The Endocrine System PDF

The Endocrine System Objectives  A messenger system in an organism comprising feedback loops of hormones that are released by internal glands directly into the circulatory system and that target and Endocrine...

The Endocrine System Objectives  A messenger system in an organism comprising feedback loops of hormones that are released by internal glands directly into the circulatory system and that target and Endocrine regulate distant organs. System In vertebrates, the hypothalamus is the neural control center for all endocrine systems. In the human body, two major organ systems participate in relatively “long distance” communication: the nervous system and the endocrine system. Together, these two systems are primarily responsible for maintaining homeostasis in the body. The nervous and endocrine systems act together to coordinate functions of all body systems. Endocrine System The study of the endocrine system and its disorders is known as endocrinology. ▪ The endocrine system is composed of endocrine cells and glands located throughout the body which secrete molecules called hormones. ▪ Hormones are mediators that are released in one part of the body but regulates the activity of cells in other parts of the body by binding to receptors.  FUNCTIONS OF THE ENDOCRINE SYSTEM Regulating development, growth, and metabolism Maintaining (e.g. growth homeostasis of hormone). blood composition and volume (e.g. Controlling Insulin). digestive processes Controlling (e.g. gastrin). reproductive activities (e.g. prolactin).  Hormones ▪ This communication is possible because body cells display various receptors for specific types of hormones. ▪ Cells that have specific receptors for a hormone are called target cells. ▪ Each type of hormone has different target cells and therefore different cellular effects. Loc al Hormones ▪ Autocrine hormones act on the same cells that produced them. ▪ Paracrine hormones act on neighboring cells.  Circulating Hormones Hormones carried through the blood to act on distant target cells. Neural vs. Endocrine Signaling Endocrine system (a, b, c): Uses chemical signals (hormones). Fast or slow response (up to 48 hours). Widespread effect throughout the body. Nervous system (d): Uses electrical and chemical signals (neurotransmitters). Always fast (milliseconds). Localized specific effect. 8 In general, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—taking care of the internal environment of the body, maintaining homeostasis, and controlling reproduction How does the fight-or-flight response that was mentioned earlier happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal.  Endocrine Glands In vertebrates, the hypothalamus is the neural control center for all endocrine systems. In humans, the major endocrine glands are the thyroid, parathyroid, pituitary, pineal, adrenal glands, and the (male) testicles and (female) ovaries. The hypothalamus, pancreas, and thymus also function as endocrine glands, among other functions. (The hypothalamus and pituitary glands are organs of the neuroendocrine system. One of the most important functions of the hypothalamus—it is located in the brain adjacent to the pituitary gland—is to link the endocrine system to the nervous system via the pituitary gland.)  Pituitary Gland ▪ The pituitary gland is approximately the size of a pea. ▪ Hormones secreted by the pituitary include: Pituitary Gland Growth hormone (GH) is involved in the growth of skeletal muscles and long bones of the body. It plays an important role in determining final body size. Hyposecretion of growth hormone during childhood leads to pituitary dwarfism. Hypersecretion during childhood results in gigantism.  Pineal Gland ▪ The pineal gland is a cone-shaped structure within the brain that produces the hormone melatonin, which regulates the circadian rhythm. ▪ Circadian rhythm: the natural, internal process that regulates the sleep–wake cycle and repeats every 24 hours. ▪ During sleep, plasma levels of melatonin increase and then drop to a low level again before awakening. 22  Thyroid and Parathyroid Glands ▪ The thyroid gland is a butterfly-shaped gland anterior to the trachea and inferior to the larynx with follicles that produce thyroid hormone (T3 and T4). ▪ Thyroid hormone increases metabolism. ▪ Thyroid hormone controls the rate at which glucose is “burned,” or oxidized, and converted to body heat and chemical energy (ATP).  Hypothyroidism Hyperthyroidism Thyroid hormone levels are too low: Thyroid hormone levels are too high: Low metabolic rate Increased metabolic rate Weight gain Weight loss Lethargy Hyperactivity Feeling of being cold Heat intolerance  Thyroid and Parathyroid Glands ▪ Parafollicular cells of the thyroid gland release calcitonin, which functions to decrease blood calcium level. ▪ Parathyroid glands, found in the posterior surface of the thyroid gland, produces parathyroid hormone.  Adrenal Gland ▪ The adrenal glands are located superior to the kidneys. ▪ The adrenal glands secrete epinephrine and norepinephrine, which are released during stress and produce effects similar to sympathetic responses. ▪ The adrenal glands also produce the hormone cortisol, the primary stress hormone. The StressResponse ▪ Stress is any potential or actual threat to homeostasis. Physiological stress Psychological stress The StressResponse ▪ A stressful situation can trigger physiological changes in the body. ▪ Any stimulus that produces a stress response is called a stressor. ▪ Stress response: a physiological response to a stressor that aims to bring back homeostasis. The stress response activates: 1. Sympathetic nervous 2. Hypothalamic-pituitary- (fight–or-flight response). adrenal axis (HPA Axis). Neural Endocrine Glands that signal each other in sequence are often referred to as an axis, such as the hypothalamic–pituitary–adrenal axis  Hypothalamic-Pituitary- Adrenal Axis The hypothalamus is a major link between the Stress nervous and endocrine systems. It contains neuroendocrine cells that synthesize and release corticotrophin-releasing hormone (CRH). Secretion of CRH activates the release of CRH adrenocorticotropic hormone (ACTH) from the pituitary gland. ACTH from the pituitary enters the blood an ACTH activates the adrenal gland to produce cortisol, the major “stress hormone”. Almost all tissues in the body have cortisol receptors, therefore, cortisol can affect Cortisol nearly every organ system in the body. 1  Cortisol Effects (short-term) Acute Stress  Glucose release  Heart rate  Memory & attention from liver Cortisol  Blood pressure  Muscle catabolism  Bone breakdown  Immunity  Fat catabolism  Negative Feedback Control of Cortisol ▪ When cortisol levels in the blood get high, there are sensors in the brain which “turn off” the stress response. ▪ Chronic stress Impairs this negative feedback mechanism. ▪ In this case, the body continues to release CRH and AC TH which leads the adrenal glands to produce more and more cortisol. Negative Feedback Loop The release of adrenal glucocorticoids is stimulated by the release of hormones from the hypothalamus and pituitary gland. This signaling is inhibited when glucocorticoid levels become elevated by causing negative signals to the pituitary gland and hypothalamus. Cortisol Effects (long-term) ChronicStress Memory problems, Heart disease depression, anxiety Diabetes Prolonged Cortisol Exposure Muscle wasting Hypertension Osteoporosis Cancer Obesity 22  Pancreas ▪ The release of insulin into the blood from beta cells results in a decrease in glucose. ▪ The release of glucagon from alpha cells results in an increase in glucose in the blood. 35 Pancreas The pancreatic exocrine function involves the acinar cells secreting digestive enzymes that are transported into the small intestine by the pancreatic duct. Its endocrine function involves the secretion of insulin (produced by beta cells) and glucagon (produced by alpha cells) within the pancreatic islets. These two hormones regulate the rate of glucose metabolism in the body. The micrograph reveals pancreatic islets. LM × 760. (Micrograph provided by the Regents of University of Michigan Medical School © 201  Gonads ▪ The testes secrete testosterone. ▪ The ovaries secrete estrogen and progesterone. ▪ These hormones help control reproductive processes.