The Endocrine System

The Endocrine System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Endocrine Glands Endocrine glands are ductless Secrete hormones

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Chemical signals that influence:

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Metabolism Growth and development Homeostasis

Categories of hormones

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Peptides (proteins, glycoproteins, and modified amino acids) Steroid hormones

Endocrine Glands Hormone function

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Second messenger system

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Peptide hormone binds to a receptor protein on the plasma membrane Peptide hormone (“first messenger”) activates a “second messenger” (cyclic AMP and calcium) Second messenger sets in motion an enzyme cascade that leads to a cellular response

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Change in cellular behavior Formation of an end product that leaves the cell

Fig 10.2 Binding of a peptide hormone leads to cAMP activation of an enzyme cascade

Fig 10.2

Endocrine Glands Intracellular mechanism of hormone function

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Steroid hormones (lipids) diffuse across the plasma membrane Once inside the cell, steroid hormones bind to receptor proteins Hormone-receptor complex binds to DNA, activating particular genes Gene activation leads to production of cellular enzymes that cause cellular changes

Fig 10.3 A steroid hormone results in a hormone receptor complex that activates DNA and protein synthesis

Fig 10.3

Control of Hormonal Secretions primarily controlled by negative feedback mechanism

Negative Feedback

Endocrine Glands Hormones and Homeostasis

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Many hormones affect concentrations of certain substances in the blood Other hormones are involved in normal function of various organs Release of hormones controlled by one or more of the following:

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The nervous system The action of other hormones Negative feedback mechanisms

Hypothalamus and Pituitary Gland o

Hypothalamus •

Controls secretions of the pituitary gland Neurosecretory cells produce

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Antidiuretic hormone (ADH) Oxytocin

Pituitary Gland Control •Hypothalamic releasing hormones stimulate cells of anterior pituitary to release hormones •Nerve impulses from hypothalamus stimulate nerve endings in the posterior pituitary gland to release hormones

Hypothalamus and Pituitary Gland o

Posterior pituitary •

Stores hypothalamic hormones ADH and oxytocin ADH

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Released when neurons in the hypothalamus detect that the blood is too concentrated with salt Causes more water to be reabsorbed into kidney capillaries Raises blood pressure by vasoconstriction of blood vessels Diabetes insipidus results from the inability to produce ADH

Hypothalamus and Pituitary Gland •

Oxytocin 

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Causes uterine contraction during childbirth Causes milk letdown when a baby is nursing Release of oxytocin is controlled by positive feedback

Hypothalamus and Pituitary Gland o

Anterior pituitary • •

Controlled by hypothalamic hormones Hormones that affect other glands   

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Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Gonadotropic hormones

Effects of other hormones  

Prolactin (PRL) Growth hormone (GH)

Hypothalamic Hormones

Hormones of the Pituitary Gland

Hypothalamus and Pituitary Gland •

Effects of growth hormone  

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Affects the height of an individual Pituitary dwarfism results if too little GH is produced during childhood If too much GH is produced during childhood, a person can become a giant Acromegaly results when too much GH is secreted in adulthood

Thyroid and Parathyroid o

Thyroid Gland •

Two forms of thyroid hormone 

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Triiodothyronine (T3) contains 3 iodine atoms Thyroxine (T4) contains 4 iodine atoms

Effects of thyroid hormone  

Increase the metabolic rate Stimulate all cells of the body

Regulation of Thyroid Hormone Secretions ( )

() Fig 10.4

eg.TRH

() eg.TSH

eg.T3, T4

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Thyroid and Parathyroid • Simple goiter 

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Caused by a lack of iodine Thyroid enlarges in response to constant stimulation by the anterior pituitary

Fig 10.7

Thyroid and Parathyroid • Congenital hypothyroidism 

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Thyroid fails to develop properly Undersecretion of thyroid hormone Individuals are short and stocky

• Myxedema  

Hypothyroidism in adults Characterized by      

Lethargy Weight gain Loss of hair Slower pulse rate Lowered body temperature Thick and puffy skin

Fig 10.8

Thyroid and Parathyroid •

Hyperthyroidism (Grave’s Disease) Oversecretion of thyroid hormone Exophthalmic goiter forms

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Edema in eye socket tissues Swelling of the muscles that move the eyes

Symptoms include

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Hyperactivity Nervousness and irritability Insomnia

Thyroid and Parathyroid •

Calcitonin  

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Helps control blood calcium levels Secreted when the blood calcium levels rise Brings about the deposit of calcium in the bones

Thyroid Gland Hormones

Disorders of the Thyroid Gland

Thyroid and Parathyroid

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Parathyroid Glands • •

Posterior surface of the thyroid gland Produces parathyroid hormone (PTH) 

Causes blood phosphate (HPO42-) level to decrease

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Causes blood calcium (Ca2+) level to increase  

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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

Parathyroid Hormone Mechanism by which PTH promotes calcium absorption in the intestine

Disorders of the Parathyroid Glands

Adrenal Glands o

Adrenal Medulla • • • • •

Inner portion Under nervous control Secretes epinephrine (adrenaline) and norepinephrine (noradrenaline) Causes “fight or flight” responses Provide a short-term response to stress

Adrenal Glands o

Adrenal Cortex • • •

Outer portion Under the control of ACTH Hormones    

Provide a long-term response to stress Mineralcorticoids Glucocorticoids Male and female sex hormones

Fig 10.10 Adrenal glands. Adrenal medulla & Adrenal cortex under control of the hypothalamus and respond to stress

Left: Adrenal medulla provides a rapid but short-term stress response

Right: Adrenal cortex provides a slower but long-term stress response

Adrenal Glands •

Glucocorticoids (cortisol) Raises the blood glucose level in at least 2 ways:

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Promotes the breakdown of muscle proteins to amino acids which pass to the bloodstream. Liver then converts excess amino acids to glucose. Promotes the metabolism of fatty acids instead of carbohydrates as spares glucose for the brain

Counteracts the inflammatory response and can relieve swelling and pain e.g. arthritis and bursitis

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Can also make a person susceptible to injury and infection

Adrenal Glands •

Mineralcorticoids (aldosterone) Targets the kidney Promotes renal absorption of sodium and water Promotes renal excretion of potassium ACTH is not the primary controller for aldosterone secretion Renin-Angiotensin mechanism stimulates aldosterone secretion when the blood sodium level and blood pressure are low Renin-Angiotensin-Aldosterone system raises blood pressure in two ways:

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Angiotensin II constricts arterioles Aldosterone causes the kidneys to reabsorb sodium

Atrial natriuretic hormone (ANH) is antagonistic to aldosterone

Fig 10.11

Adrenal Glands •

Malfunction of the Adrenal Cortex 

Addison Disease  Hyposecretion of adrenal cortex hormones  Excessive (but ineffective) ACTH causes bronzing of the skin  Because glucose cannot be replenished without cortisol, individuals are susceptible to infection  Lack of aldosterone results in the development of low blood pressure and possibly severe dehydration

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Cushing Syndrome  Hypersecretion of adrenal cortex hormones  Tendency towards diabetes mellitus  Excess aldosterone leads to hypertension

Pancreas o

Composed of two types of tissue • •

Exocrine – secretes digestive juices Endocrine tissue (pancreatic islets or islets of Langerhans) produces insulin and glucagon.

Pancreas Insulin 

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Secreted by beta-cells pancreatic islets or islets of Langerhans Secreted when blood glucose level is high Stimulates the uptake of glucose by most body cells sensitive body cells are unable to take up and/or metabolize glucose

Pancreas Glucagon    

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Also secreted by the pancreas Secreted when blood glucose levels are low Targets liver and adipose tissue Stimulates liver to break down glycogen to glucose when blood glucose levels decrease, it stimulates the breakdown of glycogen in the liver which raises the glucose level in the blood

Diabetes milletus

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Characterize by elevated 2 hour serum glucose levels after intake of 75g glucose and does NOT resume below 200mg/dL within 2 hours. sometimes greatly elevated from 1 to 5 hours. Symptoms:

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Polyphagia – extreme hunger Glycosuria – glucose in the urine Polyuria – excessive water loss through urine Polydipsia – extreme thirst

Pancreas •

Blood glucose level is elevated (hyperglycemia) 

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without insulin, diabetic ketoacidosis often develops which may result in coma or death Ketoacidosis- in the starvation/absence of glucose, the body is forced to break down fat for sustenance due to their lack of outside nutrition the body fails to adequately regulate ketone production causing a severe accumulation of keto acids. pH of the blood is substantially decreased. Ketouria - ketone bodies present in the urine, produced as a normal response to a shortage of glucose, as alternate source of fuel from fatty acid

Fig 10.14

Pancreas Two forms of diabetes mellitus

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Type I – insulin-dependent diabetes mellitus (IDDM)

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Also known as childhood-onset diabetes, juvenile diabetes Pancreas does not produce insulin Immune cells destroy the pancreatic islets characterized by loss of the insulin-producing beta cells of the islets of Langerhans in the pancreas deficiency of insulin cause of this beta cell loss is a T-cell mediated autoimmune attack. Type 1 diabetes can affect children or adults but was traditionally termed "juvenile diabetes" because it represents a majority of the diabetes cases in children

Pancreas •

Type II – non-insulin-dependent diabetes mellitus (NIDDM) 

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Also known as adult-onset diabetes, obesityrelated diabetes). Characterized differently due to insulin resistance or reduced insulin sensitivity, combined with reduced insulin secretion. Involves the insulin receptor in cell membranes Receptors on the cells do not respond to insulin Normal or elevated amounts of insulin are present in the blood

Pancreas o o o o o

Treatment of Diabetes Mellitus Mainly lifestyle adjustments : diet – careful monitoring dietary intake of sugar & carbohydrates regular exercise but not strenuous exercises deliver insulin by a pump or subcutaneous injections (allows continuous infusion of insulin 24 hours a day at preset levels) BUT cannot reverse the progress of the disease.

Other Endocrine Glands o

Testes and ovaries • • •

Testes produce androgens (testosterone) Ovaries produce estrogens and progesterone Secretion is controlled by the hypothalamus and the pituitary

Other Endocrine Glands •

Androgens Increased testosterone secretion during puberty stimulates the growth of the penis and the testes Brings about and maintains the male secondary sex characteristics

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Facial, axillary, and pubic hair Enlargement of larynx and the vocal cords Muscular strength

Stimulates oil and sweat glands of the skin

Other Endocrine Glands •

Estrogen and Progesterone Required for breast development Regulation of the uterine cycle Estrogens

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Stimulate the growth of the uterus and the vagina during puberty Necessary for egg maturation Responsible for secondary sex characteristics Female body hair Fat distribution

Other Endocrine Glands o

Thymus Gland • • •

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Most active during childhood Transforms lymphocytes into thymusderived lymphocytes (T-lymphocytes) Epithelial cells secrete hormones called thymosins

Pineal Gland • • •

Located in the brain Produces the hormone melatonin Melatonin is involved in daily sleepwake cycle

Other Endocrine Glands o

Hormones from Other Tissues •

Leptin  

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Produced by adipose tissue Signals satiety in hypothalamus

Growth Factors – stimulate cell division and mitosis 

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Granulocyte and macrophage colonystimulating factor Platelet-derived growth factor Epidermal growth factor and nerve growth factor Tumor angiogenesis factor

Other Endocrine Glands •

Prostaglandins Potent chemical signals Act locally Some effects of prostaglandins include:

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Uterine contractions Mediate the effects of pyrogens Reduce gastric secretion Lower blood pressure Inhibit platelet aggregation

The Importance of Chemical Signals o

Cells and organs communicate with one another using chemical signals Chemical signals between individuals

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Called pheromones Humans produce airborne chemicals from a variety of areas

Homeostasis o

The endocrine system and the nervous system work together to maintain homeostasis The endocrine system helps regulate the following:

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Digestion Fuel metabolism Blood pressure and volume Calcium balance Response to the external environment