Concept 3 Notes - Endocrine System

Endocrine System Overview

• Interacts with the nervous system to coordinate and control bodily activities.

• Utilizes hormones as chemical messengers secreted into extracellular fluids.

• Hormones travel through blood to regulate various functions in cells such as:

  • Reproduction

  • Growth and development

  • Blood nutrient balance

  • Cellular metabolism

  • Mobilizing body defenses for immune system.

Signal Types

Autocrine Signaling

• Involves the signaling molecule affecting the same cell that produces it.

Paracrine Signaling

• Involves signaling that affects nearby target cells.

Endocrine Signaling

• Hormones are released into the bloodstream to affect distant target cells.

Systems Comparison

Nervous System

• Uses action potentials (APs) and neurotransmitters.

• Provides immediate and short-term responses, acting at specific locations with short-distance signals.

Endocrine System

• Utilizes hormones released into the blood.

• Provides delayed and long-term responses, acting at target locations via blood diffusion.

Interaction

• Both systems work together to regulate, integrate, and coordinate body functions at a cellular level.

Key Structures of the Endocrine System

• Glands: Secretes hormones distributed throughout the body, including:

  • Pituitary Gland

  • Thyroid Gland

  • Parathyroid Gland

  • Adrenal Gland

  • Pineal Gland

• Other Key Structures: Hypothalamus, pancreas, gonads, and placenta.

Hormone Regulation

• Negative Feedback Mechanisms: Control the synthesis and release of hormones; target organ's response inhibits further hormone production.

Types of Stimuli Triggering Hormone Release

1. Humoral Stimuli: Hormone release due to altered levels of critical ions/nutrients.

   • Example: Low calcium levels stimulate parathyroid gland to secrete parathyroid hormone (PTH), increasing blood calcium levels.

2. Neural Stimuli: Hormone release due to neural input.

   • Example: Sympathetic nervous system triggers adrenal gland to release epinephrine in response to stress.

3. Hormonal Stimuli: Hormone release due to other hormones.

   • Example: Hormones from the hypothalamus stimulate the anterior pituitary gland.

Target Cell Interaction

• Hormones alter activity only in target cells with specific receptors.

• Factors Affecting Target Cell Response:

  • Blood levels of the hormone

  • Number of receptors for the hormone

  • Affinity of binding between hormone and receptor.

Hormonal Interactions

• Permissiveness: One hormone's function depends on another hormone's presence.

  • Example: Thyroid hormone stimulation is necessary for reproductive system hormones.

• Synergism: More than one hormone produces the same effect at target cells.

  • Example: Glucagon and epinephrine combined increase glucose release from the liver.

• Antagonism: One hormone opposes another's effect.

  • Example: Insulin lowers blood glucose while glucagon raises it.

Hormone Chemistry

• Hormones' chemical structure determines:

  • Solubility in water

  • Blood transportation methods

  • Duration before degradation

  • Types of receptors they can bind to.

• Main Types of Hormones:

  • Amino-Acid Based: Most hormones, generally water-soluble (excludes thyroid hormone).

  • Steroid Hormones: Lipid soluble.

Hormonal Characteristics

Lipid-Soluble Hormones

• Includes all steroid hormones and thyroid hormone.

• Secreted by adrenal cortex, gonads, and thyroid gland.

• Not stored in vesicles; relies on plasma proteins for blood transport.

• Long half-life in blood; receptor sites are usually intracellular.

Water-Soluble Hormones

• Involves all amino acid-based hormones (excluding thyroid hormone).

• Stored in secretory vesicles; transported freely in the blood.

• Shorter half-life; receptors are located on plasma membranes.

Hormonal Actions

• Lipid-soluble hormones activate genes to synthesize new proteins.

• Water-soluble hormones utilize second messengers to initiate signal transduction pathways.