Physiology Lecture 1 and 2 -- Kidney Function Flashcards
4 hormones secreted by the kidney
- Renin
- Angiotensin II
- Erythropoietin
- Activated vitamin D
Renal blood flow
1 L/min (20% of CO)
3 layers in the glomerular endothelium that separate the circulating blood from the capillary space
- Innermost = capillary endothelial cell
- Middle = collagenous basement membrane
- Outer = podocyte
6 constituents of the ultrafiltrate
- Water
- Electrolytes
- Urea
- Cr
- Sugar
- Amino acids
3 substances that should not appear in the ultrafiltrate
- Cells
- Proteins
- Fats
Charges of the capillary basement membrane and podocytes
Negative
2 conditions in which abnormal amount of albumin will cross the glomerular basement membrane
- Damage to the structure of the filtration barrier
- Loss of negative charge of the filtration barrier
Define GFR
The amount of plasma filtered through the glomeruli per unit time (most often refers to all nephrons collectively)
How to dose medication excreted by the kidneys if a patient has low GFR
Los GFR = less medication or simply avoid certain meds
3 forces involved in glomerular filtration
- Pgc = capillary pressure
- Pt = tubular pressure
- πgc = oncotic presusre (from unfiltered proteins in tubule)
3 characteristics of creatinine that make it a good estimate of GFR
- Has stable plasma concentration
- Freely filtered at the glomerulus
- Stays in the tubule to be excreted (is not reabsorbed)
Normal GFR
~100 mL/min
Equation for creatinine clearance
Relationship between plasma Cr concentration and GFR
Inverse
If a patient’s GFR falls by 50%, what happens to the newly produced creatinine?
Newly produced creatinine accumulates until the filtered load again equals the rate of production (i.e. when the plasma Cr has increased by x2)
4 formulae to estimate creatinine clearance in the hospital
- CKD epi
- MDRD
- COckroft Gault
- Schwartz
Cockroft Gault equation
NOTE: GFR must be in a steady state to use formula
What is a specific application for the Schwartz formula?
Can be used for children
What is a specific application for CKD-epi?
Screening (can estimate GFR without taking any actual measurements like weight)
What is a specific application for the Cockroft Gault equation?
More specific estimate of GFR since it takes into account body mass, so can use on patients who are amputees or have abnormal body weight (i.e. low BCM)
How does Cr clearance overestimate GFR?
- It is secreted in the proximal tubule
- –> Cr excretion exceeds Cr filtration by 10 - 20%
Perfect molecule to estimate GFR
Inulin (filtered, not reabsorbed, not secreted)
2 limitations to creatinine clearance as an estimate of GFR
- Cr production varies with muscle mass (sarcopenia in older people ages 50 - 90)
- Varies with meat intake (may have high Cr due to eating steak 3x per day)
Urea levels in hypovolemic states
Rises out of proportion to Cr, so can be useful in assessing patients with acute renal failure
3 autoregulation mechanisms to maintain GFR
- Myogenic reflex = vasospasm of afferent arteriole (50% of autoregulation)
- Tubuloglomerular feedback
- Glomerular vasoconstrictors and vasodilators
Describe the myogenic reflex
- BP too high = afferent arteriole sm contracts
- BP low = afferent arteriole stretches/dilates
Describe tubuloglomerular feedback
- Macula densa samples fluid (senses Cl concentration –> low = glomerular blood flow is low)
- Signals to arteriole to open up (possibly using adenosine)
- Juxtaglomerular cells also release renin
Describe the effect of glomerular vasoconstrictors and vasodilators
In situations of a major drop in systemic BP:
- Activation of sympathetic nervous system
- Increase production of angiotensin II
Angiotensin II constricts efferent arterioles more than afferent arteriole to preserve glomerular capillary P
Main purpose of the tubules
Reabsorb most of the water and solute of the filtrate back from the tubules into the circulation
Define resorption
Move from urine back to the body (apical –> basolateral)
Define secretion
Moves from the blood to the urine (basolateral –> apical)
Define excretion
Leaves the body in the urine
Only place in the body where a capillary does not drain into a vein
Glomerulus (drains into efferent arteriole)
General effect of ADH on kidney
Induce collecting duct water permeability by insertion of aquaporins
6 functions of the proximal tubule
- Na+ reabsorption
- Reabsorption of almost all glucose, phosphate, amino acids
- Reabsorption of bicarbonate
- Secretion of uric acid and organic acids (i.e. drugs)
- Water reabsorption
- Calcium, potassium and chloride movement along their concentration gradients
How does reabsorption occur in the proximal tubule?
Basically, it is all coupled to their respective Na+ cotransporters
3 functions of the Loop of Henle
Separation of Na and water movement –> countercurrent mechanism
- Thin descending limb permeable to water
- Thick ascending = Na+ out of lumen via Na-K-2Cl cotransporter in apical side
+ Ca and Mg reabsorption
2 functions of the distal convoluted tubule
- Reabsorption of Na via Ca-Cl cotransporter
- Calcium reabsorption (via channel in apical side and exchanger in basolateral side)
4 functions of collecting duct
Principals cells
- Na and water reabsorption
- K+ secretion
- Selective Na+ channels under aldosterone control
Intercalated cells
- Water transport (site of ADH action)
