Vanders Renal Ch5

Question 1

What is the physiological utility of excreting or reabsorbing organic solutes?

Answer 1

To maintain homeostasis by discriminating between useful metabolites to retain and waste products to eliminate.

Question 2

Where does active reabsorption or secretion of organic nutrients primarily occur in the kidneys?

Answer 2

In the proximal tubular systems.

Question 3

What is glycosuria and under what conditions is it likely to occur?

Answer 3

Glycosuria is the presence of glucose in urine, likely occurring when plasma glucose exceeds the renal threshold (Tm).

Question 4

How are proteins and small peptides handled by the kidneys?

Answer 4

They are filtered, endocytosed, degraded into amino acids, and reabsorbed into the bloodstream.

Question 5

What is the role of para-aminohippurate in renal handling?

Answer 5

It is secreted by the kidneys as a model organic anion for studying renal clearance.

Question 6

What is the process involved in the handling of urate?

Answer 6

It involves both reabsorption and secretion in the proximal tubule.

Question 7

What are organic cations and how are they secreted in the kidneys?

Answer 7

Organic cations are positively charged molecules secreted via transporters in the proximal tubule, typically using antiporters.

Question 8

How does tubular pH affect the excretion and reabsorption of weak acids and bases?

Answer 8

Changes in tubular pH can convert charged species to neutral forms, affecting their reabsorption and excretion.

Question 9

What is the significance of urea in renal function?

Answer 9

Urea is a waste product that must be excreted to prevent accumulation but also helps regulate water balance.

Question 10

What are the general properties of organic solute transport in the kidneys?

Answer 10

Transport proteins are promiscuous, and most organic solutes are handled in the proximal tubule.

Question 11

What are the main transporters involved in glucose reabsorption?

Answer 11

Sodium-glucose symporters (SGLT-2 and SGLT-1) and glucose uniporters (GLUT).

Question 12

What happens to glucose reabsorption when plasma glucose exceeds 200 mg/dL?

Answer 12

Filtered glucose begins to spill into urine as the transport maximum (Tm) is reached.

Question 13

What is the typical Tm for glucose transport?

Answer 13

375 mg/min.

Question 14

What occurs if the endocytic mechanism for protein uptake is saturated?

Answer 14

Increased filtered protein leads to significant protein excretion in urine.

Question 15

True or False: The glomerular filtrate is completely free of proteins.

Answer 15

False.

Question 16

What is the primary method for reabsorbing larger proteins in the kidneys?

Answer 16

Endocytosis followed by degradation in lysosomes.

Question 17

How are organic anions secreted in the kidneys?

Answer 17

By using α-ketoglutarate (αKG) as a tool for active transport.

Question 18

Fill in the blank: The kidneys perform a triage by reabsorbing _______, eliminating waste products, and partially reabsorbing a few special solutes.

Answer 18

metabolites.

Question 19

What is the unique role of urea in the context of renal handling?

Answer 19

It is both a waste product and a regulator of water balance.

Question 20

What is the consequence of glucose not being reabsorbed in the renal tubule?

Answer 20

It acts as an osmole, leading to osmotic diuresis.

Question 21

What is the role of α-ketoglutarate (αKG) in the secretion of organic anions in the proximal tubule?

Answer 21

αKG is actively taken up from both the lumen and interstitium by a sodium-αKG symporter, raising cellular levels of αKG, which then effluxes across the basolateral membrane via an antiporter that imports an organic anion.

Question 22

What is the function of the OAT family of transporters?

Answer 22

The OAT family facilitates the transport of organic anions across the cell membranes, playing a crucial role in the secretion of various organic anions in the kidney.

Question 23

What happens if the plasma concentration of an organic anion is too high?

Answer 23

It will not be efficiently removed from the blood by the kidneys.

Question 24

What are some examples of organic anions actively secreted by the proximal tubule?

Answer 24

• Uric acid
• Glucuronate conjugates
• Sulfate conjugates

Question 25

How is uric acid produced in the body?

Answer 25

Uric acid is continuously produced by the metabolic catabolism of purines.

Question 26

What is the pK of uric acid?

Answer 26

5.75.

Question 27

What clinical condition can arise from increased plasma urate concentration?

Answer 27

Gout, due to the deposition of sodium urate crystals in joints.

Question 28

What is the typical reabsorption rate of filtered urate in the kidneys?

Answer 28

About 90% of the filtered load is reabsorbed, and only 10% is excreted.

Question 29

What determines whether urate is secreted or reabsorbed in the kidneys?

Answer 29

The expression levels of relevant transporters in the apical or basolateral membranes.

Question 30

Fill in the blank: At low pH, weak acids are predominantly in their ______ form.

Answer 30

neutral

Question 31

Fill in the blank: At high pH, weak acids dissociate into an ______ and a proton.

Answer 31

anion

Question 32

True or False: Acidification of urine favors the passive reabsorption of weak bases.

Answer 32

False.

Question 33

What is the primary role of urea in the kidney?

Answer 33

Urea is an end product of protein metabolism and plays a crucial role in regulating water excretion.

Question 34

How does urea differ from other organic solutes in the proximal tubule?

Answer 34

Urea easily permeates tight junctions and is reabsorbed paracellularly, unlike other solutes that require specific transport mechanisms.

Question 35

What is the normal level of urea in the blood?

Answer 35

3–9 mmol/L.

Question 36

What is the net result of urea handling by the kidney?

Answer 36

About half of the filtered urea is reabsorbed and half is excreted.

Question 37

Fill in the blank: Urea is freely filtered and about half is reabsorbed passively in the ______ tubule.

Answer 37

proximal

Question 38

What happens to urea as it passes through the loop of Henle?

Answer 38

An amount equal to that reabsorbed in the proximal tubule is secreted back into the loop of Henle.

Question 39

What transporters handle urea beyond the proximal tubule?

Answer 39

Urea uniporters (UT family).

Question 40

How does dehydration affect urea concentration in the renal medulla?

Answer 40

It increases the urea concentration gradient favoring reabsorption.

Question 41

True or False: Urea is toxic to the body at normal levels.

Answer 41

False.

Question 42

What is the significance of tubular fluid pH on the reabsorption of weak organic acids and bases?

Answer 42

The pH affects the ionization state, influencing solubility and permeability.

Question 43

What is the relationship between tubular fluid concentration and urine flow rate?

Answer 43

Increasing urine flow increases the excretion of both weak acids and bases.

Question 44

What condition can result from a combination of excess plasma urate and low urinary pH?

Answer 44

Uric acid precipitation and the formation of uric acid kidney stones.

Question 45

What is the fate of important organic metabolites in renal reabsorption?

Answer 45

Reabsorbed almost completely (saved) ## Footnote This indicates that the kidneys efficiently reclaim essential organic metabolites to maintain homeostasis.

Question 46

Where in the nephron are most organic solutes transported?

Answer 46

Proximal tubule ## Footnote This region primarily utilizes multiporters for the transport of organic solutes.

Question 47

What happens to glucose reabsorption in pathological hyperglycemia?

Answer 47

Transport saturates, leading to glucose appearance in urine ## Footnote This condition is known as glycosuria.

Question 48

How are peptides reabsorbed in the proximal tubule?

Answer 48

By endocytosis or as individual amino acids ## Footnote Peptides undergo enzymatic degradation on the brush border before reabsorption.

Question 49

What can facilitate the passive reabsorption of some organic solutes?

Answer 49

Changes in tubular pH ## Footnote Neutral forms of organic solutes can be reabsorbed passively in the distal nephron.

Question 50

What is the overall result of urea reabsorption and recycling in the nephron?

Answer 50

Net excretion of about half the filtered load ## Footnote This process involves urea being reabsorbed and secreted multiple times.

Question 51

When does glucose appear in the urine?

Answer 51

When plasma glucose reaches high levels ## Footnote This is a characteristic of conditions like diabetes mellitus.

Question 52

How are useful small organic metabolites generally handled by the kidneys?

Answer 52

Reabsorbed transcellularly ## Footnote This means they are transported through the cells rather than between them.

Question 53

What is a key characteristic of organic anion secretion?

Answer 53

Involves a step of active influx across the basolateral membrane ## Footnote This process differs from passive secretion mechanisms.

Question 54

A high urinary pH favors which of the following?

Answer 54

Low excretion of drugs that are weak acids ## Footnote This is due to the increased ionization of weak acids at higher pH levels.

Question 55

Where does the tubular concentration of urea exceed plasma concentration?

Answer 55

At the hairpin turn of the loop of Henle ## Footnote This indicates a critical point for urea concentration dynamics in the nephron.

Question 56

Where is urea secreted into the tubules?

Answer 56

At any of these sites depending on hydration status: * Proximal tubules * Thin descending limbs * Medullary collecting ducts ## Footnote Urea secretion is influenced by the body's hydration needs.

Question 57

What is the normal range for blood urea nitrogen (BUN) levels?

Answer 57

8.4 to 25.2 mg/dL ## Footnote This measurement reflects plasma urea concentration, which is critical for assessing kidney function.