Test 2 - Renal Physiology Flashcards
What is the most common cause of elevated plasma creatinine?
Dehydration
What does it mean to say that the kidney (like the heart and lungs) have reserve capacity?
This means that the kidney can do more work than it does at basal levels, Which means that filtering, reabsorptive and secretory function is not at its maximimal in the basal resting state; all can be increased.
This masks nephron loss to an extent.
What is meant by transport maximum?
There is a maximum rate at which nephron epithelium can remove substances from plasma and secrete them into filtrate. Similarly, there is a maximum rate at which nephron epithelium can reabsorb substances from filtrate.
What is the cause of transport maximum?
Transport is dependent on number/density of membrane transporters that allow for secretion or reabsorption.
Exceeding the transport maximum may occur in disease states and accumulation of substances in plasma may occur OR substances that should be reabsorbed are lost in urine.
What will happen if the rate of glucose getting filtered is greater than the rate of glucose getting reabsorbed?
You will see glucose in the urine.
TRUE/FALSE.
Rate of filtration is not variable.
FALSE.
The rate of filtration is variable and is dependent on GFR (flow) and concentration.
Amount/Time = Concentration x Flow
If concentration increases, more gets filtered.
Once Tmax is reached, the rate of excretion is ___ ( >, <, = ) the rate of filtration.
Equal to
Where does glucose get reabsorbed?
Proximal tubule ONLY.
This is also true for lactate, amino acids, VFAs, Ketones, glycerol, all organic molecules that cells obtain energy from.
TRUE/FALSE.
Tmax differes from one substance to another.
TRUE.
e.g. Tmax is high for glucose, but low for ketones. You only need to have a slight increase for it to appear in the urine.
TRUE/FALSE.
All cells have the same transport maximum.
FALSE.
Each cell has a different number of transporters. This difference accounts for the “splay” in the quantitative representation.
Once glucose passes the proximal tubule, can it be reabsorbed?
NO. It will not go to another nephron to get reabsorbed. It will be excreted.
Describe bovine glucose Tmax.
Bovine has low glucose Tmax. There is less plasma glucose to begin with because not a lot of glucose reabsorption is done in the GI tract. Virtually all roughage is acted on by microbes in the forestomachs to convert them into VFAs. This then undergoes gluconeogenesis. Therefore, the kidney does not have a lot of glucose transporters. Stress causes increase in glucose production.
THE USUAL SUSPECTS:
Urine Glucose
- Too much plasma glucose - Tmax will be exceeded
- Proximal Tubular Disease - Since this is the only place glucose gets reabsorbed
- Hemorrahge into the urinary tract
- Endocrine Problem:
a. Too much Hormones that increase glucose
b. Not enough insulin
What hormones increase plasma glucose?
What causes their increase?
Growth Hormone
Cortisol
Thyroid
Epinephrine
*Too much is a result of stress
In the graph, what is does it mean when the filtered and reabsorption graphs are superimposed?
During this time, the kidney can keep up. The amount being filtered is equal to the amount being absorbed.
When the reabsorption curve begins to deviate from the filtered curve, what is happening in the kidney?
Some nephrons are beginning to reach their transport maximum, while others are not. At the same time, there is a proportional increase in the amount of glucose excreted.
What do filtration, secretion, reabsorption and excretion measure?
Amount of a given substance per time in units mg/min.
GFR and clearance are expressed in units of ______ and measure _____.
ml/min
volume of plasma per time
The slope of the graph is a reflection of ______.

GFR
If the filtration curve were to increase its slope, what would this indicate? Decrease its slope?

An increase in slope would indicate an increase in GFR (it would lean more to the left). A decrease in slope would indicate a decrease in GFR (it would lean more to the right).
What does the amount of reabsorbtion depend on?
- The number of cells in the PCT that can reabsorb
- The number of transporters in each cell.
*Each of these is variable.
TRUE/FALSE.
When filtration = reabsorption of a particular substance, none is seen in the urine.
TRUE.
Only when filtration > reabsorption, is a substance excreted.
How does specific gravity relate to water balance as a whole?
It is a measure of how well the kidney is reabsorbing.
It is a measure of the density of urine in regards to all of the solutes present.
If an animal is dehydrated, you expect the kidney to _____ (increase, decrease, not change) water reabsorption at the PCT.
Increase