Endocrine System: Anatomy & Physiology Study Notes

Endocrine System Overview

Comparison: Nervous System vs. Endocrine System

The nervous and endocrine systems are the two major control systems of the body. They differ in their methods, speed, and duration of action.

Feature	Nervous System	Endocrine System
Response Initiation	Rapid (milliseconds)	Slow (seconds to days)
Response Duration	Short	Long
Communication Method	Action potentials & neurotransmitters	Hormones released into the blood
Target Specificity	Specific locations (axon pathways)	Diffuse locations (anywhere blood reaches)
Messenger Distance	Very short	Long
Signal Strength	Frequency of action potentials	Hormone concentration

Functions of Hormones

• Reproduction

• Growth and development

• Electrolyte, water, and nutrient balance

• Cellular metabolism and energy balance

• Mobilization of body defenses

Endocrine Glands and Hormone-Producing Tissues

Endocrine vs. Exocrine Glands

• Endocrine glands: Ductless; secrete hormones into tissue fluid, with rich vascular and lymphatic drainage.

• Exocrine glands: Have ducts; secrete non-hormonal substances onto a membrane surface (e.g., sweat, saliva).

Major Endocrine Glands

• Pituitary

• Thyroid

• Parathyroid

• Adrenal

• Pineal

Additional Hormone-Producing Tissues

• Hypothalamus (neuroendocrine organ)

• Pancreas

• Gonads

• Other tissues: heart, kidneys, skeleton, skin, thymus, adipose tissue

Chemical Classification of Hormones

Types of Hormones

• Amino acid-based: Most hormones; water-soluble; cannot cross the plasma membrane (except thyroid hormone).

• Steroid hormones: Synthesized from cholesterol; lipid-soluble; can cross the plasma membrane.

• Eicosanoids: Prostaglandins; generally act as paracrines and autocrines.

Hormone Solubility and Its Implications

	Lipid-Soluble Hormones	Water-Soluble Hormones
Consist of	All steroid hormones and thyroid hormone	All amino acid-based hormones except thyroid hormone
Sources	Adrenal cortex, gonads, thyroid gland	All other endocrine glands
Stored in Secretory Vesicles	No	Yes
Transport in Blood	Bound to plasma proteins	Usually free in plasma
Half-Life in Blood	Long (most need to be metabolized by liver)	Short (most can be removed by kidneys)
Location of Receptors	Usually inside cell	On plasma membrane
Mechanism of Action at Target Cell	Activate genes, causing synthesis of new proteins	Usually act through second-messenger systems

Mechanisms of Hormone Action

Target Cells

Cells with specific receptors for a particular hormone.

Hormone Effects

• Alter the rates of normal cellular processes in target cells.

Stimuli for Hormone Release

• Humoral Stimuli: Release caused by altered levels of certain critical ions or nutrients in the blood.

• Neural Stimuli: Release caused by neural input.

• Hormonal Stimuli: Release caused by another hormone (tropic hormone).

Regulation

• Primarily through negative feedback mechanisms.

• Rising hormone levels cause target organ effects, which inhibit further hormone release.

Nervous System Modulation

• The nervous system can modify "turn-on" and "turn-off" factors affecting the endocrine system.

Factors Influencing Target Cell Activation

• Blood levels of the hormone

• Number of receptors on/in the target cell

• Affinity (strength) of binding between hormone and receptor

Regulation of Receptors

• Up-regulation: Target cells form more receptors in response to low hormone levels.

• Down-regulation: Target cells lose receptors in response to prolonged high hormone levels.

Hormone Interactions

• Permissiveness: One hormone cannot exert its full effects without another hormone being present.

• Synergism: More than one hormone produces the same effects, and their combined effects are amplified.

• Antagonism: One hormone opposes the action of another.

Half-life, Onset, and Duration of Hormone Activity

• Half-life: Time required for hormone blood levels to decrease by half (varies from minutes to hours).

Major Endocrine Glands and Their Hormones

Pituitary Gland

Posterior Pituitary (Neurohypophysis)

• Composed of neural tissue.

• Stores and releases neurohormones made in the hypothalamus.

• Hormones:

  • Oxytocin: Uterine contractions, milk ejection, neurotransmitter (social behavior).

  • Antidiuretic hormone (ADH): Water retention, vasoconstriction.

Anterior Pituitary (Adenohypophysis)

• Composed of glandular tissue.

• Synthesizes and releases several hormones in response to hypothalamic hormones.

• Hormones:

  • Growth hormone (GH): Tissue building, metabolic effects.

  • Thyroid-stimulating hormone (TSH): Thyroid hormone secretion.

  • Adrenocorticotropic hormone (ACTH): Release of adrenal cortex hormones.

  • Follicle-stimulating hormone (FSH): Gamete production.

  • Luteinizing hormone (LH): Production of gonadal hormones.

  • Prolactin (PRL): Milk production.

• Tropic Hormones: Stimulate hormone secretion by other endocrine glands (TSH, ACTH, FSH, LH).

Thyroid Gland

• Located in the anterior neck, on the trachea.

• Hormones:

  • Thyroid hormone (TH) (T3 and T4):

    • Increase basal metabolic rate and body heat production

    • Regulate tissue growth and development

    • Maintain blood pressure

  • Calcitonin: Lowers blood calcium levels (in children)

Parathyroid Glands

• Located on the posterior aspect of the thyroid gland.

• Secrete parathyroid hormone (PTH):

  • Primary regulator of blood calcium levels

  • Increases blood Ca2+ levels by targeting bone, kidneys, and intestine

Adrenal Glands

• Located on top of the kidneys.

• Regions:

  • Adrenal Cortex:

    • Zona Glomerulosa: Mineralocorticoids (Aldosterone): Regulation of electrolytes and blood pressure

    • Zona Fasciculata: Glucocorticoids (Cortisol): Regulation of metabolism and resistance to stress

    • Zona Reticularis: Gonadocorticoids (Androgens): Production of sex hormones

  • Adrenal Medulla:

    • Catecholamines (Epinephrine and Norepinephrine): Sympathetic nervous system response

Regulation of Aldosterone Secretion

• Renin-angiotensin-aldosterone system: Activated when blood pressure or volume falls; stimulates aldosterone release.

• Atrial natriuretic peptide (ANP): Released by the heart; inhibits aldosterone secretion, decreases blood pressure.

Regulation of Cortisol Secretion

• CRH (corticotropin-releasing hormone) triggers ACTH release

• ACTH (adrenocorticotropic hormone) promotes cortisol secretion

• Rising cortisol levels inhibit further release of CRH and ACTH (negative feedback)

Excessive Levels of Glucocorticoids

• Depress cartilage and bone formation

• Depress immune system

• Disrupt normal cardiovascular, neural, and gastrointestinal function

• Inhibit inflammation by decreasing release of inflammatory chemicals

Regulation of Gonadocorticoid Secretion

• ACTH stimulates their release but gonadocorticoids do not appear to exert feedback inhibition on ACTH release

• Hypersecretion causes adrenogenital syndrome (masculinization)

Adrenal Medullary Hormones and Stress Response

Hormone	Stimulus	Effects	Disorders
Glucocorticoids (mainly cortisol)	Stimulated by ACTH; inhibited by increased blood cortisol	Body cells promote gluconeogenesis, mobilize fats, depress immune response, resist stress	Cushing's syndrome, Addison's disease
Gonadocorticoids (mainly androgens)	Stimulated by ACTH; mechanism of inhibition not well understood	Contribute to axillary/pubic hair, sex drive in females, source of estrogens after menopause	Masculinization of females (adrenogenital syndrome)
Adrenal Medullary Hormones (catecholamines)	Stimulated by preganglionic fibers from the sympathetic nervous system	Sympathetic nervous system effects: increased heart rate, blood pressure, metabolic rate	Prolonged fight-or-flight response, hypertension

Stress Response

• Short-term stress: Triggers the fight-or-flight response; mediated by the sympathetic nervous system and adrenal medulla hormones.

• Long-term stress: Triggers a slower resistance reaction; mediated by hormones from the adrenal cortex.

• Exhaustion: Occurs if long-term stress is too prolonged; stress hormones disrupt homeostasis.

Pineal Gland

• Hangs from the roof of the third ventricle

• Secretes melatonin

• Controls sleep and mood

Pancreas, Gonads, and Other Hormone-Secreting Organs

Pancreas

• Type I Diabetes: Insulin absent

• Type II Diabetes: Insulin present but deficient

• Three signs of diabetes:

  • Polyuria (excessive urine)

  • Polydipsia (excessive thirst)

  • Polyphagia (excessive eating)

Gonads

• Ovaries: Estrogen, Progesterone

• Testes: Testosterone

Other Hormone-Producing Organs

• Heart: ANP (lowers blood pressure and blood volume)

• Kidneys: Erythropoietin (increases RBC production)

• Skeleton: Osteocalcin

• Skin: Cholecalciferol/Vitamin D (bone strength)

• Thymus: Thymosins (T-lymphocyte development)

• Adipose Tissue: Leptin (regulates stored energy/fat)

Key Terms and Concepts

• Hormone: Chemical messenger secreted by endocrine glands, travels in blood to target organs.

• Target cell: Cell with specific receptor for a hormone.

• Negative feedback: Mechanism that inhibits further hormone release when adequate levels are reached.

• Permissiveness, Synergism, Antagonism: Types of hormone interactions.

• Half-life: Time for hormone concentration in blood to decrease by half.

Important Equations

• Hormone Half-life: $\text{Half-life} = \frac{\ln 2}{k}$ (where $k$ is the rate constant for hormone removal)

Additional info: Some details, such as the mechanism of hormone action and feedback regulation, have been expanded for academic completeness.