Renal Plasma Flow

What is renal plasma flow? Why measure RPF?

RPF: the amount of plasma that perfuses the kidneys per unit time. Important determinant of GFR (increased RPF -> more filtrate formed, decreased RPF -> less filtration). Can be used to estimate renal blood flow (RBF), both indicate renal health

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What is used as an indicator substance for estimating RPF?

An appropriate indicator substance for estimating RPF is one where mass excreted = mas delivered to kidneys e.g. PAH (para-amiohippuric acid)

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What is PAH a good indicator substance?

PAH is freely filtered & secreted - secretion removes all PAH from peritubular fluid in proximal tubule provided tubular transport maximum (Tm) is not exceeded. At low [plasma] nearly all PAH is removed

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What equation is used to calculate excreted PAH?

Total mass PAH excreted = Total mass PAH presented to kidney (= plasma concentration x plasma volume per unit time = RPF)

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What is filtration fraction? What is the equation? What is the FF link to colloid osmotic pressure & tubular reabsorption?

FF = proportion of plasma that forms filtrate = GFR/RPF. The greater the filtration fraction, the higher the colloid osmotic pressure in peritubular capillaries, hence the greater the forces for tubular reabsorption

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What is renal blood flow (RBF) derived from? What is its equation?

RBF can be derived from RPF & packed cell volume (PCV). Haematocrit (PCV) = fraction of blood volume occupied by RBCs. RBF = RPF/1-haematocrit

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What makes up the juxtaglomerular apparatus?

Macula densa & granular cells/juxtaglomerular cells

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There is some independence of GFR & RBF from arterial BP, why is this? How is this brought about?

Maintains excretion during BP fluctuations. Mechanisms: myogenic (afferent arteriole contracts in response to pressure/stretch) & tubuloglomerular feedback ([NaCl] sensed by macula dense of JGA, signal produced to alter GFR, less [NaCl] -> more GFR)

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Autoregulation is absent when arterial pressure is...? What arterial factors affect RBF & GFR?

Absent below arterial pressures of 90 mmHg & above 180. Afferent resistance: decreased glomerular capillary hydrostatic pressure decreases RPF/GFR. Efferent resistance: increase glomerular capillary hydrostatic pressure decreases RPF & increases GFR

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Which vasoconstrictors affect RBF & GFR?

CONSTRICTORS increase vascular activity, decrease RBF/GFR. Sympathetic: aff/eff arterioles innervated by decrease BP/strong emotional stimuli. Angiotensin II: activated by decrease BP, granular cells release renin, efferent constriction (low conc)

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Which vasodilators affect RBF & GFR?

DILATORS decrease vascular resistance, increase RBF/GFR. Prostaglandins: produced in kidney, afferent/efferent vasofilatation, counter excessive vasoconstrictor effects of sympathetic nerves & Ag II to preserve RBF/GFR

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What does the clearance of a substance compared to inulin indicate?

For a substance that is freely filtered: clearance = inulin indicates only filtered, clearance > inulin indicates secretion, clearance < inulin indicates reaborption

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What is the equation to calculate filtered glucose load?

GFR x plasma glucose concentration

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What is the equation to calculate glucose excretion?

Urine flow (V) x urine glucose concentration

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What is the equation to calculate glucose reabsorbed?

Glucose filtered - glucose excreted

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Renal Plasma Flow

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