The Endocrine System: Hormone Production and Regulation

• The endocrine system (ES) and the nervous system are the body's two control systems. The nervous system controls the endocrine system.

Ductless System

• The endocrine system lacks vessels for hormone transport, using blood instead.
• It regulates diverse metabolic processes through hormone feedback loops.
• Hormones are released by internal glands/organs directly into the circulatory system, targeting and regulating distant organs.
• The hypothalamus serves as the neural control center for the endocrine system.

Hormone Production

• Hormones are chemical substances produced in one area of the body and transported to another to exert an effect.
• The system comprises glands and organs that synthesize and secrete hormones.
• Endocrine glands lack ducts, releasing hormones into the blood for transport throughout the body.

Hormone Producing Organs

• Endocrine glands produce only hormones including the pituitary, pineal, thyroid, adrenal, and parathyroid glands.
• Endocrine organs produce hormones and fulfill other functions including includes the skin, thymus, heart, liver, stomach, pancreas, small intestine, kidney, gonads, placenta, adipose tissue, and hypothalamus.

Target Cell Specificity

• Some hormones target a few specific cells, while others affect many cell types.
• Target cells have receptors for hormones, located on the cell surface or inside the cell.
• Cells with specific hormone receptors are called target cells.

Homeostasis, Reproduction, Development & Digestion

• The endocrine system regulates blood volume, cellular concentration (WBC, RBC, platelets), and reproductive activities.
• It controls the development of the reproductive system, gamete formation, and sexual characteristics/behaviors.
• The system regulates development, growth, and metabolism, including embryonic cell division, differentiation, and anabolic/catabolic processes.
• Hormones influence secretory processes, movement of materials, and chemistry within the digestive tract.

Hormone Chemistry

• Hormones are classified as either proteins or steroids.
• Sex hormones (gonadal) and adrenal cortex hormones are steroid hormones.
• All other hormones are proteins or protein derivatives.

Hormone Action

• Hormones affect membrane permeability, stimulate protein synthesis, activate/deactivate enzymes, influence secretions, and impact mitosis.

Movement of Hormones

• Protein-based hormones exert effects through intracellular second messengers activated upon binding to membrane receptors.
• Lipid-based hormones diffuse into cells, bind to intracellular receptors, and migrate to the nucleus due to structural similarity with the plasma membrane.
• Examples of second messengers include cyclic AMP, cyclic GMP, inositol triphosphate, diacylglycerol, and calcium.

Target Cell Response

• Target cell specificity is due to intracellular receptors.
• Target cell response depends on hormone levels, number of receptors, and receptor affinity.
• Target cells can change sensitivity by altering receptor numbers.

Hormone Concentration

• Hormone concentration reflects release rate, inactivation rate, and removal rate.
• Half-life is the time needed to reduce hormone concentration by 50%.
• Water-soluble hormones like insulin and glucagon have the shortest half-lives and transient effects.

Interactions of Hormones

• Permissiveness: a hormone exerts full effects in the presence of another (e.g., thyroid hormone enhances epinephrine, cortisol enhances growth hormone).
• Synergism: multiple hormones produce the same effect, amplifying the combined effect (e.g., testosterone and growth hormone for size, epinephrine and norepinephrine for heart rate, estrogen, prolactin, oxytocin and cortisol for lactation).
• Antagonism: one hormone opposes another (e.g., insulin and glucagon regulate blood sugar, parathyroid hormone and calcitonin regulate blood calcium).

Regulation of Hormone Release

• Negative feedback mechanisms have the most control over hormone release, where the product inhibits its own production.
• Hormonal control: Gonadotropin-releasing hormone (GnRH) determines the secretion pattern of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), regulating endocrine function and gamete maturation.
• Nervous system: Directly stimulates endocrine glands to release hormones via neural stimuli (e.g., epinephrine and norepinephrine for short-term stress response).

Major Endocrine Glands

• Major endocrine glands that secrete only hormones include the pituitary, pineal, thyroid, adrenal, and parathyroid glands.
• Endocrine organs secrete hormones and perform other functions, including the skin, thymus, heart, liver, stomach, pancreas, small intestine, kidney, gonads, placenta, adipose tissue, and hypothalamus.

Pineal and Posterior Pituitary Glands

• Pineal gland: Regulates the body’s circadian rhythm using melatonin.
• Posterior pituitary gland: Stores and releases oxytocin and antidiuretic hormone produced in the hypothalamus.
• Oxytocin (OT): causes Uterine contractions, and breast milk release.
• Antidiuretic hormone (ADH): regulates Fluid balance.

Anterior Pituitary Gland

• Thyroid-stimulating hormone (TSH): stimulates the thyroid gland to release thyroid hormone.
• Prolactin (PRL): is used for breast milk production.
• Follicle-stimulating hormone (FSH): affects the development of gametes and follicles in females.
• Luteinizing hormone (LH): impacts the development of gametes and ovulation in females.
• Adrenocorticotrophic hormone (ACTH): stimulates the adrenal cortex to release corticosteroids (= steroid hormones).
• Growth hormone (GH): stimulates cell growth and division.

Thyroid and Parathyroid Glands

• Thyroid gland: impacts metabolism using the thyroid hormone (TH) and blood calcium levels by using calcitonin which decreases blood calcium levels.
• Parathyroid glands: regulates blood calcium levels using the parathyroid hormone (PTH) which increases blood calcium levels.

Adrenal Glands

Adrenal medulla

• Uses catecholamines (epinephrine and norepinephrine) to prolong the fight-or-flight response. Adrenal cortex
• Mineralocorticoids e.g aldosterone: regulates blood sodium and potassium levels.
• Glucocorticoids e.g cortisol: participates in stress response.
• Gonadocorticoids e.g androgens: stimulates the maturation and functioning of the reproductive system.

Organs containing endocrine cells

• Hypothalamus: Oxytocin and antidiuretic hormones are produced in the hypothalamus and stored and released from the posterior pituitary, with Oxytocin (OT) causing uterine contractions, and breast milk release and the Antidiuretic hormone (ADH) regulating the fluid balance.
• Skin: Produces Cholecalciferol, which is an inactive form of vitamin D3, which is later converted to calcitriol through enzymes in the liver and kidney and promotes absorption of calcium from the gastrointestinal(GI) tract into the blood
• Thymus: Produces Thymosin, thymulin, and thymopoietin and stimulates maturation of T-lymphocytes and the development of the immune response.
• Heart: Produces Atrial natriuretic peptide (ANP) which regulates blood sodium levels, blood volume, and blood sodium concentration.
• Stomach: Produces Gastrin, which is a peptide hormone that increases secretions and motility of the stomach, and pepsin which is the mature active form of pepsinogen, which is used for protein digestion and stimulates secretion of gastric acid (HCl).
• Liver: Produces Angiotensinogen, which regulates blood volume and blood pressure, and Erythropoietin (EPO) which increases the production of erythrocytes.
• Pancreas: Produces Insulin, which decreases blood glucose, and Glucagon, which increases blood glucose.
• Small Intestine: Secretes Secretin and Cholecystokinin which regulate digestive processes of the small intestine.
• Kidney: Secretes Erythropoietin (EPO) which Increases the production of erythrocytes.
• Gonads: Testes and Ovaries; Testes secrete androgens (testosterone) and inhibin, which stimulates maturation and function of the male reproductive system.
• Gonads: Testes and Ovaries; Ovaries produce Estrogen, progesterone and inhibin, which stimulate maturation and function of the female reproductive system.
• Placenta: Secretes Estrogens, Progesterone and Human chorionic gonadotropin(hCg), which act on the uterus to influence pregnancy.
• Adipose Tissue: Secretes Leptin which acts on the CNS to produce a feeling of satiety, and Resistin which is an insulin antagonist.