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Chapter 25 (Early Filtrate Processing)

Once the filtrate is formed, the early tubular segments

of the nephron reabsorb solutes and water back into the blood to restore its volume and composition

Once the filtrate is formed, some solutes are removed from

the blood and are secreted into the filtrate to fine tune the bloods composition

Analogy for reabsorption

the cleaning of a child's room by throwing the contents in the trash is like the glomerular filtration, reabsorption claims the valuables that have been filtered and the unclaimed are tossed as waste

What are the two reabsorption pathways

transcellular and paracellular

Transcellular pathway

the absorptive pathway in the renal tubules that permits the movement of water and solutes through cell membranes and cytosol

Paracellular pathway

the absorptive pathway in the renal tubules that permits the diffusion of water and solutes between cells rather than through cells

Paracellular pathway through

the tight junctions

Transcellular pathway through the

luminal and basolateral membrane

What is the driving force for filtration

blood pressure

How can we cause water to diffuse from the lumen through the tight junctions of tubular cells into the interstitial space

increase osmolarity; water will move from its higher concentration in the tubule through the tight junctions to its lower concentration level in the interstitial space; water will also move through the plasma membranes of the cells that are permeable to water

How can we increase the osmolarity of the interstitial space

transport sodium ions form the cell into the interstitial space; water will then diffuse from the tubule through the tight junctions and permeable plasma membranes into the interstitial space, equilibrating the two osmolarities

How will the reduction in the intracellular Na+ concentration resulting from the basolateral transport affect the activity at the luminal membrane

sodium ions will move from a higher concentration in the filtrate into the lower intracellular concentration

Basolateral transport of Na+ ions: interstitial osmolarity increases

causing water to diffuse out of the tubular lumen

Basolateral transport of Na+ ions: lowered intracellular concentration causes

additional Na+ ions to be reabsorbed through the luminal membrane

Sodium ion transport is the driving force that

enables reabsorption of most substances in the nephron


the total concentration of all solute particles in a solution


the space within a tubular structure through which fluid flows

K+ ion channel allows K+ to return to the interstitial space by

diffusion, preventing K+ depletion in the blood and accumulation in the cells

Glucose carrier molecules bind only to glucose and transport glucose

out of the cell by a passive mechanism called facilitated diffusion

Glucose can move in either direction, depending

on it concentration; the concentration is normally higher in the cell

Facilitated diffusion

a passive transport process that uses a carrier molecule to enable the passage of a complex molecule across a membrane


a monosaccharide sugar; the principal sugar in the blood

Na+/K+ ATPase ion pump actively transports sodium ions (3 at a time)

out of the cell and potassium ions (2 at a time) into the cell


reclaiming substances from the renal tubules and returning them eventually to the blood

Basolateral membrane

the entire, combined portion of the renal cell's plasma membrane, which lies in contact with the interstitial space

As ATP yields its energy for active transport, it converts to

ADP and Pi (inorganic phosphate ion)

Sodium/Potassium pump is located

in the basolateral membrane of many regions of the nephron

The interstitial fluid becomes more concentrated when

there is a decrease in the intracellular sodium concentration, therefore the interstitial fluid achieves an increase in osmolarity

Name two trans-membrane proteins

potassium ion channel and glucose carrier molecule

The luminal membrane contains many transport proteins including

Na+--H+ counter transport and Na+--glucose co-transport carrier molecules

Luminal membrane

the plasma membrane of a cell that lies exposed to the contents of the tubular lumen

Na+/H+ counter transport carrier molecule and the Na+/glucose co-transport carrier molecule, which are transport mechanisms, depend on

activity of Na+/K+ ATPase ion pumps in the basolateral membrane

Na+/H+ counter transport carrier molecule transports one sodium ion out and

one hydrogen ion into the cell

Na+/glucose co-transport carrier molecule transports

one sodium and one glucose into the cell


a secondary active transport process in which two or more substances move in the same direction through the plasma membrane; most commonly, one substances is a sodium ion


a secondary active transport process in which two or more substances move through the plasma membrane in opposite directions; most commonly, one of the substances is the sodium ion

Co-transport molecule can only function due to the concentration

gradient created by the primary active transport pumps of the basolateral membrane


the condition in which the blood glucose level is elevated above normal

Diabetes mellitus

a medical condition characterized by high blood glucose levels resulting in the presence of glucose in the urine; this condition results in insulin deficiency or lack of cellular response to insulin

In hyperglycemia, a condition that accompanies diabetes mellitus, excessive glucose

accumulates in the blood

What will happen if the number of co-transport molecules is not sufficient to handle an abnormally high concentration of glucose in the filtrate

not all glucose will be reabsorbed; the transport maximum will have been exceeded; the extra glucose will be excreted in the urine

Transport maximum (Tm) reflects the number of

carriers in a renal tubule available to transport a particular substance across the plasma membrane

There are many carriers (high Tm) for nutrients like amino acids and

glucose, but few or no carriers for substances not useful to the body

When carriers are saturated, excesses are

excreted in the urine

Renal threshold

plasma concentration at which a substance begins to spill into the urine because its Tm has been exceeded

Renal threshold for glucose


Average blood glucose is


Uncontrolled diabetes can result in glucose levels as high as

400mg/100ml and large amounts of glucose are lost in the urine; water follows osmotically

Tubular reabsorption returns most of filtered water and many

of filtered solutes to the bloodstream

Water and solutes move from tubule

lumen back into the blood

Virtually all of the glucose and amino acids are completely

reabsorbed in healthy kidneys

Reabsorption of water and many ions is continuously

regulated and adjusted in response to hormonal signals

Paracellular pathway (2nd reabsorption route)

in the PCT, water diffuses through the tight junction down its concentration gradient; Na+, Cl and K+ may also follow in a process called solvent drag

Na+/K+ ATPase pumps drive the reabsorption of water and

solutes by increasing sodium ion concentration of the interstitium

H+ ions are secreted into the filtrate to provide

an acid/base balance

Various types of transport molecules provide

passage for solutes to diffuse between filtrate and cytosol

Bulk flow

the movement of substances all moving in the same direction as a result of pressure

Water and solutes move by

bulk flow from interstitial space into peritubule capillaries

The net result of reabsorption in the PCT is that valued substances are reclaimed, 65% of

filtrate is reabsorbed, including 100% of glucose and amino acids

Epithelium transitions from cuboidal epithelial cells to

simple squamous epithelial cells in the descending loop of Henle

Descending limb of the loop of Henle is permeable to

water but not to NaCl

In the descending limb of the loop of Henle, few membrane proteins serve as channel or

transport molecules; net result is an increased osmolarity of filtrate

Descending limb of the loop of Henle has a high filtrate

osmolarity due to water loss since it is highly permeable to water

Ascending limb of the loop of Henle transitions from simple squamous cells to

simple cuboidal cells

Ascending limb of loop of Henle: water permeability is restricted by

tight junctions and a glycoprotein covering of the luminal membrane

In the ascending limb, carrier molecule co-transports

one potassium ion and two chloride ions and one sodium ion

In the ascending limb, intracellular potassium concentration changes little because

potassium returns to the filtrate and interstitium through its channels

In the ascending limb, imported chloride ions follow sodium

ions to the basolateral membrane and diffuse into the interstitium

Basolateral membranes in ascending loop of Henle possess the same sodium/potassium ATPase pumps and

potassium channels as does the PCT; chloride channel permits chloride to diffuse with movement of sodium

Filtrate becomes more dilute as it

travels up the ascending limb

The osmolarity in the interstitium is 200mOsm greater than

the filtrate at the same level in the loop of Henle

Osmolarity of deeper medullary regions is greater than regions

close to the cortex (medullary osmotic gradient)

Countercurrent multiplier

the role played by the opposing flow of filtrate within the ascending and descending loop of Henle to form the medullary osmotic gradient

The complex interplay of the ascending and descending limbs forms and

maintains the interstitial osmolarity gradient in the medulla; this medullary gradient is essential for the concentration of urine

The ascending limb of the loop actively transports sodium chloride into the interstitial space, increasing

concentration; the interstitium surrounding the tubule becomes more concentrated while the fluid inside becomes more dilute

The highest concentration of solute is found at what point in the loop of Henle

near the bottom (1,200mOsm)

65% of filtrate is reabsorbed in the PCT through

active and passive transport processes

The filtrate is concentrated in the descending loop of Henle; water is

lost while solutes are retained

The filtrate is diluted in the ascending loop of Henle; solutes are

lost while water is retained

The asymmetrical pattern of water and NaCl reabsorption in the ascending/descending limbs creates

an osmotic gradient within the medullary region

The vasa recta circulate blood through the medulla to provide

nutrients without removing solutes and weakening the osmotic gradient

In the descending limb the

-NaCl remains
-H20 moves out
-Filtrate is concentrated

In the ascending limb the

-NaCl moves out
-H20 remains
-Filtrate is dilute

In the descending limb

-H20 high permeability
-NaCl impermeable

In the ascending limb

-H20 is impermeable
-NaCl is high permeability

In the PCT

-H20 is highly permeable
-NaCl is highly permeable
-Glucose is highly permeable

PCT cells are the most active re-absorbers along the tubule; all of the glucose,

amino acids and vitamins are reabsorbed here; plus 65% of all Na+ and water & 90% of filtered bicarbonate and 50% of Cl- and K+

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