Chapter 26 Hormones and the Endocrine System

I. The Nature of Chemical Regulation

1. The endocrine system consists of ductless glands that secrete hormones, and the molecular receptors on target cells that respond to the hormones.

2. In the endocrine system, chemical signals called hormones are released into the bloodstream by endocrine cells and carried to all locations in the body.

3. Certain cells and signals are shared by the endocrine system and the nervous system.

4. Hormones affect target cells using 2 signaling mechanisms:

(1)  Water-soluble hormones: they function by binding to membrane receptors and trigger signal transduction pathway, such as amines and proteins/polypeptides.

(2)  Lipid-soluble hormones: they pass through the phospholipid bilayer, bind to nuclear receptors, and trigger gene regulation pathway, such as steroids.

II. The Vertebrate Endocrine System

1. The pineal gland is a pea-size mass of tissue that secretes melatonin, a hormone that links environmental light conditions with biological rhythms.

2. The hypothalamus is the main control center between the nervous and the endocrine systems.

3. The pea-size pituitary gland (master gland) contains 2 distinct parts:

(1)  The anterior lobe: is composed of endocrine cells that secrete thyroid-stimulating hormone (TSH), prolactin (PRL), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and growth hormone (GH)

(2)  The posterior lobe: is composed of nervous tissue extended from the hypothalamus that secretes antidiuretic hormone (ADH) and oxytocin (OXT).

4. There are feedback controls between the hypothalamus, anterior pituitary, and endocrine glands (e.g., thyroid gland).

(1)  The hypothalamus secretes TSH-releasing hormone (TRH) to stimulate the anterior pituitary to produce TSH.

(2)  Under the influence of TSH, the thyroid gland secretes thyroid hormone (T₃ and T₄) into the blood.

(3)  When thyroid hormone increases in the blood, negative feedback mechanisms control the hypothalamus and anterior pituitary, inhibiting TRH and TSH secretion.

III. Hormones and Homeostasis

1. Growth hormone promotes the development and enlargement of all parts of the body in young mammals.

    (1)  Continually high levels of GH during childhood can lead to gigantism.

    (2)  Too little GH in childhood can lead to dwarfism.

2. The thyroid hormone regulates development and metabolism.

(1)  The thyroid gland secretes two kinds of thyroid hormones, 3-5-3'-triiodothyronine (T₃) and 3-5-3'-5'-tetraiodothyronine (thyroxine, T₄).

(2)  Thyroid hormones play the important role in development and maturation in amphibians.

(3)  Thyroid hormones are important in mammals, especially in bone and nerve cell development.

(4)  Throughout the body, thyroid hormones tend to increase the rate of oxygen consumption and cellular metabolism.

(5)  Hyperthyroidism (an excess of thyroid hormones in the blood) can make a person overheat, sweaty, high blood pressure, and loss weight, for example, Graves' disease causes protruding eyes.

(6)  Hypothyroidism (insufficient amounts of thyroid hormones) can cause weight gain, sleepy, and intolerance to cold. Mild hypothyroidism is often diagnosed in elderly people.

(7)  In adults, insufficient iodine in the diet can cause goiter (an enlargement of the thyroid).

3. The gonads (sex glands, ovaries in the female and testes in the male) stimulated by FSH and LH from the anterior pituitary secrete sex hormones, including estrogen and progesterone (maintains the uterus to support an embryo) in the female and androgen (testosterone) in the male.

4. The pancreas is a gland with dual functions:

(1)  It secretes digestive enzymes into the small intestine.

(2)  It secretes two protein hormones, insulin and glucagon (antagonistic hormones), into the blood. Three antagonistic hormone pairs provide precise regulation.

    1) blood glucose: insulin and glucagon

          2) blood calcium: calcitonin and parathyroid hormone

(3)  Diabetes mellitus (hyperglycemia) caused by the bodyꞌs inability to produce (type I) and/or use insulin (type II, insulin resistance) decreases the absorption of glucose from the blood.

(4)  Hypoglycemia: some people have hyperactive beta cells that secrete too much insulin into the blood. It may be accompanied by hunger, weakness, sweating, and nervousness.

5. The adrenal glands responses to stress:

(1)  Adrenal medulla: epinephrine (adrenaline) and norepinephrine (noradrenaline)

(2)  Adrenal cortex: corticosteroids (two main types in humans are the mineralocorticoids and the glucocorticoids)

(3)  Mineralocorticoids (e.g., aldosterone) stimulate the kidneys to reabsorb sodium ion and water.

(4)  Glucocorticoids (e.g., cortisol, cortisone, and corticosterone) promote the synthesis of glucose from noncarbohydrates, such as proteins and fats.

6. Prolactin performs many functions in different animals:

(1)  During late pregnancy, PRL stimulates mammary glands to grow and produce milk.

(2)  Suckling by a newborn stimulates further release of PRL, which increases the milk supply.

(3)  In some nonhuman mammals, PRL stimulates nest building.

(4)  In birds, PRL regulates fat metabolism and reproduction.

(5)  In amphibians, PRL stimulates movement toward water during breeding and affects metamorphosis.

(6)  In fish, PRL helps regulate salt and water balance in the gills and kidneys.