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ENDOCRINE SYSTEM

ENDOCRINE SYSTEM. George Liapakis , PhD. ENDOCRINE SYSTEM AND HOMEOSTASIS . Ability of our body to maintain a relatively stable internal environment. The endocrine system is a complex network of glands, the endocrine glands.

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ENDOCRINE SYSTEM

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  1. ENDOCRINE SYSTEM George Liapakis, PhD

  2. ENDOCRINE SYSTEM AND HOMEOSTASIS Ability of our body to maintain a relatively stable internal environment

  3. The endocrine system is a complex network of glands, the endocrine glands. • The endocrine glands secrete specific substances, which are called hormones

  4. Examples: Anterior pituitary gland : Peptide hormones that promote growth and control the secretion of other hormones from other endocrine glands. Thyroid gland : Thyroid hormones that control the basal metabolic rate of our body. Adrenal glands: Catecholamines and steroid hormones which play an important role in adaptation to stress, and in the maintenance of salt balance. Pancreas: Peptide hormones important in metabolizing nutrient molecules Parathyroid glands: Peptide hormone important in Ca2+ metabolism.

  5. Tropic hormones Their primary function is to act at different endocrine glands than those from which are secreted, maintaining their integrity and regulating the secretion of other hormones. In the absence of these hormones their endocrine target tissues lose their structural integrity and stop secreting their hormones. Example: Thyroid-stimulating hormone (TSH) Non-tropic hormones Theydo not regulate the secretion of other hormones. They directly stimulate target cells to induce effects. Example: Insulin, catecholamines.

  6. The function of some endocrine glands, such as the anterior pituitary, is only to secrete hormones. • Other glands have mixed function. For example testes, secrete the male sex hormone testosterone as well as they perform a non-endocrine function- that is sperm production.

  7. Where are the endocrine glands located? The endocrine glands are scattered throughout the body.

  8. Where do the hormones act? They act far away from the sites where they are produced and released. How do they accomplish this? Once secreted from a gland, a hormone travels in the blood to its distant target cells, where it functions.

  9. It takes a while for hormone’s action to reach full amplitude When a hormone stops secreting it lasts in blood circulation for a while • Endocrinesystemcontrols activities that require durationrather than speed. • Example: Growth is a long-term procedure. • In contrast, the nervous system coordinates rapid, precise responses.

  10. In contrast, when the neurotransmitters in the nervous system are secreted from one cell they act immediately in the neighboring cell.

  11. Many different hormones are distributed throughout the body, through the circulation. • Only specific target cellscan respond to each hormone. How is this accomplished? Only the target cells for a particular hormone carry receptors that specifically bind this hormone.

  12. Two classes of receptors Receptors located on the surface of the plasma membrane of cell. Receptors located inside the cell.

  13. Receptors located on the surface of the plasma membrane of cell. They bind the hydrophilic hormones, which are the peptidesand catecholamines.

  14. Binding of a hydrophilic hormone to its specific receptor triggers a series of biochemical steps within the cell to bring about a biological response.

  15. Receptors located inside the cell. They bind the lipophilic hormones, which are the steroid and thyroid hormones.

  16. Binding of a lipophilic hormone to its specific receptor activate genes, which are responsible for the formation of new proteins in the target cell, that carry out the desired biological response.

  17. How do hormones reach their target cells after their secretion? Hydrophilic hormones: By being dissolved into the hydrophilic environment of the blood, and traveling from their sites of secretion to their targets. Lipophilic hormones : By binding reversibly to plasma proteins in the blood and circulating at this form from their sites of secretion to their targets, where they are released from plasma proteins and get into cells where they act.

  18. Important feature of hormones • They are not released continuously from an endocrine gland. • Their secretion rate fluctuates up and down as a function of time. • The most common endocrine rhythm is the diurnal or circadian rhythm: regular, repetitive oscillations in hormone levels that cycle once every 24 hours.

  19. Some endocrine cycles operate on time scales other than a circadian rhythm, such as the monthly menstrual cycle in women

  20. The endocrine rhythms appear to be based on negative feedback loops. Hormone 1 triggers the release of hormone 2 and 3. When the plasma concentration of hormone 3 falls below a set point then hormone 1 is released and triggers the release of hormone 2 and 3. Hormone 3 in turn inhibits the secretion of hormones 1 and 2. The inhibition of the release of hormones 1 and 2 by hormone 3 is a negative feedback mechanism.

  21. Negative feedback mechanisms maintain the plasma concentration of a hormone at a given level The plasma concentration of a hormone is also influenced by its rate of renal excretion and inactivation by enzymes(liver, kidneys, blood, or target cells). Patients with liver or kidney diseases = altered levels of circulating hormones in the blood

  22. ABNORMAL LEVELS OF HORMONES IN BLOOD Patients with liver or kidney diseases Inappropriate rate of its secretion. 1. Too little hormone hyposecretion. 2. Too much hormone hypersecretion

  23. Abnormalities in hormonal secretion Endocrine dysfunction Endocrine Disorders Abnormalities in the responsiveness of target-cell to a hormone even though its plasma concentration is normal.

  24. Responsiveness of a target cell is closely associated with the number of hormone receptors. Down regulation : Decrease of the number of receptors on cell surface after the sustained elevation of the concentration of a hormone.

  25. A hormone could affect the activity of another hormone at a given target cell. • It is required for one hormone to be present in adequate amounts for the full exertion of the effect of another hormone. This phenomenon is called permissiveness. • Example: In the absence of thyroid hormone, epinephrine is only marginally effective.

  26. SUMMARY Endocrine system, network of glands, secretion of hormones. Tropic and non-tropic hormones Location of endocrine glands and distribution of hormones in our body Hormone receptors : Lipophilic and hydrophilic hormones Diurnal or circadian rhythm Negative feedback mechanisms Inactivation of hormones Abnormal blood levels of hormones and diseases Down-regulation of hormone receptors Permissiveness

  27. NEXT LECTURE HYPOTHALAMUS AND PITUITARY

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