Endocrine and Exocrine Glands

• The body has two types of glands: exocrine and endocrine.
• Exocrine glands secrete products through ducts into body cavities or onto body surfaces, examples include: sudoriferous, sebaceous, and digestive glands.
• Endocrine glands secrete hormones into extracellular spaces around secretory cells.
• Secretions from endocrine glands diffuse into capillaries and are carried away by the blood.
• The endocrine system includes endocrine glands and organs with endocrine tissue.
• Endocrinology is the study of endocrine gland structure, function, and related disorders.

Comparison of Nervous and Endocrine Systems

• Both nervous and endocrine systems coordinate body functions to maintain homeostasis.
• The nervous system uses nerve impulses (action potentials) along neuron axons such as neurotransmitters.
• The endocrine system releases hormones into the bloodstream, delivering them throughout the body.
• The nervous system can stimulate or inhibit hormone release and hormones can promote or inhibit nerve impulse generation.
• The nervous system causes muscle contraction and glandular secretion.
• The endocrine system affects most tissues, altering metabolism, regulating growth and development, and guiding reproduction.
• Nerve impulses produce rapid but brief effects.
• Hormones produce slower but more prolonged effects.

Hormone Effects

• Hormones regulate internal environment, metabolism, and energy balance.
• Hormones also help regulate smooth and cardiac muscle contraction, glandular secretion, and immune responses.
• Hormones integrate growth and development, maintain homeostasis during environmental disruptions, and contribute to reproduction.

Hormones

• Hormones affect specific target cells with matching receptors.
• Receptor numbers are regulated by the hormone concentration.
• Down-regulation decreases receptor numbers in response to excess hormone.
• Up-regulation increases receptor numbers in response to hormone deficiency.
• Circulating hormones (or endocrines) act on distant target cells via the bloodstream.
• Local hormones (paracrines or autocrines) act on nearby target cells.
• Hormones are chemically classified as steroids, biogenic amines, proteins/peptides, and eicosanoids.
• Water-soluble hormones circulate freely in the blood.
• Lipid-soluble hormones (steroid and thyroid) are transported by proteins.

Mechanisms of Hormone Action

• Hormone response depends on both the hormone and the target cell.
• Steroid and thyroid hormones bind to intracellular receptors and alter gene expression.
• Water-soluble hormones activate plasma membrane receptors, triggering intracellular events.
• Water-soluble hormones act as first messengers, delivering a message to the plasma membrane.
• Second messengers, like cyclic AMP (cAMP), relay the message inside the cell.
• G-proteins are common in second messenger systems.
• Cyclic AMP activates protein kinases, which trigger physiological responses.
• Target cell responsiveness depends on hormone concentration, receptor number, and interactions with other hormones.
• Three types of hormonal interactions are the permissive effect, synergistic effect, and antagonistic effect.

Control of Hormone Secretions

• Hormones are typically released in short bursts.
• Regulation of hormone secretion maintains homeostasis.
• Dysregulation leads to disorders.
• Hormone secretion is controlled by the nervous system, blood chemistry, and other hormones.
• Negative feedback systems commonly regulate hormone secretion.

Hypothalamus and Pituitary Glands (Hypophysis)

• The hypothalamus integrates the nervous and endocrine systems.
• The hypothalamus and pituitary regulate growth, development, metabolism, and homeostasis.
• The pituitary gland is located in the sella turcica of the sphenoid bone.
• The pituitary gland has two lobes: anterior (adenohypophysis) and posterior (neurohypophysis), with the pars intermedia in between.

Anterior Pituitary Gland (Adenohypophysis)

• Adenohypophysis hormones are controlled by releasing or inhibiting hormones from the hypothalamus.
• The superior hypophyseal arteries supply blood to the anterior pituitary, carrying releasing and inhibiting hormones.
• The anterior pituitary consists of five glandular cell types: somatotrophs, lactotrophs, corticotrophs, thyrotrophs, and gonadotrophs.
• Somatotrophs produce human growth hormone (hGH).
• Lactotrophs produce prolactin (PRL).
• Corticotrophs secrete adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH).
• Thyrotrophs secrete thyroid-stimulating hormone (TSH).
• Gonadotrophs secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
• Negative feedback decreases secretion by corticotrophs, thyrotrophs, and gonadotrophs when target gland hormone levels rise.
• Human growth hormone (hGH) stimulates growth via somatomedins and is controlled by GHIH (somatostatin) and GHRH.
• hGH-related disorders include pituitary dwarfism, giantism, and acromegaly.
• Thyroid-stimulating hormone (TSH) regulates the thyroid gland and is controlled by TRH.
• Follicle-stimulating hormone (FSH) regulates ovaries and testes and is controlled by GnRH.
• Luteinizing hormone (LH) regulates ovaries and testes and is controlled by GnRH.
• Prolactin (PRL) initiates milk secretion and is controlled by PIH and PRH.
• Melanocyte-stimulating hormone (MSH) increases skin pigmentation and is controlled by MRH and MIH.
• Adrenocorticotropic hormone (ACTH) regulates the adrenal cortex and is controlled by CRH.

Posterior Pituitary Gland (Neurohypophysis)

• The neurohypophysis stores and releases hormones made by the hypothalamus.
• The supraopticohypophyseal tract connects the hypothalamus and neurohypophysis.
• Hormones stored and released by the posterior pituitary are oxytocin (OT) and antidiuretic hormone (ADH).
• Oxytocin (OT) stimulates uterine contractions and milk ejection.
• OT secretion is controlled by uterine distention and nursing.
• Synthetic OT (Pitocin) induces labor or controls postpartum hemorrhage.
• Antidiuretic hormone (ADH) stimulates water reabsorption by the kidneys and arteriolar constriction.
• ADH decreases urine volume and conserves body water.
• ADH is controlled by blood osmotic pressure.
• Diabetes insipidus results from ADH hyposecretion, causing excessive dilute urine excretion and dehydration.

Thyroid Gland

• The thyroid gland is located below the larynx, with right and left lateral lobes.
• The thyroid consists of follicles with follicular cells that secrete thyroxine (T4) and triiodothyronine (T3).
• Parafollicular cells secrete calcitonin (CT).
• Thyroid hormones are synthesized from iodine and tyrosine within thyroglobulin (TGB) and are transported by thyroxine-binding globulin (TBG).
• Thyroid hormones regulate metabolism, growth, development, and nervous system reactivity.
• Thyroid activity is initiated by TRH and TSH.
• Thyroid secretion is controlled by iodine levels and negative feedback involving the hypothalamus and anterior pituitary.
• Thyroid-related disorders include cretinism, myxedema, Graves’ disease, and goiter.
• Calcitonin (CT) lowers blood calcium levels.
• Calcitonin secretion is controlled by blood calcium levels.

Parathyroid Glands

• The parathyroid glands are located on the posterior surfaces of the thyroid lobes.
• The parathyroids contain principal (chief) and oxyphil cells that secrete parathyroid hormone (PTH).
• Parathyroid hormone (PTH) regulates calcium and phosphate homeostasis by increasing blood calcium and decreasing blood phosphate.
• PTH secretion is controlled by blood calcium levels.
• Tetany and osteitis fibrosa cystica are disorders associated with the parathyroid glands.
• Tetany is caused by calcium deficiency due to hypoparathyroidism.
• Osteitis fibrosa cystica is characterized by demineralized, weak bones due to hyperparathyroidism.

Adrenal (Suprarenal) Glands

• The adrenal glands are located superior to the kidneys.
• The adrenal glands consist of the outer cortex and inner medulla.

Adrenal Cortex

• The adrenal cortex consists of the zona glomerulosa, zona fasciculata, and zona reticularis.
• Cortical secretions include mineralocorticoids, glucocorticoids, and gonadocorticoids.
• Mineralocorticoids (e.g., aldosterone) increase sodium and water reabsorption and decrease potassium reabsorption.
• Mineralocorticoid secretion is controlled by the renin-angiotensin pathway and blood potassium levels.
• Aldosteronism is caused by aldosterone hypersecretion, leading to muscular paralysis and hypertension.
• Glucocorticoids (e.g., cortisol) promote normal metabolism, help resist stress, and act as anti-inflammatory substances.
• Glucocorticoid secretion is controlled by CRH and ACTH.
• Addison’s disease is adrenocortical insufficiency (hyposecretion of glucocorticoids and aldosterone).
• Cushing’s syndrome is glucocorticoid hypersecretion.
• Gonadocorticoids (sex hormones) (e.g., estrogens and androgens) usually have minimal effects.
• Congenital adrenal hyperplasia (CAH) results in virilism (masculinization).
• Virilism or gynecomastia can result from adrenal gland tumors.

Adrenal Medulla

• The adrenal medulla contains chromaffin cells, which surround blood-filled sinuses.
• Medullary secretions are epinephrine and norepinephrine (NE).
• Epinephrine and norepinephrine produce sympathetic-like responses.
• Epinephrine and norepinephrine release is triggered by stress via the autonomic nervous system.
• Pheochromocytomas (tumors of chromaffin cells) cause hypersecretion of epinephrine and norepinephrine.
• Hypersecretion of epinephrine and norepinephrine puts individuals in a prolonged fight-or-flight state, causing fatigue and weakness.

Pancreas

• The pancreas is located posterior and inferior to the stomach and functions as both an endocrine and exocrine gland.
• The pancreas consists of pancreatic islets (islets of Langerhans) and acini (exocrine cells).
• Four types of hormone-secreting cells compose the pancreatic islets.
• Alpha cells secrete glucagon, which increases blood glucose levels.
• Glucagon secretion is stimulated by low blood glucose levels.
• Beta cells secrete insulin, which reduces blood glucose levels.
• Insulin secretion is stimulated by high blood glucose levels.
• Disorders associated with insulin production are diabetes mellitus and hyperinsulinism.
• Delta cells secrete somatostatin (GHIH), which inhibits insulin and glucagon secretion.
• F-cells secrete pancreatic polypeptide, regulating the release of pancreatic digestive enzymes.

Ovaries and Testes

• Ovaries are located in the pelvic cavity and produce estrogens and progesterone.
• Sex hormones from the ovaries relate to female sexual characteristics, reproduction, pregnancy, and lactation.
• The ovaries produce inhibin and relaxin.
• Testes lie inside the scrotum and produce testosterone.
• Sex hormones from the testes support male sexual characteristics and reproductive functions.
• The testes produce inhibin.

Pineal Gland (Epiphysis Cerebri)

• The pineal gland is attached to the roof of the third ventricle in the brain.
• The pineal gland consists of pinealocytes, neuroglial cells, and sympathetic fibers.
• The pineal gland secretes melatonin in a diurnal rhythm linked to the light-dark cycle.
• Seasonal affective disorder (SAD) is linked to melatonin overproduction during winter.
• Bright light therapy can relieve symptoms of SAD and jet lag.

Thymus Gland

• The thymus gland secretes hormones related to immunity.
• Thymosin, THF, TF, and thymopoietin promote T cell proliferation and maturation.

Aging and the Endocrine System

• The endocrine system undergoes various changes with age.
• Most endocrine disorders are related to pathology, not age.
• Diabetes mellitus and thyroid disorders are common endocrine problems.

Developmental Anatomy of the Endocrine System

• Endocrine organs develop in separate regions of the embryo.
• The pituitary, adrenal medulla, and pineal gland develop from ectoderm.
• The adrenal cortex develops from mesoderm.
• The thyroid, parathyroids, pancreas, and thymus develop from endoderm.

Other Endocrine Tissues

• Several tissues other than endocrine glands secrete hormones.
• The GI tract synthesizes gastrin, secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP).
• The placenta produces hCG, estrogens, progesterone, relaxin, and hCS during pregnancy.
• The kidneys release erythropoietin and synthesize calcitriol (active vitamin D).
• The skin synthesizes vitamin D from inactive precursors with sunlight.
• The heart atria produce atrial natriuretic peptide (ANP) to lower blood pressure.

Eicosanoids

• Prostaglandins (PGs) and leukotrienes (LTs) act as paracrines and autocrines.
• They alter the production of second messengers, such as cyclic AMP.
• Prostaglandins affect normal physiology and pathology.
• NSAIDs, like aspirin, ibuprofen, and acetaminophen, inhibit prostaglandin synthesis.

Growth Factors

• Growth factors are hormones that stimulate cell growth and division.
• Examples: epidermal growth factor (EGF), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), nerve growth factor (NGF), tumor angiogenesis factors (TAFs), insulin-like growth factor (IGF), and cytokines.

Stress and the General Adaptation Syndrome

• Homeostatic mechanisms counteract everyday stresses.
• Extreme, unusual, or long-lasting stress triggers the stress response or general adaptation syndrome (GAS).
• The general adaptation syndrome prepares the body to meet an emergency.
• Stressors are stimuli that produce the GAS.
• Stressors include heat/cold, surgery, poisons, infections, fever, and emotional responses.
• Stressors stimulate the hypothalamus to initiate the GAS through the alarm reaction and the resistance reaction.

Alarm Reaction

• The alarm reaction is initiated by nerve impulses from the hypothalamus to the sympathetic nervous system and adrenal medulla.
• Responses are immediate fight-or-flight reactions.
• These immediate fight-or-flight reactions increase circulation, promote catabolism, and decrease nonessential activities.

Resistance Reaction

• The resistance reaction is initiated by regulating hormones secreted by the hypothalamus.
• Regulating hormones: CRH, GHRH, and TRH.
• CRH stimulates the adenohypophysis to secrete ACTH, which stimulates the adrenal cortex.
• Resistance reactions are long-term and accelerate catabolism to provide energy to counteract stress.
• High concentrations of glucocorticoids are produced during the resistance stage.

Exhaustion

• Exhaustion results from dramatic changes during alarm and resistance reactions.
• Exhaustion is caused by loss of potassium, depletion of adrenal glucocorticoids, and weakened organs.

Stress and Disease

• Stress can lead to certain diseases.
• Stress-related conditions include gastritis, ulcerative colitis, irritable bowel syndrome, peptic ulcers, hypertension, asthma, rheumatoid arthritis, migraine headaches, anxiety, and depression.
• Stressed individuals are at greater risk of chronic disease or premature death.
• Interleukin-1 (IL-1), produced by macrophages, links stress and immunity by stimulating ACTH secretion.

Explore the two types of glands in the human body: exocrine, which secrete through ducts, and endocrine, which secrete hormones into the bloodstream. Understand the roles of these glands and the basics of endocrinology, the study of their structure, function, and disorders.