Endocrine System Overview

Chapter 9: The Endocrine System

• The Endocrine System is the body's second controlling system, working alongside the nervous system to coordinate and direct cellular activity.

Endocrine System Composition

• The system consists of glands with clusters of hormone-secreting cells found in the brain, heart, and small intestine, among other organs.

Hormone Secretion and Transport

• Endocrine glands are ductless and secrete hormones directly into the bloodstream.
• Blood transports hormones to their target sites.
• These hormones regulate the activity of other cells.

Major Processes Controlled by Hormones

• Reproduction
• Growth and development
• Mobilization of body defenses
• Homeostasis maintenance
• Regulation of metabolism

Endocrine System vs. Nervous System

• The endocrine system uses hormones distributed via the bloodstream.
• The nervous system uses neurotransmitters secreted into a synapse.
• The endocrine system responds slowly with long-lasting effects and adapts slowly to continual stimulation.
• The nervous system responds quickly with short-lived effects and adapts quickly to continual stimulation.
• The actions of both systems work together to maintain homeostasis.

Hormone Chemistry

• Hormones are chemically classified as amino acid-based, steroids, or prostaglandins.
• Amino acid-based hormones include proteins, peptides, and amines.
• Steroids are derived from cholesterol.
• Prostaglandins are made from highly active lipids acting as local hormones.

Hormone Action

• Hormones affect specific tissues or organs, known as target cells or target organs.
• Target cells possess specific protein receptors.
• Hormone interactions with target cell receptors are very specific.
• Hormone binding alters cellular activity.

Stimuli for Hormone Release Control

• Hormone levels in the blood are primarily maintained through negative feedback mechanisms.
• A stimulus or low hormone levels in the blood triggers further hormone release.
• Hormone release ceases once appropriate blood levels are reached.

Categories of Endocrine Gland Stimuli

• Hormonal: Endocrine organs are activated by other hormones.
• Humoral: Changing blood levels of ions and nutrients stimulate hormone release. Humoral refers to body fluids like blood and bile.
• Neural: Nerve fibers stimulate hormone release, often under sympathetic nervous system control.

Hormonal Stimuli Example

• Hormones from the hypothalamus stimulate the anterior pituitary to secrete its hormones.

Humoral Stimuli Examples

• Parathyroid hormone and calcitonin are produced in response to changing blood calcium levels.
• Insulin is produced in response to changing blood glucose levels.

Neural Stimuli Example

• Sympathetic stimulation of the adrenal medulla causes the release of epinephrine and norepinephrine.

Major Endocrine Organs

• Hypothalamus
• Pituitary gland
• Pineal gland
• Thyroid gland
• Parathyroid glands
• Thymus
• Adrenal glands
• Pancreas
• Gonads (testes and ovaries)

Purely Endocrine Glands

• Anterior pituitary
• Thyroid
• Adrenals
• Parathyroids

Mixed Glands (Endocrine and Exocrine Functions)

• Pancreas
• Gonads

Endocrine Gland Characteristics

• Endocrine glands are ductless, hormones are released directly into the blood or lymph.

Pituitary Gland

• The pituitary is a pea-sized gland connected to the hypothalamus via a stalk.
• It's protected by the sella turcica of the sphenoid bone.
• It has two functional lobes: the anterior pituitary (glandular tissue) and the posterior pituitary (nervous tissue).
• It's often called the "master endocrine gland" due to its control over other endocrine glands.

Hypothalamus Hormones

• The hypothalamus produces releasing and inhibiting hormones which are released into the portal circulation connected to the anterior pituitary.
• These hypothalamic hormones control anterior pituitary hormone release.
• It also produces oxytocin (OT) and antidiuretic hormone (ADH).
• OT and ADH are transported to the posterior pituitary for storage in neurosecretory cells.
• They are released from the posterior pituitary in response to nerve impulses from the hypothalamus.

Hypothalamus Hormones & Their Functions

• Gonadotropin-releasing hormone: Promotes secretion of FSH and LH.
• Thyrotropin-releasing hormone: Promotes TSH secretion.
• Corticotropin-releasing hormone: Promotes ACTH secretion.
• Prolactin-releasing hormone: Promotes prolactin secretion.
• Prolactin-inhibiting hormone: Inhibits prolactin secretion.
• Growth hormone-releasing hormone: Promotes GH secretion.
• Growth hormone-inhibiting hormone (Somatostatin): Inhibits GH and TSH secretion.

Posterior Pituitary

• The posterior pituitary does not produce hormones; it stores hormones made by the hypothalamus.
• Two hormones released are oxytocin and antidiuretic hormone (ADH).

Oxytocin

• Oxytocin is released in significant amounts during childbirth and nursing.
• It stimulates uterine contractions during labor, sexual relations, and breastfeeding.
• It causes milk ejection in a nursing woman.
• The target organs are the uterus and mammary glands.

Antidiuretic Hormone (ADH)

• ADH inhibits urine production by promoting water reabsorption by the kidneys. The kidneys are the target organs.
• ADH causes increased blood volume and pressure, in large amounts, ADH causes arteriole constriction, increasing blood pressure and thus it is also known as vasopressin.
• Diabetes insipidus results from ADH deficiency, leading to excessive urine production due to the inability to conserve water.
• Diabetes Insipidus is not related to insulin.

Anterior Pituitary Hormones

• Six hormones: growth hormone (GH), prolactin (PRL), follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyrotropic hormone/thyroid-stimulating hormone (TSH), and adrenocorticotropic hormone (ACTH)
• GH and PRL affect nonendocrine targets.
• FSH, LH, TSH, and ACTH regulate other endocrine glands and are tropic hormones

Characteristics of Anterior Pituitary Hormones

• They are proteins or peptides
• They act through second-messenger systems.
• They are regulated by hormonal stimuli.
• They are mainly regulated by negative feedback.

Growth Hormone (GH)

• GH is a general metabolic hormone with major effects directed to the growth of skeletal muscles and long bones (target organs).
• GH plays a role in determining final body size.
• It causes amino acids to be built into proteins.
• It causes fats to be broken down for energy.

Prolactin (PRL)

• PRL stimulates and maintains milk production in females post childbirth.
• The target organ is the mammary glands.
• The function in males is unknown.

Gonadotropic Hormones

• Gonadotropic hormones regulate hormonal activity of the gonads (ovaries and testes).

Follicle-Stimulating Hormone (FSH)

• FSH stimulates follicle development in ovaries and sperm development in testes.

Luteinizing Hormone (LH)

• LH triggers ovulation in females and stimulates testosterone production in males.

Thyrotropic Hormone (TH)

• TH (also called thyroid-stimulating hormone or TSH) influences growth and activity of the thyroid gland (target organ).
• It stimulates the release of thyroid hormones (T3 and T4) from the thyroid gland.

Adrenocorticotropic Hormone (ACTH)

• ACTH regulates endocrine activity by regulating the release of hormones of the adrenal cortex.

Pineal Gland

• The pineal gland hangs from the roof of the brain's third ventricle.
• It secretes melatonin, believed to trigger the body's sleep/wake cycle, and believed to coordinate human fertility hormones and inhibit the reproductive system until maturity occurs.

Thyroid Gland

• The thyroid is based at the front of the throat, just below the Adam's apple.
• It has two connected lobes.
• Its follicles store a colloidal material from which thyroid hormone is taken.
• It produces thyroid hormones (T3 and T4), and calcitonin.

Thyroid Hormone

• It is secreted by the follicular cells in the thyroid follicles.
• It is the major metabolic hormone, it controls the rate of glucose oxidation for body heat and chemical energy (ATP).
• It is crucial for tissue growth and development.
• It consists of two active iodine-containing hormones including: Thyroxine/ Tetraiodothyronine (T4), and Triiodothyronine (T3).

Calcitonin

• It is produced by parafollicular cells between the follicles.
• It decreases blood calcium levels by stimulating calcium deposition on bone and is released in response to rising blood calcium levels.
• It is antagonistic to parathyroid hormone.

Thyroid Hormone Disorders

• Hypothyroidism involves a thyroid hormone deficiency.
• Hyperthyroidism involves excess thyroid hormone.
• If hypothyroidism occurs in later life, it is characterized by low metabolic rate, resulting in weight gain, hair loss, and fatigue.
• Severe hypothyroidism in infancy and/or fetal life may result in cretinism, characterized by low birth weight, stunted growth, and mental retardation.
• Hyperthyroidism is characterized by increased metabolism, weight loss, and high heart rate.
• Graves disease is an autoimmune disease that causes hyperthyroidism.
• Goitre a term that refers to an enlarged thyroid gland and is usually caused by iodine deficiency in the diet, TSH levels are high.

Parathyroid Glands

• They are tiny masses on the posterior of the thyroid gland that secrete parathyroid hormone (PTH).
• The most important regulator of calcium ion (Ca+2) homeostasis of the blood is parathyroid hormone (PTH).
• It increases calcium and promotes bone removal and kidney and intestinal absorption, blood calcium levels.

Characteristics of PTH (parathyroid hormone)

• It increases blood calcium levels
• Stimulates osteoclasts to remove calcium from bone
• Stimulates the kidneys and intestine to absorb more calcium
• Parathyroid Hormone Disorders:
• Hyperparathyroidism is caused by hypersecretion from one or more parathyroid glands.

Hyperparathyroidism

• It is characterized by hypercalcemia, kidney stones, osteoporosis, and various other symptoms.
• Hypercalcemia results in muscle weakness and sluggish reflexes.
• In severe cases, it is known to result in cardiac arrest.

Thymus

• It is located in the upper thorax, posterior to the sternum.
• It is largest in infants and children.
• It decreases in size throughout adulthood and produces the hormone thymosin.
• It causes some types of white blood cells to mature.
• It is crucial in developing the immune system.

Adrenal Glands

• The adrenal glands sit on top of the kidneys.
• The adrenal cortex is the outer glandular region that has three layers that produce corticosteroids.
• The adrenal medulla is the inner neural tissue.

Regions of the Adrenal Cortex

• Mineralocorticoids are secreted by the outermost layer.
• Glucocorticoids are secreted by the middle layer.
• Sex hormones are secreted by the innermost layer.

Mineralocorticoids

• They are mainly aldosterone and are produced in the outer adrenal cortex.
• They regulate mineral (salt) content in blood, particularly sodium and potassium ions, regulating water and electrolyte balance.
• The target organ is the kidney.

Stimuli for Aldosterone Release

• Humoral factors (fewer sodium ions or too many potassium ions in the blood).
• Hormonal stimulation (ACTH).
• Renin and angiotensin II in response to a drop in blood pressure.
• Aldosterone production is inhibited by atrial natriuretic peptide (ANP), a hormone produced by the heart when blood pressure is too high.

Glucocorticoids (Cortisol)

• They are produced by the middle layer of the adrenal cortex, promoting normal cell metabolism.
• They help resist long-term stressors by increasing blood glucose levels (hyperglycemic hormones).
• They have anti-inflammatory properties (decrease edema and pain) and are released in response to increased blood levels of ACTH.

Sex Hormones

• They are produced in the inner layer of the adrenal cortex, with only small amounts that are made throughout life.
• Most of the hormones produced are androgens (male sex hormones), but some estrogens (female sex hormones) are also formed.

Adrenal Hormone Disorders

• Cushings Syndrome is caused by hypersecretion of Cortisol from the adrenal cortex, often accompanied by hypersecretion of adrenal androgens.

Characteristics of Cushings Syndrome

• Moon face
• Buffalo hump
• Hyperglycemia
• Hypertension
• Edema due to sodium and water retention and muscle wasting

Characteristics of Addison’s Disease

• Fluid and electrolyte loss
• Weakness
• Muscle wasting
• Dehydration
• Lowered BP
• Weight loss and can be life-threatening

Adrenal Medulla

• The adrenal medulla deals with 2 hormones, epinephrine and norepinephrine (also known as catecholamines).
• The hormones help prepare the body to deal with short-term stress (“fight or flight”)

Events that these hormones (Adrenal Medulla) cause in the body

• They cause an increase in heart rate, an increase of blood pressure, and an increase in glucose levels within the blood.
• They also make the small airways of the lungs (bronchioles) dilate.

Pancreatic Islets

• The pancreas, located in the abdomen is a mixed gland, and has both endocrine and exocrine functions.
• The pancreatic islets (islets of Langerhans) produce hormones including insulin and glucagon.

Insulin

• The hormone Insulin is produced by beta cells
• It is released when blood glucose levels are high
• It increases the rate of glucose uptake and metabolism by body cells
• Insulin makes the body hypoglycemic (not enough glucose in the blood)

Glucagon

• The hormone Glucagon is produced by alpha cells
• It's released when blood glucose levels are low
• Stimulates the liver causing a release of glucose in the blood, thus increasing blood glucose levels
• Glucagon makes the body hyperglycemic (too high blood glucose levels)

Pancreatic Hormone Disorders

• One of the pancreatic problems which can occur is Diabetes mellitus.
• Diabetes mellitus – Type 1 is due to low insulin production caused by damage to beta cells of islets.
• Diabetes mellitus – Type 2 is due to problems because of decreased insulin receptors on the surface of the target cells,resulting in insulin resistance.

Gonads

• The gonads consist of testes (males) and ovaries (females)
• They both create egg cells (ovaries) or sperm (testes) and they control and produce sex hormones that affect endocrine functions.

Ovaries

• These are the female form of gonads,which are located in the pelvic cavity.
• They produce two groups of steroid hormones called Estrogens and Progesterone.

Important actions with Ovaries

• They trigger the creation of female characteristics.
• They make the reproductive organs of females mature.
• Breast development with Progesterone.
• They regulate the female menstrual cycle too.

Progesterone

• Acts to bring about the menstrual cycle Helps in the implantation of an embryo into the uterus Helps to prepare the breasts for lactation and breast feeding

Testes

• They create male specific androgens Testosterone helps to cause the secondary sex characteristics of males Are important for sperm production of healthy functioning sperm Helps to boost growth and maturation of the male reproductive system

Non-Endocrine Organs That Produce Hormones

• Stomach: Produces Gastrin.
• Small Intestine: it is the location for the secretion of secretin and cholecystokinin (CCK).
• Kidneys: location for the secretion of Erythropoietin.
• Heart: location for the hormone atrinatriuretic peptide (ANP).

Placenta

• It creates human chorionic gonadotropin (hCG) alongside estrogen and progesterone.
• It produces hormones that maintain the stage of pregnancy.
• Some of the hormones released have play a important role in the baby’s delivery.
• Relaxin works to cause relaxation in the pelvic ligaments and pubic symphysis readying for the childbirth.