61. Physiology - Proximal Tubule Tm Mechanism

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Questions and Answers

What is the estimated time for a 70 kg individual with a GFR of 125 ml/min to completely excrete his total body water without any reabsorption?

8.2 hours

5.6 hours (correct)

10.5 hours

2.4 hours

What percentage of the ATP used by the kidney is dedicated to sodium reabsorption?

40%

100%

60%

80% (correct)

How much sodium is filtered per minute at a GFR of 125 ml/min with a sodium concentration of 150 mM in the extracellular volume?

18.75 mmol (correct)

12.5 mmol

9.2 mmol

25 mmol

What is the approximate volume of fluid typically excreted by humans in a day?

1 - 2 liters Signup and view all the answers

Which of the following is NOT a function of the proximal tubule?

Active secretion of all solutes Signup and view all the answers

What is the composition of total body water (TBW) in relation to a person's weight?

0.6 times body weight Signup and view all the answers

In the context of fluid balance, what does 'Tm' stand for?

Transport maximum Signup and view all the answers

Which compartment contains 1/3 of total body water (TBW)?

Extracellular volume Signup and view all the answers

What percentage of filtered fluid and solute may be reabsorbed by the proximal tubule under optimal conditions?

90% Signup and view all the answers

Which of the following substances is not typically one of the organic bases that undergo Tm secretion?

Penicillin Signup and view all the answers

In the proximal tubule, which transport mechanism does not exhibit a maximum transport rate?

Both A and B Signup and view all the answers

What is the primary driving force for water reabsorption in the proximal tubule?

Active solute reabsorption Signup and view all the answers

Which of the following statements about aquaporin 1 in the proximal tubule is true?

It facilitates passive reabsorption of water. Signup and view all the answers

What happens when Rglu equals Tmglu in the proximal tubule?

Glucose appears in urine. Signup and view all the answers

In which condition is the reabsorption of water in the proximal tubule described as isotonic?

When it follows solute reabsorption Signup and view all the answers

Which of the following ionic substances is reabsorbed without a tubular transport maximum in the proximal tubule?

Sodium (Na+) Signup and view all the answers

What is the relationship between GFR and the threshold for glucose?

As GFR increases, the glucose threshold decreases. Signup and view all the answers

Which of the following is TRUE about the threshold for glucose during pregnancy?

The threshold for glucose decreases. Signup and view all the answers

What happens to the filtration rate line when GFR decreases?

It becomes less steep. Signup and view all the answers

Which of the following correctly describes the characteristic of Na+ in relation to glucose transport?

Na+ concentration is always saturated while transporting glucose. Signup and view all the answers

What is the primary role of osmolality in the context of water movement into the cell?

To induce osmotic pressure differential Signup and view all the answers

What does Tm represent in the context of glucose transport?

The point at which glucose reabsorption reaches its maximum rate. Signup and view all the answers

Which of the following is NOT an example of a substance that competes for glucose transport?

Mannose Signup and view all the answers

How does the movement of water from the tubular fluid (TF) into the peritubular capillary occur?

Due to high protein concentration creating oncotic pressure Signup and view all the answers

What does the concept of saturability in glucose transport imply?

Transporters can become saturated, creating a limit for glucose transport. Signup and view all the answers

What dictates the threshold plasma concentration for renal glucose reabsorption?

The saturation point of glucose reabsorption mechanism Signup and view all the answers

What effect does chronic kidney disease have on glucose threshold?

Increases the glucose threshold. Signup and view all the answers

Which statement about Tm (transport maximum) for glucose is accurate?

Tm is directly proportional to the threshold concentration Signup and view all the answers

What effect would doubling the GFR have on the glucose threshold?

It would halve the threshold value Signup and view all the answers

What is a notable misconception regarding the definitions of threshold and Tm?

Both refer to the same physiological mechanism Signup and view all the answers

How does aquaporin 1 contribute to the water movement in the proximal tubule?

By facilitating osmosis across the cell membrane Signup and view all the answers

Which of the following statements accurately describes isotonic movement at a macro level?

It reflects a balance of water flow between the cell and interstitial spaces. Signup and view all the answers

What effect does poisoning the Na,K-ATPase have on intracellular sodium concentration?

Increases ICF [Na+] and stops sodium coupled transport Signup and view all the answers

In untreated diabetes mellitus, what happens to the renal handling of glucose?

Filtered glucose exceeds the reabsorptive capacity Signup and view all the answers

How does SGLT2 function in glucose and sodium transport?

Transports Na+ and glucose at a ratio of 1:1 Signup and view all the answers

What has changed regarding the concept of the Tm for glucose with the advent of SGLT2 inhibitors?

Tm for glucose varies significantly among patients with diabetes Signup and view all the answers

What is a notable effect of SGLT2 inhibitors on cardiovascular health in diabetic patients?

They decrease cardiovascular risk and renal disease progression Signup and view all the answers

What is the ratio of sodium to glucose transport for SGLT1?

2:1 Signup and view all the answers

What happens to the cell to TF glucose ratio when SGLT2 functions effectively?

Establishes a ratio of 70 Signup and view all the answers

What is the primary consequence when the maximum reabsorptive capacity for glucose (Tm) is exceeded?

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

What is the primary role of the Na+-glucose co-transporter in the proximal tubule?

To facilitate the movement of glucose against its gradient using Na+ Signup and view all the answers

How does the concentration of Na+ influence the reabsorption of glucose in the proximal tubule?

It remains consistently high, preventing Na+ from being rate limiting Signup and view all the answers

Which carbohydrate does NOT compete with glucose for the same transporter in the proximal tubule?

Mannose Signup and view all the answers

Which mechanism provides energy for the secondary active transport of glucose in the proximal tubule?

Coupled movement of Na+ down its gradient Signup and view all the answers

What is the normal condition for glucose reabsorption in the proximal tubule?

Filtered glucose is completely reabsorbed Signup and view all the answers

What is the significance of the Na,K-ATPase in the cellular transport mechanisms of the proximal tubule?

It establishes Na+ and K+ gradients necessary for transport Signup and view all the answers

Why is glucose reabsorbed by secondary active transport rather than primary active transport?

Because energy is derived from another solute moving against its gradient Signup and view all the answers

What potential difference exists across the membranes in the proximal tubule, primarily due to permeability characteristics?

Negative potential difference for K+ Signup and view all the answers

Study Notes

Proximal Tubule Mechanisms

Proximal tubules reabsorb 60-80% of filtered solutes and water, isotonically.
Reabsorption occurs through both Tm (tubular maximum) and non-Tm mechanisms.
Secretion also takes place, but is not as substantial as reabsorption.
Glomerulo-tubular balance refers to the proportional changes in proximal tubule reabsorption related to changes in glomerular filtration rate.

Transport Types

Primary and secondary active transport, facilitated diffusion, passive transport, and endocytosis are different transport methods.
Primary active transport directly uses ATP. Secondary active transport uses the electrochemical gradient created by another ion. Facilitated diffusion requires carrier proteins. Passive transport does not need energy. Endocytosis involves taking in material into the cell via vesicles.
Co-transport (symport) and counter-transport (antiport) are specific types of assisted transport. Cotransport involves moving two molecules in the same direction; countertransport moves two molecules in opposite directions.
Ion channels facilitate direct movement of ions across cell membranes.

Tm Reabsorption

Threshold and Tm are concepts relating to maximum transport rates.
Threshold is the plasma concentration at which reabsorption mechanism saturates.
Tm is the maximum rate of transport at which a substance is reabsorbed.
The glomerular filtration rate influences the threshold concentration.

Tm Glucose

Glucose reabsorption in the proximal tubule has a transport maximum (Tm).
Threshold represents the plasma glucose concentration at which the reabsorption rate reaches its maximum (Tm).
If the plasma glucose concentration surpasses the threshold, glucose appears in the urine.
Splays indicate that tubules may differ in their transport capabilities, so the threshold is not a single definitive point.
Factors affecting glucose threshold include glomerular filtration rate.

H₂O Reabsorption

Water reabsorption in proximal tubules occurs passively by osmosis.
Osmotic gradients, established by solute reabsorption, drive water movement.
The high water permeability of the proximal tubule cells and aquaporin channels regulate this process.

Other Substances with Tm Reabsorption

Amino acids exhibit specific, saturable, and competitive characteristics related to transport.
Metabolic intermediates such as lactate and ketone bodies are often reabsorbed if their concentration levels are relatively low. High concentrations result in excretion.

Cellular Transport: Glucose

Glucose transport involves Na+-coupled glucose transporters (SGLTs) and glucose transporters (GLUTs).
SGLTs are responsible for moving glucose into the cell and GLUTs are for transport out of the cell.
Na+-K+-ATPase is crucial in creating the electrochemical gradient that drives secondary active transport of glucose.

Tm Secretory Mechanisms

Secretion occurs when substances are transported from the peritubular capillaries into the tubular fluid.
For secretion, no threshold exists.
Organic anions are secreted using secondary active transport coupled with other ions.
Organic cations also show similar mechanisms.

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Description

Test your understanding of the proximal tubule mechanisms involved in reabsorption and secretion. Explore different transport types and their roles in renal function, including primary and secondary transport methods. This quiz will reinforce the concepts of glomerulo-tubular balance and various transport processes.