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Chapter 34Coordination by Hormone

Signaling

The Endocrine System Utilizes Chemical Signals

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34.1 The endocrine and nervous systems work together

Nervous system sends nerve impulses via nerve fiber directly to a target organ

Endocrine system uses blood vessels to send hormones, chemical messengers, to target organs Functions more slowly than nervous system

Negative Feedback Mechanisms Systems use negative feedback mechanisms Example: Sensory receptors signal a control center in

the brain, which then sends nerve impulses to the arteriole walls so that they constrict, and blood pressure rises Now the sensory receptors are no longer stimulated, and no

nerve impulses are generated34-3

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Figure 34.1 Modes of action of the nervous and endocrine systems

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34.2 Hormones affect cellular metabolism

Peptide hormone - includes peptides, proteins, glycoproteins, or modified amino acids Action of Peptide Hormones

Hormone epinephrine binds to a receptor in the plasma membrane

Formation of cyclic adenosine monophosphate (cAMP), a molecule that contains one phosphate group attached to adenosine

cAMP activates an enzyme cascade, a series of enzymes that amplify the effect of the molecule

Glycogen is broken down to glucose, which enters the bloodstream

Because a peptide hormone never enters cell, it is called first messenger

cAMP, which sets the metabolic machinery in motion, is second messenger

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Figure 34.2A Action of peptide hormones

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Action of Peptide Hormones

Steroid hormone - always same complex of four carbon rings, but each one has different side groups Do not bind to plasma membrane receptors, but are

able to enter the cell because they are lipids Once inside, steroid hormones bind to receptors,

usually in the nucleus Inside the nucleus, the hormone-receptor complex

binds with DNA and activates transcription of certain portions of DNA

Translation of messenger RNA (mRNA) transcripts at ribosomes results in enzymes and other proteins that can carry out a response to the hormonal signal

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Figure 34.2B Action of steroid hormones

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34.3 The vertebrate endocrine system includes diverse hormones

Locations of the major endocrine glands Hypothalamus, a part of the brain, close proximity to pituitary Pineal gland in the brain Thyroid and parathyroids are in the neck Thymus gland lies just beneath the breastbone in the thoracic

cavity Adrenal glands and pancreas are in the abdominal cavity Gonads include ovaries, located in the pelvic cavity, and testes,

located outside this cavity in the scrotum Secretion of hormones is controlled by negative

feedback, in conjunction with antagonistic hormonal actions Example: An antagonistic hormone called calcitonin, produced

by the thyroid gland, lowers the Ca2+ below levels set by the

hormone PTH 34-10

Figure 34.3 The human endocrine system

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34-12

The Hypothalamus and Pituitary Are Central to the Endocrine System

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34.4 The hypothalamus is a part of the nervous and endocrine systems

Hypothalamus controls secretions of posterior pituitary and anterior pituitary Stalk-like structure connects the hypothalamus to the pituitary

Hypothalamus and Posterior Pituitary Neurosecretory cells produce hormones antidiuretic hormone

(ADH) and oxytocin When ADH reaches the kidneys it causes water to be reabsorbed

Oxytocin causes uterine contractions during childbirth and milk letdown when a baby is nursing

Controlled by positive feedback—the stimulus continues to bring about an effect that ever increases in intensity

Hypothalamus and Anterior Pituitary Hypothalamus controls anterior pituitary by producing

hypothalamic releasing hormones Example: One hypothalamic-releasing hormones stimulates the

anterior pituitary to secrete a thyroid-stimulating hormone, and a hypothalamic-inhibiting hormone prevents the anterior pituitary from secreting prolactin 34-14

Figure 34.4 The hypothalamus and pituitary gland

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34.5 The anterior pituitary produces nontropic and tropic hormones

Nontropic Hormones: Some hormones produced by the anterior pituitary are controlled by the hypothalamus Prolactin (PRL) causes the mammary glands in the breasts to

develop and produce milk after child birth Growth hormone (GH) promotes skeletal and muscular growth Melanocyte-stimulating hormone (MSH) causes skin-color

changes in vertebrates having melanophores Tropic Hormones: Four hormones produced by the anterior

pituitary are called tropic hormones because they stimulate the activity of other endocrine glands Thyroid-stimulating hormone (TSH) stimulates the thyroid to

produce thyroxine and triiodothyronine Adrenocorticotropic hormone (ACTH) stimulates the adrenal

cortex to produce glucocorticoid Gonadotropic hormones (follicle-stimulating hormone, FSH,

and luteinizing hormone, LH) stimulate the gonads—the testes in males and the ovaries in females—to produce gametes and sex hormones 34-16

Figure 34.5 The hypothalamus-anterior pituitary-target gland control system

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Hormones Regulate Metabolismand Homeostasis

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34.6 The adrenal glands respond to stress

Two adrenal glands sit atop the kidneys Inner portion called the adrenal medulla and an outer portion called

the adrenal cortex Adrenal Medulla

The hypothalamus initiates nerve impulses that travel to the adrenal medulla, which then secretes its hormones

Epinephrine (adrenaline) and norepinephrine (noradrenaline) bring about all the body changes that occur when an individual reacts to an emergency situation

Adrenal Cortex Secretes small amounts of male and female sex hormones Produces the glucocorticoid and the mineralocorticoid hormones

that provide a long-term response to stress Cortisol - raises the blood glucose level by two means

It promotes the breakdown of muscle proteins to amino acids Promotes metabolism of fatty acids rather than carbohydrates

Aldosterone - most important of the mineralocorticoids, targets the kidney, where it promotes renal absorption of sodium (Na+) and excretion of potassium (K+)

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Figure 34.6 Stress responses of the adrenal gland

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

34.7 Glucocorticoid therapy can lead to Cushing syndrome

When the level of adrenal cortex hormones is high due to hypersecretion, a person develops Cushing syndrome Excess cortisol results in a tendency toward diabetes mellitus as

muscle protein is metabolized and subcutaneous fat is deposited in the midsection

Excess aldosterone and reabsorption of sodium and water by the kidneys lead to a basic blood pH and hypertension

Some people develop Cushing syndrome due to a tumor in the adrenal cortex or anterior pituitary Normally, cortisol secretion normally fluctuates over the course of

the day, but it stays high in Cushing patients Others develop Cushing syndrome because they are

taking glucocorticoid drugs, which are often prescribed for medical conditions such as asthma, rheumatoid arthritis, and lupus

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Figure 34.7 Patient with Cushing syndrome (left) before and (right) after treatment

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34.8 The pancreas regulates the blood sugar level

Pancreas - slender, pale-colored organ between the kidneys and near the duodenum of the small intestine Exocrine tissue produces and secretes digestive juices that

travel to small intestine Endocrine tissue (pancreatic islets) secretes insulin and

glucagon directly into the blood Insulin is secreted when the blood glucose level is high,

which usually occurs just after eating Stimulates uptake of glucose by cells

Glucagon is secreted from the pancreas, usually between meals, when the blood glucose level is low Stimulates the liver to break down glycogen to glucose and to

use fat and protein in preference to glucose as energy sources

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Figure 34.8 Regulation of blood glucose level

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

34.9 Diabetes is becoming a very common ailment

21 million people have now been diagnosed with diabetes in the U.S. Investigate whether a close relative has diabetes

Symptoms Frequent urination, especially at night Unusual hunger and/or thirst Unexplained change in weight Blurred vision Sores that do not heal Excessive fatigue

Diabetes Type 1 and Type 2 Diabetes Type 1: Pancreas fails to secrete insulin because the cells that

make insulin have died off Usually occurs after a viral infection

Diabetes Type 2: Cells are resistant to insulin Caused by poor health habits associated with physical inactivity and excessive

food intake leading to obesity Treatment - People with diabetes must regularly check their blood

sugar level, usually before meals and at bedtime Medication is available for diabetes type 2, but a healthy diet and exercise can also help

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Figure 34.9 Testing the blood sugar level

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34.10 The pineal gland is involved in biorhythms

Pineal gland, which is located in the brain produces the hormone melatonin, primarily at night Melatonin is involved in our daily sleep-wake cycle

Normally we grow sleepy at night when melatonin levels increase and awaken once daylight returns and melatonin levels are low

Because light suppresses melatonin secretion, the duration of secretion is longer in the winter than in the summer

Daily 24-hour cycles such as this are called circadian rhythms Circadian rhythms are controlled by an internal timing

mechanism called a biological clock Research indicates that melatonin also regulates sexual

development Researchers have noted that children whose pineal gland has

been destroyed due to a brain tumor experience early puberty34-27

Figure 34.10 Melatonin production

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34.11 The thyroid regulates development and increases

the metabolic rate Thyroid gland - located in the neck and consists of two

distinct lobes connected by a slender isthmus Each lobe has a large number of follicles filled with the thyroid

hormones triiodothyronine (T3) and thyroxine (T4), which increase metabolic rate

Hypothyroidism Low levels of T3 and T4 results in hypothyroidism and slowing of

metabolism Children with congenital hypothyroidism (cretinism) are short and stocky In adults it can result due to lack of iodine in the diet resulting in a

simple goiter Myxedema, a type of hypothyroidism that results in skin puffiness,

lethargy and weight gain Hyperthyroidism - Graves disease

Thyroid is overactive, and exophthalmic goiter forms Eyes protrude (exophthalmos) because of edema in the eye socket Patient becomes hyperactive, nervous, and irritable, with insomnia 34-29

Figure 34.11A Congenital Figure 34.11B Simple goiter

hypothyroidism 34-30

Figure 34.11C Exophthalmic goiter

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34.12 The thyroid and the parathyroids regulate the blood calcium level

Blood calcium level is regulated in part by calcitonin, a hormone secreted by the thyroid gland Primary effect of calcitonin is to bring about the deposit of

calcium in the bones when the blood calcium level rises Thereafter, the blood calcium level falls

Parathyroid glands are embedded adjacent to the thyroid gland Parathyroid hormone causes the blood phosphate (HPO4

2+) level to decrease and the blood calcium level to increase

PTH promotes the release of calcium from the bones when the blood calcium level lowers

Also promotes the reabsorption of calcium by the kidneys, where it activates vitamin D

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Connecting the Concepts:Chapter 34

Nervous and endocrine systems are structurally and functionally related Hypothalamus, a portion of the brain, controls the pituitary, an endocrine gland

Even produces the hormones that are released by the posterior pituitary Also stimulates the release of hormones by the adrenal medulla

Neurosecretory cells in the hypothalamus produce chemical signals that control the activity of the anterior pituitary

Nervous system is well known for bringing about an immediate response to environmental stimuli Chemical signals released by the nervous system, called neurotransmitters,

help maintain homeostasis Heart rate, breathing rate, and blood pressure are all regulated to stay

relatively constant Hormones released by endocrine glands also help maintain

homeostasis by keeping the levels of calcium, sodium, glucose, and other blood constituents within normal limits Endocrine system is slower acting than the nervous system and often

regulates processes that occur over days or even months Hormones secreted into the bloodstream control whole-body processes, such

as growth and reproduction34-33