The Endocrine System: Endocrine Glands and Hormone Actions

The Endocrine System: Overview

General Characteristics

The endocrine system is responsible for long-distance communication within the body, primarily through the release of hormones. It is a slow-acting system compared to the nervous system and consists of glands derived from epithelial tissue. Endocrine glands are classified as either primary or secondary, depending on their main function.

• Primary endocrine organs: Main function is hormone secretion.

• Secondary endocrine organs: Hormone secretion is secondary to other functions.

Primary Endocrine Organs

Organs Located Within the Brain

• Hypothalamus

• Pituitary gland

• Pineal gland

Organs Located Outside the Nervous System

• Thyroid gland and parathyroid glands

• Thymus

• Adrenal glands

• Pancreas

• Gonads (testes and ovaries)

• Placenta (in pregnant females)

Secondary Endocrine Organs

Organs and Functions

• Heart: Secretes atrial natriuretic peptide (ANP), regulates sodium reabsorption.

• Liver: Secretes insulin-like growth factors (IGFs), promotes tissue growth.

• Stomach, small intestine: Secrete hormones regulating digestion.

• Kidney: Secretes erythropoietin, stimulates red blood cell production.

• Skin: Involved in vitamin D metabolism.

Hypothalamus and Pituitary Gland

Structure and Connection

The hypothalamus and pituitary gland are located in the brain and are connected by the infundibulum, a thin stalk of tissue. The pituitary gland is divided into the anterior lobe (epithelial tissue) and posterior lobe (neural tissue). Hormones secreted by the hypothalamus regulate pituitary gland function.

Neural Connection: Posterior Pituitary

• Neurons from the hypothalamus project to the posterior pituitary.

• Secrete two peptide hormones: antidiuretic hormone (ADH) and oxytocin.

• ADH increases water reabsorption in kidneys; oxytocin increases uterine contraction and milk letdown.

Blood Connection: Anterior Pituitary

The hypothalamic-pituitary portal system is a specialized blood vessel arrangement linking two capillary beds in series. Neurosecretory cells in the hypothalamus secrete tropic hormones into this system, which travel to the anterior pituitary and regulate hormone release.

Hypothalamic and Anterior Pituitary Tropic Hormones

Types and Functions

• The hypothalamus secretes seven tropic hormones: five stimulating (–RH) and two inhibiting (–IH).

• Tropic hormones regulate the secretion of other hormones, either stimulating or inhibiting.

• Anterior pituitary tropic hormones act on other endocrine glands to stimulate hormone release.

Feedback Regulation of Hormone Release

Negative Feedback Loops

Feedback loops regulate hormone release in multistep pathways:

• Short loop negative feedback: Anterior pituitary tropic hormone inhibits hypothalamic tropic hormone.

• Long loop negative feedback: Hormone from the endocrine gland inhibits secretion of tropic hormones by hypothalamus/anterior pituitary.

Example: Regulation of Cortisol Release

• Cortisol inhibits its own secretion by decreasing CRH and ACTH release.

Pineal Gland

Structure and Function

The pineal gland is located in the brain and composed of epithelial tissue. It secretes melatonin, which regulates circadian rhythms, enhances immune function, and is used therapeutically for sleep disorders and jet lag.

Thyroid and Parathyroid Glands

Thyroid Gland

• Butterfly-shaped, located on the ventral surface of the trachea.

• Secretes T4 (thyroxin) and T3 (triiodothyronine) to regulate metabolic rate and growth.

• Secretes calcitonin to regulate blood calcium levels.

Parathyroid Gland

• Four small glands on the posterior surface of the thyroid.

• Secrete parathyroid hormone (PTH) to increase blood calcium levels by acting on bones, kidneys, and intestines.

Thymus

Structure and Function

• Located near the heart.

• Secretes thymosin, which regulates T-cell function and maturation.

Adrenal Glands

Structure

The adrenal glands are located above the kidneys and are divided into two regions: the inner medulla (neural tissue) and the outer cortex (epithelial tissue). The cortex contains three layers: zona glomerulosa, zona fasciculata, and zona reticularis.

Hormones of the Adrenal Cortex

• Mineralocorticoids (aldosterone): Regulate sodium and potassium levels.

• Glucocorticoids (cortisol): Regulate stress response and metabolism.

• Sex hormones (androgens): Regulate reproductive function.

Adrenal Medulla

• Contains chromaffin cells.

• Secretes catecholamines: 80% epinephrine, 20% norepinephrine, <1% dopamine.

• Epinephrine is released during stress, increasing heart rate and mobilizing energy stores.

Pancreas

Structure and Dual Function

The pancreas is located below the stomach and functions as both an exocrine and endocrine gland. Exocrine cells secrete digestive enzymes, while endocrine cells (islets of Langerhans) secrete hormones.

Endocrine Pancreas: Islets of Langerhans

• Alpha cells: Secrete glucagon, increase blood glucose.

• Beta cells: Secrete insulin, decrease blood glucose.

• Delta cells: Secrete somatostatin, regulate digestion and hormone secretion.

• F cells: Secrete pancreatic polypeptide, inhibit exocrine pancreas and gallbladder contraction.

Gonads and Placenta

Gonads

• Produce gametes (sperm and oocytes).

• Secrete sex hormones: androgens (testosterone, androstenedione) in males; estradiol and progesterone in females.

Placenta

• Functions as an endocrine gland in pregnancy.

• Secretes estrogens, progesterone, and human chorionic gonadotropin (hCG).

• Supports pregnancy and fetal development.

Control of Hormone Levels in Blood

Factors Affecting Hormone Concentration

• Rate of hormone secretion

• Amount of hormone bound to plasma proteins

• Rate of hormone metabolism

Rate of Hormone Secretion

• Endocrine cells respond to neural and humoral signals, which may be stimulatory or inhibitory.

Transport of Hormones

• Hydrophobic hormones are transported bound to carrier proteins, increasing their half-life.

• Only free hormone can bind to receptors and produce effects.

Rate of Hormone Metabolism

• Hormones are metabolized in target cells, blood, and liver.

• Hydrophobic hormones are metabolized more slowly than hydrophilic hormones.

Abnormal Secretion of Hormones

Types and Consequences

• Hyposecretion: Too little hormone (e.g., diabetes mellitus type 1).

• Hypersecretion: Too much hormone (e.g., acromegaly—excess growth hormone in adults causes bone thickening and organ growth).

Primary vs. Secondary Hormone Secretion Disorders

• Primary disorder: Abnormality in the endocrine gland itself.

• Secondary disorder: Abnormality in hypothalamic or anterior pituitary tropic hormone secretion.

Primary Hypersecretion of Cortisol

• Abnormal adrenal gland secretes excess cortisol.

• Strong negative feedback decreases CRH and ACTH release.

Secondary Hypersecretion of Cortisol

• Abnormal anterior pituitary secretes excess ACTH.

• Stimulates excess cortisol production.

• Increased negative feedback decreases CRH levels.

Hormone Interactions

Types of Interactions

• Antagonism: Hormones have opposing effects (e.g., glucagon vs. insulin).

• Additive: Net effect equals the sum of individual effects.

• Synergistic: Net effect exceeds the sum of individual effects.

• Permissiveness: One hormone enables another to exert its effect (e.g., thyroid hormones enable epinephrine-induced bronchodilation).

Tables: Endocrine Organs and Their Hormones

Table: Primary Endocrine Organs and Hormones

Organ	Hormones	Functions
Hypothalamus	Releasing/inhibiting hormones	Regulate secretion of anterior pituitary hormones
Anterior pituitary	GH, ACTH, TSH, PRL, FSH, LH	Growth, metabolism, stress, thyroid function, reproduction
Posterior pituitary	ADH, oxytocin	Water balance, uterine contraction, milk letdown
Pineal gland	Melatonin	Circadian rhythms, immune function
Thyroid gland	T3, T4, calcitonin	Metabolic rate, growth, calcium regulation
Parathyroid gland	PTH	Calcium regulation
Thymus	Thymosin	T-cell maturation
Adrenal cortex	Aldosterone, cortisol, androgens	Electrolyte balance, stress response, reproduction
Adrenal medulla	Epinephrine, norepinephrine	Fight-or-flight response
Pancreas	Insulin, glucagon, somatostatin, pancreatic polypeptide	Blood glucose regulation, digestion
Gonads	Testosterone, estradiol, progesterone	Reproduction
Placenta	Estrogens, progesterone, hCG	Pregnancy maintenance

Table: Secondary Endocrine Organs and Hormones

Organ	Hormones	Functions
Heart	ANP	Sodium regulation
Kidney	Erythropoietin	Red blood cell production
GI tract	Cholecystokinin, secretin, gastrin	Digestion regulation
Liver	IGFs	Tissue growth
Skin, liver, kidney	Vitamin D3	Calcium regulation

Additional info: Academic context was added to clarify hormone functions, feedback mechanisms, and hormone interactions. Tables were inferred and expanded for completeness.