The glomerulus Flashcards
Which substances are recovered by the kidney?
- Water (<99%)
- Na+ and Cl+ (<99%)
- Bicarbonate (100%)
- Glucose and amino acids (100%)
Describe the electrolyte composition of the extracellular fluid
- Low K+
- High Na+
- Main anions are Cl- and HCO3-
- Failure to control these will affect transport and electrical functions.
Describe the electrolyte composition of intracellular fluid
- High K+
- Low Na+
- Many large organic anions
What is the RBF?
- Renal blood flow = 1l of blood flowing through glomeruli per minute
What is the RPF?
- Renal plasma flow = amount of plasma flowing through glomeruli at any one time
- 600 ml/min
- RPF = RBF x (1-haemoatocrit)
What is the filtration fraction?
- Proportion of fluid reaching the kidneys that passes into the renal tubules.
- FF = GFR/RPF
- Normally = ~20%
What is the composition of the glomerular filtrate?
- It is composed of mostly organic solutes with a low molecular weight and inorganic ions.
- Contains no platelets
- Contains virtually no proteins
How can you distinguish between the PCT and the DCT in a histology specimen?
- PCT has a brush border to help increase reabsorption.
- DCT can be seen snuggled up to the glomerulus
What is the function of the podocytes?
- They help create a selectively permeable barrier so that not all molecules can pass into the Bowman’s capsule.
- They have a negative charge which allows them to repel other negatively charged molecules like albumin.
What results in proteinuria?
- The negative charge on the filtration barrier between the glomerulus and the Bowman’s capsule is lost.
- This means that proteins are more readily filtered.
What molecular weight is too big to pass through the filtration barrier of the nephron?
- Anything bigger than 68000
What is the GFR?
- Glomerular filtration rate is used to measure kidney filtration function.
- It is the amount of filtrate produced from the blood flow per unit time.
- Measured in mL/min
- Determined by the average volume of filtrate produced by each nephron.
What does a decrease in GFR mean?
- Kidney function has worsened
- There has been a decline in number of nephrons or a decline in GFR within individual nephrons.
- Slow decline in kidney function can cause individual nephrons to hypertrophy, so kidney function may not fall until significant damage has occurred.
What is renal clearance?
- A surrogate marker for GFR
- The volume of blood plasma that is cleared of a substance in a unit of time.
What is the equation for renal clearance?
- Clearance = [urine concentration x urine flow rate] / plasma concentration
What are the units for the equation for renal clearance?
- Clearance is measured in mL/minute
- Urine concentration of x = mg/mL
- Urine flow rate measured in mL/min
- Plasma concentration of x = mg/mL
How is filtration rate calculated?
- Filtration rate = Plasma concentration x GFR
How is excretion rate calculated?
- Excretion rate = Urine concentration x urine flow rate
What are the factors that affect GFR?
- Young age - due to lower nephron numbers
- Old age - due to loss of functioning nephrons
- Pregnancy - causes GFR and kidney size to increase
What are the ideal qualities of a substance that is used to measure kidney clearance?
- Be produced at a constant rate
- Be freely filtered across the glomerulus
- Not be reabsorbed in the nephron
- Not be secreted into the nephron
- E.g. inulin
Why don’t we use inulin to measure renal clearance?
- It requires continuous IVI to maintain a steady state
- Requires catheter and timed urine collections
What substance do we use to measure renal clearance?
- 51 Cr-EDTA
- A radioactive labelled marker
- Given as a timed injection with blood samples taken 2, 3 and 4 hours afterwards.
What is creatinine?
- The end product of muscle breakdown
- Can be used to measure GFR (in pregnancy)
- Measured by urine creatinine over 24 hours or serum creatinine
What are the disadvantages of using creatinine as a measure of GFR?
- Cumbersome - you have to carry a bottle of urine
- Frequently inaccurate
- Overestimates GFR by 10-20% due to creatinine secretion (i.e. creatinine is secreted into the nephron from the blood plasma).
What is the normal value range for serum creatinine?
- 70-150 micromoles/L
What factors may cause serum creatinine to increase?
- Large muscle bulk
- Young
- Male
- Creatine supplements
- High intake of meat
- Certain drugs e.g. trimethoprim
What factors may cause serum creatinine to decrease?
- Reduced muscle mass
- Muscle cell breakdown
- Old
- Female
- Vegetarian
What are some of the problems with estimating GFR?
- It’s inaccurate in mild kidney disease
- Reduced nephron number leads to nephron hypertrophy so there’s no change in GFR.
- Reduced filtration of creatinine causes increased secretion of creatinine into the tubule.
