Solute Reabsorption in the Tubules - Dr. Bechtold

The ultrafiltrate formed in the glomerulus is modified by two things:

1.) secretion of substances into the ultrafiltrate

2.) reabsorption of substances from the ultrafiltrate

The content of what is actually excreted in the urine is equal to...

filtration in the glomerulus - (reabsorption + secretion in the tubules)

renal tubule secretion

secretion into the renal tubules from the blood that may augment filtration or

What is the purpose of renal tubule secretion?

it allows for excretion of poorly filtered compounds in the urine

Four examples of things secreted into the tubules:

1.) potassium

2.) hydrogen ions

3.) ammonium

4.) drugs and toxins

Substances that are secreted may also be ____________

reabsorbed

Which substance undergoes both extensive secretion and reabsorption?

potassium

reabsorption

renal process of moving water, ions, and essential nutrients (like glucose) out of the kidney's filtrate (tubular fluid) and back into the bloodstream via the peritubular capillaries

Reabsorption is driven by two things:

1.) energy-consuming pumps

2.) electrical gradients caused by ion charge differences

The kidney must reabsorb most filtered substances because...

the kidneys filter extremely large volumes of plasma daily; without reabsorption, enormous amounts of electrolytes and water would be lost in urine

Two pathways of reabsorption:

1.) transcellular reabsorption

2.) paracellular reabsorption

transcellular reabsorption

solutes enter the tubular epithelium from the luminal side, travel through the cytoplasm, and exit at the basolateral side into the institium and then the blood stream

for transcellular reabsorption, solutes travel __________ cells

through

transcellular reabsorption is mediated by...

carrier proteins and ATP-driven pumps

paracellular reabsorption

solutes travel from the renal ultrafiltrate back into the bloodstream by traveling between tight junctions of tubular cells

for paracellular reabsorption, solutes travel __________ cells

between

paracellular reabsorption is mediated by...

electrical/ion gradients

In what section of the nephron are tight junctions particularly "leaky"?

proximal tubule

Two transport mechanisms in the tubules:

1.) active transport

2.) passive transport

active transport

the movement of solutes through a cell membrane against their electrochemical gradient using ATP

There are two types of active transport:

1.) primary active transport

2.) secondary active transport

primary active transport

directly uses ATP hydrolysis and moves solutes against their electrochemical gradient

Four examples of primary active transporters in the kidney:

1.) Na⁺/K⁺ ATPase pump

2.) H⁺ ATPase pump

3.) H⁺/K⁺ ATPase pump

4.) Ca²⁺ ATPase pump

Na+/K+ ATPase pump

one of the most important pumps in nephron function; moves 3 Na+ out of cell and 2 K+ into cell

The Na+/K+ ATPase pump is located on the ___________ surface of renal cells

basolateral

The Na+/K+ ATPase pump creates low __________ Na+ levels. This allows...

intracellular; allows luminal Na+ within renal tubules to enter the cell using other mechanisms

The Na+/K+ ATPase pump ultimately helps drive...

secondary active transport; because Na⁺ wants to move down its gradient into the cell, it can drag other molecules with it

secondary active transport

uses a pre-existing gradient created by primary active transport to drive transport of solute

Example of how secondary active transport works with Na⁺/K⁺ ATPase pump

1.) Na⁺/K⁺ ATPase pump lowers intracellular Na+

2.) Na+ moves into the cell down the gradient

3.) as Na+ moves along its gradient, another molecule will move with Na+ against its own gradient

Three examples of secondary active transporters in the kidney:

1.) Na+/Glucose cotransporter

1.) Na+/Amino acid cotransporter

2.) Na+/H+ counter-transporter

cotransporters

carrier protein that transports two different solutes across a membrane at a time in the same direction

counter-transporters

carrier protein that transports two different solutes across a membrane at a time in the opposite direction

Passive transport

the movement of substances across a cell membrane without the use of energy by the cell

Three types of passive transport:

1.) simply diffusion

2.) facilitated diffusion

3.) osmosis and solvent drag

Simple diffusion

Movement of a solute down its concentration gradient; does not require a carrier

Two examples of simple diffusion in the kidney:

1.) Chloride paracellular diffusion in proximal tubule

2.) Urea transcellular diffusion

Facilitated diffusion

Movement of a solute down its concentration gradient; does require a carrier

Example of facilitated diffusion in the kidney:

Urea transport in the collecting ducts

Osmosis and Solvent Drag

the process by which water movement can carry solvents

Mechanism of osmosis and solvent drag

water moves through aquaporins or tight junctions, and solutes like K+ and Na+ move with it

Water will especially follow _________ reabsorption

sodium

*water and sodium are best friends :)

Pinocytosis

specialized transport mechanism in the kidney where a cell engulfs extracellular fluid and its dissolved solute

Pinocytosis is a form of _________ transport

active

Mechanism of pinocytosis

1.) Cell membrane engulfs substances in the lumen

2.) this forms a vesicle inside the cell

3.) the products of the vesicle are then digested within the cell

pinocytosis occurs mainly in what segment of the renal tubules?

proximal tubule

Reabsorption vs conservation

Reabsorption is the process by which substances move from the tubular fluid back into the blood

Conservation is how the kidneys adjust reabsorption so very little of a substance is excreted in urine

Two solutes that are highly conserved in the tubules:

1.) sodium

2.) chloride

In which segment is most sodium reabsorbed?

proximal tubule (65%)

*then ascending loop of Henle, then distal tubule, then collecting duct

Which segment of the renal tubules is the major site or reabsorption?

the proximal tubule

Approximately _____ of the ultrafiltrate produced in the glomerulus gets

reabsorbed in the proximal tubule

2/3rds

What structure on the proximal tubule aids with it’s reabsorption?

brush border

The proximal tubule has a high ________ demand. Why?

energy; due to strong Na+/K+ ATPase activity

In proximal tubule, luminal Na+ transport (through the Na+/K+ ATPase pump) is coupled with reabsorption of other _________

solutes (aka secondary active transport)

In the early proximal tubule, three solutes are coupled with Na+ transport:

1.) glucose

2.) amino acids

3.) phosphates

In the late proximal tubule, one solutes is coupled with Na+ transport:

Cl-

Six substances reabsorbed in the proximal tubule:

1.) sodium

2.) chloride

3.) BUN

4.) glucose

5.) amino acids

6.) bicarbonate

Two mechanisms through which chloride is reabsorbed in the proximal tubule:

1.) Na+/Cl- cotransport

2.) passive paracellular diffusion

The movement of chloride in the proximal tubule is driven by....

gradients created during sodium reabsorption

Mechanism through which urea is reabsorbed in the proximal tubule:

As water is reabsorbed from the tubule, urea concentration increases inside the tubule, which creates a concentration gradient favorable to reabsorption into the blood

The initial glucose concentration in the ultrafiltrate will be equal to that of _______. Why?

plasma; it is a small molecule that is freely filtered at the glomerulus

Two mechanisms through which glucose is reabsorbed in the proximal tubule:

1.) Na⁺-glucose cotransport into cell

2.) Facilitated diffusion across basolateral membrane

Na⁺-glucose cotransport into cell

1.) Na⁺ moves down its electrochemical gradient into the cell

2.) Glucose is pulled into the cell along with Na⁺

Facilitated diffusion of glucose across basolateral membrane

Once inside the cell, glucose moves into the bloodstream down its concentration gradient through facilitated diffusion

Glucose reabsorption uses ______ proteins

transport

Since glucose is reabsorbed using transport proteins, it is a __________ process.

Saturable

What happens if all the glucose transport proteins are saturated?

some glucose will not be reabsorbed and appears in urine

Transport threshold

the point at which a solute that uses transport proteins starts to appear in the urine due to saturation of transport proteins

Mechanism through which amino acids are reabsorbed in the proximal tubule

secondary active transport coupled with sodium reabsorption

There are several different types of Na+-_______ ______ carriers, each of which can...

amino acid; transport amino acids of similar type

Mechanism through which proteins are reabsorbed in the proximal tubule

pinocytosis

A very ________ amount of protein is reabsorbed. Why?

small; proteins are usually very large and only a small amount will be able to make it into the ultrafiltrate

Transport maximum

The maximum transport capacity of protein carriers

If the filtered load exceed transport maximum, solute remains in the _________ and appears in _______

lumen; urine

Transport threshold

the tubular load at which the transport maximum is exceeded in some nephrons, but not all

True or false: Not all nephrons have the same transport maximum

true!

Therefore, some nephrons will leave substances in urine at ________ concentrations than others if they have a lower ________ ________

lower; transport threshold

The loop of Henle has three parts:

1.) Descending thin limb

2.) Ascending thin Limb

3.) Ascending thick limb

The descending thin limb of Henle is highly permeable to __________ and moderately permeable to __________

higher permeable: water

moderately permeable: most solutes

Why is water reabsorbed so avidly in the descending limb of Henle?

it follows the high solute gradient that is in the interstitium

The reabsorption in the descending limb of Henle is passive or active?

passive

The ultrafiltrate becomes _____________ as it travels through the descending loop of Henle

concentrated

The ascending thin limb of Henle is highly permeable to __________ and impermeable to __________

permeable: solutes

impermeable: water

The reabsorption in the ascending thin limb of Henle is passive or active?

passive

The ultrafiltrate becomes _____________ as it travels through the ascending loop of Henle

dilute

The ascending thick limb of the loop of Henle reabsorbs three solutes:

1.) sodium

2.) chloride

3.) potassium

The reabsorption in the ascending thick ascending limb of Henle is passive or active?

active

Which pump is responsible for the resorption in the ascending thick ascending limb of Henle?

Na+ K+ 2Cl- cotransporter

The Na+ K+ 2Cl- pump uses _________ active transport

secondary

How does the Na+ K+ 2Cl- cotransporter work?

1.) first, Na+/K+ ATPase creates a Na+ gradient (low intracellular Na+, higher Na+ concentration in the tubular lumen)

2.) Then, since Na+ wants to move down its gradient, the Na+ K+ 2Cl- cotransporter will transport Na+ along with K+ and 2Cl- inside the cell from the lumen

3.) When all 4 binding sites are occupied, the pump undergoes a conformation change that transports the ions into the cell

The Na+ K+ 2Cl- cotransporter is ___________

electroneutral

Water and sodium frequently get reabsorbed in tandem but in the loop of Henle...

they do not move in concert!!

The thin segments of the Loop of Henle do or do not use much energy?

do not! thin and are not able contain large amount of mitochondria for ATP production

The thick segment of the Loop of Henle does or does not use much energy?

does! the cells are thick and contain many mitochondria to fuel the Na+K+ATPase pump

Distal convoluted tubule

Sits between the ascending thick limb of the loop of Henle and connecting tubule/collecting tubule

The distal tubule can be broken into two sections:

1.) early distal tubule

2.) late distal tubule

early distal tubule is aka...

the diluting segment if the distal tubule

Two solutes resorbed in the early distal tubule:

1.) Na+

2.) Cl-

Sodium and chloride as resorbed in the early distal tubule through what pump?

Na⁺Cl⁻ cotransporter

the early distal tubule is more similar to...

the ascending thick limb of the loop of Henle

The late distal tubule is more similar to...

the connecting/collecting tubule