endocrine system

18.1 Overview of the Endocrine System The main regulatory functions of the endocrine system are metabolism, control of food intake and digestion, tissue maturation, ion regulation, water balance, heart rate and blood pressure regulation, control of blood glucose and other nutrients, control of reproductive functions, uterine contractions and milk letdown, and immune system regulation.

18.2 Pituitary Gland and Hypothalamus 1. The pituitary gland secretes at least nine hormones that regulate numerous body functions as well as other endocrine glands. 2. The hypothalamus regulates pituitary gland activity through neurohormones and action potentials. Structure of the Pituitary Gland 1. The posterior pituitary develops from the floor of the brain and consists of the infundibulum and the neurohypophysis. 2. The anterior pituitary develops from the roof of the mouth. Relationship of the Pituitary Gland to the Brain: The Hypothalamus 1. The hypothalamohypophysial tract connects the hypothalamus and the posterior pituitary. ■ Neurohormones are produced in hypothalamic neurons. ■ The neurohormones move down the axons of the tract and are secreted from the posterior pituitary. 2. The hypothalamohypophysial portal system connects the hypothalamus and the anterior pituitary. ■ Neurohormones are produced in hypothalamic neurons. ■ Through the portal system, the neurohormones inhibit or stimulate hormone production in the anterior pituitary. Hormones of the Pituitary Gland 1. ADH promotes water retention by the kidneys. 2. Oxytocin promotes uterine contractions during delivery and causes milk letdown in lactating women. 3. GH is sometimes called somatotropin. ■ GH stimulates growth in most tissues and regulates metabolism. ■ GH stimulates the uptake of amino acids and their conversion into proteins and stimulates the breakdown of lipids and the synthesis of glucose. ■ GH stimulates the production of somatomedins; together, they promote bone and cartilage growth. ■ GH secretion increases in response to an increase in blood amino acids, low blood glucose, or stress. ■ GH is regulated by GHRH and somatostatin. 4. TSH, or thyrotropin, causes the release of thyroid hormones. 5. ACTH is derived from proopiomelanocortin; it stimulates cortisol secretion from the adrenal cortex and increases skin pigmentation. 6. Several hormones in addition to ACTH are derived from proopiomelanocortin. ■ Lipotropins cause lipid breakdown. ■ β endorphins play a role in analgesia. ■ MSH increases skin pigmentation. 7. LH and FSH are major gonadotropins. ■ Both hormones regulate the production of gametes and reproductive hormones (testosterone in males, estrogen and progesterone in females). ■ GnRH from the hypothalamus stimulates LH and FSH secretion. 8. Prolactin stimulates milk production in lactating females. Prolactinreleasing hormone (PRH) and prolactin-inhibiting hormone (PIH) from the hypothalamus affect prolactin secretion. Summary van72198_ch18_603-647.indd 643 1/7/19 2:49 PM 644 PART 3 Integration and Control Systems 18.3 Thyroid Gland 1. The thyroid gland is just inferior to the larynx. 2. The thyroid gland is composed of small, hollow balls of cells called follicles, which contain thyroglobulin. 3. Parafollicular cells are scattered throughout the thyroid gland. Thyroid Hormones 1. T3 and T4 synthesis occurs in thyroid follicles. ■ Iodide ions are taken into the follicles by active transport, oxidized, and bound to tyrosine molecules in thyroglobulin. ■ Thyroglobulin is secreted into the follicle lumen. Tyrosine molecules with iodine combine to form T3 and T4 thyroid hormones. ■ Thyroglobulin is taken into the follicular cells and broken down; T3 and T4 diffuse from the follicles to the blood. 2. T3 and T4 are transported in the blood. ■ T3 and T4 bind to thyroxine-binding globulin and other plasma proteins. ■ The plasma proteins prolong the half-life of T3 and T4 and regulate the levels of T3 and T4 in the blood. ■ Approximately one-third of the T4 is converted into functional T3. 3. T3 and T4 bind with nuclear receptor molecules and initiate new protein synthesis. 4. T3 and T4 affect nearly every tissue in the body. ■ T3 and T4 increase the rate of glucose, lipid, and protein metabolism in many tissues, thus increasing body temperature. ■ Normal growth of many tissues is dependent on T3 and T4. 5. TRH and TSH regulate T3 and T4 secretion. ■ Increased TSH from the anterior pituitary increases T3 and T4 secretion. ■ TRH from the hypothalamus increases TSH secretion. TRH increases as a result of chronic exposure to cold, food deprivation, and stress. ■ T3 and T4 inhibit TSH and TRH secretion. Calcitonin 1. An increase in blood calcium levels stimulates calcitonin secretion by the parafollicular cells. 2. Calcitonin decreases blood calcium and phosphate levels by inhibiting osteoclasts. 18.4 Parathyroid Glands 1. The parathyroid glands are embedded in the thyroid gland. 2. PTH increases blood calcium levels. ■ PTH stimulates osteoclasts. ■ PTH promotes calcium reabsorption by the kidneys and the formation of active vitamin D by the kidneys. ■ Active vitamin D increases calcium absorption by the intestine. 3. A decrease in blood calcium stimulates PTH secretion. 18.5 Adrenal Glands 1. The adrenal glands are near the superior poles of the kidneys. 2. The adrenal medulla arises from neural crest cells and functions as part of the sympathetic nervous system. The adrenal cortex is derived from mesoderm. 3. The adrenal medulla is composed of closely packed cells. 4. The adrenal cortex is divided into three layers: the zona glomerulosa, the zona fasciculata, and the zona reticularis. Hormones of the Adrenal Medulla 1. Epinephrine accounts for 80% and norepinephrine for 20% of the adrenal medulla hormones. ■ Epinephrine increases blood glucose levels, the use of glycogen and glucose by skeletal muscle, and heart rate and force of contraction. It also causes vasoconstriction in the skin and viscera and vasodilation in skeletal and cardiac muscle. ■ Norepinephrine and epinephrine stimulate cardiac muscle and cause the constriction of most peripheral blood vessels. 2. The adrenal medulla hormones prepare the body for physical activity. 3. Release of adrenal medulla hormones is mediated by the sympathetic nervous system in response to emotions, injury, stress, exercise, and low blood glucose. Hormones of the Adrenal Cortex 1. The zona glomerulosa secretes the mineralocorticoids, especially aldosterone. Aldosterone acts on the kidneys to increase sodium and to decrease potassium and hydrogen levels in the blood. 2. The zona fasciculata secretes glucocorticoids, especially cortisol. ■ Cortisol increases lipid and protein breakdown, increases glucose synthesis from amino acids, decreases the inflammatory response, and is necessary for the development of some tissues. ■ ACTH from the anterior pituitary stimulates cortisol secretion. CRH from the hypothalamus stimulates ACTH release. Low blood glucose levels and stress stimulate CRH secretion. 3. The zona reticularis secretes androgens. In females, androgens stimulate axillary and pubic hair growth and sex drive. 18.6 Pancreas 1. The pancreas, located along the small intestine and the stomach, is both an exocrine and an endocrine gland. 2. The exocrine portion of the pancreas consists of a complex duct system, which ends in small sacs, called acini, that produce pancreatic digestive juices. 3. The endocrine portion consists of the pancreatic islets. Each islet is composed of alpha cells, which secrete glucagon; beta cells, which secrete insulin; and delta cells, which secrete somatostatin. Effect of Insulin and Glucagon on Their Target Tissues 1. Insulin’s target tissues are the liver, adipose tissue, muscle, and the satiety center in the hypothalamus. The nervous system is not a target tissue, but it does rely on blood glucose levels maintained by insulin. 2. Insulin increases the uptake of glucose and amino acids by cells. Glucose is used for energy or is stored as glycogen. Amino acids are used for energy or are converted to glucose or proteins. 3. Glucagon’s target tissue is mainly the liver. 4. Glucagon causes the breakdown of glycogen and lipids for use as an energy source. Regulation of Pancreatic Hormone Secretion 1. Insulin secretion increases because of elevated blood glucose levels, an increase in some amino acids, parasympathetic stimulation, and gastrointestinal hormones. Sympathetic stimulation decreases insulin secretion. van72198_ch18_603-647.indd 644 1/7/19 2:49 PM CHAPTER 18 Endocrine Glands 645 2. Glucagon secretion is stimulated by low blood glucose levels, certain amino acids, and sympathetic stimulation. 3. Somatostatin inhibits insulin and glucagon secretion. 18.7 Hormonal Regulation of Nutrient Utilization 1. After a meal, the following events take place: ■ High glucose levels inhibit glucagon, cortisol, GH, and epinephrine, which reduces the release of glucose from tissues. ■ Insulin secretion increases as a result of the high blood glucose levels, thereby increasing the uptake of glucose, amino acids, and lipids, which are used for energy or stored. ■ Sometime after the meal, blood glucose levels drop. Glucagon, GH, cortisol, and epinephrine levels increase, insulin levels decrease, and glucose is released from tissues. ■ Adipose tissue releases fatty acids, triglycerides, and ketones, which most tissues use for energy. 2. During exercise, the following events occur: ■ Sympathetic activity increases epinephrine and glucagon secretion, causing a release of glucose into the blood. ■ Low blood sugar levels, caused by the uptake of glucose by skeletal muscles, stimulate epinephrine, glucagon, GH, and cortisol secretion, causing an increase in fatty acids, triglycerides, and ketones in the blood, all of which are used for energy. 18.8 Hormones of the Reproductive System The ovaries, testes, placenta, and pituitary gland secrete reproductive hormones. 18.9 Hormones of the Pineal Gland The pineal gland produces melatonin and arginine vasotocin, which can inhibit reproductive maturation and may regulate sleep-wake cycles. 18.10 Other Hormones and Chemical Messengers 1. The thymus produces thymosin, which is involved in the development of the immune system. 2. The digestive tract produces several hormones that regulate digestive functions. 3. Autocrine and paracrine chemical messengers are produced by many cells of the body and usually have a local effect on body functions. 4. Eicosanoids, such as prostaglandins, prostacyclins, thromboxanes, and leukotrienes, are derived from fatty acids and mediate inflammation and other functions. Endorphins, enkephalins, and dynorphins are analgesic substances. Growth factors influence cell division and growth in many tissues, and interleukin-2 influences cell division in the T cells of the immune system. 18.11 Effects of Aging on the Endocrine System A gradual decrease in the secretion rate occurs for most, but not all, hormones. Some of these decreases are related to gradual decreases in physical activity

Certainly! The endocrine system includes several glands, each producing specific hormones that regulate various physiological functions. Here are some key endocrine glands and their primary functions:

1. Pituitary Gland:

- Location: Located at the base of the brain.

- Function: Often referred to as the "master gland," the pituitary gland secretes hormones that control other endocrine glands. It plays a crucial role in growth, development, and the regulation of other hormonal systems.

2. Thyroid Gland:

- Location: Found in the neck, below the Adam's apple.

- Function: Produces thyroid hormones (T3 and T4) that regulate metabolism, energy production, and overall body growth. It also secretes calcitonin, which helps regulate calcium levels in the blood.

3. Parathyroid Glands:

- Location: Four small glands located on the thyroid gland.

- Function: Secrete parathyroid hormone (PTH), which regulates calcium and phosphorus levels in the blood and bone.

4. Adrenal Glands:

- Location: Sit on top of each kidney.

- Function: Produce hormones such as cortisol (involved in stress response and metabolism), aldosterone (regulates salt and water balance), and adrenaline (involved in the "fight or flight" response).

5. Pancreas:

- Location: Located behind the stomach.

- Function: Has both endocrine and exocrine functions. The endocrine part secretes insulin (lowers blood sugar) and glucagon (raises blood sugar), regulating glucose metabolism.

6. Pineal Gland:

- Location: Located in the brain, deep within the center.

- Function: Produces melatonin, which regulates sleep-wake cycles and circadian rhythms.

7. Hypothalamus:

- Location: Part of the brain.

- Function: While not a gland, the hypothalamus plays a crucial role in endocrine function by producing releasing and inhibiting hormones that control the pituitary gland's secretion of hormones.

8. Ovaries (in females):

- Location: In the pelvic cavity.

- Function: Produce estrogen and progesterone, which regulate the menstrual cycle and are involved in reproductive processes.

9. Testes (in males):

- Location: In the scrotum.

- Function: Produce testosterone, which is essential for the development of male reproductive tissues and secondary sexual characteristics.

These glands work together to maintain the body's internal balance and respond to changes in the external environment. Hormones produced by these glands act as messengers that regulate processes such as metabolism, growth, reproduction, and response to stress.

Certainly! Let's take a closer look at the mentioned hormones and their functions:

1. Insulin:

- Produced by: Beta cells in the pancreas.

- Function: Facilitates the uptake of glucose by cells, promoting its conversion into energy. Insulin helps lower blood sugar levels.

2. Glucagon:

- Produced by: Alpha cells in the pancreas.

- Function: Stimulates the liver to convert stored glycogen into glucose, raising blood sugar levels. It works in opposition to insulin to regulate glucose balance.

3. Thyroid Hormones (T3 and T4):

- Produced by: Thyroid gland.

- Function: Regulate metabolism by influencing the body's energy production and utilization. They also play a crucial role in growth, development, and the maintenance of body temperature.

4. Calcitonin:

- Produced by: Thyroid gland.

- Function: Regulates calcium levels in the blood by inhibiting the release of calcium from bones, helping to lower blood calcium levels.

5. Parathyroid Hormone (PTH):

- Produced by: Parathyroid glands.

- Function: Increases blood calcium levels by stimulating the release of calcium from bones, enhancing calcium absorption in the intestines, and reducing calcium loss in the kidneys.

6. Cortisol:

- Produced by: Adrenal glands.

- Function: Regulates metabolism, suppresses the immune system, and helps the body respond to stress. It also plays a role in maintaining blood pressure and balancing salt and water.

7. Aldosterone:

- Produced by: Adrenal glands.

- Function: Regulates salt and water balance by promoting the reabsorption of sodium and excretion of potassium in the kidneys, helping to maintain blood pressure.

8. Adrenaline (Epinephrine):

- Produced by: Adrenal glands.

- Function: Triggers the "fight or flight" response, increasing heart rate, dilating airways, and redirecting blood flow to critical organs to prepare the body for immediate action in response to stress.

9. Melatonin:

- Produced by: Pineal gland.

- Function: Regulates the sleep-wake cycle and circadian rhythms, helping to synchronize the body's internal clock with the day-night cycle.

10. Estrogen and Progesterone:

- Produced by: Ovaries (in females).

- Function: Regulate the menstrual cycle, support the development of female reproductive tissues, and play a crucial role in pregnancy and the maintenance of the uterine lining.

11. Testosterone:

- Produced by: Testes (in males).

- Function: Promotes the development of male reproductive tissues, secondary sexual characteristics (such as facial hair and deepening voice), and plays a role in maintaining male reproductive health.

These hormones work in concert to maintain homeostasis in the body, ensuring that various physiological processes are regulated and coordinated appropriately. Any imbalance in hormone levels can lead to health issues and disruptions in normal bodily functions

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