Hormones and their Functions

Overview of the Endocrine System

• The two major regulatory systems are the nervous and endocrine systems.
• Organs synthesize and secrete chemical messengers called hormones into the blood.
• Hormones interact with cells known as target cells.
• Tissues where a hormone's target cells reside are called target tissues.
• The blood transports hormones to the heart through veins, and the heart transports them to the body through arteries.
• In capillary beds, hormones diffuse out of the blood into interstitial fluid and bind to receptors on target cells.

Functions of the Endocrine System

• Regulates metabolism and tissue maturation.
• Regulates blood pH, sodium, potassium, calcium, and other ion levels in tissues.
• Maintains water balance by regulating blood solute concentration.
• Regulates the immune system.
• Controls heart rate and blood pressure directly and through blood volume.
• Regulates blood and tissue levels of glucose and other nutrients.
• Controls reproductive functions.
• Controls uterine contractions and milk release.

Paracrine and Autocrine Signals

• Classic endocrine signals are hormones secreted into the blood to affect distant targets.
• Hormones can be secreted into the interstitial fluid and affect local cells without entering the blood.
• Paracrine signals involve chemicals secreted by cells into the extracellular fluid (ECF) to influence nearby cells.

Endocrine Organs

• Endocrine glands are organs of the endocrine system.
• Hormones can take seconds to hours to have effects, but effects last longer than the nervous system.
• Endocrine glands contain ductless glandular epithelial cells that secrete hormones into the interstitial fluid.
• Exocrine glands secrete products into ducts leading to body surfaces or cavities.
• Primary organs include the anterior pituitary gland, thyroid gland, parathyroid glands, adrenal cortices, and endocrine pancreas.
• Secondary endocrine glands produce hormones but belong to other systems, such as testes, ovaries, heart, kidneys, small intestine, and adipose tissue.

Paraneoplastic syndrome

• Cancer cells can produce hormones.
• Signs and symptoms accompanying hormone secretion from cancer cells are called paraneoplastic syndrome.
• Lung and gastrointestinal cancers are the most common causes.
• Effects include imbalances in fluid, calcium ion, and sodium ion homeostasis.
• Symptoms often precede cancer diagnosis and could help with early diagnosis.

Hormone Classes

• Amino-acid based hormones are derived from amino acids, and most are hydrophilic.
• Peptide/protein hormones consist of many amino acids linked by peptide bonds and are generally hydrophilic.
• Peptide/protein hormones bind to plasma membrane receptors (water soluble and unbound).
• Steroid hormones are cholesterol derivatives that are hydrophobic or lipid-soluble and stored in adipose tissue.

Target Cells and Receptors

• Hormone receptors can be embedded in the plasma membrane and bind hydrophilic and hydrophobic hormones.
• Hormone receptors located in the cytosol or nucleus bind only hydrophobic hormones.
• Hydrophilic hormones are usually administered via injection.
• The number of receptors in target cells varies with the body's needs, allowing tight control over interaction with hormones.
• Upregulation occurs when hormone levels in the blood decline, causing target cells to make more receptors.

Hormone Interactions

• Complementary actions occur when hormones affect different target cells to accomplish a common goal.
• Synergists are hormones act on the same target cell to exert the same effect.

Regulation of Hormone Secretion

• Hormone secretion can be initiated by hormonal stimuli, humoral stimuli, and neural stimuli.
• Hormonal stimuli involve endocrine cells that increase or decrease secretion in response to other hormones.
• The hypothalamus regulates the anterior pituitary by secreting releasing and inhibiting hormones.
• Humoral stimuli involve endocrine cells responding to the concentration of ions or compounds in the blood or ECF.
• The pancreas releases insulin in response to elevated glucose levels.
• The negative feedback loop for hormones is:
  • A regulated variable deviates from its normal range (stimulus).
  • Receptors detect the deviation (receptor).
  • An endocrine cell increases or decreases its secretion of a particular hormone (control center).
  • Target cells move the condition toward the normal range (effector/response).
  • A return to normal range occurs.

Hypothalamus and Pituitary Gland Structure

• The hypothalamus region is a region of the brain connected to the pituitary gland (hypophysis) by the infundibulum.
• The anterior pituitary gland (adenohypophysis) is a true gland composed of hormone-secreting cells.
• Capillaries merge in the hypothalamus to form portal veins that travel through the infundibulum to capillaries in the anterior pituitary (hypothalamic-hypophyseal portal system).

Hypothalamus Hormones

• Antidiuretic hormone (ADH or vasopressin) increases water retention and affects kidney tubules.
• Oxytocin stimulates uterine contractions and milk letdown from mammary glands when suckling is stimulated.

Functional Relationship of the Hypothalamus and Anterior Pituitary

• The hypothalamus produces tropic hormones stimulate or inhibit the release of other anterior pituitary hormones.
• Gonadotropin-releasing hormone (GnRH) stimulates gonadotropin release from the pituitary, for example.
• Hormone regulation is maintained by negative feedback loops with tiers of control.

Anterior Pituitary Hormones

• Thyroid-stimulating hormone (TSH) stimulates thyroid hormone secretion.
• Adrenocorticotropic hormone (ACTH) stimulates glucocorticoid hormone secretion.
• Prolactin (PRL) stimulates mammary gland tissue growth and initiates milk production after childbirth.
• Luteinizing hormone (LH)
  • In males, it stimulates testosterone production.
  • In females, it stimulates estrogen and progesterone production and triggers ovulation.
• Lipotropins increase fat breakdown.
• Beta (β) endorphins act as an analgesic in the brain and inhibit gonadotropin-releasing hormone secretion.

Thyroid and Parathyroid Structure

• The thyroid gland is in the anterior neck, with right and left lobes connected by the isthmus.
• Follicle cells produce thyroglobulin, becoming triiodothyronine (T3) and tetraiodothyronine (T4 or thyroxine).
• Thyroglobulin stored in colloid is a protein-rich, gelatinous material with iodine atoms.
• Parafollicular cells are between thyroid follicles and produce calcitonin.

Thyroid Hormones

• Thyroid hormones are hydrophobic, consisting of an amino acid core bound to iodine atoms.
• T3 (10%) and T4 (thyroxine) (90%) are transported in blood bound to thyroxine-binding globulin from the liver.
• 33-40% of T4 is converted to T3 in cells.
• Thyroid hormones promotes protein synthesis
• Thyroid hormones increases the rate of glucose, fat, and protein metabolism
• Thyroid hormones are essential for normal growth

Thyroid Disorders

• Hyperthyroidism involves overproduction of thyroid hormones.
• Hyperthyroidism symptoms include weight loss, heat intolerance, disruptions in blood pressure, and heart rhythms.

Parathyroid Hormone

• Parathyroid hormone (PTH) is secreted in response to declining blood calcium ion concentration (hypocalcemia).
• PTH increases release of calcium ions from bone by stimulating osteoclasts.
• PTH stimulates the kidneys to form active Vitamin D, increasing absorption of dietary calcium ions in the small intestine.

Adrenal Gland Structure

• Adrenal glands are pyramid-shaped and located on the superior end of the right and left kidneys.
• The outer adrenal cortex surrounds an inner adrenal medulla.
• The cortex is a typical endocrine gland made up of three zones that each produce steroid hormones from cholesterol.

Adrenal Cortex Hormones

• The outer zona glomerulosa secretes mineralocorticoid hormones.
  • Aldosterone is the most important mineralocorticoid.
• The middle zona fasciculata secretes glucocorticoids and androgenic steroids.
  • Cortisol (hydrocortisone) is the most potent glucocorticoid.
• Aldosterone regulates concentration of sodium, potassium, and hydrogen ions in the body.
• Aldosterone maintains extracellular concentrations of sodium and potassium and maintains blood pressure with other organs through the renin-angiotensin-aldosterone system (RAAS). Cortisol is important to help mediate the body's response to stress.
• Cortisol increases fat and protein breakdown and glucose synthesis and decreases inflammation.
• Androgenic steroids are steroid sex hormones that affect gonads and other tissues.

Hormones of the Adrenal Medulla

• Consists of neuroendocrine cells called chromaffin cells, derived from nervous tissue.
• Chromaffin cells secrete epinephrine and norepinephrine. -They:
  • Increase rate and force of heart contraction
  • Dilate bronchioles in lungs
  • Constrict blood vessels supplying skin, digestive organs, and urinary organs

Pancreas Structure

• The pancreas is a club-shaped organ in the abdominal cavity, posterior to the stomach, with a rounded head, middle body, and thin tail.
• Pancreatic islets (islets of Langerhans) secrete hormones into the bloodstream.
  • Alpha cells secrete the hormone glucagon.
  • Beta cells secrete the hormone insulin.
  • Delta cells secrete the hormone somatostatin (inhibits secretion of glucagon and insulin).

Hormones of the Endocrine Pancreas and Glucose Homeostasis

• Glucagon promotes reactions that increase glucose levels in the blood, targets the liver, muscle, and adipose tissue, and causes:
  • Glycogenolysis: breakdown of glycogen into glucose.
  • Formation of new glucose by gluconeogenesis.
  • Protein breakdown in muscle tissue to release amino acids for gluconeogenesis.
  • Release of fats from adipose tissue for gluconeogenesis and energy.

Hormones Effect on Glucose Homeostasis

Promotes uptake of ingested nutrients, which lowers the glucose levels in the blood:

• Promotes storage of nutrients in target cells when not immediately needed.
• Promotes lipid, amino acid, and glucose uptake.
• Synthesis of glycogen in the liver and skeletal muscle.
• Hypoglycemia is when blood glucose levels are too low
  • Hypoglycemia includes weakness, dizziness, rapid breathing, nausea, and sweating.
  • Severe hypoglycemia includes confusion, hallucinations, seizures, coma, and death.
• Hyperglycemia is when blood glucose levels are too high

Thymus Gland

• Thymus is an irregularly shaped organ in the mediastinum providing a site for T lymphocyte maturation.
• Thymosin and thymopoietin are secreted to function as paracrine signals assisting T lymphocyte maturation.
• Both hormones are larger and more active in infants and children who have developing immune systems.

Gonads

• The gonads include the testes in males and ovaries in females, produce gametes (sperm or ova), and secrete sex steroid hormones.
  • The testes secrete testosterone, stimulating development of secondary male sex characteristics, with multiple effects on other tissues and organs.
  • The ovaries' major hormones are estrogens and progesterone, considered female hormones but are produced in small amounts in males.
  • The ovaries stimulate development of secondary sex characteristics such as breasts and the menstrual cycle, and effect other organs.

The Pineal Gland

• This gland is part of the epithalamus of the diencephalon that secretes the neurohormone melatonin.
• The melatonin trigger appears related to light and dark cycles; secretion begins in the evening.
• Melatonin target tissues are the sleep-regulating centers in the reticular formation of the brainstem, adjusting the sleep phase of the sleep/wake cycle.

Adipose Tissue

• Adipocytes produce the hormone leptin.
• Leptin can cross the blood-brain barrier to interact with neurons in the hypothalamus controlling feeding.
• Leptin acts on neurons to induce satiety, which is a preventative in overeating.
• The amount of hormone leptin is related to the amount of adipose tissue

The Heart

• Cardiac muscle cells produce atrial natriuretic peptide (ANP) in response to stretch-sensitive ion channels.
• ANP targets smooth muscle cells lining blood vessels and causes vasodilation increasing vessel diameter.
• ANP also targets kidney tubules and enhances excretion of sodium called natriuresis enhancing water excretion, lowering blood volume.

Importance of The Kidneys

• Secretes erythropoietin in response to a decreased level of oxygen in the blood.
• Stimulates development of erythrocytes.
• Renin secretion happens when renin is converting the plasma protein angiotensinogen to angiotensin-1.
• it is a Important factor in the renin-angiotensin-aldosterone system maintaining normal blood pressure.

Hormone Like Substances

• Eicosanoids:
  • Mediate inflammatory responses.
  • Types include: Prostaglandins, Prostacyclins, Thromboxanes, Leukotrienes
• Endorphins and Enkephalins Reduces the feeling of pain
• Growth Factors Stimulates the division of cells.

Explore the differences between steroid and peptide hormones, their mechanisms, and receptor locations. Learn about hormone regulation, the effects of luteinizing hormone (LH), and synergistic hormone actions. Understand the functions of lipotropins and beta-endorphins.