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Published on January 20, 2009

Author: lalballer08

Source: authorstream.com

Endocrine Physiology : Endocrine Physiology Endocrine system maintains homeostasis : Endocrine system maintains homeostasis The concept that hormones acting on distant target cells to maintain the stability of the internal milieu was a major advance in physiological understanding. The secretion of the hormone was evoked by a change in the milieu and the resulting action on the target cell restored the milieu to normal.The desired return to the status quo results in the maintenance of homeostasis Sensing and signaling : Sensing and signaling Endocrine “glands” synthesize and store hormones. These glands have a sensing and signaling system which regulate the duration and magnitude of hormone release via feedback from the target cell. Endocrine vs. Nervous System : Endocrine vs. Nervous System Major communication systems in the body Integrate stimuli and responses to changes in external and internal environment Both are crucial to coordinated functions of highly differentiated cells, tissues and organs Unlike the nervous system, the endocrine system is anatomically discontinuous. Nervous system : Nervous system The nervous system exerts point-to-point control through nerves, similar to sending messages by conventional telephone. Nervous control is electrical in nature and fast. Hormones travel via the bloodstream to target cells : Hormones travel via the bloodstream to target cells The endocrine system broadcasts its hormonal messages to essentially all cells by secretion into blood and extracellular fluid. Like a radio broadcast, it requires a receiver to get the message - in the case of endocrine messages, cells must bear a receptor for the hormone being broadcast in order to respond. Principal functions of the endocrine system : Principal functions of the endocrine system Maintenance of the internal environment in the body (maintaining the optimum biochemical environment). Integration and regulation of growth and development. Control, maintenance and instigation of sexual reproduction, including gametogenesis, coitus, fertilization, fetal growth and development and nourishment of the newborn. Response vs. distance traveled : Response vs. distance traveled Endocrine action: the hormone is distributed in blood and binds to distant target cells.Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.Autocrine action: the hormone acts on the same cell that produced it. Major hormones and systems : Major hormones and systems Top down organization of endocrine system. Hypothalamus produces releasing factors that stimulate production of anterior pituitary hormone which act on peripheral endocrine gland to stimulate release of third hormone Specific examples to follow Posterior pituitary hormones are synthesized in neuronal cell bodies in the hypothalamus and are released via synapses in posterior pituitary. Oxytocin and antidiuretic hormone (ADH) Types of hormones : Types of hormones Hormones are categorized into four structural groups, with members of each group having many properties in common: Peptides and proteins Amino acid derivatives Steroids Fatty acid derivatives - Eicosanoids Peptide/protein hormones : Range from 3 amino acids to hundreds of amino acids in size. Often produced as larger molecular weight precursors that are proteolytically cleaved to the active form of the hormone. Peptide/protein hormones are water soluble. Comprise the largest number of hormones Peptide/protein hormones Amine hormones : Amine hormones There are two groups of hormones derived from the amino acid tyrosine Thyroid hormones and Catecholamines Thyroid hormone is produced by the thyroid gland and is lipid soluble Epinephrine and norepinephrine are produced by the adrenal medulla both are water soluble Amine Hormones : Amine Hormones Two other amino acids are used for synthesis of hormones: Tryptophan is the precursor to serotonin and the pineal hormone melatonin Glutamic acid is converted to histamine Steroid hormones : All steroid hormones are derived from cholesterol and differ only in the ring structure and side chains attached to it. All steroid hormones are lipid soluble Steroid hormones Types of steroid hormones : Types of steroid hormones Glucocorticoids; cortisol is the major representative in most mammals Mineralocorticoids; aldosterone being most prominent Androgens such as testosterone Estrogens, including estradiol and estrone Progestogens (also known a progestins) such as progesterone Steroid hormones : Steroid hormones Are not packaged, but synthesized and immediately released Are all derived from the same parent compound: Cholesterol Enzymes which produce steroid hormones from cholesterol are located in mitochondria and smooth ER Steroids are lipid soluble and thus are freely permeable to membranes so are not stored in cells Steroid hormones : Steroid hormones Steroid hormones are not water soluble so have to be carried in the blood complexed to specific binding globulins. Corticosteroid binding globulin carries cortisol Sex steroid binding globulin carries testosterone and estradiol In some cases a steroid is secreted by one cell and is converted to the active steroid by the target cell: an example is androgen which secreted by the gonad and converted into estrogen in the brain 1,25-Dihydroxy Vitamin D3 : 1,25-dihydroxy Vitamin D3 is also derived from cholesterol and is lipid soluble Not really a “vitamin” as it can be synthesized de novo Acts as a true hormone 1,25-Dihydroxy Vitamin D3 Fatty Acid Derivatives - Eicosanoids : Fatty Acid Derivatives - Eicosanoids Arachadonic acid is the most abundant precursor for these hormones. The principal groups of hormones of this class are prostaglandins, prostacyclins, leukotrienes and thromboxanes. Control of Endocrine Activity : Control of Endocrine Activity The physiologic effects of hormones depend largely on their concentration in blood and extracellular fluid. Almost inevitably, disease results when hormone concentrations are either too high or too low, and precise control over circulating concentrations of hormones is therefore crucial. Feedback Control of Hormone Production : Feedback Control of Hormone Production Feedback loops are used extensively to regulate secretion of hormones in the hypothalamic-pituitary axis. An important example of a negative feedback loop is seen in control of thyroid hormone secretion Episodic secretion of hormones : If the periodicity is approximately 24 hours, the rhythm is referred to as circadian usually referred to as diurnal because the increase in secretory activity happens at a defined period of the day. Episodic secretion of hormones Circadian (chronotropic) control : Circadian (chronotropic) control Physiological importance of pulsatile hormone release : Physiological importance of pulsatile hormone release Demonstrated by GnRH infusion If given once hourly, gonadotropin secretion and gonadal function are maintained normally Faster, or continuous infusion inhibits gonadotropin secretion and blocks gonadal steroid production Clinical correlate : Clinical correlate Long-acting GnRH analogs (such as leuproline) have been applied to the treatment of precocious puberty, to manipulate reproductive cycles Hormone-Receptor interactions : Hormone-Receptor interactions A tissue becomes a target for a hormone by expressing a specific receptor for it. Hormones circulate in the blood stream but only cells with receptors for it are targets for its action. A cell is a target because is has a specific receptor for the hormone : A cell is a target because is has a specific receptor for the hormone Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for the hormone. Types of receptors : Types of receptors Receptors for the water soluble hormones are found on the surface of the target cell, on the plasma membrane. These types of receptors are coupled to various second messenger systems which mediate the action of the hormone in the target cell. Receptors for the lipid soluble hormones reside in the nucleus (and sometimes the cytoplasm) of the target cell. Because these hormones can diffuse through the lipid bilayer of the plasma membrane, their receptors are located on the interior of the target cell Second messenger systems : Second messenger systems Receptors for the water soluble hormones are found on the surface of the target cell, on the plasma membrane. These types of receptors are coupled to various second messenger systems which mediate the action of the hormone in the target cell Classes of hormones : Classes of hormones The hormones fall into two general classes based on their solubility in water. The water soluble hormones are the catecholamines (epinephrine and norepinephrine) and peptide/protein hormones. The lipid soluble hormones include thyroid hormone, steroid hormones and Vitamin D3 Mechanisms of endocrine disease : Mechanisms of endocrine disease Endocrine disorders result from hormone deficiency, hormone excess or hormone resistance Almost without exception, hormone deficiency causes disease Mechanism of Hormone Action Summary of the Differences Between the Two Major Classes of Hormones : Mechanism of Hormone Action Summary of the Differences Between the Two Major Classes of Hormones

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