Endocrine System PDF

# Endocrine System - **Endocrine System** - the cells, tissues and organs that secrete hormones into the body fluids (ie. extracellular fluid, blood) - **Endocrine glands** - glands that secrete hormones directly into the blood or tissue fluid. - **Exocrine glands** - glands that secrete su...

# Endocrine System - **Endocrine System** - the cells, tissues and organs that secrete hormones into the body fluids (ie. extracellular fluid, blood) - **Endocrine glands** - glands that secrete hormones directly into the blood or tissue fluid. - **Exocrine glands** - glands that secrete substances into ducts that open onto an internal or external surface. May be single cell (eg. Goblet cell), or multicellular (sweat glands in skin). - **Function of endocrine system** - main regulator of metabolism, of growth and development, help regulate water and electrolyte balance, critical in reproductive processes and in the response to stress. Extremely important in maintaining homeostasis. Excesses or deficiencies of hormones make the difference between normalcy and all kinds of abnormalities (dwarfism, gigantism, sterility), and even in some instances determines whether an individual will live or die. - **The endocrine system interacts with the nervous system to coordinate and integrate the activity of body cells.** ## Endocrine Glands: | Location | Gland | | -------- | -------------------------------------- | | In brain | Hypothalamus | | | Pituitary gland | | | Pineal gland | | In neck | Thyroid gland | | | Parathyroid gland | | Above heart | Thymus | | Abdomenal | Adrenal gland | | | Kidney | | | Pancreas | | Pelvis | Ovary | | | Testis | ## Hormones and Their Actions: - A hormone is a substance secreted by a cell that affects the function of another cell. The hormone is released into the space around a cell. It can either diffuse a short distance and affect other cells in the immediate area (having a local effect) or it can enter the bloodstream and be transported throughout the entire body (having a generalized effect). - The hormone is only going to affect cells that have receptors for that particular hormone on their cell surfaces. These cells that are affected by a particular hormone are called its **target cells**. - Whereas the nervous system sends electrochemical signals to other cells and those cells respond very quickly to the signals (msecs), hormones act more slowly. Hormones must be transported through the blood so cells respond more slowly (seconds or days). Once initiated, the response to hormones is usually longer lasting than the responses invoked by the nervous system. - Hormones are organic substances that are very potent and can cause changes in their target cells even in extremely low concentrations. ### Physiological concentrations vs. pharmacological concentrations - Hormones may be steroids, amines, peptides, proteins, or glycoproteins. ## Steroid Hormones: - Steroids are the most common forms of hormones. Steroids are compounds made of complex rings of C and H atoms. They are synthesized from cholesterol. - Steroid hormones include sex hormones (estrogen and testosterone), hormones secreted by the adrenal cortex (aldosterone and cortisol). ### Mechanism of Action of Steroid Hormones: - Steroid hormones are soluble in lipids so they can cross the cell membranes of target cells easily by diffusion. Once inside the cell, they enter the nucleus and combine with receptors (proteins) inside the nucleus. - The resulting steroid-protein complex binds to a specific region of the target cell's DNA molecules and activates certain genes. The activated genes then cause mRNA to be synthesized (transcription), which causes certain proteins to be synthesized (translation). - The proteins that are synthesized in response to the hormone can - act as enzymes and alter rates of metabolic processes - act as parts of membrane transport systems (pores, carrier proteins) or - activate or inhibit other enzymes. - The degree of cellular response induced by a steroid hormone is proportional to the number of hormone-receptor complexes formed. ## Nonsteroidal Hormones: - Hormones that are amines (eg. norepinephrine and epinephrine) are produced from amino acids. - Peptide hormones are made of short chains of amino acids. - Protein hormones are made of many amino acids. - Glycoproteins are made of a protein attached to a carbohydrate. ### Mechanism of Action of Nonsteroidal hormones: - Nonsteroidal hormones act through "second messengers". - These nonsteroidal hormones usually act by combining with specific receptors on the membrane of the target cell (they are not lipid soluble so cannot cross cell membrane). Each receptor is a protein with a binding site and an activity site. - The hormone binds with the binding site of its receptor. This stimulates the receptor's activity site to interact with other membrane proteins. This interaction causes other membrane bound enzymes to do certain actions, or it changes transport mechanisms and the concentrations of other substances changes. - These "other substances" serve as "second messengers" which cause certain changes in the cell. ### Several types of second messengers: - cAMP - cGMP - Calcium - Polyphosphoinositides