Glands * Consists of one or more cells that produce and secrete a product * Most are composed primarily of epithelium * Exocrine glands – secrete their product onto the outer surface or into a cavity (have a duct) * Endocrine glands are ductless and secrete their product internally to be transported by the bloodstream * The pancreas has both endocrine and exocrine functions

Endocrine Glands

The endocrine system consists of glands and tissues that secrete hormones.

Endocrine Glands: * Are ductless; they secrete their hormones directly into tissue fluid * From there the hormones diffuse into the blood stream for distribution throughout the body
Hormones:
* Are chemical signals that affect the behavior of other glands or tissues. * Hormones influence the metabolism of cells, the growth and development of body parts, and homeostasis.

Hormones can be categorized into peptide hormones and steroid hormones: * Peptide hormones: (non-lipid soluble) * Include proteins, glycoproteins, and modified amino acids. (non-lipid soluble) * The peptide hormones can not enter the cell * Peptide hormones bind to a receptor protein in the plasma membrane and activate a "second messenger" system. * Second messenger sets in motion an enzyme cascade that leads to a cellular response * The second messenger causes the cellular changes for which the hormone is credited. (the peptide hormone stays outside the cell and directs activities within) * Common second messengers found in many body cells include cyclic AMP and calcium. * Peptide hormone action is regulated by the regulation of the number of receptors.- if no receptors, there will be no reaction * There are specific receptors for a specific type of hormone (they have to fit) * Most hormones are peptide hormones: * E.g. Insulin is a peptide hormone

* Steroid hormones (fat soluble) * Intracellular mechanism * All steroid hormones are made of lipids (cholesterol) that have the same four-carbon ring complex as peptides, but have different side chains. * Because they are lipids they diffuse across the plasma membrane and other cellular membranes. * Once inside the cell, steroid hormones bind to receptor proteins * Hormone-recepter complex binds to DNA, activating particular genes (need to move inside the nucleus to bind to a receptor) * Gene activation leads to production of cellular enzymes that cause cellular changes.
Afterwards the hormones are broken down (no recycling)

The figure depicts the locations of the major endocrine glands in the body:

The table lists the hormones they release:

Hormone Control * Many hormones affect concentrations of certain substances in the blood * Other hormones are involved in normal function of various organs * Release of hormones is controlled by one or more of the following: * The nervous system * The action of other hormones * Humoral control (Concentration of dissolved molecules or ions in the blood) * Negative feedback mechanisms * e.g parathyroid hormone which functions in calcium homeostasis. * Hormones can be influenced by more than one control mechanisms. * E.g. Insulin is regulated by all three mechanisms. * When you eat, sensory information (nervous control) makes signals to the brain which signals parasympathetic motor neurons to cause the release of insulin from the pancreas. * The pancreas secretes insulin as response to higher blood glucose, and stores it as glycogen in liver an muscles (humoral control) * Insulin release is inhibited by glucagon release from the pancreas (action of other hormones)

Hypothalamus and Pituitary Gland

Hypothalamus * Controls secretions of the pituitary gland * Neurosecretory cells produce * Antidiuretic hormone (ADH) * Oxytocin

Posterior Pituitary: * Neurons in the hypothalamus called neurosecretory cells produce the hormones antidiuretic hormone (ADH) and oxytocin. * These hormones pass though axons into the posterior pituitary where they are stored in axon endings.

Antidiurectic Hormone and Oxytocin: * Antidiurectic hormone: * Targets the kidney and blood vessels * Released when neurons in the hypothalamus detect that the blood is too concentrated with salt * Is released from the posterior pituitary. * Causes more water to be reabsorbed into kidney capillaries * Raises blood pressure by vasoconstriction (makes the blood vessels more narrow) of blood vessels * Diabetes insipidus results from the inability to produce ADH (inability to hold back on the water) * Negative feedback * Alcohol suppresses the secretion of ADH and thus decreases the reabsorption of water. * Oxytocin: * Causes uterine contraction during childbirth and milk letdown when a baby is nursing. * Positive feedback
The hypothalamus and the pituitary (figure):

Anterior Pituitary: * A portal system between the hypothalamus and the anterior pituitary * The hypothalamus controls the anterior pituitary by producing hypothalamic-releasing hormones and hypothalamic-inhibinting hormones. * The hypothalamus secretes releasing and inhibiting hormones that regulate the production and secretion of hormones from the anterior pituitary gland.
Hormones that affect other glands: (Hormones produced by the anterior pituitary that affect other glands)
Controlled by hypothalamic hormones * Hormones that affect other glands: * Thyroid-stimulating hormone (TSH) * Adrenocorticotropic hormone (ACTH) * Gonadotropic hormones (FSH) * Luteinizing hormone (LH) * Thyroid-stimulating hormone (TSH or thyrotropin): Stimulates the thyroid to produce the thyroid hormones; * adrenocorticotropic hormone (ACTH or corticotropin) which stimulates the adrenal cortex to produce its hormones; * and gonadotopic hormones (follicle-stimulationg hormone - FSH) * and luteinizing hormone LH) which stimulates the gonads (the testes in males and the ovaries in females, to produce gametes and sex hormones.
Effects of other hormones: (does not affect other endocrine glands) * Effects of other hormones * Prolactin (PRL) * Growth hormone (GH) * Other hormones produced by the anterior pituitary do not affect other endocrine glands. * Prolactin (PRL) is produced beginning at about the fifth month pregnancy - it causes mammary glands in the breast to develop and produce milk. * Growth hormone (GH) stimulates protein synthesis within cartilage, bone and muscle. It also stimulates the rate at which amino acids enter cells and protein synthesis occurs. It also promotes fat metabolism.
Effects of Growth Hormone (GH): * Growth hormone: * Promotes fat metabolism * Stimulates protien eynthesis in bone and cartilage * GH affects the height of an individual * If too little GH is produced during childhood, the individual has pituitary dwarfism * If too much GH is secreted, a person can become giant * Overproduction of GH in an adult is causing a condition called acromegaly results.

Thyroid and Parathyroid Glands

Thyroid Gland
The anterior pituitary gland produces thyroid-stimulating hormone (TSH) which signals to the thyroid gland to produce its hormones. * Two forms of thyroid hormone * Triiodothyronine (T3) contains 3 iodine atoms * Thyroxine (T4) contains 4 iodine atoms * Produced with the aid of TSH (what stimulates the gland to produce the hormones) * Iodine is needed for thyroxine production (because it contains iodine) * Effects of thyroid hormone (Thyroxine) * Increase the metabolic rate * Stimulate all cells of the body * Lack of iodine in the diet can lead to simple or endemic goiter (caused by continuous anterior pituitary stimulation without being able to produce the thyroid hormones) * Can be treated by adding iodine to the diet.
Calcitonin (released from the thyroid gland) * Produced by the thyroid gland * Helps control blood calcium levels (calcium is important for bones and in the nervous system - transmission of nerve signals and it is important for muscle contraction) * Secreted when the blood calcium levels rise * Causes blood calcium to decrease * Make the bones take up calcium from the blood * Lower the numbers and activity of osteclasters * The antagonistic hormone to calcitonin is parathyroid hormone (PTH)

Endocrine regulation of bone metabolism:

Parathyroid Glands * Posterior surface of the thyroid gland * Produces parathyroid hormone (PTH) * Causes blood phosphate (HPO42-) level to decrease * Causes blood calcium (Ca2+) level to increase * Promotes the release of calcium from the bones * Promotes the reabsorption of calcium by the kidneys * Activates vitamin D in the kidneys, which stimulates the reabsorption of calcium from the intestines * Hypocalcemic tetany results when there is insufficient secretion of PTH, because of increased excitability of the neurons * Calcium homeostasis is important in relation to neurons and muscle cells

Adrenal Glands

The adrenal glands sit atop the kidneys and consists of an inner portion called the adrenal medulla and an outer portion called adrenal cortex.

Adrenal Medulla * Is the inner portion of the adrenal glands * Under nervous control (sympathetic system) * Secretes epinephrine (adrenaline) and norepinephrine (noradrenaline) * Causes “fight or flight” responses (same reaction as the sympathetic nervous system) * Provide a short-term response to stress * Increase: heart rate & force, respiration * Decrease digestive activity

Adrenal Cortex * Outer portion – 3 layers, each produces a different set of steroid hormones * Under the control of ACTH (adrenocorticotropic hormone) * The adrenal cortex regulates the levels of glucose and minerals in the blood * Hormones: * Provide a long-term response to stress * Mineralcorticoids (aldosterone) (regulate salt and water balance --> increase blood volume and pressure) * Glucocorticoids (Cortisol) (Regulate carbohydrate, protein and fat metabolism --> increase blood glucose level) * Male and female sex hormone * Malfunction of the adrenal cortex can lead to Addison's disease and Cushing's syndrome. * Hypersecretion of adrenal cortex hormones can cause Cushing syndrome.

Glucocorticoids (cortisol)
One of the major hormones produced by the adrenal cortex * Raises the blood glucose level in at least 2 ways: * By breakdown of (muscle) protein to amino acids * Promotes fatty acid metabolism * Can lead to an increase in blood glucose levels * Medical treatment with cortisone * Counteracts the inflammatory response and can relieve swelling and pain * Can also make a person susceptible to injury and infection (affects

Endocrine regulation of blood pressure:
Mineralcorticoids (aldosterone)
One of the major hormones produced by the adrenal cortex * Released in response to low sodium level (low blood pressure) * Targets the kidney * Promotes renal absorption of sodium and water --> increase blood volume and blood pressure. * Decreasing urine volume (but increase in urine concentration) * Promotes renal excretion of potassium * ACTH is not the primary controller for aldosterone secretion * Renin-Angiotensin mechanism (RAS) stimulates aldosterone secretion when the blood sodium level and blood pressure are low * Renin-Angiotensin-Aldosterone system (RAAS) raises blood pressure in two ways: * Angiotensin II constricts arterioles * Aldosterone causes the kidneys to reabsorb sodium * Atrial natriuretic hormone (ANH) is antagonistic to aldosterone * ANH is produced by the right atrium of the heart * Released in response to high blood volume (high level of sodium) * Affects the kidney to excrete sodium and water to decrease blood pressure. * Increases urine volume (decreases concentration)

Renin-angiotensin-aldosterone mechanism (RAAS) * Juxtaglomerular apparatus secrets renin, when blood pressure is low. * Renin changes angiotensinogen into angiotensin I * Angiotensin I is converted to angiotensin II by an enzyme found in the lungs. * Angiotensin II constricts blood vessels --> increase blood pressure * Angiotensin II also stimulates these release of aldosterone * Aldosterone causes the tubules of the kidneys to increase the reabsorption of sodium and water into the blood. This increases the volume of fluid in the body, which also increases blood pressure.

Pancreas
Endocrine regulation of Carbohydrate Metabolism metabolism: * Composed of two types of tissue * Exocrine – secretes digestive juices * Endocrine tissue (pancreatic islets or islets of Langerhans) produces: * Insulin (storage hormone) * Secreted when blood glucose level is high * Stimulates the uptake of glucose by most body cells (Liver, muscle, adipose tissue - where it is stored as glycogen ) - Brain and red blood cells do not require insulin for glucose transport. * Lowers blood glucose * Glucagon (releasing hormone – catabolic) * Target tissues are, adipose tissue and liver * Secreted when blood glucose levels are low * Stimulates liver to break down glycogen to glucose (in the liver), stimulates lipolysis in adipose tissue (break down of fat to glycerol and fatty acids that the liver can use for glucose formation) * Stimulates gluconeogenesis * Raises blood glucose level * Glucagon and inulin is antagonistic hormones to each other.

Function of insulin:
Insulin is secreted from the pancreas and is released into the bloodstream. It binds to insulin receptors on the cell membrane and regulates the GLUT4 receptors, which are responsible for moving glucose inside the tissue.

* Diabetes Mellitus * Insulin-sensitive body cells are unable to take up and/or metabolize glucose * Blood glucose level is elevated (hyperglycemia) * Symptoms: * Polyphagia – extreme hunger * Glycosuria – glucose in the urine * Polyuria – excessive water loss through urine * Polydipsia – extreme thirst

Two forms of diabetes mellitus * Type I – insulin-dependent diabetes mellitus * Pancreas does not produce insulin * Immune cells destroy the pancreatic islets * Type II – non-insulin-dependent diabetes mellitus * Normal or elevated amounts of insulin are present in the blood * Receptors on the cells do not respond to insulin

Other Endocrine Glands

Testes and ovaries * Testes produce androgens (testosterone) - growth causing hormone. * Ovaries produce estrogen and progesterone * Secretion is controlled by the hypothalamus and the pituitary
Androgens (testoterone) * Increased testosterone secretion during puberty stimulates the growth of the penis and the testes * Testeterone is controlled from the anterior pituitary gland. * Brings about and maintains the male secondary sex characteristics * Facial, axillary, and pubic hair * Enlargement of larynx and the vocal cords * Muscular strength through increased protein synthesis * Can cause baldness (if genetically predisposed) * Stimulates oil and sweat glands of the skin

Estrogen and Progesterone * Progesterone and estrogen: * Required for breast development * Regulation of the uterine cycle (menstrual periods) * Estrogens: * Stimulate the growth of the uterus and the vagina during puberty * Necessary for egg (ovum) maturation * Responsible for secondary sex characteristics * Female body hair * Fat distribution

Thymus Gland * Most active during childhood * Important part of the immune system * Thymosin * Thymosins are necessary for the activation of T-lymphocytes in the thymus gland. * Transforms lymphocytes into thymus-derived lymphocytes (T-lymphocytes) * Epithelial cells secrete hormones called thymosins

Pineal Gland (circadian clock) - biological watch * Located in the brain * Produces the hormone melatonin * Regulation of sexual development. * Melatonin is involved in daily sleep-wake cycle. * Darkness stimulates the secretion of melatonin, which improves sleep. * Light/brightness decreases the secretion of melatonin and causes you to wake up.

Hormones from Other Tissues * Leptin (signal hormone) * Produced in the adipose tissue * Signals satiety in hypothalamus * The higher the concentration of leptin in the blood the more signals are send to the hypothalamus indicating fullness. * The more adipose tissue the more leptin. --> Obese people have a high level of leptin in the blood. * We can be insensitive to the signals of leptin --> causes the brain to think that we starve. --> causes people to continue eating, and further increase body mass. * Ghrelin: * Signals hunger * Produced by the stomach * Antagonist to Leptin.

* Growth Factors – stimulate cell division and mitosis * Granulocyte and macrophage colony-stimulating factor * Platelet-derived growth factor * Epidermal growth factor and nerve growth factor * Tumor angiogenesis factor

Prostaglandins * Hormone like compounds * Potent chemical signals - produced within cells from arachidonate (a fatty acid) * Act locally (not distributed in the blood) * Some effects of prostaglandins include: * Uterine contractions * Mediate the effects of pyrogens (chemicals that reset the temperature regulatory center in the brain) * Reduce gastric secretion * Lower blood pressure * Inhibit platelet aggregation

The Importance of Chemical Signals

Chemical signals are molecules that affect the behavior of those cells that have receptor proteins to receive them. * Cells and organs communicate with one another using chemical signals * Chemical signals between individuals * Called pheromones * Humans produce airborne chemicals from a variety of areas

Homeostasis

The endocrine system and the nervous system work together to maintain homeostasis * The endocrine system helps regulate the following: * Digestion * Fuel metabolism * Blood pressure and volume * Calcium balance * Response to the external environment