Glomerular filtration and tubular reabsorption Flashcards
What is Glomerular Filtration?
the bulk flow of fluid from the glomerular capillaries into Bowman’s capsule
What are freely filtered substances? Examples (6)?
Substances that are at the same concentration in the plasma and the filtrate
e.g. Na, K, Cl, HCO3, glucose, urea
Glomerular capillaries are like other capillaries in that filtration depends on Starling forces. What is the major difference in these forces in the kidney compared to other capillaries?
hydrostatic pressure is much higher than in other capillaries
Is there any reabsorption in the glomerulus?
No
What are the layers that a substance must pass through to be filtered from the glomerulus? (3)
Endothelial cell layer
BM
Podocytes
What are the characteristics of the endothelial cell layer in Bowman’s capsule?
Spaces between the cells are larger than in a normal capillary
What are the three basic nephron mechanisms?
Filtration
Reabsorption
Secretion
What are the two barriers to protein filtration through the endothelial cell membrane in the glomerulus?
- Small size of endothelial pores
2. Negative charges on endothelium and podocytes (proteins usually negatively charged)
What is nephrotic syndrome? How is it caused?
Increased permeability of the glomerular capillaries to protein
Disruption of structure of podocytes via mutation in Nephrin gene
What is the function of mesangial cells in the glomerulus?
Contract and change the surface area of the capillary
A mutation in what gene leads to a steroid resistant nephrotic syndrome?
Podocin gene
What happens to the net filtration pressure as you move along the capillary? Why?
Oncotic pressure rises and net filtration pressure decreases. However, NFP >0
What is the oncotic pressure in Bowman’s capsule (normally)?
0
What is the filtration fraction?
the percent of the renal plasma flow (RPF) that is filtered at the glomerulus
If the FF is increased then when will happen to the oncotic pressure at the efferent end of the glomerular capillaries?
Increased
If the FF is lowered then the oncotic pressure at the efferent end of the glomerular capillaries does what?
Decreases
What happens during severe sweating to the GFR? How?
Colloid osmotic pressure in the blood increases since sweat has no proteins, leading to lower GFR
What happens during severe diarrhea, emesis to the GFR? How?
Colloid osmotic pressure in the blood increases since diarrhea/emesis has no proteins, leading to lower GFR
What happens to the GFR during renal stones? How?
Decreases, since there is a higher osmotic pressure in the glomerulus
What are the four possible outcomes of Filtration/reabsorption in the kidney tubule?
- Filtration only
- Filtration, partial reabsorption
- Filtration, complete reabsorption
- Filtration, secretion
Why do we filter 100% of plasma so often?
Difficult to control plasma volume and composition if only filtered once a week or even once a day
What is transcytosis? Why is this important for renal tubule function?
Coupled endocytosis and exocytosis through a cell
Brings in proteins lost from filtration and brings out Ig
What are the two ways in which proteins lost in the urine are reabsorbed?
- Through transcytosis
2. Breakdown into AA by attached enzymes, then reuptaken
How easily can the processes the take back in protein be saturated? What is the consequence of this?
Easily, thus proteinuria
Why can renal disease result in increased levels of peptide hormones?
Because they are the major place peptide hormones are broken down
Why can only a small change in Na excretion/reabsorption have a large effect?
Filtered so many times
What specialization do the cells that face the renal lumen have to aid in reuptake?
Brush border
What is the function of the Na/K pump on the lumen cells?
Establishes low intracellular [Na], allowing to easy reuptake
How is glucose brought back in from the tubule lumen?
Via SLGT (Na symporter)
How are AAs brought back in from the tubule lumen?
Via AA/Na symporter
What is the effect of the NHE (Na, H exchanger)?
Brings Na into the cell,
Pushes H out into the lumen
Because the tight junctions of the proximal tubule are permeable to Na+, Na+ transport is a gradient limited system. What does this mean?
If the concentration of Na+ became higher in the interstitial fluid than it is in the tubular lumen, the Na+ would back-leak into the tubule.
What are the two forces acting to pull in Na?
Chemical and electric gradient (note these are in the same direction–into the cell)
What allows for water to move so freely from the nephron tubules to the capillaries? What is this called?
Aquaporins on both sides, and Na gradient
isosmotic reabsorption
How is Cl maintained in the cell?
Cl/anion antiporters on the apical side of the cell
Cl/K symporters (goes out to capillaries) on the basal surface
What prevents a negative charge from building up in the tubule lumen (since Na is being brought out)?
Cl is being taken up with Na
What is secretion?
The active process or pumping out chemicals from the capillaries
Why does glucose appear in the urine of DM pts?
The plasma concentration of glucose is higher than the transport maximum of a nephron. Thus the filtered glucose that cannot be reabsorbed in the proximal tubule passes through the rest of the nephron into the urine.
What happens to water reabsorption in DM? What symptom does this cause?
Decreases b/c there is more water taken by glucose in the urine
This causes the polyuria/polydipsia seen in DM
What causes the “Splay” of nephrons, (referring to the amount of glucose that is lost in the urine prior to reaching the maximum threshold of reabsorption)?
Some nephrons are worse at handling the loads than others.
