Renal II Flashcards
Which of the following substances will and will not pass freely through glomerular filtration?
a) Water
b) Proteins
c) Fatty acids
d) Low-molecular weight substances
e) Calcium ions
f) Cells
a) Yes
b) No
c) No
d) Yes
e) 1/2 yes, 1/2 no (those bound to albumin)
f) No
What are the major forces responsible for glomerular filtration? What direction do they act in with regards to filtration?
Glomerular capillary blood pressure (PGC) - favours filtration
Fluid pressure in Bowman’s space (PBS) - opposes filtration
Osmotic force due to protein in plasma (piGC) - opposes filtration
What is PGC?
It is the pressure exerted by the blood flow on the glomerular capillary, favouring filtration of the blood into Bowman’s space.
What is PBS?
PBS is the fluid pressure in Bowman’s space, which pushes fluid back into the glomerular capillaries.
What is piGC?
It is the osmotic force created by the proteins in the plasma in the glomerular capillaries, pulling fluid back into the capillaries from Bowman’s space.
What is the formula for net glomerular filtration pressure? What is the typical value?
Net glomerular filtration pressure = PGC - PBS - piGC
Typical value: around 16
What is glomerular filtration rate?
GRF is the volume of fluid filtered from the glomeruli into Bowman’s space per unit time.
GFR is regulated by what three factors? Which is/are the most important?
- Net filtration pressure
- Membrane permeability
- Surface area available for filtration
Net filtration pressure is the biggest contributor.
The others, membrane permeability and surface area available for filtration, are not as significant in normal physiology. In pathological conditions, these get lower.
What is the standard GFR value?
180 L/day
How often does plasma get filtered at the glomeruli? Why?
Plasma is filtered 51 times a day, as most of it gets reabsorbed after it initially gets filtered.
Will the following conditions lead to an increase or decrease in GRF?
a) Constriction of afferent arteriole
b) Dilation of afferent arteriole
c) Constriction of efferent arteriole
d) Dilation of efferent arteriole
a) Decrease GFR
b) Increase GFR
c) Increase GFR
d) Decrease GFR
What is filtered load? What is the formula?
Filtered load is the total amount of any freely filtered substance per unit time.
Filtered load = GFR x plasma concentration of the substance
If filtered load > amount excreted in urine, […] has occurred.
net reabsorption
If filtered load < amount excreted in urine, […] has taken place
net secretion
The tubular lumen is lined by […] cells connected by […] junctions
The tubular lumen is lined by epithelial cells connected by tight junctions
What are the two routes by which reabsoption can occur? Explain the difference between them.
Paracellular: fluid passes through tight junctions between epithelial cells.
Transcellular: fluid passes through epithelial cell.
Compare the reabsorption rates for water, sodium, glucose, urea, and potassium.
Water: 99%
Sodium: 99.5%
Glucose: 100%
Urea: 44%
Potassium: 86.1%
How does the size of filtered loads compare to the amount of that substance in the body?
Filtered loads are enormous, generally greater than the amounts of the substance in the body. This is because most gets reabsorbed and then filtered again.
What determines the reabsorption percent for a given substance?
Its usefulness to the body. Reabsorption of waste products is relatively incomplete (e.g. urea), while reabsorption of important ions and water is relatively complete.
What types of substances have heavily regulated tubular reabsorption? Which don’t?
Not regulated: glucose, amino acids
Highly regulated: water, inorganic ions
What are the two mechanisms of reabsorption?
Diffusion and mediated transport
How does reabsorption by diffusion occur? Is this process usually paracellular, transcellular, or both?
Almost paracellular. Occurs across the tight junctions connecting the tubular epithelial cells, following the electrochemical gradient of the substance.
The reabsorption of urea in the proximal tubule occurs by what mechanism? Explain how.
It occurs via diffusion.
- Urea is freely filtered at the glomerulus
- In the proximal tubule, water reabsorption occurs
- Urea concentration in the tubular fluid becomes higher
- Urea diffuses into the interstitial fluid and peritubular capillaries.
How does reabsorption by mediate transport occur? Is this process usually paracellular, transcellular, or both?
Reabsorption by mediated transport occurs across tubular cells (transcellular epithelial transport).
Requires the participation of transport proteins in the plasma membrane of tubular cells. Usually coupled to the reabsorption of sodium.
The reabsorption of glucose usually occurs by what mechanism? Explain how it works.
It occurs by mediated transport.
1. Along the edge of tubular epithelial cells, there is an Na+/K+ ATPase that transport Na+ out of the cell.
2. Na+ from the tubular lumen wants to enter the cell because its concentration has decreased there. A symporter transports it inside the cell along its concentration gradient, bringing glucose along with it.
3. The glucose in the tubular epithelial cell then leaves via a glucose transporter.
The reabsorption of amino acids usually occurs by what mechanism? Explain how it works.
It occurs by mediated transport.
1. Along the edge of tubular epithelial cells, there is an Na+/K+ ATPase that transport Na+ out of the cell.
2. Na+ from the tubular lumen wants to enter the cell because its concentration has decreased there. A symporter transports it inside the cell along its concentration gradient, bringing amino acids along with it.
3. The amino acids in the tubular epithelial cell then leaves via an amino acid transporter.
What is transport maximum?
When the membrane transport proteins become saturated, the tubule can not reabsorb the substance any more. This limit is called transport maximum.
Give an example of a disease for which transport maximum is relevant and explain how.
Diabetes mellitus: the plasma concentration of glucose can become very high and the filtered load of glucose exceeds the capacity of the tubules to reabsorb glucose. As a result, glucose appears in the urine (glucosuria).
What are the two mechanisms of tubular secretion?
Diffusion and transcellular mediated transport.
What are the most important substances secreted by the tubules?
Hydrogen ion and potassium
By what mechanism does tubular secretion usually work?
It is usually coupled to the reabsorption of sodium.
Describe the division of labour between the proximal tubule, Henle’s loop, and DCT/CD.
Proximal tubule: reabsorbs most filtered water and solutes. It is also a major site of secretion for various solutes EXCEPT K+.
Henle’s loop: also reabsorbs relatively large quantities of ions (less water)
Distal convoluted tubule/collecting duct: relatively small amounts of secretion and reabsorption (fine tuning) volume of water and solutes. More homeostatic controls are exerted here.
What is the definition of clearance?
The volume of plasma from which that substance is completely removed (“cleared”) by the kidneys per unit time.
What is the formula for clearance? What is it used for?
Clearance of S = mass of S excreted per unit time/plasma concentration of S
Where mass of S = urine concentration of S (Us) x urine volume per unit time (V)
Cs = UsV/Ps
This can be used to estimate a person’s glomerular filtration rate.
What is inulin? What is it used for?
Inulin is a polysaccharide that would be administered intravenously. It is freely filtered at the glomerulus and is NOT reabsorbed, secreted, or metabolized.
Clearance of inuli = GFR
It is the most accurate marker of GFR.
What is the most accurate way to estimate GFR? What is a disadvantage of this method?
Inulin clearance. It is the most accurate, but is also a highly cumbersome method.
What is most commonly used as a clinical marker for GFR? Why?
Creatinine. It is freely filtered at the glomerulus and is NOT reabsorbed or metabolized. It is secreted slightly but the amount is small. It is less cumbersome than inulin, which must be administered intravenously.
What is the formula for creatinine clearance?
Creatinine clearance = UcrV/Pcr = GFR
V = urine volume
Ucr = urine concentration of creatinine
Pcr = plasma concentration of creatinine
For para-amino-hippurate, how does clearance compare to GFR?
Since there is net secretion, clearance > GFR
For water, how does clearance compare to GRF?
Since there is net reabsorption, clearance < GFR
How does clearance vary between individuals?
Each person has their own unique clearance value.
How does GFR vary between individuals?
It varies even among individuals for different substances.
What is the significance of clearance being bigger or smaller than GFR?
If clearance > GFR, secretion has happened at some point in the tubules.
If clearance < GFR, reabsorption is occuring in the tubule.
