Renal Physiology II Flashcards
*Understand Renal Clearance
Clearance of substance: the Volume of plasma from which the substance is completely cleared, by kidneys per unit time.
*Know the general principles of tubular reabsorption and secretion
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*Understand the concept of transport maxima
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*Understand the specific mechanisms of Solute transport in different nephron segments
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*Understand how the nephron concentrates urine
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What does principle of clearance emphasize? What is the formula for clearance??
Principle of Renal clearance: emphasizes Excretory function of kidneys. It considers only rate at which substance is excreted into urine
formula; Cx= UxV/Px: Cx= clearance of substance x
Ux= urine concentration of x; V= urine volume (mL/min)
Px= plasma concentration of x (mg/ml)\
clearance units: mL/min
What is another important factor in Renal system that clearance can be used to calculate? What is the formula and when will GFR equation only be true?
Renal clearance can be used to estimate GFR (glomerulus filtration rate)/ Formula: GFR= UwV/Pw. Equation only true if W is: 1. Freely filterable at the renal corpuscle 2. Not reabsorbed 3. Not secreted 4. Not synthesized by tubules 6. Not broken down by tubules
What substance fits the description of special properties needed to use clearance for GFR?
INULIN (polysaccharide)- fits the description and can be used to determine GFR. GFR= renal clearance of inulin.
Why is measuring GFR NOT convenient? What substance is frequently used instead? What are its properties and how does it affect GFR?
Measuring GFR by inulin is inconvenient because Inulin is NOT a naturally occurring substance
Clinic CREATINE is frequently used instead.
Clinic creatine is freely filterable (like inulin), and NOT reabsorbed, but a SMALL amount is SECRETED.
hence, Creatine is slightly HIGHER than GFR>
What is the normal value of plasma creatine? What happens to creatine levels when GFR decreases? What occurs when plasma creatine is raised (increased) ?
Normal Plasma Creatine= 10 mg/L.
If GFR Decreases 50%, Plasma creatine rises but stabilizes at 20 mg/L. Hence plasma creatine increases as GFR decreases?
large increase in creatine, can be small drop in GFR
When plasma creatine is raised, it indicates that RENAL function, especially GFR is IMPAIRED (less substance filtered)
What is the role of Para-aminohippurate (PAH)? what are its properties and what does it indicate? What is the equation for clearance of PAH? What is the RBF equation in terms of RPF (renal plasma flow)?
PAH- substance Filtered at Glomerulus and COMPLETELY SECRETED. NOT reabsorbed.
The clearance of PAH approximates Renal Plasma Flow.
10-15% of total renal plasma flow supplies non-filtering and non-secreting portions of kidney, such as (peripelvic fat) and this plasma cannot lose its PAH by secretion.
Clearance of PAH (CPAH)= UPAHV/PPAH= ERPF (effective renal plasma flow)
ERPF= 0.9RPF (90% of RPF)
RBF=RPF(1-hematocrit)
What are the 5 main types of transport mechanisms in nephron tubules ?
- Diffusion
- Facilitated Diffusion
- Primary active transport (requires energy; ATPASE)
- Secondary active transport (cotransport and countertransport)
- Bulk flow or Solvent drag
Define the term Diffusion?
Diffusion of uncharged solutes occur from area of HIGHER concentration to one of LOWER concentration. Diffusion of ions (Na+, K+) is affected by Electrical potential difference across cell membrane of renal tubule cells
What is facilitated diffusion? What proteins are involved?
Facilitated diffusion- depends on interaction of Solute with specific protein in the membrane that facilitates it movement across membrane.
The membrane proteins involved in facilitated transport are TRANSPORTERS
What is primary active transport? What is needed for this form of transport? How does active transport occur in renal tubule cells?
Primary Active Transport- movement of molecules through mechanism directly coupled to energy derived from metabolic processes (it consumes ATP)
Active transport of solutes- occurs against concentration gradient (from LOW [ ] to HIGH [ ]) Primary active transport in tubule cells use ATP-dependent transporters or through ENDOCYTOSIS.
Describe the process of endocytosis?
Endocytosis- transport process where there is an invagination of a part of cell membrane until it pinches off and forms a vesicle in cytoplasm.
Compare and contrast secondary active transport and solvent drag
Secondary active transport in kidney cells, occur when the energy from downhill movement of solute provides energy for uphill movement of another solute
Solvent drag- when water is reabsorbed and solutes which have been dissolved in the water is reabsorbed with water.
What are the main routes of reabsorption?
Routes of reabsorption:
1. Transcellular - goes across the tubular cell;
susbtance must cross two membranes (must cross lumenal into cell, and then cross basolateral membrane into interstitial.
2. Paracellular- goes between the two cells
-go out of LUMEN between cells.
What must occur for substance to be reabsorbed transcellularly? What about transport?
For any substance to be reabsorbed transcellularly, ONE of its two movements have to be ACTIVE.
2 modes of transport must be DIFFERENT
ex: ubiquitous Na K+ ATPASE- form of PRIMARY Active transport:; transports sodium out of cell and potassium into the cell (3:2 ratio)
Na+ go into cell by facilitated diffusion and Na+ go Out of cell by primary active transport.
How does Glucose get transported in the cell? What about sodium?
Glucose goes OUT of cell through facilitated diffusion and glucose goes IN to the cell by SECONDARY active transport COUPLED with SODIUM (Na+).
Na+ out of cell by Na/K+ Atpase pump and sodium goes into cell by diffusion
Distinguish between how lipid soluble and poor-lipid soluble susbtances moves across cell?
Lipid soluble substances can transverse BOTH membrane (and cytosol) by Diffusion and Net Passive reabsorption occurs by transcellular route
- transcellular movement for substances that are poorly lipid-soluble is active. Generalization is active net reabsorption of substance requires:
1. that luminal and basolateral membranes are asymmetrical for substance (contain different channels/transporters)
2. that energy be used for movement of substance either from lumen into cell or form cell into interstitial fluid.
What kind of transport is seen throughout entire nephron?
active transport mechanism: Na/K+ Atpase in basolateral membrane is present throughout nephron to keep sodium levels within cell LOW. This gives energy for sodium to be reabsorbed into cell.
What is transport maxima? How does it relate to kidneys?
many active reabsorptive systems in renal tubule have tm.
Tm- maximum amount that can be reabsorbed or secreted (moved in or out) is dependent on how saturated all transporters are.
if all transporters are saturated, the substance CANNOT be removed or reabsorbed by the kidneys anymore
Tm represent the maximum transport capacity of both kidneys. Also tm- sum of transport capacities of individual functional nephrons.
Describe the relationship between amount of substance filtered vs reabsorbed.
There is a LINEAR relationship between the amount of substance filtered and amount reabsorbed.
As Tm is approached, some nephrons reached their capacity and some of the substance appears in the urine.
What is renal threshold? How much glucose that is filtered is reabsorbed?
Renal threshold- the plasma concentration where the substance occurs in the urine. As more nephrons exceed their capacity the relationship between amount filtered and reabsorbed is not LINEAR (the Splay)
100% of glucose that is filtered is reabsorbed; hence renal clearance of glucose = 0. so Urinary concentration of glucose= 0
What happens when glucose is filtered? What about when there is too much glucose filtered? Is there glucose present in normal conditions of urine?
As glucose is filtered, it is reabsorbed (hence no glucose in urine0
When too much glucose is filtered, renal threshold is met and reabsorption PLATEAUS and glucose is EXCRETED (now glucose is seen in urea)
under normal circumstances, Glucose NEVER in urine because renal threshold is 200 mL (renal threshold is double normal limit of plasma glucose).
What happens to glucose for diabetic person that is controlled?
In diabetic person, glucose can be seen in urine which means glucose level is so HIGH, that kidneys cannot reabsorb any more (you can pee it out in urine). It causes no damage to kidneys.
