Renal Function

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Physiology: Renal Function
Labs: Chemistry Measures of Renal Function
Labs: Other changes in the absence of Renal Function
References
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II. Physiology: Renal Function

Background: Core Renal Functions
1. Sodium and Water Homeostasis
2. Acid-Base Homeostasis
3. Blood Pressure Physiology (including Renin-Angiotensin System)
4. Metabolic Waste Product Excretion
  1. Urea (from Amino Acid metabolism)
  2. Creatinine (from Muscle breakdown)
  3. Uric Acid (Nucleic Acid breakdown)
  4. Conjugated Bilirubin as Urobilinogen (from Hemoglobin breakdown)
5. Synthetic function (e.g. Erythropoietin, Vitamin D, Glucose)
Renal Filtration (Glomerulus)
1. Glomerular Membrane (3 layers)
  1. Capillary endothelium (outer layer)
  2. Glomerular Basement Membrane
    1. Main barrier to leakage of Proteins and other large molecules
    2. Openings within basement membrane are much smaller than those in capillary endothelium or podocytes
    3. Openings within the basement membrane are negatively charged
      1. Repels moderate to large Proteins which are typically negatively charged
  3. Podocytes (epithelial foot processes, inner layer adjacent to bowman's space)
2. Mesangial Cells
  1. Modulates glomerular filtration (when contracted, reduces glomerular surface area and filtration)
  2. Contract in response to Norepinephrine, Epinephrine, Angiotensin II (decreases filtration)
  3. Relax in response to atrial natriuretic factor (increases filtration)
Renal Reabsorption (Renal Tubule)
1. Non-charged molecules (non-polar) are more easily reabsorbed
2. Sodium reabsorption (active transport) occurs primarily in the proximal tubule
3. Carrier molecules facilitate reabsorption of various molecules (e.g. Amino Acids)
4. Other molecule reabsorption (Active co-transport with Sodium)
  1. Water
  2. Glucose, Lactic Acid and Ketones
  3. Water soluble Vitamins
  4. Amino Acids (see below)
Renal Secretion (Peritubular Capillary)
1. Peritubular capillary secretion of molecules (e.g. ammonia, Hydrogen Ion, Potassium) into the renal tubules
Renal Synthesis
1. Erythropoetin (stimulates erythrocyte production)
2. Renin
3. Vitamin D
4. Prostaglandins (e.g. PGE2)
5. Glucose
6. Bicarbonate
7. Ammonia

III. Labs: Chemistry Measures of Renal Function

Serum Creatinine (Cr)
1. Creatinine is a Muscle breakdown product
2. Typically filtered by the normal Kidney at a constant rate, and rises with renal dysfunction
3. However varies with Muscle mass and Muscle activity
Blood Urea Nitrogen (BUN)
1. Protein breakdown product
2. Increases in renal dysfunction, but also with increased Protein ingestion or tissue breakdown
BUN to Serum Creatinine Ratio
1. Prerenal Failure: Ratio >20:1
Glomerular Filtration Rate (GFR, Creatinine Clearance)
1. More accurate Renal Function measure than either Serum Creatinine or BUN alone
2. Rate of fluid filtered through glomerular membrane
3. GFR = uCr/sCr * uVol/t
  1. where uCr = Urine Creatinine concentration (excreted Creatinine concentration)
  2. where sCr = Serum Creatinine concentration (equal to filtered Creatinine concentration)
  3. where uVol/t = volume of urine collected over measured time period (rate of urine production)
Fractional Excretion of Sodium (FENa)

IV. Labs: Other changes in the absence of Renal Function

Serum Potassium increases 0.3-0.5 mEq/L/day
Serum Bicarbonate decreases 1-2 meq/L/day
Serum Calcium decreases
Serum Phosphorus increases
Serum Magnesium increases
Complete Blood Count changes in chronic insufficiency
1. Anemia
2. Platelet Dysfunction

V. References

Goldberg (2014) Clinical Physiology, Medmaster, p. 21-23, 33-4
Guyton and Hall (2006) Medical Physiology, Elsevier, p. 307-47

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