Intro to renal physiology

Question 1

Functions of the renal system

Answer 1

• Maintain composition and volumes of body fluids
• Excrete metabolic waste
• Regulate blood pressure
• RBC production (EPO)
• Regulate vit. D production
• Gluconeogenesis

Question 2

Urine formation

Answer 2

• Filtration: transport from glomerular capillaries into renal tubular lumen, occurs in glomerulus (small solutes enter filtrate by convection)
• Reabsorption: transport from tubular lumen fluid back into circulation across or between epithelial cells
• Secretion: transport from non-glomerular capillaries or interstitial fluid into tubular lumen across or btwn epithelial cells
• All of these together lead to excretion (F+S-R)

Question 3

Relationship of excretion and filtration for various compounds

Answer 3

• Inulin: excretion = filtration (all that is filtered is excreted)
• Glc: excretion < filtration (net reabsorption)
• PAH: excretion > filtration (net secretion)

Question 4

GFR

Answer 4

• The rate (ml/min) of ultra filtrate across the glomerular capillaries into bowman’s space (protein free)
• Sum of all glomeruli in the body, normal is 120ml/min (provides estimate of functioning renal mass)
• 20% of plasma is filtered thru glomerulus, the other 80% continues in circulation to the peritubular network
• RPF (ml/min): renal plasma flow (amount of plasma going to the kidneys)
• Filtration fraction: GFR/RPF = 120/600 = .2

Question 5

Clearance

Answer 5

• Refers to rate of removal of a substance from a volume of plasma/blood (vol/time)
• Renal clearance: rate of the blood/plasma to be cleared of a substance by its excretion in urine over time
• ml of plasma cleared of substance per unit time (ml/min)
• Best estimate is the GFR for a substance (inulin, creatinine)

Question 6

Calculating clearance/GFR

Answer 6

• Need 3 things to solve for clearance (U, V, P): [substance in urine], (vol of urine produced/min, urine flow rate), and [substance in plasma]
• Cl = UxV/P
• When there is high concentration of the substance in urine, there will be lower plasma concentration of the substance
• This leads to a high clearance, which is also based on urine production rate

Question 7

Clearance for inulin and creatinine

Answer 7

• Since the amount of inulin filtered is equal to the amount excreted (amount in plasma will equal amount in urine), the only thing that matters it is the urine formation rate (GFR = clearance)
• For creatinine, the plasma concentration varies inversely with GFR (at high GFR there is low [P]) because Cr excretion is equal to Cr production
• Therefore, we can estimate GFR based on the [P] of Cr, similarly to inulin

Question 8

Limitations to Cr GFR estimation

Answer 8

• Cr levels can be elevated by high meat diet, or low in malnourished pts
• Cr and GFR can both be low in cirrhotic pts w/ normal [P]cr
• When GFR is low there is high secretion of Cr

Question 9

Blood urea nitrogen (BUN)

Answer 9

• 90% of urea is excreted in kidneys, so if BUN is elevated there must be a decrease in GFR
• Can also be elevated w/ dehydration ;)

Question 10

Reabsorption

Answer 10

• Transport of substance from tubular fluid across/btwn epithelia back to blood
• Glc reabsorption: reabsorbed via SGLT (couples Na and glc together)
• Relies on Na gradient (favors reabsorption), due to Na/K ATPase on the basolateral surface of the epithelial cell
• Glc excretion is less than the filtered load of glc, thus the difference is the reabsorption rate
• Normally all glc is reabsorbed (none in urine)

Question 11

Secretion 1

Answer 11

• Transport of substance from (non glomerular) capillaries across the basolateral then apical epithelial membrane into the urine
• PAH: rate of PAH excretion > rate of PAH filtration, and this difference is PAH secretion
• PAH is brought into the epithelial cell (secretion) via PAH/aKG antiporter (using aKG gradient favoring reabsorption) on the basolateral membrane

Question 12

Secretion 2

Answer 12

• PAH then uses a channel on the apical membrane to exit the epithelial cell
• aKG is brought into the cell from apical Na/aKG cotransporter (reabsorption), thus everything is dependent on the Na/K ATPase to generate Na gradient
• Since PAH is mostly excreted from secretion (not filtration), it is a good indicator of RPF
• Measuring RPF is the same equation as GFR, when looking at PAH: [U]xV/[P]