Renal Blood Flow, GFR, and Autoregulation Flashcards
Major principle
Both renal blood flow (RBF) and GFR are autoregulated
Autoregulation: property of maintaining constant flow despite changes of input pressure
Autoregulatory range: range of pressure over which RBF remains nearly constant
Accomplished by changing resistance of afferent and efferent arterioles
Renal vascular system
Blood passes from glomerular capillaries-> efferent arterioles -> peritubular capillaries (adjacent to proximal tubule) -> bed that lies next to nephron structures
Vasa recta: separate loop that descends into medulla next to Loop of Henle
Juxtaglomerular apparatus: tubule bends around ascending loop comes back near glomerulus and afferent and efferent arterioles
Myogenic mechanism
Activates contraction and narrows diameter of afferent arteriole
Decreasing pressure allows arteriole to dilate
High blood flow through the kidneys means changes in vascular resistance and contributes to TPR
If BP increases, myogenic mechanism constricts afferent arterioles which increase TOR and increase arterial BP, low BP has opposite effect
Tubuloglomerular feedback
Changes in afferent and efferent diameter have opposite effects on GFR
Sensitive to small changes in GFR
Inc GFR -> constricts afferent and dilates efferent and vise versa
Macula dense of JGA detects changes in tubular flow
Change in GFR -> change in TL of Na in early distal tubule
Nitric oxide may be released if RBF declines, and causes vasodilation
Renin-Angiotensin System
Angiotensin II is potent vasoconstrictor but has little effect on afferent arteriole than on the efferent, except at high levels
Potent systemic constrictor that makes a potent contribution to control TPR
Autoregulates renal blood flow and exerts tight control over GFR
Renin released by JGA when
Arterial BP down
TL of Na at macula dense decreases
Effective circulating volume decreases
Response to hypotension-induced decreases in renal blood flow and GFR
Low renal blood flow and GFR -> decreases ionic concentration in ascending loop of Henle and lowers Na and Cl in macula densa -> causes afferent arteriolar dilation -> restores glomerular capillary pressure and GFR returns upward toward normal and decreases renal vascular resistance and restores RBF upward toward normal
Low GFR -> decreased TL at macula densa -> secretion of renin -> acts on angiotensinogen produced in liver -> converts AI to AII -> vasoconstriction effect on efferent arteriole-> GFR up toward normal -> increase vascular resistance -> reduces RBF
Response to pressure-induced increases in renal blood flow and GFR
Increased MAP -> increased RBF and GFR -> macula densa stimulates afferent arteriole -> increases resistance -> lowers RBF toward normal -> GFR returns toward normal range
Increased GFR -> increases filter load of Na -> increases TL of Na at JGA -> suppresses renin secretion -> less AII produced -> efferent arteriole dilates -> lowers glomerular capillary pressure -> reduces GFR toward normal
Effective circulating volume (ECF)
Ties together cardiovascular factors that involve both blood volume and effectiveness of the heart
Measuring GFR in order of liability
Clearance of inulin
Clearance of creatinine
Serum creatinine
BUN
Urine flow
Is NOT AUTOREGULATED
Pressure diuresis
Urinary output increases as MAP increases
Described by renal function curve
Pressure natriuresis
MAP increases -> increases Na excretion
Parallels pressure diuresis
How can we have dramatically increased water and sodium excretion without an increase in GFR
Reabsorption of water and salt must have decreased
