Endocrine System: Anatomy & Physiology Study Notes

Endocrine System Overview

Introduction

The endocrine system is a network of glands and organs that produce, store, and secrete hormones. These chemical messengers regulate various physiological processes, including growth, metabolism, and homeostasis.

Classification of Endocrine Organs

Primary, Secondary, and Neuroendocrine Organs

• Primary endocrine organs are those whose main function is hormone secretion (e.g., pituitary gland, thyroid gland).

• Secondary endocrine organs have other primary functions but also secrete hormones (e.g., heart, kidneys).

• Neuroendocrine organs integrate neural and endocrine functions (e.g., hypothalamus).

Location of Major Endocrine Organs

• Hypothalamus: Base of the brain, above the pituitary gland.

• Pituitary Gland: Below the hypothalamus, in the sella turcica of the sphenoid bone.

• Pineal Gland: Deep in the brain, near the center.

• Thyroid Gland: Anterior neck, below the larynx.

• Parathyroid Glands: Posterior surface of the thyroid gland.

• Thymus: Upper chest, behind the sternum.

• Adrenal Glands: On top of each kidney.

• Pancreas: Upper abdomen, behind the stomach.

• Ovaries/Testes: Pelvic cavity/scrotum.

Hormone Classification

Amino Acid/Protein-Based vs. Steroid-Based Hormones

• Amino acid/protein-based hormones are generally hydrophilic (water-soluble), e.g., insulin, growth hormone.

• Steroid-based hormones are hydrophobic (lipid-soluble), e.g., cortisol, aldosterone.

Hormone Signaling Mechanisms

Paracrine, Autocrine, and Endocrine Signals

• Paracrine signals: Affect nearby cells.

• Autocrine signals: Affect the same cell that secreted the hormone.

• Endocrine signals: Travel through the bloodstream to distant target cells.

Water-Soluble vs. Lipid-Soluble Hormone Action

• Water-soluble hormones bind to cell surface receptors, activating second messenger systems (e.g., cAMP) to elicit a response.

• Lipid-soluble hormones diffuse through the cell membrane and bind to intracellular receptors, directly affecting gene transcription.

Free vs. Bound Hormones

• Free hormones circulate unbound in the blood and are usually water-soluble.

• Bound hormones are attached to carrier proteins, typically lipid-soluble hormones.

Up-Regulation vs. Down-Regulation

• Up-regulation: Increase in receptor number in response to low hormone levels.

• Down-regulation: Decrease in receptor number in response to high hormone levels.

Hypothalamic-Pituitary Axis

Control of Pituitary Glands

• The hypothalamus controls the anterior pituitary via releasing/inhibiting hormones through the hypophyseal portal system.

• The posterior pituitary is controlled by direct neural connections from the hypothalamus.

Anterior vs. Posterior Pituitary

• Anterior pituitary: Also called adenohypophysis.

• Posterior pituitary: Also called neurohypophysis.

Hormones Stored in Posterior Pituitary

• Oxytocin and Antidiuretic Hormone (ADH) are produced by the hypothalamus and stored/released by the posterior pituitary.

Tropic Hormones

Definition

• Tropic hormones are hormones that stimulate other endocrine glands to secrete their hormones (e.g., TSH, ACTH).

Thyroid and Adrenal Gland Structure

Thyroid Cell Types

• Follicular cells: Secrete T3 and T4 (thyroid hormones).

• Parafollicular cells (C cells): Secrete calcitonin.

Adrenal Cortex Zones and Hormones

• Zona glomerulosa: Secretes mineralocorticoids (e.g., aldosterone).

• Zona fasciculata: Secretes glucocorticoids (e.g., cortisol).

• Zona reticularis: Secretes androgens.

Pancreatic Cell Types

Endocrine vs. Exocrine Pancreatic Cells

• Exocrine cells: Acinar cells, secrete digestive enzymes.

• Endocrine cells: Islets of Langerhans, secrete hormones.

• Endocrine vs. Exocrine: Endocrine glands secrete hormones into the blood; exocrine glands secrete substances into ducts.

Alpha, Beta, Delta Cells

• Alpha cells: Secrete glucagon.

• Beta cells: Secrete insulin.

• Delta cells: Secrete somatostatin.

Endocrine Disorders

Major Disorders, Causes, and Characteristics

Disorder	What is caused by?	Important characteristics
Gigantism	Excess growth hormone (GH) before epiphyseal plate closure	Abnormal height, large hands/feet
Acromegaly	Excess GH after epiphyseal plate closure	Enlarged facial bones, hands, feet
Pituitary Dwarfism	GH deficiency in childhood	Short stature, normal body proportions
Graves' Disease	Autoimmune hyperthyroidism	Goiter, exophthalmos, increased metabolism
Hypothyroidism	Thyroid hormone deficiency	Fatigue, weight gain, cold intolerance
Cushing's syndrome	Excess cortisol	Moon face, buffalo hump, hyperglycemia
Addison disease	Adrenal cortex hormone deficiency	Hyperpigmentation, hypotension
Hypoglycemia	Low blood glucose	Shakiness, confusion, sweating
Hyperglycemia	High blood glucose	Polyuria, polydipsia, fatigue
Type 1 Diabetes Mellitus	Autoimmune destruction of beta cells	Insulin deficiency, early onset
Type 2 Diabetes Mellitus	Insulin resistance	Adult onset, obesity-related

Major Hormones and Their Effects

Secreting Organ	Hormone	Effect
Posterior Pituitary	Oxytocin (OT)	Uterine contraction, milk ejection
Posterior Pituitary	Antidiuretic Hormone (ADH)	Water reabsorption in kidneys
Anterior Pituitary	Follicle Stimulating Hormone (FSH)	Stimulates gamete production
Anterior Pituitary	Luteinizing Hormone (LH)	Stimulates ovulation/testosterone production
Anterior Pituitary	Adrenocorticotropic Hormone (ACTH)	Stimulates adrenal cortex
Anterior Pituitary	Thyroid Stimulating Hormone (TSH)	Stimulates thyroid hormone release
Anterior Pituitary	Prolactin (PRL)	Milk production
Anterior Pituitary	Growth Hormone (GH)	Stimulates growth, metabolism
Thyroid	T3 and T4	Regulate metabolism
Thyroid	Calcitonin	Lowers blood calcium
Parathyroid	Parathyroid Hormone (PTH)	Raises blood calcium
Adrenal Cortex	Aldosterone	Regulates sodium/potassium
Adrenal Cortex	Cortisol	Stress response, increases glucose
Adrenal Medulla	Epinephrine/Norepinephrine	Fight-or-flight response
Pancreas	Glucagon	Raises blood glucose
Pancreas	Insulin	Lowers blood glucose
Thymus	Thymosin/Thymopoietin	T cell development
Pineal	Melatonin	Regulates circadian rhythms
Heart	Atrial Natriuretic Peptide (ANP)	Lowers blood pressure
Kidney	Erythropoietin	Stimulates red blood cell production

Feedback Loops in Endocrine Regulation

Negative Feedback Loops

• Most endocrine responses are regulated by negative feedback, maintaining homeostasis.

• Example: Blood glucose regulation by insulin and glucagon.

Feedback Loop Structure

1. Stimulus: Change in physiological variable (e.g., low blood glucose).

2. Receptor: Detects the change.

3. Control Center: Processes information and initiates response (e.g., pancreas).

4. Effector: Executes the response (e.g., release of insulin).

5. Response: Restores variable to normal range.

Examples of Feedback Loops

• Low Blood Glucose: Stimulus → Pancreatic alpha cells (receptor) → Pancreas (control center) → Glucagon release (effector) → Increased blood glucose (response).

• High Blood Glucose: Stimulus → Pancreatic beta cells (receptor) → Pancreas (control center) → Insulin release (effector) → Decreased blood glucose (response).

• Low Blood Calcium: Stimulus → Parathyroid gland (receptor) → Parathyroid gland (control center) → PTH release (effector) → Increased blood calcium (response).

• High Blood Calcium: Stimulus → Thyroid gland (receptor) → Thyroid gland (control center) → Calcitonin release (effector) → Decreased blood calcium (response).

Key Equations and Concepts

Hormone-Receptor Binding

• Water-soluble hormone action: $\text{Hormone} + \text{Receptor} \rightarrow \text{Second Messenger Activation} \rightarrow \text{Cellular Response}$

• Lipid-soluble hormone action: $\text{Hormone} + \text{Intracellular Receptor} \rightarrow \text{Gene Transcription} \rightarrow \text{Protein Synthesis}$

Blood Glucose Regulation

• Insulin lowers blood glucose: $\text{Glucose uptake} \uparrow$

• Glucagon raises blood glucose: $\text{Glycogen breakdown} \uparrow$

Blood Calcium Regulation

• PTH increases blood calcium: $\text{Bone resorption} \uparrow$

• Calcitonin decreases blood calcium: $\text{Bone deposition} \uparrow$

Additional info: Some details, such as specific symptoms and feedback loop steps, were inferred based on standard Anatomy & Physiology curriculum.