Endocrine Systems and Hormones

Definition: The endocrine system is composed of organs that secrete hormones into the bloodstream.
Hormones:
- Definition: A chemical substance secreted into the blood which communicates signals between organs or organ systems.
- General Impact: Most hormones affect the entire body, while some have specific functions.

Characteristics:
  - Easily transported in the blood as they are soluble in water.
  - These hormones are all proteins, synthesized through the pathway of DNA → RNA → proteins.
  - Stored in the gland from which they are secreted.
Mechanism of Action:
  - Act on receptors located on the cell membrane.
  - Magnify signals within the cell leading to a quick response.
  - Effects: Rapidly observed and can be immediate.

Characteristics:
- Require a protein carrier since lipids do not dissolve in blood.
- Derive from cholesterol, with specific enzymes in the organ converting cholesterol into hormones.
Mechanism of Action:
  - Bind to target cells initiating the endocytosis of the hormone.
  - The hormone then travels to the nucleus to regulate protein synthesis.
  - Effects: Responses are slower compared to water-soluble hormones.

Water-Soluble:
  - Easily transported; do not require carriers.
  - Composed of proteins made and stored in the cell.
  - Act on the cell surface to amplify signals.
  - Fast-acting.
Lipid-Soluble:
  - Challenging to transport in the blood; require a protein carrier.
  - Cholesterol-derived.
  - Function within the nucleus to regulate protein synthesis.
  - Slow-acting.

Function:
- Acts solely on the pituitary gland, regulating its hormonal output.
Mechanisms:
- Blood: Hormones travel to the pituitary through the hypothalamic portal system via arterial blood.
- Neural: Nuclei send axons to stimulate the posterior pituitary directly.

Location: Positioned inferior to the hypothalamus, located outside the central nervous system (CNS).

The hypothalamus secretes 5 hormones that influence the secretion of 6 anterior pituitary hormones.
Anterior Pituitary Hormones:
  - Follicle-stimulating hormone (FSH)
  - Luteinizing hormone (LH)
  - Adrenocorticotropic hormone (ACTH)
  - Thyroid-stimulating hormone (TSH)
  - Prolactin
  - Growth hormone (GH)

Mechanism of Secretion: The hypothalamus sends neural stimuli to the posterior pituitary either autonomously or in response to sensory input.
Hormones Secreted:
- Vasopressin (ADH)
- Oxytocin

Concept: Utilizes negative feedback mechanisms.
Process:
  - Pituitary gland secretes hormones targeting specific organs.
  - Target organs release chemicals that feedback to the hypothalamus and pituitary to inhibit further hormone release.
  - Activation of the hypothalamus occurs when the levels of these feedback chemicals in the blood are low.

Function: Stimulates the thyroid gland to release thyroid hormones (T3 and T4).
Characteristics of T3 and T4:
- Named based on the number of iodine atoms.
- Both are water-soluble hormones.
Effects of T3 and T4:
  - Regulates basal metabolic rate of the body.
  - Increases basal body temperature.
  - Stimulates lipolysis (catabolism of fats).
  - Enhances catecholamine action, especially in cardiac tissue.
  - Accelerates growth in nervous and skeletal systems in conjunction with Growth Hormone (GH).

Process:
  - Low levels of T3 and T4 stimulate the hypothalamus to secrete a stimulating hormone.
  - This stimulates the pituitary to produce TSH.
  - TSH then encourages the release of T3 and T4 from the thyroid gland.
  - Increased levels of T3 and T4 feedback to inhibit the hypothalamus and pituitary to maintain homeostasis.

Function: Stimulates the adrenal cortex to secrete cortisol.
Hormones Produced by Adrenal Cortex:
  - Cortisol: Full actions to be discussed in detail later.
  - Aldosterone: Discussed in kidney context.
  - Sex hormones: Estrogens, progesterone, testosterone, produced by both genders albeit in different levels.

Actions of Cortisol:
  - Immune system depression.
  - Anti-inflammatory effects.
  - Increases glucose production.
  - Promotes protein and amino acid catabolism.
  - Stimulates gluconeogenesis and lipolysis, enhancing stress resistance.
Control Mechanism: Feedback inhibition similar to T3 and T4 control process:
  - Low levels of cortisol stimulate the hypothalamus.
  - Stimulates the pituitary to produce ACTH.
  - ACTH stimulates the adrenal cortex for cortisol synthesis.
  - High levels of cortisol feedback to inhibit the hypothalamus.

Target Tissues: Liver, skeletal muscles, cartilage, and bones.
Function: Stimulates secretion of Insulin-like Growth Factor (IGF), which is a water-soluble hormone.
Actions of IGF:
  - Increases uptake of amino acids and lipolysis.
  - Triggers glycolysis.
  - Overall effect: Ensures adequate amino acids and energy for protein synthesis, promoting cell division and growth.

Prolactin:
- Target: Mammary glands for milk production (requires cortisol, GH, T3 & T4, insulin, and high progesterone levels).
FSH & LH: To be discussed in the reproduction section.
Posterior Pituitary Hormones:
  - Oxytocin: Contracts the uterus and promotes milk release in mammary glands.
  - Lactation relies on both prolactin and oxytocin functionalities.
  - Vasopressin (ADH): Further details provided in the kidney section.

Stimulus: Low blood glucose levels.
Origin: Produced by α-cells in the pancreas.
Function: Stimulates liver (and muscle cells) to generate and release glucose via gluconeogenesis.

Stimulus: High blood glucose levels.
Origin: Produced by β-cells in the pancreas.
Function: Promotes liver and muscle cells to absorb glucose and convert it into glycogen.

Glucagon Function: Stimulated by low blood sugar to increase blood glucose levels.
Insulin Function: Stimulated by high blood sugar to decrease blood glucose levels.