Endocrine System: Structure, Function, and Regulation

16.1 Overview of the Endocrine System

Comparison of Endocrine and Nervous System Control

The endocrine and nervous systems are the primary regulators of body functions, each using distinct mechanisms to communicate and coordinate physiological processes.

• Nervous System:

  • Uses electrical signals and neurotransmitters

  • Fast responses but short-lived

  • Usually affects a specific area (e.g., muscle contraction)

• Endocrine System:

  • Uses hormones carried in the blood

  • Slower responses but longer lasting

  • Can affect many parts of the body (e.g., growth or metabolism)

Main Structures and Functions of the Endocrine System

The endocrine system consists of glands that secrete hormones to regulate various body functions.

• Major glands: Pituitary, pineal, thyroid, parathyroid, adrenal

• Other hormone-producing organs: Pancreas, gonads, hypothalamus, thymus, heart, kidneys, digestive tract, adipose tissue, placenta

• Functions:

  • Maintain homeostasis

  • Control metabolism

  • Support growth and development

  • Manage reproduction

Types of Chemical Signaling Used by the Body

• Endocrine: Hormones travel through blood to distant cells

• Paracrine: Signals act locally on nearby cells

• Autocrine: Cells signal themselves

• Juxtacrine: Direct contact between cells

• Neuroendocrine: Neurons release hormones into blood

16.1 Chemical Classes of Hormones and Receptors

Major Chemical Classes

• Amino acid-derived: Epinephrine, melatonin; water-soluble, bind to surface receptors

• Peptide/protein: Insulin, glucagon, GH; water-soluble, bind surface receptors, activate second messengers

• Steroid: Cortisol, estrogen, testosterone; lipid-soluble, bind intracellular receptors, change gene activity

Stimuli Controlling Hormone Secretion & Feedback

• Humoral stimuli: Changes in blood levels of ions/nutrients (e.g., low Ca2+ → PTH release)

• Hormonal stimuli: One hormone triggers another (TRH → TSH → thyroid hormone)

• Neural stimuli: Nervous system signals glands (sympathetic nerves → adrenal medulla → epinephrine)

• Feedback:

  • Negative: Most common, brings things back to balance

  • Positive: Amplifies response (rare; e.g., oxytocin during childbirth)

16.2 Hypothalamus and Pituitary Gland

Anatomical and Functional Relationship

• Hypothalamus: Links nervous and endocrine systems

• Posterior pituitary: Stores hypothalamic hormones (not endocrine itself)

• Anterior pituitary: Makes hormones under hypothalamic control (through hypophyseal portal system)

Topic: Stimuli and Order of Effects

• Examples:

  • TSH → thyroid

  • ACTH → adrenal cortex

  • LH & FSH → gonads

Posterior Pituitary Hormones

• ADH: High blood osmolarity → kidneys → retain water

• Oxytocin: Cervix stretch or sucking → uterus & mammary glands → contractions & milk ejection

Anterior Pituitary Hormones

• GH: Stimulated by GHRH → liver, muscle, bone → growth & protein synthesis

• TSH: Stimulated by TRH → thyroid → thyroid hormone release

• ACTH: Stimulated by CRH → adrenal cortex → cortisol release

• FSH/LH: Stimulated by GnRH → gonads → gamete production & sex hormone release

• Prolactin: Stimulated by PRH → mammary glands → milk production

Negative Feedback Loops

• High thyroid hormone → stops TRH & TSH release

16.3 Thyroid and Parathyroid Glands

Anatomy

• Thyroid: Butterfly-shaped, front of neck; follicles store thyroglobulin (colloid)

• Parathyroid: Four small glands behind thyroid

Thyroid Gland Cells

• Follicular cells: Make T3 and T4

• Parafollicular cells: Make calcitonin → lowers blood calcium

Thyroid Hormones

• Stimulus: TSH from anterior pituitary

• Targets: Most body cells

• Functions: Increase metabolism, growth, development

• Regulation: High thyroid hormone inhibits TRH & TSH

Parathyroid Hormone (PTH)

• Stimulus: Low blood calcium

• Target: Bones, kidneys, intestines

• Effect: Raises blood calcium by breaking down bone, conserving calcium in kidneys, and increasing intestinal absorption (via vitamin D)

16.4 Adrenal Glands

Anatomy

• Cortex (outer): Makes steroid hormones

• Medulla (inner): Nervous tissue, makes catecholamines

Cortex Hormones

• Mineralocorticoids (aldosterone): Low Na+ / high K+ → kidneys → retain Na+ & excrete K+

• Glucocorticoids (cortisol): Stimulated by ACTH → most cells → stress response, raise glucose, reduce inflammation

Medulla Hormones

• Epinephrine & norepinephrine: Sympathetic activation → fight-or-flight (↑HR, ↑BP, ↑glucose)

16.5 Endocrine Pancreas

Structure

• Alpha cells: Make glucagon

• Beta cells: Make insulin

Hormones

• Glucagon: Raises blood sugar

  • Target: Liver

  • Effect: Break down glycogen → raise blood sugar

• Insulin: Lowers blood sugar

  • Target: Most cells

  • Effect: Glucose uptake & storage → lower blood sugar

Interaction

• Glucagon and insulin work opposite each other → keep blood sugar steady (78–110 mg/dL)

Diabetes Mellitus

• Type 1: Beta cells destroyed → no insulin

• Type 2: Insulin resistance, often linked to obesity

• Symptoms:

  • Polyuria = very dilute urine

  • Polydipsia = very thirsty

  • Polyphagia = very hungry

• Treatments: Type 1 = insulin, Type 2 = lifestyle changes + meds

16.6 Other Endocrine Glands & Hormone-Secreting Tissues

• Pineal gland: Melatonin → darkness → brain → regulates sleep/wake cycles

• Thymus: Thymosin, T cells → immune system maturation

• Gonads:

  • Ovaries: Estrogen & progesterone → menstrual cycle & female traits

  • Testes: Testosterone → sperm production & male traits

• Adipose tissue: Leptin → hypothalamus → suppress appetite & control energy balance

• Heart: ANP → kidneys → lowers BP by getting rid of sodium/water

• Kidneys:

  • EPO → makes RBCs

  • Renin → raises BP

  • Calcitriol → activates vitamin D → ↑ calcium absorption

16.7 Endocrine Control of Physiological Variables

Examples of Homeostasis

• Blood sugar: insulin vs glucagon

• Calcium: PTH vs calcitonin

• Blood pressure: aldosterone, ANP, ADH

Stress Response

• Short-term: Sympathetic nerves → adrenal medulla → epinephrine/norepinephrine → fight-or-flight

• Long-term: Hypothalamus → CRH → pituitary → ACTH → adrenal cortex → cortisol

Table: Major Endocrine Glands, Hormones, and Functions

Gland	Hormone(s)	Main Function(s)
Pituitary (anterior)	GH, TSH, ACTH, FSH, LH, Prolactin	Growth, metabolism, stress response, reproduction, lactation
Pituitary (posterior)	ADH, Oxytocin	Water balance, uterine contraction, milk ejection
Thyroid	T3, T4, Calcitonin	Metabolism, calcium regulation
Parathyroid	PTH	Raises blood calcium
Adrenal cortex	Aldosterone, Cortisol	Salt/water balance, stress response
Adrenal medulla	Epinephrine, Norepinephrine	Fight-or-flight response
Pancreas	Insulin, Glucagon	Blood sugar regulation
Pineal	Melatonin	Sleep/wake cycles
Gonads	Estrogen, Progesterone, Testosterone	Reproduction, secondary sex traits

Key Equations

• Blood Glucose Homeostasis: $\text{Insulin} \uparrow \Rightarrow \text{Blood Glucose} \downarrow$ $\text{Glucagon} \uparrow \Rightarrow \text{Blood Glucose} \uparrow$

• Calcium Homeostasis: $\text{PTH} \uparrow \Rightarrow \text{Blood Ca}^{2+} \uparrow$ $\text{Calcitonin} \uparrow \Rightarrow \text{Blood Ca}^{2+} \downarrow$

Additional info: Expanded explanations and table for clarity and completeness; some hormone functions and feedback mechanisms inferred from standard academic sources.