QUESTIONS

1. What is Bowman�s space & what cells comprise the filtration barrier?
    
2. What is the Macula Densa?
    
3. Describe the branches of blood supply to the kidney.
    
4. What do interstitial cells do?
    
5. What three cell types make up the juxtaglomerular apparatus?
    
6. Describe urea excretion.
    
7. If a substance is protein-bound, does this interfere with its filtration by the glomerulus?  How about any tubular secretion?
    
8. What urine production rate is defined as �Oliguria?�
    
9. There are two causes of Acute Renal Failure which stand out (2 causes of  sustained GFR reduction).  What might they be?
    
10. How is Acute Tubular Necrosis a vicious cycle?
     
11. Mannitol has more to offer you in oliguric ARF besides diuresis. What are some other positive effects mannitol administration  might have?
     
12. In oliguric ARF, it is VIP to beware of overhydration.  Watch how much fluid comes out & only put that much in.  Why shouldn�t you give mannitol to an overhydrated patient?
    
    
    
     

ANSWERS

1. Okay.  Think of the glomerular capillaries as a fist and Bowman�s capsule as a loosely filled water balloon into which the fist is punching.  Bowman�s space is the space between the wall of the balloon which is in contact with the fist and the opposite wall of the water balloon.  (It is not the space between the fist and the water balloon.) 
   
   The filtration barrier consists of the capillary endothelium, basement membrane of the capillary endothelium, and a single layer of capsular epithelium.  The capillary endothelial cells are separated by large fenestrations. The basement membrane is a mesh of mucopolysaccharides and glycoproteins.  The capsular epithelial cells are called �Podocytes.�  Podocytes have numerous extensions called �foot processes� which are embedded in the capillary basement membrane.  There are slits in between the podocytes for the filtrate to go through to get into Bowman�s space.  The slits are bridged by diaphragms for control.
   
   The order to remember is:  Blood, capillary endothelial cells, basement membrane, foot processes, podocytes, Bowman�s space.
   
   
   
   
    
2. The macula densa is an area of the ascending loop of Henle where the loop passes between the afferent and efferent arterioles.  (Imagine a V of arterioles with the loop passing between the arms.)  This area of the loop up to the distal tubule is the macula densa.  The macula densa plus the V of arterioles make up the juxtaglomerular apparatus.
   
   
   
   
    
3. Renal artery >> interlobar artery >> arcuates >> interlobular arteries >> afferent arterioles >> glomerular capillaries >> efferent arterioles >> peritubular capillaries >> venous system.
   
   
   
   
    
4. Interstitial cells become more numerous the deeper you go.  They are located between the tubules & capillaries & they produce prostaglandins.
   
   
   
   
    
5. The juxtaglomerular apparatus is composed of:
   
   a) Granular cells (smooth muscles cells in the arterial walls which secrete renin).
   
   b) Extraglomerular mesangial cells (function unknown).
   
   c)  Macula Densa cells (involved in control of renin secretion & in controlling GFR).
   
   There are also lots of sympathetic neurons here, too.
   
   
   
   
    
6. Water is happily & readily resorbed in the prox. convoluted tubule.  With water being resorbed, this means that the concentrations of all solutes in the tubule is higher than their corresponding concentrations in the peritubular capillaries.  Urea readily diffuses out into the peritubular capillaries.  By the time we get to the distal tubule, membranes are not very urea permeable & urea cannot diffuse out any more.  When we reach the medullary collecting ducts, urea diffusion out is very high again. It sounds like urea just gets resorbed & doesn�t get excreted at all but here�s the cute part.  Urea diffusion out of the tubule is so high in the area of the medullary collecting ducts that there ends up being a ton of urea outside the tubule here.  The loop of Henle dips into this area & when it does, the urea diffusion gradient reverses & urea ends up being secretied back into the tubule in this area!  Net result:  60% of the originally filtered urea is reabsorbed back & 40% is excreted in urine.
   
   
   
   
    
7. Protein bound stuff cannot be filtered by the glomerulus because the protein is too big.  This does not interfere with tubular secretion, though.
   
   
   
   
    
8. Oliguria is 0.5 ml/kg BW urine production per hour.
   
   
   
   
    
9. They be:  ischemia & toxins.
   
   
   
   
    
10. Tubular damage leads to  a filtrate with too much Na+.  (Na+ resorption will be impaired - patient may well be hyponatremic!)  The juxtaglomerular apparatus reads this & constricts the afferent arteriole which leads to ischemia & more tubular damage etc.  etc.  Also, increases in Na+ inside the tubular cells leads to cell swelling as H20 gets sucked in which partially obstructs the tubular lumen & doesn�t help.
    
    
    
    
     
11. Mannitol has a free radical scavenging effect & also causes renal vasodilation.  As a diuretic, mannitol has a  special advantage over other diuretics, too.  Mannitol works on the entire tubule, not just part of it.  This helps push out any debris plugging the tubule all the way through the tubule.
    
    
    
    
     
12. A fluid overloaded patient is already hypertonic to begin with.  If you add a hypertonic agent like mannitol, you will suck in even more fluid from the periphery & overload the circulatory system.  The patient will drown.   :(