Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 31Endocrine Control

(Sections 31.6 - 31.11)

31.6 Thyroid and Parathyroid Glands

• The thyroid gland regulates metabolic rate, and the adjacent parathyroids regulate calcium levels

• thyroid gland • Endocrine gland at the base of the neck• Produces thyroid hormone, which increases metabolism

• parathyroid glands • Four small endocrine glands whose hormone product

increases the level of calcium in blood

Thyroid Function

• The thyroid secretes thyroid hormone (two iodine-containing molecules: triiodothyronine and thyroxine) which increases metabolic activity of tissues throughout the body

• The thyroid gland also secretes calcitonin, a hormone that causes deposition of calcium in bones of growing children

• The anterior pituitary gland and hypothalamus regulate thyroid hormone secretion by a negative feedback loop

Feedback Control of Thyroid Function

1. A low level of thyroid hormone causes the hypothalamus to secrete thyroid-releasing hormone (TRH)

2. TRH causes the anterior pituitary to secrete thyroid-stimulating hormone (TSH)

3. TSH stimulates the secretion of thyroid hormone

4. When the blood level of thyroid hormone rises, secretion of TRH and TSH declines

Negative Feedback Loop

Fig. 31.7, p. 511Thyroid hormone is secreted.

Thyroid Gland

TSHRise of thyroid hormone level in blood inhibits the secretion of TRH and TSH.

Anterior Pituitary

TRH

RESPONSE

HypothalamusBlood level of thyroid hormone falls below a set point.

STIMULUS

1

2

3

4

Negative Feedback Loop

TRH

STIMULUS + HypothalamusBlood level of

thyroid hormone falls below a set point.

Anterior Pituitary

Thyroid hormone is secreted.Stepped Art

Thyroid Gland

TSH

RESPONSE

Rise of thyroid hormone level in blood inhibits the secretion of TRH and TSH.

Fig. 31.7, p. 511Negative Feedback Loop

Hypothyroidism

• A diet deficient in iodine can cause thyroid hormone deficiency (hypothyroidism)

• Hypothyroidism can also arise when the body’s immune system mistakenly attacks the thyroid

• In either case, ongoing stimulation of the thyroid can lead to thyroid enlargement, or goiter

Goiter

• Enlarged thyroid (goiter) caused by a dietary iodine deficiency

Parathyroid Glands and Calcium Levels

• Four parathyroid glands located at the rear of the thyroid gland are the main regulators of calcium levels in the blood

• When blood calcium level declines, the glands release parathyroid hormone (PTH), which increases breakdown of bone – blood calcium level rises

• PTH also encourages calcium reabsorption by kidneys and activation of vitamin D, which helps the intestine take up calcium from food

Rickets

• Rickets caused by a lack of vitamin D

• Parathyroid hormone softens the bones, causing bowed legs

ANIMATION: Parathyroid hormone action

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31.7 The Adrenal Glands

• There are two adrenal glands, one above each kidney

• An adrenal gland has two functional zones, controlled by different mechanisms: • The outer cortex secretes steroid hormones• The inner medulla releases molecules that function as

neurotransmitters

Key Terms

• adrenal gland • Endocrine gland that is located atop the kidney

• adrenal cortex • Outer portion of adrenal gland• Secretes aldosterone and cortisol

• adrenal medulla • Inner portion of adrenal gland• Secretes epinephrine and norepinephrine

The Adrenal Cortex

• The adrenal cortex secretes two steroid hormones:

• Aldosterone controls sodium and water reabsorption in the kidneys

• Cortisol helps maintain blood glucose available to the brain by inducing the liver to break down glycogen, adipose cells to degrade fats, and skeletal muscles to degrade proteins

Negative Feedback and Cortisol

• Cortisol secretion is governed by a negative feedback loop to the anterior pituitary gland and hypothalamus

• In times of stress, the central nervous system overrides the feedback controls so that cortisol levels rise

• Over the long term, excess cortisol has negative impacts on health

Negative Feedback Mechanism

1. A decrease in cortisol triggers secretion of CRH (corticotropin-releasing hormone) by the hypothalamus

2. CRH stimulates secretion of ACTH by the anterior pituitary

3. ACTH causes release of cortisol from the adrenal cortex

4. Cortisol level increases, causing hypothalamus and anterior pituitary to secrete less CRH and ACTH, and cortisol secretion slows

Negative Feedback Mechanism

Fig. 31.9, p. 512kidney

Cellular uptake of glucose from blood slows in many tissues, especially muscles (but not in the brain). Protein breakdown accelerates, especially in muscles.

Some of the amino acids freed by this process get converted to glucose.

Fats in adipose tissue are degraded to fatty acids and enter blood as an alternative energy source, indirectly adrenal cortex adrenal medulla kidney conserving glucose for the brain.

Cortisol secretion increases and has the following effects:

Adrenal Cortex

Rise of cortisol level in the blood inhibits the secretion of CRH and ACTH.

ACTH

Anterior Pituitary

CRH

Hypothalamus

adrenal cortex

Blood level of cortisol declines.

RESPONSESTIMULUS

adrenal medulla

1

2

3

4

Negative Feedback Mechanism

STIMULUS +

A Blood level of cortisol falls below a set point.

Hypothalamus

Stepped Art

Protein breakdown accelerates, especially in muscles. Some of the amino acids freed by this process get converted to glucose.Fats in adipose tissue are degraded to fatty acids and enter blood as an alternative energy source, indirectly conserving glucose for the brain.

C Cortisol is secreted and has the following effects:

Cellular uptake of glucose from blood slows in many tissues, especially muscles (but not in the brain).

RESPONSE

D Hypothalamus and pituitary detect rise in blood level of cortisol and slow its secretion.

B CRH

Anterior Pituitaryadrenal cortexadrenal medulla

Adrenal Cortex

ACTH

Fig. 31.9, p. 512

Negative Feedback

Mechanism

The Adrenal Medulla

• The adrenal medulla contains specialized neurons of the sympathetic division that release norepinephrine and epinephrine that enter the blood and function as hormones

• Like sympathetic stimulation, norepinephrine and epinephrine cause a fight-flight response: they dilate the pupils, increase breathing, and make the heart beat faster

Stress, Elevated Cortisol, and Health

• When an animal is frightened or under physical stress, commands from the nervous system trigger increased secretion of cortisol, epinephrine, and norepinephrine

• Physiological responses to chronic stress interfere with growth, the immune system, sexual function, and cardiovascular function

• Chronically high cortisol levels harm cells in the hippocampus, a brain region central to memory and learning

Elevated Cortisol: Cushing Syndrome• Before and after removal of a adrenal gland tumor

Hypocortisolism: Addison’s Disease

• Tuberculosis and other infectious diseases can damage adrenal glands, resulting in adrenal insufficiency

• President John F. Kennedy had an autoimmune form of Addison’s Disease

31.8 Pancreatic Hormones

• The pancreas has both exocrine and endocrine functions:

• Exocrine cells secrete digestive enzymes into a duct to the small intestine

• Endocrine cells are grouped in pancreatic islets; each islet contains three types of hormone-secreting cells

• pancreas • Organ that secretes digestive enzymes into the small

intestine and hormones into the blood

The Pancreas and Digestion

• Delta cells in pancreatic islets secrete somatostatin

• Somatostatin helps control digestion and nutrient absorption

Fig. 31.12a, p. 514

The Pancreas and Digestion

Fig. 31.12a, p. 514

smallintestine

stomach

pancreas

The Pancreas and Digestion

The Pancreas and Blood Sugar

• Two pancreatic hormones with opposing effects work together to regulate the level of sugar in the blood

• Insulin (secreted by beta cells) stimulates glucose uptake by muscle and liver cells and thus lowers the blood glucose

• Glucagon (secreted by alpha cells) stimulates the release of glucose, which increases blood levels

Regulating High Blood Sugar

1.Blood glucose rises2.Glucagon is blocked3. Insulin is secreted4.Glucose is taken up5.Blood glucose level

decreases

Fig. 31.12b, p. 514

Decrease in blood glucose

FAT CELLSMUSCLELIVER

insulin

PANCREAS

Increase in blood glucose

Body cells, especially in muscle and adipose tissue, take up and use more glucose. Cells in skeletal muscle and liver store glucose in the form of glycogen.

Response

Stimulus

glucagon

1

2 3

4

5

Regulating High Blood Sugar

Regulating Low Blood Sugar

6.Low blood glucose7.Glucagon is secreted8. Insulin is blocked9.Liver breaks down

glycogen into glucose 10.Blood glucose

increases

Fig. 31.12c, p. 514Increase in blood glucose

Cells in liver break down glycogen faster. The released glucose monomers enter blood.

Response

Stimulus

insulinglucagon

Decrease in blood glucose6

7 8

9

10

Regulating Low Blood Sugar

ANIMATION: Hormones and glucose metabolism

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31.9 Diabetes

• Diabetes mellitus is a disorder in which the body does not make or does not respond to insulin

• When cells do not take up and store glucose as they should, high blood sugar (hyperglycemia) disrupts normal metabolism

• Cells have to use proteins and fats for energy – breakdown of these substances yields harmful waste products

Some Complications of Diabetes

Type 1 Diabetes

• Type 1diabetes develops after white blood cells wrongly identify insulin-secreting beta cells as foreign (nonself) and destroy them (autoimmune response)

• All affected individuals require injections of insulin, and must monitor their blood sugar level carefully

• When fats and proteins are used as energy sources, ketones accumulate in the blood and urine (ketosis)

Insulin Pump

• An insulin pump helps smooth out fluctuations in blood sugar, lowering risk of complications

Type 2 Diabetes

• In type 2 diabetes insulin levels are normal or even high

• However, target cells do not respond to the hormone as they should, and blood sugar levels remain elevated

• Western diets and sedentary life-styles are contributing factors in type 2 diabetes – diet, exercise, and oral medications can control most cases

31.10 Gonads, Pineal Gland, and Thymus• Outputs from gonads, pineal gland, and thymus all change as

an individual enters puberty

• gonads • Primary reproductive organs (ovaries or testes) that

produce gametes and sex hormones

• pineal gland • Endocrine gland deep inside the brain that secretes

melatonin when the retina is not stimulated by light

• thymus • Endocrine gland beneath the breastbone; secretes

hormones that encourage maturation of T lymphocytes

The Gonads

• Gonads are primary reproductive organs, which produce gametes (eggs or sperm)

• Gonads produce steroid sex hormones with roles in reproduction and development of secondary sexual traits• Male gonads (testes) secrete mainly testosterone• Female gonads (ovaries) secrete mainly estrogens and

progesterone

Location of Human Gonads

Fig. 31.14a, p. 516

testis (where sperm originate)

Location of Human Gonads

Fig. 31.14b, p. 516

ovary (where eggs develop)

Location of Human Gonads

Control of Sex Hormone Secretion

• The hypothalamus and anterior pituitary control secretion of sex hormones

The Pineal Gland

• The pineal gland lies deep within the vertebrate brain

• Melatonin secreted by the pineal gland affects the daily sleep/wake cycle and the onset of puberty

• Melatonin also protects against some cancers

• Melatonin secretion declines when the retina detects light and sends action potentials along the optic nerve to the brain

The Thymus

• The thymus lies beneath the breastbone

• It secretes thymosins that encourage maturation of infection-fighting white blood cells (T lymphocytes, or T cells)

• At puberty, the surge of sex hormones causes the thymus to shrink, and its secretions decline – this can be a problem for people with HIV infection, because AIDS kills T cells

Key Concepts

• Other Hormone Sources • Endocrine glands throughout the body respond to signals

from the hypothalamus and the pituitary• Others secrete hormones in response to internal changes

such as a shift in blood glucose level• Poor diet, immune problems, and genetic factors can

cause hormone disorders

31.11 Invertebrate Hormones

• Vertebrate hormone receptor proteins often resemble similar receptor proteins in invertebrates and probably evolved from them

• Invertebrates also have hormones with no vertebrate counterpart, such as the steroid hormone ecdysone, which regulates molting in arthropods such as crabs and insects

Evolution of Receptor Diversity

• Genetic analysis has revealed the invertebrate ancestry of some vertebrate hormone receptors

• Sea anemones have receptors that are structurally similar to vertebrate receptors for TSH, LH, FSH, and other signaling molecules

• Genes that encode these receptors have similar nucleotide sequences in vertebrates and invertebrates, and have the same number and type of introns in similar regions

Control of Molting

• In arthropods, hormones regulate the periodic molting of the cuticle that allows the animal to grow

• Details vary, but in all cases, molting is largely controlled by ecdysone, a steroid hormone unique to invertebrates

• A molting gland produces and stores ecdysone and releases it at molting time

Molting in Crustaceans

Fig. 31.16, p. 517

No molting

Y organ makes and releases

ecdysone

MIH prevents Y organ from

making ecdysone

Signals from brain inhibit

release of MIH

X organ releases molt-inhibiting hormone (MIH)

Presence of suitable stimuli

Absence of suitable stimuli

Molting

Molting in Crustaceans

Fig. 31.16c, p. 517

Molting in Crustaceans

Key Concepts

• Invertebrate Hormones • Hormones control molting and other events in invertebrate

life cycles• Vertebrate hormones and receptors for them first evolved

in ancestral lineages of invertebrates

Hormones in the Balance (revisited)

• The photo shows breast development in a girl less than two years old

• Exposure to high levels of synthetic chemicals (phthalates) may cause such premature breast enlargement