HormonesIN THIS CHAPTER:
Hormones
Negative and Positive Feedback
Endocrine Glands and Their Secretions
Solved Problems
The endocrine system consists of endocrine glands that secrete specific chemicals called hormones into the blood or surrounding interstitial fluid. The endocrine system functions closely with the nervous system in regulating and integrating body processes. More specifically, hormones cause changes in the metabolic activities in specific cells, and nerve impulses cause muscles to contract or glands to secrete. In general, the action of hormones is relatively slow and the effects are prolonged, whereas the action of nerve impulses is fast and the effects are of short duration.
A hormone is a chemical messenger secreted by an endocrine gland. Its chemical composition is such that it has its effect on specific receptor sites on target cells causing a series of biochemical events that leads to a specific response. Hormones are classified according to chemical structure and the location of the cell membrane receptors on their target cells.
Table 13.1 Classes and Composition of Hormones
Hormones are divided into two groups, based on the location of the receptors on the target cells. Group I hormones bind to intracellular receptors and are lipophilic. These include the steroid hormones. Group II hormones bind to cell surface receptors and are hydrophilic. These hormones include polypeptide, glycoprotein, and catecholamine hormones.
Negative feedback involves a chain of biochemical or physiological events. Generally, an increased amount of end product inhibits the production, mechanism, or action of a starting substance to prohibit further synthesis of the end product. For example: A > B > C > D. As A progresses through B and C to D the amount of D increases. Substance D, however, is an inhibitor of substance A. As levels of substance D increase, substance A receives “negative feedback” to prevent the process from continuing to produce more D. An example of this mechanism is the production of cortisol and its regulation along the hypothalamic-pituitary-adrenal axis.
In the case of positive feedback, D, in the example above, would stimulate A to further produce increased amounts of B, and so on to D. This is less common in the body. One example is the secretion of oxytocin stimulating contraction of uterine muscles during labor. As labor progresses pressure on the cervix continues to stimulate the release of oxytocin, which continues to stimulate uterine muscles to contract.
Homeostatsis is maintained by the continual adjustments of endocrine function in response to changes in our environment. Negative feedback inhibits the factors that produced a product, so as to maintain a normal range of values. Positive feedback increases the deviation from normal values, and thus is not homeostatic.
Endocrine organs are widely scattered throughout the body with no anatomical continuity. In addition to the discrete endocrine organs, several other organs, referred to as mixed organs, have an endocrine function. These include the thymus, stomach, duodenum, placenta and the heart.
Location and structure: Located on the inferior side of the brain in the sella turcica of the sphenoid bone. The pituitary stalk attaches the gland to the hypothalamus portion of the brain. The pituitary gland is divided into an anterior lobe, the adenohypophysis, and a posterior lobe, the neurohypophysis.
Secretions and effects:
Secreted by the adenohypophysis:
• Human growth hormone (HGH). Targets the bones and soft tissues. Accelerates rate of body growth by 1) stimulating amino acid uptake by cells; 2) increasing synthesis of tRNA; and 3) increasing the number and aggregation of ribosomes, thus promoting protein synthesis.
• Thyroid-stimulating hormone (TSH). Targets the thyroid gland. Stimulates synthesis and release of thyroid hormones.
• Adrenocorticotropic hormone (ACTH). Targets the adrenal cortex. Stimulates secretion of glucocorticoids.
• Prolactin (PRL). Targets the mammary glands. Promotes development of mammary glands and stimulates milk production. Regulated by production of placental hormones during pregnancy and stimulation of the nipple during lactation.
• Follicle stimulating hormone (FSH). Targets the ovaries and testes. Stimulates growth of ovarian follicles and spermatogenesis in females and males respectively.
• Luteinizing hormone (LH). Targets the ovaries and testes. In females, stimulates maturation of follicles, promotes ovulation and stimulates corpus luteum to secrete estrogens and progesterone. In males, stimulates interstitial cells to secrete testosterone.
Secreted by the neurohypophysis:
• Antidiuretic hormone (ADH). Targets the kidney tubules. Facilitates water reabsorption in the distal convoluted tubules and collecting ducts. Release stimulated by dehydration and increased plasma osmolarity. Controlled by negative feedback.
• Oxytocin. Targets the uterus and mammary glands. Stimulates contraction of uterine muscles and secretion of milk. Release stimulated by stretching of uterus late in pregnancy and by mechanical stimulation of the nipple during nursing. Controlled by positive feedback.
Location and structure: Located in the neck on either side of the thyroid cartilage at the top of the trachea.
Secretions and effects: Thyroid hormones triodothyronine (T3) and tetraiodotyrosine (T4 or thyroxine) are secreted under the stimulation of TSH from the pituitary. These hormones accelerate metabolic rate, oxygen consumption, and glucose obsorption; increase body temperature; affect growth and development; and enhance the effects of the sympathetic nervous system.
Location and structure: Small glands embedded in the posterior surface of the thyroid gland.
Secretion and effects: Parathyroid hormone (PTH) increases plasma calcium levels by 1) stimulating the formation and activity of osteoclasts, which break down bone tissue and release calcium from the bones into the blood; 2) acting on kidney tubules to increase calcium reabsorption; and 3) increasing synthesis of 1/25-dihydroxycholecalciferol, which increases calcium absorption from the GI tract. Release stimulated by decreased plasma calcium concentration.
Location and structure: Triangular shaped gland embedded in adipose tissue at the superior borders of the kidneys. Consists of an outer adrenal cortex and an inner adrenal medulla.
Secretions and effects:
Secreted by the adrenal cortex:
• Glucocorticoids (corticosterone, cortisol). 1) Regulate carbohydrate and lipid metabolism, accelerate breakdown of proteins; 2) in large doses, inhibit inflammatory responses; 3) promote vasoconstriction; and 4) help the body resist stress. Secretion controlled by ACTH from the anterior pituitary and negative feedback mechanisms.
• Mineralocorticoids (deoxycorticosterone, aldosterone). Regulate concentration of extracellular electrolytes, especially sodium and potassium. Aldosterone secretion controlled by the reninangiotensin system (Figure 13-1), plasma K+ concentration, and ACTH.
Figure 13-1. The sequence of events in aldosterone production.
Secreted by the adrenal medulla:
• Amine hormones (epinephrine and norepinephrine). Results in sympathetic response (see Chap. 11). Increases release of ACTH and TSH from the adenohypophysis. Release stimulated by sympathetic stimulation.
Location and structure: In the abdomen, inferior to the stomach. Endocrine portion consists of scattered clusters of cells called pancreatic islets (islets of Langerhans).
• Glugagon is secreted by alpha cells innervated by cholinergic fibers. It stimulates glycogenolysis and maintains blood glucose levels during fasting or starvation.
• Insulin is secreted by beta cells innervated by adrenergic fibers. It stimulates movement of blood glucose across cells, stimulates glycolysis and lowers blood glucose levels.
• Somatostatin, secreted by delta cells, stimulates incorporation of sulfur into cartilage and stimulates collagen formation.
Endocrine secretions from mixed glands
• Thymus. Secretes thymosin; stimulates T-lymphocyte activity.
• Pineal gland. Secretes melatonin; affects secretion of gonadotropins and ACTH from the anterior pituitary.
• Gastric mucosa. G cells secrete gastrin; stimulates gastric juice secretion and gastric motility.
• Duodenal mucosa. Secretes secretin; stimulates secretion of pancreatic juice.
• Placenta. Secretes human chorionic gonadotropin (hCG), human somatomammatropin (hCS), estrogens, and progesterone.
You Need to Know 
• Name, location, and secretion of each endocrine organ.
• Target tissue and effect for each endocrine secretion.
True or False
_____1. Two hormones are never present in the blood at the same time. (False)
_____2. The cells of the parathyroid gland respond directly to the glucose concentration in the blood. (False)
_____3. The posterior pituitary is not composed of true glandular tissue. (False)
