62. Physiology - Proximal Tubule non-Tm Mechanisms and Osmotic Diuresis

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What effect does acetazolamide have on carbonic anhydrase activity?

It only affects the secretion of H+

It decreases carbonic anhydrase activity (correct)

It increases carbonic anhydrase activity

It has no effect on carbonic anhydrase activity

How does a decrease in carbonic anhydrase activity affect urine flow rate?

Increases urine flow rate (correct)

Has no effect on urine flow rate

Makes urine more concentrated

Decreases urine flow rate

What is the relationship between partial pressure CO2 (pCO2) and HCO3- reabsorption?

Increased pCO2 decreases HCO3- reabsorption

Increased pCO2 increases HCO3- reabsorption (correct)

Decreased pCO2 has no effect on HCO3- reabsorption

HCO3- reabsorption is independent of pCO2 levels

What determines the extent of H+ secretion in the proximal tubule?

The amount of HCO3- filtered Signup and view all the answers

How much solute does the proximal tubule reabsorb every eight minutes?

70 mosmol Signup and view all the answers

What is the volume of water reabsorbed alongside the solute in the proximal tubule?

230 mL Signup and view all the answers

What prevents Cl- from being readily reabsorbed in the early proximal tubule?

Unfavorable electrochemical gradient Signup and view all the answers

How does the reabsorption of Na+ largely relate to HCO3- reabsorption?

HCO3- reabsorption is coupled with Na+ reabsorption Signup and view all the answers

What is the net result of HCO3- reabsorption in the proximal tubule?

For each HCO3-, one Na+ is reabsorbed. Signup and view all the answers

Which substance plays a crucial role in the catalytic conversion of CO2 and H2O to H2CO3 in the proximal tubule?

Carbonic anhydrase type II Signup and view all the answers

What mechanism is primarily responsible for the movement of HCO3- into the interstitial fluid in the proximal tubule?

Na-HCO3 co-transporter Signup and view all the answers

What percentage of the filtered HCO3- is reclaimed by the proximal tubule?

80% Signup and view all the answers

Which mechanism represents the primary active transport of H+ in the proximal tubule?

H-ATPase Signup and view all the answers

What is the initial concentration of HCO3- in the filtrate entering the proximal tubule?

25 mM Signup and view all the answers

During HCO3- reabsorption, what happens to the concentration of HCO3- in the lumen by the end of the second part of the proximal tubule?

It decreases to ~5 mM. Signup and view all the answers

What primarily drives the secretion of H+ at the apical side of the proximal tubule?

Electrochemical gradient Signup and view all the answers

What drives the reaction to the right in the context of intracellular bicarbonate reabsorption?

Removal of H+ and HCO3- from ICF Signup and view all the answers

Which processes occur in the proximal tubule regarding bicarbonate?

Hydrogen ions are secreted but do not reach the urine. Signup and view all the answers

What is the role of carbonic anhydrase in the brush border of renal tubular cells?

To catalyze the conversion of H2CO3 into H2O and CO2. Signup and view all the answers

What has recent research indicated about bicarbonate reabsorption in the kidney?

Carbonate may be reabsorbed on the basolateral side instead of bicarbonate. Signup and view all the answers

What happens to CO2 and H2O upon entering the renal tubular cell?

They trigger further production of bicarbonate. Signup and view all the answers

Which statement correctly describes the difference between secretion and excretion related to the kidneys?

Secretion refers to the transport of substances into a body compartment, while excretion removes them from the body. Signup and view all the answers

Which factor primarily limits the rate of bicarbonate reabsorption in renal physiology?

Activity level of carbonic anhydrase Signup and view all the answers

What is the primary reason for hydrogen ions not being excreted into the urine during bicarbonate reabsorption?

They are secreted into the tubular fluid but subsequently reabsorbed. Signup and view all the answers

What primarily causes Cl- to move from the tubular fluid into the interstitial fluid in the early proximal tubule?

Increase in tubular fluid water reabsorption Signup and view all the answers

In the late proximal tubule, how does the positive lumen potential influence Cl- reabsorption?

It facilitates the exchange of an intracellular anion for Cl- Signup and view all the answers

What is the relationship between Cl- reabsorption and Na+ in the proximal tubule?

Cl- reabsorption is coupled with Na+ transport Signup and view all the answers

What occurs to Cl- concentration as the proximal tubule is traversed?

It increases as water is reabsorbed and concentration rises Signup and view all the answers

What type of transport mechanism is primarily involved in Cl- reabsorption in the late proximal tubule?

Transcellular transport involving active exchange Signup and view all the answers

Which statement accurately describes the electrochemical gradient for Cl- in the early proximal tubule?

The net electrochemical gradient does not favor Cl- movement into the cell. Signup and view all the answers

What occurs during tertiary active transport related to Cl- reabsorption in the late proximal tubule?

H+ is exchanged in the process of transporting Cl- into the cell. Signup and view all the answers

What is the primary reason more Na+ and Cl- are reabsorbed in the proximal tubule than all other solutes combined?

They rely on active transport mechanisms coupled with other ions. Signup and view all the answers

What is the primary mechanism for K+ reabsorption in the early proximal tubule?

Solvent drag due to water reabsorption Signup and view all the answers

Which factor contributes to increased proximal Na+ reabsorption?

Renal sympathetic nerve activity Signup and view all the answers

In the late proximal tubule, how does Cl- reabsorption primarily occur?

Passive movement down an electrochemical gradient Signup and view all the answers

What role does angiotensin II play in renal physiology?

Stimulates Na+ reabsorption in the proximal tubule Signup and view all the answers

Which of the following statements about K+ reabsorption is incorrect?

A higher concentration of K+ in the proximal tubule encourages K+ to move into the cells. Signup and view all the answers

How does atrial natriuretic hormone affect proximal Na+ reabsorption?

It decreases proximal Na+ reabsorption. Signup and view all the answers

What is the total osmolarity of solutes that must be reabsorbed in the proximal tubule given a GFR of 125 ml/min and a total filtered load of 1,000 ml providing 300 mosm?

170 mosmol Signup and view all the answers

What initiates the passive reabsorption of K+ in the proximal tubule?

High permeability of proximal tubular membranes to K+ Signup and view all the answers

What is the minimum TF [K+] required for K+ to achieve electrochemical equilibrium across the apical membrane?

4 mM Signup and view all the answers

What percentage of filtered solutes and water are normally reabsorbed in the proximal tubule?

60-80% Signup and view all the answers

What is the impact of mannitol on water reabsorption in the proximal tubule?

Decreases water reabsorption Signup and view all the answers

How does the osmotic pressure of the tubular fluid (TF) change after mannitol is administered?

It increases to 400 mosm/kgH2O Signup and view all the answers

What occurs to the rate of Na reabsorption when mannitol is present in the tubular fluid?

Decreases due to lower [Na] in the TF Signup and view all the answers

What is defined as diuresis in the context of renal physiology?

Increase in excretion of both salt and water Signup and view all the answers

What amount of solute remains at the end of the proximal tubule if 400 mosmol is filtered over 8 minutes?

60 mosmol Signup and view all the answers

Which of the following correctly describes the composition of the tubular fluid after filtration?

Components include Na, Cl, HCO3, and mannitol Signup and view all the answers

Study Notes

Proximal Tubule Non-Tm Mechanisms and Osmotic Diuresis

Proximal tubules reabsorb >80% of filtered solutes and water.
Non-Tm mechanisms handle most of the transport.
Bicarbonate reabsorption occurs via a mechanism involving carbonic anhydrase.
Bicarbonate concentration, carbon dioxide partial pressure, and volume status influence reabsorption.
Chloride reabsorption is influenced by electrochemical forces.
The rate of chloride reabsorption varies along the length of the tubule.
Sodium is reabsorbed with chloride and other solutes.
Major factors influencing proximal tubular reabsorption include catecholamines, angiotensin II, and atrial natriuretic hormone.
Potassium is handled differently in the proximal tubule than other solutes.
Osmotic diuresis is defined as increased excretion of salt and water due to non-reabsorbables substances.
Substances like mannitol, glucose, and radiologic contrast can cause osmotic diuresis.
The reabsorption of bicarbonate, chloride, and other electrolytes in the proximal tubule is crucial for maintaining body fluid balance and pH.
The proximal tubule actively secretes or reabsorbs various substances based on electrochemical gradients and gradients themselves.

Non-Tm Mechanisms

Quantitatively, majority of transport in proximal tubule is via non-Tm mechanisms.
Non-Tm mechanisms handle majority of filtered solutes in proximal tubule (~210 mmol).
Tm mechanisms handle ~10-15 mosm of solute.
300 mOsm/L filtrate is formed, with 80% of filtrate being reabsorbed.

HCO3 Reabsorption

Bicarbonate (HCO3-) in the filtrate is reabsorbed by the proximal tubule.
For every HCO3- reabsorbed, one sodium (Na+) ion is reabsorbed.
The reaction relies on carbonic anhydrase (CA).
CO2 + H2O → H2CO3 → H+ + HCO3-.
H+ is secreted into the filtrate, while HCO3- enters the blood.

Chloride Reabsorption

Early in the proximal tubule, chloride is not readily reabsorbed because the electrical gradient is not favorable.
Later in the proximal tubule, reabsorption is facilitated by a positive lumen potential, allowing chloride to move into the cell.
The movement of chloride is coupled to exchange with a base. This base neutralizes and is coupled to the sodium-hydrogen (Na+/H+) exchanger.

Control of Proximal Na+ Reabsorption (The Big 3)

Catecholamines: Renal sympathetic nerves and adrenal medulla increase proximal sodium reabsorption.
Angiotensin II: Increases proximal sodium reabsorption.
Atrial Natriuretic Hormone: Decreases proximal sodium reabsorption.

K+ Reabsorption

Potassium (K⁺) is passively reabsorbed in proximal tubule.
The electrochemical gradient drives potassium reabsorption.

Osmotic Diuresis

Osmotic diuresis increases salt and water excretion due to a non-reabsorbable substance.
Substances like mannitol, glucose prevent water reabsorption.
This leads to increased urine volume and electrolyte excretion.

Clinical Questions

Question 1: (Low tubular fluid / plasma concentration ratio for which substance in the end proximal tubule samples ?)
Bicarbonate
Question 2: (Treatment for brain edema after head trauma?)
Mannitol (because it's a non-reabsorbable substance, causing osmotic diuresis, which helps decrease brain edema).

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Explore the crucial functions of the proximal tubule in kidney physiology, focusing on non-transporter mechanisms and their role in osmotic diuresis. Discover how various factors like bicarbonate concentration and hormonal influences affect solute reabsorption. Test your knowledge on key substances involved in these processes.