Endocrine System: Glands, Hormones, and Regulation – Study Notes

Endocrine System Overview

Main Endocrine Glands

The endocrine system consists of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. The major endocrine glands include:

• Pancreas

• Adrenal Glands

• Thyroid

• Parathyroid

• Thymus

• Gonads (Ovaries/Testes)

• Pineal Gland

• Pituitary Gland

Exocrine Glands

Exocrine glands release their secretions through ducts to the surface of an organ or tissue. Examples include:

• Sweat glands

• Salivary glands

• Liver

• Mammary glands

Glands with Both Endocrine and Exocrine Functions

Some glands have both endocrine and exocrine roles:

• Pancreas

• Gonads

• Liver

Hormone Secretion and Mechanisms

Paracrine Secretion

Paracrine secretion involves the release of substances that affect nearby cells. Example:

• Exocrine enzymes released into the small intestine to aid digestion.

Key digestive enzymes:

• Amylase: Breaks down carbohydrates (e.g., starch) into smaller sugars.

• Lipase: Digests fats into smaller molecules like free fatty acids.

• Proteases: Group of enzymes that digest proteins.

Hormone Solubility and Receptor Location

Hormones can be classified by their solubility, which determines their receptor location:

• Fat-soluble hormones (e.g., Testosterone, Estrogen, Aldosterone, T3): Receptors are intracellular.

• Water-soluble hormones (e.g., Insulin, Oxytocin, Epinephrine): Receptors are on the cell membrane.

Second Messenger Utilization

Water-soluble hormones typically utilize second messenger systems (e.g., cAMP):

• Insulin

• Oxytocin

• Epinephrine

Adrenal Gland Function

Adrenal Medullary Hormones

The adrenal medulla releases hormones in response to stress and exercise (fight or flight response). Effects include:

• Increased heart rate

• Increased blood pressure

• Bronchodilation (opens airways)

• Increased metabolic rate

• Increased blood glucose levels

• Redistribution of blood flow

• Enhanced alertness

• Decreased fatigue

The adrenal medulla is controlled by sympathetic nerve signals from the hypothalamus, not by hormones.

Eicosanoids

Eicosanoids are local signaling molecules derived from arachidonic acid. They regulate inflammation, immunity, blood flow, clotting, and smooth muscle activity.

• Prostaglandins: Pain, inflammation, fever, uterine contractions, blood flow regulation.

• Leukotrienes: Allergic and inflammatory responses (e.g., asthma).

• Thromboxane: Platelet aggregation and blood clotting.

Pituitary Gland and Hypothalamic Control

Anterior Pituitary (Adenohypophysis)

The anterior pituitary is controlled by the hypothalamus via the hypophyseal portal system, a unique blood supply connecting two capillary beds.

Key tropic hormones produced:

• Luteinizing Hormone (LH)

• Follicle Stimulating Hormone (FSH)

• Thyroid Stimulating Hormone (TSH)

• Adrenocorticotropic Hormone (ACTH)

Tropic hormones stimulate other endocrine glands to release their hormones.

Hormone	Target Tissue
FSH	Gonads
ACTH	Adrenal Glands (Kidneys)
TSH	Thyroid

Posterior Pituitary (Neurohypophysis)

The posterior pituitary does not synthesize hormones; it stores and releases hormones produced by the hypothalamus (ADH and oxytocin).

Hypothalamic Hormones

The hypothalamus produces several releasing and inhibiting hormones:

• Growth hormone releasing hormone (GHRH)

• Thyrotropin releasing hormone (TRH)

• Corticotropin-releasing hormone (CRH)

• Gonadotropin releasing hormone (GnRH)

• Somatostatin/Growth hormone-inhibiting hormone (GHIH)

Growth Hormone and Related Factors

Human Growth Hormone (GH)

GH stimulates growth and metabolism in various tissues, especially liver, muscle, and adipose tissue.

• Fat metabolism: Stimulates fat breakdown in adipose tissue.

• Protein metabolism: Increases amino acid uptake and protein synthesis.

• Glucose metabolism: Increases blood glucose by reducing uptake and stimulating gluconeogenesis.

GH secretion is regulated by a negative feedback loop involving blood glucose and somatostatin.

Hypersecretion in adulthood causes acromegaly; in children, it leads to gigantism.

Somatomedins (Insulin-like Growth Factors, IGFs)

IGFs are produced mainly by the liver in response to GH and promote growth of bones, muscles, and other tissues.

• Bone growth

• Muscle growth

• Organ and tissue growth

• Metabolic effects

Other Key Hormones and Their Effects

Prolactin and Oxytocin

• Prolactin: Stimulates milk production in mammary glands (anterior pituitary).

• Oxytocin: Causes milk ejection (posterior pituitary).

Antidiuretic Hormone (ADH)

• Target tissue: Kidneys

• Effect: Decreases urine output, increases water retention in blood.

• Stimulus: High blood osmolarity

• Disease: Lack of ADH causes central diabetes insipidus (large volumes of dilute urine, dehydration, excessive thirst).

Thyroid Gland Function

Thyroid Hormone Production

• Produced by: Follicular cells

• Required element: Iodine

• Storage: As thyroglobulin in colloid of follicles

• Duration: Sufficient for 80–100 days

Functions of Thyroid Hormone

• Metabolism: Increases basal metabolic rate (BMR)

• Nervous system development: Supports myelination, synapse formation, neural growth

• Growth and development: Works with GH and IGFs for normal growth

Thyroid hormone accelerates cellular respiration and increases heart rate and cardiac output. It requires a transport protein in the blood.

Hormone	Solubility	Receptor Location	Second Messenger?
Testosterone	Fat-soluble	Intracellular	No
Insulin	Water-soluble	Cell membrane	Yes
Oxytocin	Water-soluble	Cell membrane	Yes
Estrogen	Fat-soluble	Intracellular	No
Aldosterone	Fat-soluble	Intracellular	No
T3	Fat-soluble	Intracellular	No
Epinephrine	Water-soluble	Cell membrane	Yes

*Additional info: Some explanations and definitions have been expanded for clarity and completeness. Tables have been recreated to summarize hormone properties and target tissues.*