24.1 Hormones

GEORGE B. JOHNSON

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PowerPoint® Lectures prepared by Johnny El-Rady

24 Chemical SignalingWithin theAnimal Body

Essentials of

The Living World

First Edition


24.1 Hormones

A hormone is a chemical signal that is produced in one part of the body and that acts at a distant site

Three advantages to using chemical signals

1. Can spread to all tissues via the blood

2. Can persist much longer than electric signals

3. Many can act as hormones

Different hormones can target different tissues


Hormones are produced by endocrine glands

These glands do not have ducts

Hormones are secreted directly into the blood

Hormones are under the control of the neuroendocrine system

The hypothalamus is the main switchboard

Issues commands to the nearby pituitary gland

Pituitary sends chemical signals to the various hormone-producing glands in body


Fig. 24.1 Major glands of the human endocrine system


Hormones are effective because they are recognized by specific receptors

These are on or in target cells

The path of communication of a hormone signal

1. Issuing the commandBy the hypothalamus

2. Transporting the signalBy the blood

3. Hitting the targetBinding to the receptor

4. Having an effectA change in cell activity


Fig. 24.2 How hormonal communication works


24.2 How Hormones Target Cells

Steroid hormones

Manufactured from cholesterol

Lipid-soluble and thus can across the lipid bilayer of the cell membrane

Bind to receptors within the nucleus usually

The hormone-receptor complex binds to DNA causing changes in gene activity

Anabolic steroids

Synthetic compounds resembling testosterone

Cause muscle cells to produce more protein


Fig. 24.3 How steroid hormones work


Peptide hormones

Short peptides or full-sized proteins

Bind to receptors embedded in the cell membrane

Induce a change in cytoplasmic end of the receptor protein

Triggers events within the cytoplasm

Fig. 24.4


The change in cellular activity is facilitated by second messengers such as cAMP

Trigger a cascade of enzymic activation within cell

Fig. 24.5


24.3 The Hypothalamus and the Pituitary

The pituitary gland is located in a bony recess in the brain below the hypothalamus

It produces nine major hormones

It is actually two glands

Posterior lobe

Anterior lobe


Stores and releases two short peptide hormones

Vasopressin (antidiuretic hormone, ADH)Regulates water conservation by the kidney

OxytocinInitiates uterine contractions during childbirthCauses milk letdown

Both synthesized within hypothalamus neuronsTransported down nerve axons to the posterior lobe

The Posterior Pituitary


Fig. 24.6 Posterior pituitary


A complete gland that produces seven major hormones

1. Thyroid-stimulating hormone (TSH)2. Luteinizing hormone (LH)3. Follicle-stimulating hormone (FSH)4. Adrenocorticotropic hormone (ACTH)5. Growth hormone (GH)6. Prolactin 7. Melanocyte-stimulating hormone (MSH)

The Anterior Pituitary


Fig. 24.7 The role of the pituitary


The hypothalamus controls the anterior pituitary

Fig. 24.8Secretes releasing and inhibiting hormones into capillaries

Hypothalamo-hypophyseal portal system carries the hormones to the pituitary


Fig. 24.9

Hypothalamus and anterior pituitary are themselves controlled by negative feedback inhibition

Regulated by the hormones whose secretion they stimulate!

Thyrotropin-releasing hormone

Corticotropin-releasing hormone

Gonadotropin-releasing hormone


24.4 The Pancreas

The pancreas gland is located behind the stomachSecretes a number of digestive enzymes into the small intestine through a narrow tube

Islets of Langerhans govern blood glucose levels

InsulinSecreted by beta cells

Storage hormone that promotes accumulation of glycogen in liver and triglycerides in fat cells

GlucagonSecreted by alpha cells

Cause release of stored glucose and fat breakdown


Fig. 24.10


Diabetes mellitusAffected individuals cannot take up glucose from the blood

Type I

~ 5-10% of diabetics

Autoimmune disorder in which immune systems attacks the Islets of Langerhans

Develops before age 20 (juvenile-onset diabetes)

Treated by daily injections of insulin

Type II

~ 90-95% of diabetics

Caused by abnormally low number of insulin receptors

Develops after age 40 (adult-onset diabetes)Typically in obese individuals

Treated by diet and exercise


24.5 The Thyroid, Parathyroid and Adrenal Glands

The thyroid gland lies just below the Adam’s apple in front of the neck

Its two most important hormones are

ThyroxineIncreases metabolic rate and promotes growth

Contains iodine

Lack of iodine in diet causes goiters

Calcitonin

Stimulates calcium deposition in the bone


Fig. 24.11 The thyroid gland secretes thyroxine

Goiter


The parathyroid glands are four small glands attached to the thyroid

Produce parathyroid hormone (PTH)One of only two hormones essential for survival

Acts as a fail-safe mechanism ensuring that calcium levels never fall too low

Calcium ions are key to muscle contractions

When levels are low, PTH stimulates osteoclast cells to break down bone matrix

This raises calcium levels in the blood

PTH also acts on kidneys to reabsorb calcium ions from the urine


Fig. 24.12 Maintenance of proper calcium levels in the blood


The adrenal glands are two glands, one located just above each kidney

Each composed of two parts

Medulla – Inner core

Adrenaline and norepinephrine

Released in times of stress to stimulate rapid deployment of body fuel

Cortex – Outer shell

Cortisol – Stimulates carbohydrate metabolism and reduces inflammation

Aldosterone – Stimulates the kidney to uptake sodium and other ions from the urine

This is the other hormone that is absolutely essential for survival


24.6 A Host of Other Hormones

Steroid sex hormones Estrogen, progesterone and testosterone

Produced by gonads to regulate sexual development

Atrial natriuretic hormoneSecreted by the right atrium of the heart

Stimulate kidneys to excrete salts and water in the urine

ErythropoietinSecreted by the kidney

Stimulates the bone marrow to produce RBC


Molting is triggered by the molting hormone, also called ecdysone

The production of ecdysone is itself controlled by the brain hormone

These two hormones are balanced by a third hormone, the juvenile hormone

If present in high levels, it inhibits the formation of the pupa and adult forms

Molting and Metamorphosis in Insects


Fig. 24.13 The hormonal control of metamorphosis in the silkworm moth, Bombyx mori

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24.1 Hormones