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lecture_notes_25_(ta) - 26 27 Renal Physiology Reading...

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Lectures 24, 25, 26, 27: Renal Physiology Reading: Chapter 14, pgs 501-542 (pgs 511-553, if using the 5 th edition) Glomerular filtration rate ( GFR ) – depends on 1) the net filtration pressure, 2) the amount of glomerular surface area available for penetration, and 3) the permeability of the glomerular membrane. The most common factor resulting in a change in the GFR is an alteration in the glomerular capillary blood pressure (plasma-colloid pressure and Bowman’s capsule hydrostatic pressure do not typically change under normal conditions). GFR can be adjusted by two mechanisms (both work by changing caliber and resistance of afferent arterioles) : 1) sympathetic control , and 2) autoregulation Sympathetic control (Figure 14-12) of GFR involves a baroreceptor reflex. Baroreceptors - located in the arterial carotid sinus and the aortic arch detect rises and falls in arterial blood pressure. These receptors send signals to the cardiovascular control center in brainstem which then adjusts levels of sympathetic activity. ( DECREASE BP, INCREASE S YMPATHETIC ACTIVITY ) ( SEE FIG 10-14 ) Example: If plasma volume decreases because of hemorrhage, baroreceptors will detect the drop in blood pressure and communicate this drop to the cardiovascular control center. The cardiovascular control center will then coordinate an increase in sympathetic activity. Sy mpathetic activity will not only increase cardiac output and total peripheral resistance but will also decreases GFR (to maintain plasma volume by constricting afferent arterioles). (note: this response will override the myogenic response) Autoregulation - helps maintain a constant blood flow into the glomerular capillaries. This is accomplished by changing the caliber of the afferent arterioles (note: sympathetic input can override autoregulation) . The mechanism(s) for autoregulation are unknown, but they may include: (1) a myogenic mechanism , and/or (2) a tubuloglomerular feedback mechanism . ( MAINTAIN WITHIN ACCEPTABLE HOMEOSTATIC RANGES )
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( I NTRINSIC REGULATORY MECHANISMS ) ( 1 ) Myogenic mechanism - a common property of vascular smooth muscle is to contract automatically in response to increased stretch and to relax in response to decreased stretch. ( STRETCH A SMOOTH MUSCLE AND IT WILL CONTRACT ) ( 2 ) Tubuloglomerular feedback mechanism ( CONSTRICTS OR DIALATES AFFERENT ARTERIOLE TO MAINTAIN ACCEPTABLE BLOOD FLOW THROUGH GLOMERULUS ) (Figures 14-11, 14-12) a feedback process that involves the
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