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Renal Blood flow

• 20-25% cardiac output (1 - 1.2 l/min)
• Determinants of renal blood flow

1. Autoregulated between 80-180mmHg - (1) Myogenic mechanism: increased wall tension stimulates vasoconstriction (2)  Tubuloglomerular feedback - alterations in flow of blood occurs with alterations in arterial pressure leading to stimulation of juxtraglomerular apparatus.
2. SNS: alpha-1 stimulation - afferent arteriole contraction: reduced blood flow
3. Angiotensin II: efferent arteriole constriction (ACEi cause dilation, and reduced blood flow)
4. PGE₂ PGI₂: efferent arteriole constriction (NSAIDs cause renal failure by inhibiting PG production)

• Measured by para-aminohippuric acid (PAH): - completely eliminated through processof filtration and secretion by tbubules (PAH clearance = Renal plasma flow)
• Renal blood flow = Renal plasma flow / (1 - Haematocrit) 

Renal Clearance

• Volume of plasma from which all of a substance has been removed and excreted in urine per unit time
• Clearance = [Urine] x Volume / [Plasma]
• Substance

1. Freely filtered (see below)
2. Not secreted / reabsorbed / metabolised
3. Must not inherently alter GFR

Glomerular Filtration Determinants

1. Molecular size - cut off 40Angstroms
2. Molecular charge (BM is negatively charged)

Measurement of GFR

1. Inulin clearance (must undergo continous iv infusion)
2. 24hour urinary creatinine (anhydride of creatine - ie without the water) excretion (some secretion of creatinine into tubules)
3. 51CrEDTA

Estimation of GFR

1. Cockcroft-Gault formula

The Nephron

1. Glomerulus
2. Proximal convoluted tubule
   • Major site of reabsorping solutes (70%): - Na, Cl, K, glucose, amino acids + phosphate, lactate
3. Loop of Henle
   • Resorption of solute (20%): - Na, Cl, K
   • Water resorption in thin descending loop (Thich ascending loop is impermeable to water)
   • Forms counter-current mutiplication system - concentrates urine
     1. Fluid enters LOH which is isotonic with plasma
     2. Decending limb permable to water; water progressively absorbed down limb (into nephron) making interstitium more concentrated
     3. Ascending limb impermeable to water but permeable to sodium - passive diffusion of NaCl down concentration gradient, this dilutes tubular fluid
4. Distal convoluted tubule
   • Resorption of solute (10%) - Na, K
   • Secretion: variable amounts of K / H
   • Reabsorption of water - distal portions
5. Cortical collecting duct
   • Water reabsorption (via Aquaporin-mediated V2 receptor: Vasopressin, produced in supraoptic and paraventricular nuclei, stored in posterior pituitary)
   • Also leads to increased NaCl reabsorption by thick ascending limb - by increasing concentration of interstitium around loop of Henle.

Glucose and the Nephron

• Filtered glucose normally completely resorbed by kidney
• Above filtration load, glucose starts to appear in urine (saturated resorptive capacity)