Endocrinology Quiz: Hormones and Glands

Chapter 10: The Endocrine System

• The endocrine system maintains homeostasis through chemical messengers called hormones.
• Hormones are released into the bloodstream and affect target cells with longer-lasting effects.
• The nervous system, in contrast, is faster with specific destinations and shorter-lived effects.
• Both systems use chemical messengers that bind to specific receptors on target cells.
• Example chemical messengers: epinephrine (E) and norepinephrine (NE), which are also hormones when released from the adrenal medulla, and neurotransmitters when released across synapses.

Intercellular Communication

• The endocrine and nervous systems coordinate distant communication.
• The endocrine system is slower, releasing hormones into the bloodstream for longer-lasting effects on target cells.
• The nervous system is faster, using neurotransmitters for specific and short-lived effects.
• Both systems work together to maintain homeostasis.

The Endocrine System

• Includes all endocrine cells and tissues.
• Cells are glandular and secretory; secretions enter the extracellular fluid.
• Local hormones, such as prostaglandins, affect adjacent cells.
• Hormones act as chemical messengers, released in one tissue and transported by the bloodstream to target cells in other tissues.

Organs and Tissues of the Endocrine System

• Hypothalamus produces ADH, oxytocin, and regulatory hormones.
• Pineal gland secretes melatonin.
• Pituitary gland (anterior lobe): ACTH, TSH, GH, PRL, FSH, LH, and MSH; posterior lobe: ADH and oxytocin.
• Thyroid gland: thyroxine (T4), triiodothyronine (T3), and calcitonin (CT).
• Adrenal glands (medulla): epinephrine (E) and norepinephrine (NE); (cortex): cortisol, corticosterone, aldosterone, androgens.
• Pancreas (pancreatic islets): insulin and glucagon.
• Ovaries: estrogens, progesterone, inhibin.
• Testes: androgens (especially testosterone), inhibin.
• Heart (secondary): ANP
• Thymus: (undergoes atrophy in adulthood) thymosins
• Adipose tissue: leptin
• Digestive tract: various hormones for coordination, glucose metabolism and appetite.
• Kidneys: erythropoietin (EPO), calcitriol

Three Groups of Hormones

• Amino acid derivatives: Structurally similar to amino acids; include epinephrine, norepinephrine, thyroid hormones, and melatonin.
• Peptide hormones: Consist of chains of amino acids; the largest class; include ADH, oxytocin, hypothalamic, pituitary, and pancreatic hormones.
• Lipid derivatives:
• Steroid hormones: Structurally similar to cholesterol, transported bound to plasma proteins; released by reproductive organs and adrenal glands.
• Eicosanoids: Fatty acid-based, derived from arachidonic acid; coordinate local cellular functions and affect enzymatic processes (e.g., prostaglandins).

Hormone Action

• Hormones alter operations in target cells by changing the types, activities, locations, or quantities of structural proteins and enzymes.
• Sensitivity to a hormone depends on the presence or absence of hormone receptors on the target cell.
• Receptors are on the plasma membrane or inside the cell.

Plasma Membrane Receptors

• Required for hormones that are not lipid soluble (e.g., epinephrine, norepinephrine, peptide hormones, and eicosanoids).
• Hormones cannot diffuse through the plasma membrane.
• These hormones are first messengers that activate second messengers (e.g., cyclic AMP, calcium ions, cyclic GMP) in the cytoplasm.
• Action is linked by a G protein, an enzyme complex.

Second Messenger Systems

• A small number of hormone molecules can activate thousands of second messengers.
• This process, called amplification, magnifies the effect of the hormone on the cell.
• Examples of second messengers include cyclic AMP, calcium ions, and cyclic GMP.

Intracellular Receptors

• Receptors are located inside the cytoplasm or nucleus.
• Used for lipid-soluble hormones (e.g., thyroid and steroid hormones).
• Hormones cross the plasma membrane.
• The hormone-receptor complex activates or inactivates specific genes.
• This alters the rate of mRNA transcription and changes the structure or function of the cell.

Hormone Secretion and Distribution

• Hormones enter the bloodstream and are distributed to target cells throughout the body.
• Freely circulating hormones have short lives and are deactivated through diffusion to target cells, absorption and breakdown in the liver and kidneys, and enzymatic breakdown in the plasma or interstitial fluid.
• Hormones bound to transport proteins stay in circulation longer (steroid and thyroid hormones).

Control of Endocrine Activity

• Hormonal secretion is based on three types of stimuli:
• Humoral stimuli: Changes in the composition of the extracellular fluid (e.g., hormonal control of blood calcium levels).
• Hormonal stimuli: Changes in circulating hormone levels.
• Neural stimuli: Neural stimulation of a neuroglandular junction through neurotransmitters.

Hypothalamus and Endocrine Control

• The hypothalamus provides the highest level of endocrine control.
• Hypothalamic coordinating centers regulate nervous and endocrine systems through:
  1. Production of ADH and oxytocin (OXT).
  2. Secretion of regulatory hormones to control the activity of the anterior lobe of the pituitary gland.
  3. Controlling sympathetic output to the adrenal medullae.

The Pituitary Gland

• Also called the hypophysis.
• Protected by the sella turcica.
• Hangs from the hypothalamus by the infundibulum.
• Anterior and posterior lobes have very different structures.
• Secretes nine hormones, all unique peptides or small proteins, and all using a cAMP second messenger mechanism.

The Anterior Lobe of the Pituitary Gland

• Contains endocrine cells surrounded by a complex capillary bed(part of the hypophyseal portal system).
• Two capillary networks and the linking blood vessels are a portal system, named after their destinations.

The Hypophyseal Portal System

• Regulatory hormones diffuse onto target cells in the anterior lobe.
• Many of these hormones are tropic hormones targeting other endocrine glands.

The Seven Anterior Lobe Hormones

• Thyroid-stimulating hormone (TSH)
• Adrenocorticotropic hormone (ACTH)
• Follicle-stimulating hormone (FSH)
• Luteinizing hormone (LH)
• Prolactin (PRL)
• Growth hormone (GH)
• Melanocyte-stimulating hormone (MSH)

Thyroid-stimulating Hormone

• Also called thyrotropin.
• Released in response to thyrotropin-releasing hormone (TRH) from the hypothalamus.
• Targets the thyroid gland.
• Triggers the release of thyroid hormones.
• Increased thyroid hormones decrease TRH and TSH secretion.

Adrenocorticotropic Hormone (ACTH)

• Also called corticotropin.
• Released in response to corticotropin-releasing hormone (CRH) from the hypothalamus.
• Targets the adrenal cortex.
• Stimulates secretion of glucocorticoids.
• Increased glucocorticoids decrease ACTH and CRH.

Gonadotropins

• Released in response to gonadotropin-releasing hormone (GnRH) from the hypothalamus.
• Target the male and female gonads.
• Include follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Follicle-stimulating Hormone (FSH)

• Promotes follicle development in females.
• Promotes sperm production in males.
• Cells of ovaries and testes produce inhibin, which inhibits FSH production through negative feedback.

Luteinizing Hormone (LH)

• Induces ovulation and estrogen and progesterone secretion in females.
• In males, also known as interstitial cell-stimulating hormone (ICSH).
• Stimulates interstitial cells/endocrine cells of the testes to produce androgens like testosterone.
• Estrogens, progesterone, and androgens inhibit GnRH (thus decreasing LH levels).

Prolactin (PRL)

• Released in response to prolactin-releasing factor (PRF) from the hypothalamus.
• Targets female mammary glands.
• With other hormones, stimulates mammary gland development.
• During pregnancy and nursing, stimulates milk production.
• Circulating PRL stimulates prolactin-inhibiting hormone (PIH) and inhibits PRF.

Growth Hormone (GH)

• Also called human growth hormone (hGH) and somatotropin.
• Regulated by growth hormone-releasing hormone (GH-RH) and growth hormone-inhibiting hormone (GH-IH) from the hypothalamus.
• Stimulates cell growth and replication (especially of skeletal muscle and chondrocytes).
• Indirect action: stimulates liver to release somatomedins (or insulin-like growth factors [IGFs]), triggering increased amino acid uptake and protein synthesis by cells after a meal.
• Somatomedins stimulate GH-IH and inhibit GH-RH.

Direct Actions of Growth Hormone

• In epithelial and connective tissues: Stimulates stem cell division and differentiation.
• In adipose tissue: Stimulates breakdown of stored fats and releases fatty acids, a glucose-sparing effect.
• In the liver: Stimulates breakdown of glycogen reserves and releases glucose into the bloodstream.

Melanocyte-stimulating Hormone (MSH)

• Increases activity of melanocytes in skin to increase melanin production.
• Appears to be non-functional in adults.

The Posterior Lobe of the Pituitary Gland

• Contains axons from two groups of hypothalamic neurons.
• Produces two hormones: antidiuretic hormone (ADH) and oxytocin (OXT).
• Hormones are transported within the axons and then into capillaries of the posterior pituitary lobe.

Antidiuretic Hormone (ADH)

• Also called vasopressin.
• Released when body water is low.
• Stimulated by an increase in the concentration of solutes in the blood or a decrease in blood volume and/or pressure.
• Primary target is the kidney to decrease water loss.
• Triggers vasoconstriction to increase blood pressure.

Oxytocin (OXT)

• In women: Stimulates contraction of uterine muscles during labor and delivery, and contraction of cells surrounding milk secretory cells in mammary glands.
• In men: Stimulates smooth muscle in walls of sperm duct.
• In women: May stimulate smooth muscle contractions in the uterus and vagina, promoting sperm transport.

The Thyroid Gland

• Located anterior to the trachea and inferior to the thyroid cartilage.
• Has two lobes connected by an isthmus.
• Contains many spherical thyroid follicles lined by simple cuboidal epithelium.
• Follicles are filled with viscous colloid containing many proteins and thyroid hormones.

The Thyroid Follicles

• Follicular cells make thyroid hormones and store them in colloid.
• TSH causes thyroid hormone release.
• Most thyroid hormones are transported bound to plasma proteins.
• Derived from amino acid tyrosine and iodine.
• Thyroxine (T4) has four iodine atoms; triiodothyronine (T3) has three.
• T3 is more potent than T4.

The Effects of Thyroid Hormones

• Affect nearly every cell in the body.
• Increase the rate of ATP production in mitochondria.
• Activate genes coding for enzymes involved in glycolysis and energy production.
• Enzymes increase the rate of metabolism.
• Calorigenic effect: Cell uses more energy, measured in calories, and heat is produced.
• Important for the development of skeletal, muscular, and nervous systems.

The C Cells of the Thyroid Gland

• Also called parafollicular cells; located between follicles.
• Produce calcitonin (CT).
• Stimulated by increases in plasma Ca2+.
• Inhibits osteoclasts in bone.
• Stimulates calcium excretion by kidneys.
• Important for normal bone growth in children and during the last trimester of pregnancy.

Calcium Balance and Calcitonin

• Hypercalcemia (high blood calcium level) results in decreased sodium permeability of excitable membranes and less responsive muscles and nerves.

The Parathyroid Glands

• Two pairs of small glands embedded in the posterior surface of the thyroid gland.
• Parathyroid principal, or chief, cells produce parathyroid hormone (PTH).
• Stimulated by a decrease in plasma Ca2+.
• Activates osteoclasts and inhibits osteoblasts in bone.
• Reduces calcium excretion by kidneys.
• Stimulates the kidneys to secrete calcitriol, increasing Ca2+ and PO43- absorption in the digestive tract.

Calcium Balance and PTH

• Hypocalcemia (low blood calcium level) results in increased sodium permeability of excitable membranes, highly excitable spasmodic muscles and neurons, convulsions, or muscular spasms.
• Parathyroid glands prevent hypocalcemia.

Hormones of the Thyroid and Parathyroid Glands

• Table summarizing the hormones of thyroid and parathyroid glands, their targets, and hormonal effects.

The Adrenal Gland

• Also called the suprarenal gland.
• Yellow, pyramid-shaped; sits superiorly on the border of each kidney.
• Two portions: adrenal cortex and adrenal medulla.

The Adrenal Cortex

• Contains high levels of cholesterol and fatty acids.
• Produces more than 24 steroid hormones called corticosteroids.
• Essential for metabolic functions and life.
• Transported in plasma bound to proteins.
• Three zones produce three types:
• Zona glomerulosa: Mineralocorticoids (e.g., aldosterone).
• Zona fasciculata: Glucocorticoids (e.g., cortisol, corticosterone, cortisone).
• Zona reticularis: Androgens.

Mineralocorticoids

• Affect electrolyte balance in body fluids.
• Aldosterone is the major mineralocorticoid.
• Secreted in response to low plasma Na+, low blood pressure, high plasma K+, or the presence of angiotensin II.
• Triggers the reabsorption of Na+ ions and the elimination of K+ ions.
• Prevents loss of Na+ in urine, sweat, saliva, and digestive secretions.
• Triggers secondary water reabsorption through osmosis.

Glucocorticoids

• Affect glucose metabolism.
• Most important are cortisol (hydrocortisone), corticosterone, and cortisone.
• Secreted in response to ACTH.
• Increase rates of glucose synthesis and glycogen formation. This results in an increase in blood glucose levels.
• Also act as anti-inflammatory.

The Androgens

• Small quantities produced in both males and females.
• Some converted to estrogens in the plasma.
• Stimulate development of pubic hair in boys and girls before puberty.
• Not important in adult men but contribute to muscle mass, blood cell formation, and sexual drive in adult women.

The Adrenal Medulla

• Highly vascular.
• Contains cells similar to sympathetic ganglia and is innervated by preganglionic sympathetic fibers.
• Two groups of secretory cells produce: epinephrine (E, or adrenaline) 75-80%, and norepinephrine (NE, or noradrenaline) 20-25%.
• Hormones trigger metabolic changes to increase energy molecule availability and support/prolong the overall sympathetic response.

The Pineal Gland

• Located on the posterior portion of the roof of the third ventricle.
• Contains neurons, glial cells, and secretory cells that produce melatonin.
• Rate of secretion affected by light and day-night cycles.
• May influence the timing of sexual maturation.
• May protect CNS neurons with antioxidant activity.
• Plays a role in maintaining circadian rhythms (day-night cycles).

The Endocrine Pancreas

• Pancreas lies between the stomach and proximal small intestine.
• Contains both exocrine and endocrine cells.
• Endocrine cells are located in pancreatic islets, or islets of Langerhans.
• Account for only about 1% of all pancreatic cells.
• Alpha cells secrete glucagon.
• Beta cells secrete insulin.

Blood Glucose and Insulin

• Increases in blood glucose levels activate beta cells to release more insulin, stimulating glucose uptake by cells (except neurons, red blood cells, and certain epithelial cells).
• Increases rates of protein synthesis and fat storage, resulting in lower blood glucose levels.

Blood Glucose and Glucagon

• Decreases in blood glucose levels activate alpha cells to release more glucagon.
• Mobilizing energy reserves; glycogen in liver and muscles broken down to glucose; adipose tissue releases fatty acids.
• Results in higher blood glucose levels.

Pancreatic Regulation of Blood Glucose

• Insulin and glucagon secretion is independent of direct neural stimulus.
• Indirectly affected by any hormone that influences blood glucose levels (e.g., cortisol, thyroid hormones).
• Also affected by ANS activity: parasympathetic stimulation enhances insulin release, and sympathetic stimulation inhibits it.

Diabetes Mellitus

• Faulty glucose metabolism causing buildup of glucose in blood and urine
• Types: Type 1 (juvenile diabetes): inadequate insulin production; Type 2: insulin resistance.
• Symptoms: Hyperglycemia, glycosuria, polyuria.

Secondary Endocrine Organs

• Many organs release hormones, but their primary function is in another system
• Examples include intestines, kidneys, heart, thymus, gonads, and adipose tissue.

The Intestines

• Release local hormones that coordinate digestive activities and major control over digestive processes.
• Can be influenced by ANS.

Hormones Released by the Kidneys

• Calcitriol: Stimulated by PTH, increasing calcium and phosphate ion absorption from the digestive tract.
• Erythropoietin (EPO): Stimulated by low oxygen levels in kidney tissues, increasing RBC production.
• Renin: An enzyme that triggers the renin-angiotensin-aldosterone system, to increase blood pressure and blood volume.

The Heart

• Endocrine cells in the right atrium respond to increased blood volume and release atrial natriuretic peptide (ANP).
• ANP promotes Na+ and water loss, inhibits renin, ADH, and aldosterone release, and results in decreased blood pressure and blood volume.

The Thymus

• Located deep to the sternum in the mediastinum.
• Very active in early childhood; atrophies in adults.
• Releases thymosins that aid in the development and maintenance of immune defenses.

The Gonads: The Testes

• Interstitial endocrine cells produce androgens.
• Most important is testosterone which promotes sperm production, maintains secretory glands of the male reproductive tract, determines secondary sex characteristics, stimulates protein synthesis in skeletal muscles.
• Sperm production balance is maintained by inhibin secreted by nurse cells in the testes.

The Gonads: The Ovaries

• Female sex cells (oocytes) are surrounded by follicles.
• FSH triggers follicular cells to produce estrogens.
• Estrogens support egg cell maturation, uterine lining growth, and have negative feedback on FSH.
• Inhibin provides negative feedback on FSH.
• Once a follicle releases an oocyte (ovulation), the corpus luteum is formed from remaining follicular cells.
• Releases a mixture of estrogens and progesterone. This accelerates fertilized egg movement through the uterine tube, prepares the uterus for a developing embryo, and contributes to mammary gland enlargement.
• All gonadal hormones are regulated by hormones from the anterior pituitary gland.

Hormones of the Reproductive System

• Table summarizing the hormones of the reproductive system, their targets, and their effects.

Adipose Tissue

• Produces peptide hormone leptin.
• Provides negative feedback control of appetite by binding to hypothalamic neurons to trigger satiation (fullness) and appetite suppression.
• Must be present for normal GnRH and gonadotropin synthesis.
• Low body fat can result in late puberty and cessation of menstrual cycles.
• Increased body fat can increase fertility.

Hormonal Interactions

• Extracellular fluid contains mixtures of hormones that may have the same target, resulting in four possibilities:
  • Antagonistic: (e.g., parathyroid hormone and calcitonin)
  • Synergistic
  • Permissive
  • Integrative

Hormones and Growth

• Six hormones required for normal growth:
  1. Growth hormone (GH)
  2. Thyroid hormones
  3. Insulin
  4. Parathyroid hormone (PTH)
  5. Calcitriol
  6. Reproductive hormones

Hormones and Stress

• Stress is triggered by physical injury/illness, emotional responses (anxiety or depression), environmental conditions (extreme cold or heat), or metabolic conditions (acute starvation).
• Stress triggers the general adaptation syndrome (GAS; or stress response).
  • Three phases: alarm, resistance, exhaustion

Hormones and Behavior

• The hypothalamus is the primary regulator and monitor of hormones.
• Behavior is also affected by hormonal abnormalities (e.g., precocious puberty – early sex hormone release, resulting in aggressive and assertive behavior, hormone imbalances affecting CNS intellectual functions like learning, memory, and emotions).

Hormones and Aging

• Most hormones generally stay the same in adulthood, but exceptions include reproductive hormones.
• Changes in target organ receptors occur more often via reduced sensitivity.

An Overview of Some Important Endocrine Disorders

• Tables summarizing important endocrine disorders, related symptoms, and tissue/hormonal effects.

Test your knowledge on the functions of hormones, the structure of the pituitary gland, and the hypothalamus' role in endocrine control. This quiz covers key concepts related to hormone transport, signaling, and the hypophyseal portal system.