Exam #4: Glomerular Filtration

Question 1

What is ultrafiltration at the glomerulus?

Answer 1

Bulk flow of fluid from the glomerular capillaries into Bowman’s Capsule

Question 2

What is tubular reabsorption?

Answer 2

Returning filtered substances back to the circulation & ECF

See p. 261

Question 3

What is tubular secretion?

Answer 3

Removal of substances from the peritubular capillary blood & adding them to the urine

See p.261

Question 4

Explain why large, negatively charged molecules are less permeable to the glomerular capillary than smaller or neutral molecules.

Answer 4

• Endothelial cells of the glomerular capillary are negatively charged with mucopolysaccharides
• Negative charges are present on the filtration slits of the epithelium in the glomerular capillary

*****Because of these negative charges, large negatively charged proteins are repelled from being filtered. Small molecules & neutral molecules are not influenced.

Question 5

List the Starling forces and tell whether each would promote filtration or not. Write the equation for GFR.

Answer 5

N/A

Filtration coefficient= water permeability of hydraulic conductance of the glomerular capillary wall

Hydrostatic pressure in glomerular capillary= favors filtration

Hydrostatic pressure in bowman’s space= pressure from the fluid in the lumen; opposes filtration

Oncotic pressure in glomerular capillary= opposes filtration; this is determined by the protein concentration in the glomerular capillary, which is higher than bowman’s space

See p. 257

Question 6

Define term filtration fraction.

Answer 6

Filtration Fraction is the percent of renal plasma flow (RPF) that is filtered at the glomerulus or,

GFR/RPF= ~20%

Question 7

How do changes in the filtration fraction affect the oncotic pressure at the efferent end of the glomerular capillaries?

Answer 7

High Filtration Fraction= High Oncotic pressure at the efferent arteriole

Low Filtration Fraction= Low Oncotic pressure at the efferent arteriole

Question 8

Describe the different transport mechanisms that are used for reabsorption.

Answer 8

Mechanisms for reabsorption involve transporters in the membranes of the renal epithelial cells

Question 9

Describe the different transport mechanisms that are used for secretion.

Answer 9

Secretin, like reabsorption, involves transporters in the membranes of epithelial cells lining the nephron

Question 10

Where is Na+ reabsorbed in the nephron?

Answer 10

• First, note that ~99% of the filtered Na+ is reabsorbed through the nephron i.e. urine contains <1% of the filtered Na+

1) 67% of filtered Na+ is reabsorbed in the proximal tubule–isosmotically
2) 25% of filtered Na+ is reabsorbed in the thick ascending limb, WITHOUT WATER
3) Distal tubule & collecting ducts reabsorb the remaining 8%

Question 11

How is glucose reabsorbed? Trace the path from tubule lumen to capillary describing the transport process used.

Answer 11

• Remember:
  a) Na+ & glucose are cotransported in the early proximal tubule (reabsorbed together)
  b) 100% of filtered glucose is normally reabsorbed by the end of the early proximal tubule

1) Secondary active transport i.e. cotransport of Na+ & Glucose into the early proximal tubule cell (Na+ downhill & Glucose uphill)
2) Glucose is transported into the peritubular capillary blood via facilitated diffusion i.e. GLUT 1 & GLUT2 transport proteins

Question 12

How is water reabsorbed? Trace the path from tubule lumen to capillary describing the transport process used.

Answer 12

• Reabsorption of solute into the proximal tubule creates a transtubular osmotic gradient that favors water reabsorption here as well

Question 13

Describe how proteins and peptides are reabsorbed.

Answer 13

• Proteins are reabsorbed in the proximal tubule
• Enzymes on the luminal surface (inside) degrade proteins into small peptides & amino acids
• These peptides & amino acids bind receptors on the liminal surface (inside) & are transported by carrier protein to the basolateral membrane
• Facilitated diffusion or “uniporters” transport these peptides & amino acids into the peritubular capillary

Question 14

Define “transport maximum.”

Answer 14

Point at which increases in concentration do not result in an increase in movement of a substance across a membrane.

Question 15

Explain why a person with a very high blood level of glucose will have a high urine volume (high urine excretion rate).

Answer 15

• Filtered load= GFR x plasma concentration of substance, in this case, glucose; Increase in plasma glucose = increase in filtered glucose
• There comes a point at which Na+/glucose cotransporters can no longer keep up & filtered glucose is excreted

Question 16

What is osmotic diuresis?

Answer 16

• Plasma concentration of glucose > transport maximum of nephron= filtered glucose will be excreted
• Increased glucose in the urine decreases water reabsorption, leading to increased water in the urine

*Net result= “osmotic diuresis”

Question 17

What does it mean to say that a substance is “freely filtered?”

Answer 17

The substance is in the same concentration in the plasma as it is in the ultrafiltrate
- Includes low molecular weight substances e.g. Na+, K+, Cl-, bicarboante…etc.

Question 18

What are the differences between the Starling forces in the glomerular capillaries & regular capillaries?

Answer 18

• Higher hydrostatic pressure b/c the glomerular capillary is between 2x high pressure circuits: afferent & efferent arterioles

Question 19

List the layers of the glomerular capillary.

Answer 19

Endothelium
Basement membrane
Epithelium

Question 20

How do the pores of the glomerular endothelium compare to the normal capillary?

Answer 20

Pores are larger, which allows fluid, dissolved solutes, and plasma proteins to the filtered across the membrane

*Note that these pores are NOT large enough to normally let RBCs through

Question 21

What specialized cells are contained within the epithelium of the glomerular capillary?

Answer 21

• Podocytes that are attached to the basement membrane by foot processes
• The gaps between podocytes are filled with “filtration slits” that is closed by a thin membrane

Question 22

Why is there high oncotic pressure in efferent arterioles & peritubular capillaries?

Answer 22

• As fluid if filtered out of the glomerular capillary, protein is left behind.
• By the end of the glomerular capillary, this oncotic pressure is so high that net ultrafiltration pressure becomes zero.
• Thus, the blood entering the peritubular capillary has a high oncotic pressure, which drives reabsorption

Question 23

How are Starling’s Forces altered in the glomerular capillary if there is a renal stone obstructing the kidney?

Answer 23

Hydrostatic pressure in Bowman’s Space is dramatically increased

Question 24

How is Na+ reabsorbed in the early proximal tubule?

Answer 24

• Remember a total of 67% of the filtered Na+ is reabsorbed in the proximal tubule–isosmotically
• Na+ moves into the early proximal tubule cell via contransport i.e. Na+ moves DOWN its electrochemical gradient, coupled with glucose, amino acids, phosphate, lactate, and citrate
• Once in the cell the concentration gradient & electrical gradient DO NOT favor the movement of Na+ into the interstitial fluid; thus, an Na+/K+ ATPase uses “primary active transport” to extrude Na+ into the interstitial fluid

p. 272

Question 25

How is Na+ reabsorbed in the late proximal tubule

Answer 25

*Remember a total of 67% of the filtered Na+ is reabsorbed in the proximal tubule–isosmotically

Question 26

What is the countertransport or exchange mechanism that exists in the luminal side of the early proximal tubule?

Answer 26

Na+/ H+ exchanger

• Na+ comes INTO the cell, DOWN its electrochemical gradient
• H+ is exchanged & moves out of the early proximal tubule cell, into the tubular lumen

p. 272

Question 27

How and where is NaCl reabsorbed?

Answer 27

• NaCl is reabsorbed in the late proximal tubule
• There are BOTH transcellular & paracellular mechanisms that exist for the reabsorption of NaCl in the late proximal tubule

p. 273

Question 28

What is splay?

Answer 28

Not all tubules have exactly the same transport maximum; thus, there is some “splay” in the actual glucose reabsorption vs. ideal

p. 264