Exam 2 A&P

Endocrine System

🔹 Definition: The endocrine system is a network of ductless glands that secrete hormones directly into the bloodstream to regulate various long-term body processes such as growth, metabolism, reproduction, and maintaining homeostasis.

Glands

🔹 Glands: Organs that produce and secrete substances.

• Endocrine glands: Type of glands that secrete hormones into the bloodstream.

• Exocrine glands: Type of glands that secrete substances into ducts (e.g., sweat, saliva).

Hormones

🔹 Hormones: Chemical messengers released into the blood that bind to specific receptors on target cells.

Target Cells

🔹 Target Cells: Cells that contain specific receptors for a hormone. They respond when the corresponding hormone binds to them.

Portal System

🔹 Portal System: A specialized system of blood vessels that connects two capillary beds directly.

• Example: Hypothalamo-hypophyseal portal system.

Comparison: Endocrine vs Nervous System

Up- & Down-Regulation (Clinically Important)

🔹 Up-regulation: Increase in the number of receptors leading to a stronger response.
🔹 Down-regulation: Decrease in the number of receptors leading to a weaker response.
🔎 Clinical importance:

• Explains hormone resistance (e.g., insulin resistance in diabetes).

• Influences medication effectiveness.

Hypothalamo-Hypophyseal Pathways

1️⃣ Hypothalamo-Hypophyseal Tract:

• Neural connection from the hypothalamus to the posterior pituitary.

• Releases hormones: ADH (Antidiuretic Hormone) & Oxytocin.

2️⃣ Hypothalamo-Hypophyseal Portal System:

• Blood vessel connection from the hypothalamus to the anterior pituitary.

• Regulates the release of various regulatory hormones.

Comparison: Endocrine vs Exocrine Glands

Functions of the Endocrine System

🔹 Regulates:

• Metabolism

• Growth & development

• Fluid/electrolyte balance

• Reproduction

• Homeostasis

• Stress response

Endocrine Organ vs Endocrine Tissue

🔹 Endocrine organ: Primarily functions in hormone secretion.

• Example: Thyroid gland.

🔹 Endocrine tissue: Contains hormone-secreting cells within another organ.

• Example: Pancreatic islets.

Major Endocrine Glands & Locations

🧠 Hypothalamus: Contains neurosecretory cells.

• Located in the base of the brain.

• Connected via the infundibulum to the pituitary gland.

• Composed of median eminence and infundibular stem.

🏛 Pituitary Gland:

• Located in the sella turcica.

• Referred to as the “master gland” of the endocrine system, because it regulates other endocrine glands.

• Divided into anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis).

🦋 Thyroid Gland:

• Located in the anterior neck.

• Functions primarily to regulate metabolism.

🔺 Adrenal Glands:

• Located on top of kidneys.

• Composed of two parts: cortex and medulla.

🍬 Pancreas:

• Located posterior to the stomach.

• Islets of Langerhans secrete insulin and glucagon.

Regulation of Hormone Release

Three Types of Regulation

1️⃣ Hormonal Regulation:

• One hormone stimulates the release of another.

• Example: Thyroid Stimulating Hormone (TSH) stimulates the thyroid gland to release thyroid hormones.

2️⃣ Humoral Regulation:

• Blood levels trigger the release of hormones.

• Example: Rising glucose levels stimulate insulin secretion.

3️⃣ Neural Regulation:

• Nerve stimulation leads to release of hormones.

• Example: Sympathetic nervous system stimulates the adrenal medulla.

Types of Hormones

Three Types of Hormones

First vs Second Messenger

🔹 First messenger: The hormone that acts outside the target cell.
🔹 Second messenger: An intracellular signal that relays the chemical signal within the cell (e.g., cyclic AMP (cAMP)).

• Water-soluble hormones utilize second messenger systems for their effects.

Receptors & Cellular Response

🔹 The number of receptors present on a target cell affects the strength of the response:

• More receptors: Stronger response.

• Fewer receptors: Weaker response.

• Response intensity is dictated by hormone concentration and receptor number.

Structure of Endocrine Glands

Common Features

🔹 Endocrine glands typically exhibit the following characteristics:

• Highly vascularized.

• Composed of secretory cells organized in cords or clusters.

• Lack ducts.

• Surrounded by a connective tissue capsule.

Hypothalamus-Pituitary Interaction

🔹 Interactions:

• The hypothalamus regulates the anterior pituitary via the portal system.

• It controls the posterior pituitary through neural tracts.

• Employs releasing and inhibiting hormones to regulate hormone release.

Hormone Pathways & Feedback Mechanisms

Feedback Mechanisms

🔹 Prolactin:

• Pathway: Hypothalamus → inhibits prolactin release via dopamine.

• Mechanism: Negative feedback.

🔹 Growth Hormone (GH):

• Pathway: GHRH → GH → Insulin-like Growth Factors (IGFs).

• Mechanism: Negative feedback.

🔹 Luteinizing Hormone (LH) & Follicle Stimulating Hormone (FSH):

• Pathway: GnRH → LH/FSH → Gonads.

• Mechanism: Negative feedback (regulated by estrogen/testosterone levels).

🔹 Thyroid Hormone:

• Pathway: TRH → TSH → T3/T4.

• Mechanism: Negative feedback.

🔹 Cortisol:

• Pathway: CRH → ACTH → Cortisol.

• Mechanism: Negative feedback.

🔹 Insulin:

• Trigger: High glucose levels stimulate insulin release (humoral regulation).

🔹 Glucagon:

• Trigger: Low glucose levels stimulate glucagon release (humoral regulation).

Special Features of Oxytocin & ADH

🔹 Oxytocin:

• Mechanism: Positive feedback mechanism during childbirth.

• Functions: Stimulates uterine contractions and milk let-down reflex.

🔹 ADH (Antidiuretic Hormone):

• Function: Regulates water balance in the body.

• Mechanism: Increases water reabsorption in the kidneys.

• Regulation: Controlled by osmolarity of the blood.