  • Stressful stimuli.

  • Physical exercise.

  • Post-protein meal intake.

  • Hormones: GHRH, estrogen, androgen, and glucagon.

• Factors Decreasing Secretion ($\downarrow\downarrow$):

  • Sufficient calorie intake: Acute (hyperglycemia, high FFA) or Chronic (obesity).

  • Hormones: Somatostatin, cortisol, and progesterone.

Prolactin (Mammotrophic Hormone)

• Functions in Females:

  • Milk Production: After the breasts are primed with estrogen and progesterone, Prolactin (PRL) causes milk formation by increasing the production of casein and lactalbumin.

  • Prevention of Ovulation (Anovulation): PRL inhibits the effect of FSH and LH on the ovaries, leading to amenorrhea and infertility during the lactation period.

• Functions in Males: No known physiological effect.

• Regulation of Prolactin Secretion:

  • Hypothalamic Inhibition: Controlled primarily by Prolactin Inhibiting Hormone (Dopamine).

  • Tonic Inhibition: Powerful dopamine secretion maintains tonic inhibition during non-lactating periods.

  • Indirect Negative Feedback: Secreted PRL stimulates the hypothalamus to secrete dopamine, which then inhibits further PRL secretion.