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Micturition - Complete notes .docx

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BIOL 153
Paul Hewitt

Micturitionaka urination or voidingis a reflex initiated by stretch receptors in the walls of the urinary bladderfiltrate from the kidneys enters the bladder via the ureters and is stored there until enough is present to distend the bladder when the stretch receptors are stimulated they send afferent impulses to two destinations1 to the brainwhere the urge to urinate is felt2 to the sacral region of the spinal card parasympathetic efferent impulses return to the internal sphincter and the muscles of the bladdercontractions of the bladder muscles coupled with relaxation of the internal sphincter cause a small amount of urine to be pushed into the beginning of the urethraSince the external sphincter is under voluntary control urine will not go any further until this last barrier is relaxedif urination does not occur at this time the reflexive bladder contractions will subside until more urine accumulates and the stretch receptors are reactivatedeventually if urination does not take place voluntarily the reflex will override and the bladder will empty anywayBody FluidsBody Water Content varies with body composition which depends on age sex and nutritional statusfat is the least hydrated tissue therefore total water content declines throughout life is lower in women than in men is lower in obese people than in leaner peoplegenerally the body is made up of 5060 waterFluid Compartments2 main fluid compartments in the body1 Intracellular fluid2 Extracellular fluida plasmab interstitial fluidthe composition of plasma and interstitial fluid are very similar except that plasma contains many proteins that do not cross the endothelium of the blood vessels th
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