Exocrine vs Endocrine Glands

Types of Glands

• There are two types of glands: exocrine and endocrine.

Exocrine Glands

• These glands secrete products into ducts.
• Ducts carry secretions into body cavities, organ lumens, or the body's outer surface.
• Examples include sudoriferous (sweat), sebaceous (oil), mucous, and digestive glands.

Endocrine Glands

• These are ductless glands that secrete hormones directly into the blood.
• Hormones influence cells with specific receptor sites.
• Examples include the hypothalamus, pituitary, thyroid, parathyroid, adrenals, pineal, thymus, endocrine pancreas (Islets of Langerhans), testes, ovaries, and placenta.
• Some books include the thymus gland, skin, heart, and kidneys as endocrine glands.
• Some glands have cells functioning in both the endocrine system and other systems like the pancreas, ovaries, testes, stomach, and heart.

Mechanism of Hormone Action

• A hormone is a secretion produced by a ductless (endocrine) gland.
• It is released into the bloodstream to reach receptors on a target organ.
• Hormone-receptor interaction has a "lock and key" fit; a hormone must fit the receptor to have an effect.
• Target tissue consists of all cells with receptor sites for a specific hormone.
• Target tissue can be found on a gland, organ, or scattered throughout the body.
• Receptor sites are located on the cell surface (protein hormone) or within the cell (steroid hormone).
• Hormones are either peptide (protein) or steroid based.

Protein (Peptide) Hormones

• Peptide hormones are composed of amino acids.
• All hormones in the body, except sex hormones and adrenal cortex hormones, are proteins.
• They must be taken by injection because stomach acid (HCl) and pepsin will break them down.
• They are too large to diffuse across the cell membrane, so they react on the cell surface, causing a rapid reaction.
• Lipid-based substances pass easily through the plasma membrane due to its lipid-soluble nature.

Steroid Hormones

• Steroid hormones are derived from cholesterol.
• They are secreted by the adrenal cortex and sex hormones.
• Steroid hormones may be taken orally or by injection.
• Being lipid-soluble, they diffuse through the cell membrane and react with receptors inside the cell, causing a slower reaction.

Control of Hormone Activity

• Small amounts of hormones are released based on the body's needs.
• Hormones can act within seconds, but most take several hours.
• Nerve impulses produce effects within milliseconds.
• Hormone release is controlled by three systems: negative feedback, tropic hormones, and nervous system stimulation.

Negative Feedback System

• This system reverses increases or decreases in hormone levels.
• Example: Islets of Langerhans secrete insulin in response to high blood sugar levels.
• Insulin decreases blood sugar.
• A decrease in blood sugar stops insulin production.

Tropic Hormones

• One hormone causes the secretion of another hormone.
• Example: TSH from the anterior pituitary gland causes the thyroid gland to secrete thyroxin.

Nervous System

• A nervous stimulus causes an endocrine gland to secrete a hormone.
• Example: The adrenal medulla secretes epinephrine in response to sympathetic nerve stimulation.

Hormone Breakdown

• After hormones achieve their goal, they are degraded by target cells, the liver, and the kidneys.
• Target cells can break down hormones but not excrete them.
• Excretion occurs via the liver and kidneys, which filter the hormones.

Pituitary Gland (Hypophysis)

• The pituitary gland, also known as the master gland, is the size of a pea.
• It's protected by bone within the sella turcica, a depression of the sphenoid cranial bone.
• The pituitary connects to the hypothalamus via the infundibulum.
• It is considered a functional extension of the hypothalamus.
• It has two lobes with different regulatory functions: the anterior and posterior lobes.

Anterior Pituitary Lobe

• The anterior pituitary lobe is responsible for stimulatory or inhibitory direction.
• Nervous stimulation goes to the hypothalamus.
• The hypothalamus secretes releasing or inhibiting factors (hormones).
• Releasing factors enter capillaries and travel to the anterior pituitary lobe.
• The lobe is made of epithelial cells (secretory cells) which secrete a hormone in response to hypothalamic releasing factors.

Posterior Pituitary Lobe

• Some hypothalamic cells have long axons terminating in the posterior pituitary lobe.
• Hormones are made in hypothalamus neuronal cells.
• Hormones travel down axons to the posterior lobe for storage.
• Release occurs when a neuron fires after stimulation.
• The anterior lobe is controlled by releasing factors/hormones from the hypothalamus.
• The posterior lobe is controlled by nerve stimulation.

Anterior Pituitary Gland (Adenohypophysis)

• "Adeno" means glandular.
• All anterior pituitary hormones are controlled by releasing factors, except for MSH.
• 7 Hormones secreted: Growth Hormone (GH), Thyroid-Stimulating Hormone (TSH), Adrenocorticotropic Hormone (ACTH), Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), Prolactin (PRL), Melanocyte-Stimulating Hormone (MSH).

Growth Hormone (GH)

• Other names: Hgh, Somatotropin, Somatotrophic hormone
• HGh (Human Growth Hormone) is the most abundant anterior pituitary hormone.
• Target cells of Hgh include most cells.

Functions of Hgh

• Growth of body cells
• Protein anabolism
• Stimulation of lipolysis
• Breakdown of triglycerides into fatty acids and glycerol to restart glucose use for ATP production
• Hypersecretion of Hgh in children results in exaggerated bone growth and gigantism.
• Acromegaly is excessive secretion after ossification, causing bone enlargement in diameter, especially in the face and hands.
• Hyposecretion of Hgh in children leads to dwarfism.
• In rare cases, deficiency may cause tissue atrophy and premature aging.

Thyroid-Stimulating Hormone (TSH)

• TSH causes the glandular cells of the thyroid to secrete thyroid hormones T3 and T4.
• Hypersecretion of TSH causes thyroid gland enlargement and excess hormone secretion.
• Hyposecretion of TSH causes thyroid gland atrophy and insufficient hormone secretion.

Adrenocorticotropic Hormone (ACTH)

• ACTH reacts with receptor sites in the adrenal cortex.

Functions of ACTH

• Stimulates secretion of adrenal gonadocorticoids (androgens), including DHEA, which converts to testosterone.
• Stimulates secretion of adrenal glucocorticoids, such as cortisol, which increases glucose levels and inflammation.
• Stimulates secretion of adrenal mineralocorticoids, such as aldosterone, which balances water and electrolytes by regulating renal sodium and water reabsorption.
• Aldosterone regulates long-term blood pressure.
• Hypersecretion and hyposecretion of ACTH affect adrenal cortex activity.

Follicle-Stimulating Hormone (FSH)

• FSH is considered a gonadotropic hormone.
• In females, it initiates development of ova (eggs) in the follicle and induces ovarian secretion of estrogen.
• In males, it stimulates the testes to produce sperm (spermatogenesis).
• Low FSH causes low LH levels.
• Female low FSH results in infertility and lack of a monthly cycle.
• Male low FSH results in infertility and sexual dysfunction.

Luteinizing Hormone (LH)

• LH is considered a gonadotropic hormone.
• In females, it stimulates ovulation and estrogen/progesterone production.
• Low LH in females causes low progesterone levels, excessive bleeding, and irritability.
• In males, it is also known as Interstitial Cell Stimulating Hormone (ICSH).
• Low LH in males causes sexual disinterest or low sperm count due to low testosterone.
• It stimulates interstitial cells in the testes (Leydig cells) to produce testosterone.
• FSH and LH are sometimes referred to as Gonadotropic Hormones because they react with receptor sites in the gonads (ovaries and testes).

Prolactin (PRL)

• In females, prolactin promotes the development of glandular tissue in the breast during pregnancy.
• It stimulates milk production after childbirth.
• Oxytocin from the pituitary gland and neuronal influences cause milk ejection.

Melanocyte-Stimulating Hormone (MSH)

• MSH stimulates dispersion of melanin granules in melanocytes.
• Target cells: most cells of the body.

Posterior Pituitary Gland (Neurohypophysis)

• This gland is an extension of the hypothalamus, made of neurons and neuroglia.
• It's controlled by nerve stimulation.
• It stores hormones produced by the hypothalamus but doesn't synthesize its own.

2 Major Hormones Secreted

• Oxytocin and Antidiuretic Hormone (Vasopressin).
• Oxytocin is produced in the hypothalamus.
• Dilation of the uterine cervix and suckling activates the hypothalamus, which activates the posterior pituitary, resulting in oxytocin secretion.
• Oxytocin stimulates contraction of smooth muscle in the pregnant uterus to induce labor.
• Oxytocin stimulates contraction of mammary gland contractile cells for milk ejection.
• Pitocin is a commercial form of oxytocin sometimes used to induce labor.

Antidiuretic Hormone (Vasopressin)

• Also known as the water conservation system.
• ADH is secreted when osmoreceptors in the hypothalamus sense a decrease in blood pressure.
• ADH stimulates kidney tubules to reabsorb water, reducing urine formation.
• ADH, in large amounts, constricts blood vessels, increasing blood pressure.
• During severe hemorrhage, ADH raises blood pressure by constricting arterioles.
• Insufficient ADH results in excessive water loss and dilute urine, a condition known as Diabetes Insipidus.
• Alcohol inhibits ADH secretion and increases urine output, leading to dehydration, thirst, and headaches.

Thyroid Gland

• Located inferior to the larynx.
• Has two lobes, one on each side of the trachea.
• The lobes are connected by the isthmus, a narrow band of tissue.
• Thyroid follicles are spherical and are the functional units of the thyroid.
• Stores hormones and releases them steadily over time.
• Contains iodine and requires iodine for hormone synthesis.

Hormones of the Thyroid Gland

• Thyroxine (T4) and Triiodothyronine (T3).
• They regulate organic metabolism, growth, development, and nervous system activity.
• T3 acts faster and is more potent than T4.
• T4 is more abundant (95%) than T3.
• Both are made from the amino acid tyrosine.
• Target cells of T3 and T4: all cells of the body (no specific target organ).
• The hypothalamus and anterior pituitary gland control activity.
• Decreased thyroid hormones stimulate the hypothalamus to produce thyroid-releasing factor (TRF).
• TRF stimulates the anterior pituitary to synthesize and secrete thyroid-stimulating hormone (TSH).
• TSH results in T3 and T4 secretion into surrounding capillaries.

Negative Feedback of the Thyroid Gland

• Elevated T3 and T4 in blood inhibit further TRF secretion by the hypothalamus and TSH by the anterior pituitary gland.

Goiter

• Iodine deficiency prevents T3/T4 production, stimulating the anterior pituitary to secrete TSH. This continues, causing an increase in the size of the thyroid gland.

Hyperthyroidism

• Caused by an enlarged gland producing too much hormone.
• Characterized by a high metabolic rate, hyperactivity, nervousness, irritability, and chronic fatigue.
• It can cause protruding eyes (exophthalmos).

Hypothyroidism

• Adults: Characterized by lethargy, weight gain, hair loss, decreased body temperature, low metabolic rate, and slow heart rate.
• Hormone therapy alleviates symptoms.
• Children: Results in cretinism; a mentally retarded dwarf with skeletal abnormalities.

Calcitonin (CT)

• Lowers serum calcium

Calcitonin and Serum Calcium

• Secreted when serum calcium levels are high (hypercalcemia).
• Target cells are primarily on bone and in the kidneys.
• Lowers blood calcium by increasing calcium absorption by bone and increasing calcium excretion by the kidneys.
• The parathyroid gland has the opposite effect – it increases calcium in the blood.
• CT secretion is inhibited by a negative feedback mechanism when serum calcium levels are normal.

Parathyroid Glands

• Humans typically have four parathyroid glands associated with the thyroid glands.
• Located posterior to the thyroid
• They are embedded within a thick connective tissue capsule.
• Has the opposite effect of calcitonin.
• Calcitonin (CT), parathyroid hormone (PTH), and vitamin D regulate calcium homeostasis.
• Parathyroid hormone (PTH) is secreted.
• PTH elevates serum calcium by stimulating bone absorption (osteoclastic activity).

Parathyroid Hormone (PTH)

• Increases blood calcium and magnesium by increasing the rate of dietary calcium and magnesium absorption.
• Increases the number and activity of osteoclasts; decreases the number of osteoblasts.
• Increases calcium resorption by kidneys.
• Promotes the formation of vitamin D.
• When calcium is at a normal state, PTH secretion is inhibited by a negative feedback system.

Adrenal (Suprarenal) Glands

• The adrenal glands are yellow, embedded in adipose tissue, and located superior to the kidneys.
• Each adrenal gland consists of two glands in one outer (cortex) and inner (medulla).

Adrenal Cortex

• The cortex is essential for life.
• The medulla may be removed with no life-threatening effects because its functions are like those of the sympathetic nervous system.
• Influenced by the hypothalamus, but use different mechanisms

Cortex

• Hypothalamus affects the cortex by secreting ACTH-releasing hormone.
• It stimulates the anterior pituitary to secrete ACTH
• It then stimulates the adrenal cortex.

Medulla

• The hypothalamus affects the medulla by direct stimulation from nerve impulses.
• Impulses originate in the hypothalamus and then travel through the brain stem, spinal cord, and sympathetic nerves.

Parts of the Adrenal Gland

• Adrenal Cortex (outer) makes up the bulk of the gland.

• The cortex is arranged in cords.

• There are 3 zones in the adrenal cortex.

• Each region produces a different type of hormone(s).

• All 3 zones of the adrenal cortex secrete a corticosteroid: aldosterone, cortisol, DHEA.

• Mineralocorticoids are secreted by the outermost region (zona glomerulosa).

• Functions in water and electrolyte balance.

• The principal mineralocorticoid is aldosterone.

• Aldosterone is a steroid synthesized from cholesterol.

• It regulates long-term hypotension.

• Hypotension stimulates sodium reabsorption from the kidneys.

• Water follows sodium, which increases blood volume and pressure.

• The renin-angiotensin system helps to regulate BP.

• Hypotension stimulates the kidneys to secrete renin into the blood.

• Renin converts angiotensinogen to angiotensin I.

• Angiotensin I is converted to angiotensin II in the lungs.

• Vasoconstriction occurs and blood pressure increases.

• Increases potassium excretion by the kidneys and sweat glands.

Glucocorticoids

• Secreted by the middle region (zona fasciculata).
• The principal hormone is cortisol (hydrocortisone).
• Glucocorticoids are steroids synthesized from cholesterol.
• Promotes normal glucose levels by:
  • Breaking down fats and proteins to increase blood sugar (gluconeogenesis).
  • Breaking down stored glycogen in the liver to glucose (glycogenolysis).
• Counter inflammatory responses.
• Glucocorticoid secretion is controlled by ACTH from the anterior pituitary gland, which is controlled by the releasing factors of the hypothalamus.

Gonadocorticoids (Sex Hormones)

• Secreted by the innermost region (zona reticularis).
• Secretes androgens, mainly DHEA (Dehydroepiandrosterone).
• DHEA is the major excretory steroidal product of the adrenal cortex, the most abundant steroidal hormone in the human body.
• Human Growth Hormone (Hgh) is the most abundant protein hormone.
• DHEA is also produced in the gonads and brain.
• DHEA acts on androgen receptors and can convert to produce testosterone and all 3 estrogens.
• Sometimes called the "Mother Hormone".

Adrenal Medulla

• Composed primarily of nervous tissue (sympathetic nervous system action.)
• Surrounds the adrenal cortex.
• Target cells: most cells of the body.

Hormones Secreted from the Adrenal Medulla

• Epinephrine (adrenaline): the majority of total secretions = cardiac stimulator.
• Norepinephrine (noradrenaline): a vasoconstrictor.
• Both have sympathomimetic effects (fight or flight, excitatory) during stress.
• Increased heart rate and force of heart contractions.
• Increased blood pressure.
• Increased liver glycogenolysis = increased blood glucose.
• Increased lipolysis = increased blood glucose.
• Increased pupil size = increased vision
• Increased sweat rate
• Decreased digestion
• Decreased urine formation
• Increased respiration rate and increased dilation of pulmonary passages

Pancreas

• Located in the abdominal cavity, posterior to the stomach.

• There are 3 parts:

  • Head: extending from the duodenum of the small intestine.
  • Body
  • Tail: extending from the spleen

• There are two types of tissue:

  • Endocrine portion
  • Digestive portion
  • Endocrine Portion: consists of clusters of cells called the Islets of Langerhans.
  • Digestive Portion: the Acini.

Islets of Langerhans

• Clustered cells organized into cords are separated by fenestrated capillaries.
• A protective connective tissue surrounds and separates the islets from the exocrine pancreas.
• The islets are innervated by the ANS (involuntary)
• 4 types of cells secrete different hormones, causing different effects.

Cells Found in Islets

• Alpha Cells (20%): secrete the hormone glucagon (polypeptide)
  • Target cells are liver cells and hepatocytes.
  • They are active when blood sugar is low (normal sugar = 80-110mg/dl).
  • A fasting serum sugar should be lower than 100mg\dl.
  • A person suffering from low blood sugar is termed "hypoglycemic"
  • Increases blood sugar by two processes:
    • Glycogenolysis: The breakdown of glycogen stored in the liver; then transporting it to the blood.
    • Gluconeogenesis: Making glucose from a non-carbohydrate source (fats and amino acids); then transporting it to the blood. - Is a resort during starvation and is costly in energy.

Beta cells (70%)

• Secrete the hormone insulin (polypeptide)
  • Target cells are most body cells excluding neurons and mature RBC's. Major target cells are hepatocytes, skeletal muscle cells, and adipocytes.
  • They are active when blood sugar is high (normal sugar = 80-110mg/dl).
    • A fasting serum sugar should be lower than 100mg\dl.
    • A person suffering from high blood sugar is termed suffering Diabetes Mellitus
  • Type 1 diabetes is characterized by a lack of, or diminished secretion of, insulin.
    • Occurs when glucose levels are high in serum and urine.
  • Type 2 diabetes is characterized by a lack of insulin receptors, (defective or even absent).
    • Causes the action of insulin to decrease blood sugar by four processes:
      • Glycogenesis: Converting the excess glucose and storing it in the liver.
      • Decreasing glycogenolysis/gluconeogenesis.
      • Increasing lipid synthesis (lipogenesis)
      • Stimulating protein synthesis.
    • Note: Insulin is not an effective oral medication because it is destroyed by digestive enzymes.
    • Do not forget insulin is a polypeptide.

Delta Cells

• Secretes Growth Hormone Inhibiting Factor. Also known as somatostatin
  • It is a polypeptide and targets the alpha, beta, and exocrine cells. The results will respectively inhibit glucagon, inhibit insulin, and inhibit exocrine secretions.
• F cells
  • Secrete pancreatic polypeptide
  • Has 3 target cells:
    • Pancreatic exocrine cells: Inhibits the secretion of enzymes and bicarbonate
    • Involves the Gall Bladder: Inhibits secretion of bile.
    • Involves the Small Intestine: Inhibits motility.

Ovaries

• Produce 5 hormones.
  • Estrogen: Mainly produced by the ovaries and the placenta. Small amounts are produced by the liver, adrenal glands, and the breasts.
  • The secondary sources are important in postmenopausal women.
    • Fat cells also produce estrogen, thus underweight persons are at risk.
    • 3 Types exist:
      • Estrone (E1): Primary estrogen, produced during menopause.
      • Estradiol (E2): produced during a non-pregnant's female.
      • Estriol (E3): produced by the placenta. It's an indicator of the infant's health.
      • Progesterone
        • Works the same way as Estrogen and maintains sexual characteristics.
      • Relaxin: Relaxes the pubis symphysis for dilation of the cervix for pregnancy.
      • Inhibin: Inhibits FSH during the final stages of the menstrual cycle.
      • Testosterone: Produced in the ovaries, though minute amounts.

Testicles

• Produce two hormones.
  • Testosterone: Created predominantly by Leydig cells, responding to the release of luteinizing hormone (LH) in the pituitary gland.
    • Causes anabolic effects: The increase of muscle and bone growth.
    • Can also cause Androgenic effects: The growth of beards; a larger penis/testicles, regulates sperm production, and deepens voice.

Inhibin

• Inhibits FSH

Pinneal Gland

• Located in the epithalamus of the diencephalon.
• In animals, it plays a role in behavioral patterns such as migration and circadian rhythms.
• 2 hormones are secreted.
  • Melotonin (derived from seratonin)
    • Inhabits reproduction and inhabits gonadotropic hormones.
    • Regulates body temperature and sleep, because pinealocytes are sensitive to day/night cycles.
    • In the absence of light, it produces melanin. With light, it produces seratonin.
  • Adrenoglomerulotropin: Stimulates the adrenal cortex to serete aldosterone.

Thymus

• The main hormone is Thymosin, which stimulates T cell production.
• 4 hormones create maturation of T cells and destroy foreign substances:
  • Thymosin
  • Thymic Humoral Factor (THF)
  • Thymic Factor (TF)
  • & Thymopoietin

Kidney

• It produces 2 hormones:
  • Erythropoietin (ERP): Which regulates red blood cell formation
  • Vitamin D (most active form is 1, 25 dihydroxycholecalciferol): Which intakes essential calcium in the small intestine
• Don't forget about enzyme Renin which interacts with angiotensinogen.
• The main enzyme that cleaves Angiotensin 1 to Angiotensin 2 is Angiotensin-converting enzyme (ACE).
  • It targets lungs and drugs such as ACE inhibitors (which are anti-hypertensive).

Heart

• Its main hormone is Atrial Natriuretic Peptide.
• It decreases blood pressure and controls blood sodium.

Stomach

• Produces hormone gastrin (producing in response to food)
• Gastrin stimulates hydrochloric acid and the enzyme pepsin.
• Hydrocloric acid and pepsin are secreted by the parietal cells of stomach.
• It produces this hormone and is also the target organ.

Small Intenstine

• It produces 2 hormones:
  • Secretin: Stimulates pancreas to produce bicarbonate. Neutralizes acid and is in the stomach.
  • Cholecystokinin: Stimulates the gallbladder and releases bile.

Leptin (white fat) & Ghrelin (stomach & pancreas)

• Leptin
• A protein hormone derived from white adipose tissue. Receptors work on the hypothalamus (satiety center).
  • White fat vs. Brown Fat:
    • White Fat: Known as the few mitochondria and a few capillaries. Known to the general public as adipose tissue and functions with an energy load and thermals.
    • Brown Fat: Called brown fat because of the large number of mitichondria.
    • Located in large amounts in children and animals, and smaller amounts in adults. Functions well as a thermal regulation.
    • Products a protein product of the OB (obesity gene).
    • Functions to increase satiety, decrease appetite, and cause weight loss.
    • Research shows that the brain creates a chemical to resist Leptin.
    • Is responsible for long term energy balance.
• Ghrelin
  • Is opposite of the hormone Leptin: It initiates meal and short term energy
  • It stimulates growth hormone and secreted anterior pituitary.
  • Hormone receptors are on the hypothalamus, pituitary gland, and GI tract.
  • Derived from growth hormone. (ghre- means to grow. - Lin- a a usual suffix for some hormones)
  • DOES NOT contribute to obesity; anorexics have been noted to have encreased levels of ghrelin