Chapter 12 Endocrine System

Organs of the Endocrine System

Organs of the Endocrine System The endocrine system consists of the glands shown here as well as clusters of hormone-secreting cells in various organs, including the brain, heart, and small intestine. The glands secrete chemicals (called hormones) that influence almost every cell and organ in the body. Endocrine glands are ductless glands: they secrete their hormones directly into the bloodstream. The cells of many different organs are exposed to a particular hormone; however, only cells having receptors for that hormone (called target cells) will respond.

Endocrine vs. Nervous System Endocrine system Nervous system Uses hormones Distributes hormones via the bloodstream Responds slowly Exerts long-lasting effects Adapts slowly to continual stimulation Uses neurotransmitters Secretes neurotransmitters into a synapse Responds quickly Exerts short-lived effects Adapts quickly to continual stimulation The actions of the endocrine and nervous systems complement one another to ensure that the body maintains homeostasis.

Hormone Classifications Steroid hormone Nonsteroid hormone

Hormone Classifications Steroid hormones – synthesized from cholesterol; pass easily through a cell’s membrane; once inside the cell, they bind to receptors in the nucleus. Nonsteroid (protein-based) hormones – synthesized from amino acids; can’t penetrate the cell wall; bind to receptors on cell surface which activates a second messenger system which then activates specific enzymes.

Question Which is a characteristic of endocrine glands? They employ neurotransmitters to relay messages. Ducts carry their secretions to the body’s surface. They exert a long-lasting effect. They respond quickly to stimuli.

Pituitary and Hypothalamus Infundibulum

Pituitary and Hypothalamus The pituitary gland influences more body processes than any other endocrine gland. The pea-sized pituitary gland sits underneath the hypothalamus. It lies in the sella turcica, a cavity within the sphenoid bone. The pituitary gland is two distinct glands: Adenohypophysis (anterior pituitary) Neurohypophysis (posterior pituitary)

Anterior Pituitary The anterior pituitary is the larger of the two pituitary glands. It consists of glandular tissue and secretes a number of important hormones under the direction of the hypothalamus. Anterior pituitary

Anterior Pituitary Neurons within the hypothalamus synthesize releasing hormones (which stimulate the anterior pituitary to secrete the hormones it produces as well as inhibiting hormones (which suppress hormone secretion by the anterior pituitary). Neurons of the hypothalamus release their hormones into a system of blood vessels called the hypophyseal portal system.

Hormones Released by the Hypothalamus Gonadotropin-releasing hormone: Promotes secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) Thyrotropin-releasing hormone: Promotes secretion of thyroid-stimulating hormone (TSH) Corticotropin-releasing hormone: Promotes secretion of adrenocorticotropic hormone (ACTH) Prolactin-releasing hormone: Promotes secretion of prolactin Prolactin-inhibiting hormone: Inhibits secretion of prolactin Growth hormone-releasing hormone: Promotes secretion of growth hormone (GH)

Hormones of the Anterior Pituitary Thyroid-stimulating hormone (TSH), or thyrotropin Prolactin Adrenocorticotropic hormone (ACTH) Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Growth hormone (GH), or somatotropin

Hormones of the Anterior Pituitary Thyroid-stimulating hormone stimulates thyroid Prolactin stimulates milk production Adrenocorticotropic hormone stimulates adrenal cortex Follicle-stimulating hormone stimulates production of eggs in the ovaries and sperm in the testes Luteinizing hormone stimulates ovulation and estrogen and progesterone synthesis and secretion of testosterone by the testes Growth hormone acts on the entire body to promote protein synthesis, lipid and carbohydrate metabolism, and bone and skeletal muscle growth.

Posterior Pituitary Oxytocin Antidiuretic hormone View animation on “Stimulation and release of hormones from anterior and posterior pituitary glands”

Posterior Pituitary The posterior pituitary is made of neural tissue (in contrast with the anterior pituitary, which is made of glandular tissue). Instead of synthesizing hormones, the posterior pituitary stores Antidiuretic hormone (ADH) Oxytocin (OT) Hypothalamic neurons synthesize hormones, which they send down to posterior pituitary to be stored until stimulated by the nervous system to release them.

Control of Pituitary Secretions By negative feedback By central nervous system Brain monitors conditions. Brain triggers hormone release as needed. Pituitary stimulates gland to release hormone. Hormone is fed back to pituitary, which halts release of tropic hormone. View animation on “Negative feedback loop”

Question A key difference between the anterior and posterior pituitary is that the posterior pituitary: consists of glandular tissue. secretes hormones under the direction of the hypothalamus. receives hormones from the hypothalamus via the hypophyseal portal system. stores hormones released from the hypothalamus.

Pineal Gland Pineal gland The pineal gland is located on the roof of the brain’s third ventricle.

Pineal Gland The pineal gland produces melatonin, a hormone that rises at night (when sunlight is absent) and falls during the day. High melatonin levels trigger sleepiness, making it a key factor in the sleep/wake cycle. The pineal gland may also regulate the timing of puberty.

Thymus The thymus lies in the mediastinum just beneath the sternum.

Thymus The thymus secretes thymosin and thymopoietin, which have a role in the development of the immune system. Because it secretes hormones, the thymus is a member of the endocrine system; the actions of the hormones make the thymus part of the immune system.

Thyroid Thyroid gland The thyroid is the largest endocrine gland; it consists of two large lobes connected by a narrow band of tissue called the isthmus. The thyroid gland resides in the neck; it wraps around the anterior and lateral portions of the trachea.

Thyroid Tissue Thyroid follicle Parafollicular cells

Thyroid Tissue Tiny sacs called thyroid follicles are filled with thick fluid called thyroid colloid and secrete the two main thyroid hormones: T3 (triiodothyronine) and T4 (thyroxine). Cells between the thyroid follicles (parafollicular cells) secrete calcitonin in response to increasing blood calcium levels.

Parathyroid Glands Parathyroid glands

Parathyroid Glands Parathyroid glands lie on the posterior surface of the thyroid; they secrete parathyroid hormone (PTH) in response to low blood levels of calcium. PTH is the main hormone used to maintain normal blood levels of calcium; inhibits new bone formation and stimulates breakdown of old bone, causing calcium (and phosphate) to move out of bone and into blood; encourages kidneys to reabsorb calcium; prompts kidneys to activate vitamin D, which is important for intestinal absorption of calcium.

View animation on “Regulation of blood calcium levels” Calcium Homeostasis View animation on “Regulation of blood calcium levels”

Question Which endocrine gland has a role in the immune system? Thymus Thyroid Pineal Pituitary

Adrenal Glands Adrenal medulla Adrenal cortex Zona glomerulosa Zona fasciculata Zona reticularis

Adrenal Glands Each adrenal gland is two distinct glands: Arenal medulla – modified neurons; part of the sympathetic nervous system secrete catecholamines (epinephrine and norepinephrine) in response to stimulation. Adrenal cortex – glandular tissue secretes steroid hormones (corticosteroids).

Adrenal Glands Catecholamines prepare the body for physical; they also boost glucose levels (glycogenolysis and gluconeogenesis). The adrenal cortex consists of three layers of glandular tissue: Zona glomerulosa (outermost) secretes mineralocorticoids Zona fasciculata (middle) secretes glucocorticoids Zona reticularis (innermost) secretes sex steroids

Classes of Adrenal Cortex Hormones Mineralocorticoids: Aldosterone Glucocorticoids: Cortisol Sex steroids: Sex steroids

Adrenal Cortex Hormones Aldosterone acts on the kidneys to promote Na+ retention and K+ excretion; in turn, it also causes water retention. Cortisol helps the body adapt to stress and repair damaged tissues; has an anti-inflammatory effect, suppresses the immune system if secreted over a long term, and is essential for maintaining a normal blood pressure. Sex steroids include a weak form of androgen (that is converted to the more potent androgen testosterone) and small amounts of estrogen.

Pancreas The pancreas contains both endocrine and exocrine tissues: the majority of the pancreas acts as an exocrine gland, but a small percentage serves an important endocrine function. Pancreas Acini Islets of Langerhans

Pancreas Exocrine cells (acini) secrete digestive enzymes into ducts that drain into the small intestine. Interspersed with the exocrine cells are clusters of endocrine cells called pancreatic islets or the islets of Langerhans. The pancreatic islets contain alpha cells, beta cells, and delta cells.

Pancreas Alpha cells secrete glucagon, when blood glucose levels decline. It stimulates liver cells to convert glycogen into glucose and fatty acids and amino acids into glucose (gluconeogenesis). The glucose is released into the bloodstream, so blood glucose levels increase. Beta cells secrete the hormone insulin. After eating, the levels of glucose and amino acids in the blood increase. Insulin stimulates cells to absorb these nutrients, causing blood glucose levels to decline. Delta cells secrete somatostatin, a hormone that works within the pancreas to regulate the other endocrine cells. (It inhibits the release of glucagon, insulin, and growth hormone.)

Regulation of Blood Glucose

Regulation of Blood Glucose View animation on “Regulation of blood glucose levels”

Gonads Ovaries Testes Primary sex organ (females) Secrete estrogen Primary sex organ (males) Secrete testosterone Sex hormones stimulate the production of eggs (in females) and sperm (in males).

Gonads Estrogen helps promote the development of female characteristics (such as breast development) and contributes to the development of the reproductive system. After ovulation, the corpus luteum (the tissue left behind after a rupture of a follicle during ovulation) secretes progesterone. Progesterone, in combination with estrogen, helps maintain the uterine lining during pregnancy. Testosterone triggers the development of male sexual characteristics; it also sustains sperm production.

Question The chief role of insulin is to: stimulate cells to take up glucose. stimulate the liver to break down stored glycogen. stimulate the pancreas to release glucagon. trigger the conversion of fatty acids and amino acids into glucose.