Sodium Balance and Reabsorption Quiz

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Questions and Answers

What occurs when Na+ excretion is less than Na+ intake?

Positive sodium balance (correct)

Dehydration

Sodium depletion

Negative sodium balance

Where does the majority of Na+ reabsorption occur?

Distal convoluted tubule

Thick ascending limb

Proximal convoluted tubule (correct)

Collecting duct

What type of mechanism links water reabsorption to Na+ in the proximal tubule?

Countercurrent multiplication

Facilitated diffusion

Isosmotic reabsorption (correct)

Active transport

What fraction of Na+ reabsorption occurs in the thick ascending limb?

1/4 Signup and view all the answers

Which part of the nephron is impermeable to water?

Thick ascending limb Signup and view all the answers

What regulates the last step of Na+ reabsorption in the distal convoluted tubule and collecting duct?

Aldosterone Signup and view all the answers

What is the physiological consequence of Na+ excretion being greater than Na+ intake?

Negative sodium balance Signup and view all the answers

How is water reabsorption impacted in the proximal tubule?

It is dependent on Na+ reabsorption Signup and view all the answers

What is the primary mechanism for sodium exit across the basolateral membrane in the distal tubule?

Na+/K+-ATPase Signup and view all the answers

Which diuretics act on the Na+/K+-ATPase to decrease sodium reabsorption?

Thiazide diuretics Signup and view all the answers

What type of channels are responsible for sodium entry in the apical membrane of the distal tubule?

Epithelial Sodium Channels (ENaC's) Signup and view all the answers

What effect do K+ sparing diuretics like amiloride have on sodium absorption?

Decrease Na absorption slightly Signup and view all the answers

How does aldosterone affect sodium reabsorption in the distal tubule and collecting duct?

Regulates sodium influx through ENaC's Signup and view all the answers

What percentage of sodium reabsorption occurs in the late distal tubule and collecting duct?

Roughly 3% Signup and view all the answers

What is the effect of thiazide diuretics on NaCl absorption?

Decrease NaCl absorption Signup and view all the answers

What is the primary purpose of K+ sparing diuretics?

To inhibit sodium reabsorption without causing potassium loss Signup and view all the answers

What gradient is essential for sodium reabsorption in the proximal tubule?

Na+ gradient Signup and view all the answers

Which of the following mechanisms is used for sodium reabsorption coupled with other solutes?

Na+-lactate cotransporter Signup and view all the answers

How is sodium transported out of the cell after reabsorption in the proximal tubule?

Na+ /K+ - ATPase Signup and view all the answers

What happens to the tubular fluid as it moves through the late proximal tubule?

It becomes devoid of glucose and amino acids. Signup and view all the answers

Which electrolyte predominantly drives the sodium reabsorption in the late proximal tubule?

Chloride Signup and view all the answers

What role do cotransport mechanisms serve in the early proximal tubule?

Reabsorb sodium alongside other essential solutes. Signup and view all the answers

Which transporter is crucial for maintaining acid/base balance in the proximal tubule?

Na+/H+ exchanger Signup and view all the answers

What is the primary function of the early proximal tubule in sodium balance?

Reabsorb most essential solutes including sodium Signup and view all the answers

What effect does aldosterone have on ENaC channel production and insertion?

Increases both production and insertion Signup and view all the answers

What is the function of Na+/K+-ATPase in relation to Na+-glucose cotransport?

Maintains the Na gradient for cotransport Signup and view all the answers

What occurs when solutes are transported above their tubular maximum (Tm)?

The solute is excreted Signup and view all the answers

Where does glucose reabsorption primarily occur in the nephron?

Proximal tubule Signup and view all the answers

What drives the secondary active transport of sodium in Na+-glucose cotransport?

Na+/K+-ATPase activity Signup and view all the answers

What happens to the filtered load of a solute below its tubular maximum?

All of it is reabsorbed Signup and view all the answers

In what manner is sodium entered into the tubular fluid during glucose reabsorption?

Via Na+-glucose cotransporters Signup and view all the answers

What occurs in the nephron when the transporters for a solute reach saturation?

All filtered solute is excreted into urine Signup and view all the answers

What does the term 'splay' refer to in the context of a glucose titration curve?

The range where reabsorption approaches full saturation but is not reached. Signup and view all the answers

Why does splay occur according to the information provided?

Because Tm reflects an average for all nephrons. Signup and view all the answers

What is the plasma glucose concentration range at which all filtered glucose is typically reabsorbed?

70-100 mg/dL Signup and view all the answers

In the context of uncontrolled diabetes mellitus, what happens when plasma glucose levels exceed Tm?

Glucose is excreted in the urine. Signup and view all the answers

What is indicated by the term 'Tm' in relation to glucose reabsorption?

The maximum reabsorptive capacity for glucose by the kidneys. Signup and view all the answers

What implication does a genetic abnormality in the SGLT have on glucose reabsorption?

It is associated with a decreased Tm, leading to glucose excretion. Signup and view all the answers

What happens to glucose excretion when plasma glucose levels are normal?

Glucose is never excreted. Signup and view all the answers

Which statement correctly describes the behavior of nephrons in relation to glucose reabsorption?

Heterogeneity among nephrons affects glucose reabsorption rates. Signup and view all the answers

Study Notes

Sodium Balance

When sodium intake is less than sodium excretion, the body is in a negative sodium balance, leading to sodium loss.
When sodium intake is more than sodium excretion, the body is in a positive sodium balance, leading to sodium retention.

Sodium Reabsorption

The majority of sodium reabsorption takes place in the proximal convoluted tubule (PCT), where 2/3 of sodium is reabsorbed.
In the PCT, water reabsorption is linked to sodium reabsorption through an isosmotic mechanism.
The thick ascending loop of Henle reabsorbs ¼ of the filtered sodium load without any water reabsorption.
The final stage of sodium reabsorption in the distal convoluted tubule (DCT) and collecting duct (CD) is regulated by aldosterone.

Early Proximal Tubule

Most essential solutes are reabsorbed in the early proximal tubule.
Many secondary active transporters are utilized, driven by the sodium gradient.
Cotransport mechanisms:
- Sodium-glucose (SGLT)
- Sodium-amino acid
- Sodium-lactate, citrate, or phosphate
Sodium moves into the cell down its electrochemical gradient coupled to these solutes (apical membrane)
Sodium is removed from the cell via the Na+/K+ - ATPase.
The solutes exit via facilitated diffusion.
Sodium/Hydrogen (Na+/H+) exchanger and bicarbonate diffusion are key for acid/base balance.

Late Proximal Tubule

This region primarily reabsorbs sodium chloride (NaCl).
Sodium/Potassium ATPase (Na+/K+-ATPase) drives NaCl reabsorption.
Thiazide diuretics act on this region, inhibiting NaCl absorption. Examples include:
- Chlorothiazide
- Hydrochlorothiazide
- Metalozone

Late Distal Tubule & Collecting Duct

Approximately 3% of sodium is reabsorbed in this region.
Epithelial sodium channels (ENaCs) are present on the apical membrane.
Sodium enters the channels down its electrochemical gradient and exits via the Na+/K+-ATPase (basolateral membrane).
Potassium-sparing diuretics like amiloride and spironolactone inhibit sodium absorption in this region, leading to mild diuresis.
Aldosterone increases sodium reabsorption in this region by:
- Increasing ENaC channel production and insertion
- Increasing Na+/K+-ATPase protein expression

Tubular Maximum (Tm)

The maximal rate of solute transport is called Tm. Measured in mg/min.
Saturation of transporters causes Tm.
Below Tm, all filtered load is reabsorbed.
Above Tm, solute is excreted.
Tms are found in the proximal tubule.

Glucose Reabsorption

Glucose is filtered across the glomerulus and reabsorbed in the proximal tubule through a two-step process:
- Sodium-glucose cotransporter (SGLT)
- Facilitated diffusion
The energy driving this process comes from the sodium/potassium ATPase.
The SGLT is a secondary active transport system that is saturated at a certain level.
Splay is the portion of the titration curve where reabsorption approaches full saturation but is not quite there yet.
Glucose excretion (glucosuria) occurs when the plasma glucose concentration exceeds the Tm.
Heterogeneity of nephrons and differences in Tm across nephrons contribute to splay.

Glucosuria (Glucose Excretion)

Normally, all filtered glucose is reabsorbed, with none excreted.
Uncontrolled diabetes mellitus leads to increased plasma glucose concentrations, resulting in a filtered load that exceeds reabsorptive capacity. Glucose is then excreted.
Genetic abnormalities in the SGLT (sodium-glucose cotransporter) can result in decreased Tm and glucose excretion.

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Description

Test your knowledge on sodium balance and the mechanisms of sodium reabsorption in the kidneys. This quiz covers concepts such as negative and positive sodium balance, the role of different tubules, and the influence of aldosterone. Perfect for students studying renal physiology.