Adelina Vlad, MD PhD

Urine formation results

from:

Glomerular filtration

Tubular reabsorption

Tubular secretion

Excretion = Filtration – Reabsorbtion + Secretion

Reabsorption and Secretion by the Renal Tubules

Tubular Reabsorption

• Is a highly selective process

• By controlling the reabsorbtion rate, the kidney adjust the excretion of specific compounds

Reabsorption by the Renal Tubules

Amount filtered = Glomerular filtration rate x Plasma concentration

Reabsorbtion and Secretion Mechanisms

Active Transport Against an electrochemical gradient

With energy consumption (ATP)

Primary active transport

Directly connected to an energy source (Na/K-ATPase, Na/H-

ATPase, H-ATPase, Ca-ATPase)

E.g.: Na+ reabsorbtion

Na/K pump in the laterobasal membrane creates an

electrochemical gradient favouring Na+ facilitated diffusion from

the tubular lumen

At the apical site: passive reabsorbtion through cotransporters

and exchangers (PT, TAL, DCT) or epithelial Na+ channels

(ENaC, in the collecting ducts)

Secondary active transport

Two or more substances interact

with a carrier molecule

The energy liberated from the

downhill movement of one of the

substances enables uphill

movement of a second substance

Co-transport: same direction

(reabsorbtion of glucose,

aminoacids in the proximal tubule,

and Na+/K+/Cl- in the TAL and

DCT)

Counter-transport: opposite

direction (secretion of H+ in the

proximal tubule)

Pynocitosis

Active transport mechanism

Characteristic to reabsorbtion of large molecules (proteins)

Proteins are incorporated in pynocitosis vesicles at the luminal

side of the tubular cell; inside the vesicles, the proteins are

digested to aminoacids that passively diffuse into the interstitial

fluid and further into the peritubular capillaries

Transport Maximum = The limit to the rate at which a

substance can be transported

- Is characteristic to active

transport

- Appears when the transport

system gets saturated

Saturation - the tubular load

exceeds the capacity of the

carrier/enzyme

Threshold – the filtered load of

substance (glucose) at which the

substance (fully reabsorbed, not

secreted) begins to be excreted

in the urine

Substances actively reabsorbed

Substances actively secreted

Substances Passively Reabsorbed

Do not demonstrate a transport maximum

Their rate of transport is determined by:

the electrochemical gradient for diffusion

the permeability of the membrane

the time that the fluid containing the substance remains within the

tubule

Transport of this type is referred to as gradient-time transport

Water Reabsorbtion

Transcellular and paracellular

Depends on the permeability of each tubule segment

High in the proximal tubule

Low in the other segments; ADH-dependent in late distal and

collecting tubules

Realised by osmosis

Follows Na+ reabsorbtion

Contributes to the reabsorbtion of other solutes through solvent

drag

Paracellular transport of Na+, passive

Governed by the transepithelial electrochemical gradient for Na+

Proximal tubule and thick ascending limb of the loop of Henle:

Na+ reabsorbtion

The other tubule segments: backleak of Na+

The leakiness of the paracellular pathway decreases along the

nephron from the proximal tubule (the most leaky) to the papillary

collecting ducts

Chloride reabsorbtion

Paracellular pathway (PT, CD), by the electrochemical gradient

Transcellular pathway, involving K+/Cl- cotransporter (PT, DCT),

Na+/Cl- cotransporter (DCT), Na+/K+/Cl- cotransporter (TAL) and

HCO3-/Cl- exchanger (CD) across the apical site, and Cl-

channels at the basolateral membrane

Urea reabsorbtion and secretion

PT: solvent drag, facilitated diffusion (paracellular and

transcellular reabsorbtion)

Thin LH: ureea secretion through urea transporter UT2

CD: ureea reabsorbtion mediated by UT1 and UT4

Water, chloride, and urea reabsorbtion is coupled with

sodium reabsorbtion

Reabsorption and Secretion Along Different Parts of

the Nephron

Proximal Tubule Reabsorbtion

very active

PT is highly permeable to water

glucose, AA reabs. in the first half

Na+: cotransport with AA, glucose,

exchanger with H+

Cl- (terminal part), HCO3-, K+, urea

HCO3- reabsorbtion depends on

carbonic anhidrase activity

Secretion

H+, bile salts, oxalate, urate,

catecholamines

toxins, drugs (penicillin,

salicylates, PAH)

The Loop of Henle

Thick ascending loop of Henle

The Descending Loop

highly permeable to water (approx. 20% of the filtered water is

reabsorbed here)

moderately permeable to most solutes (urea, sodium)

The Ascending Loop

Impermeable to water and

urea

The thick segment:

reabsorption of sodium,

chloride, potassium

(25% of the filtered

amount), calcium,

bicarbonate,

magnesium

the filtrate becomes

hypotonic

secretion of hydrogen

ions

Thick

ascending

segment

5%

Distal and Cortical Collecting Tubules Early DT

Impermeable to water and urea

Reabsorbs sodium, potassium, chloride

diluting segment

Principal cell

Late DT and Cortical CT

Impermeable to urea

ADH-dependent water permeability

Principal Cells

Na+ reabsorbtion,

K+ secretion (Na/K-ATPase

pump), controlled by aldosterone

Intercalated Cells

H+ secretion by a hydrogen-

ATPase pump, against a large

concentration gradient (1000 to 1)

for each H+ secreted, a –HCO3

is reabsorbed

reabsorbtion of K+

Medullary Collecting Duct

Reabsorbtion of 10% of the

filtered water (ADH - dependent)

and Na+, important in determining

the final urine output of water and

solutes

Reabsorption of urea into the

medullary interstitium, helps to

raise the osmolality in this region,

important for the urine

concentration process

Secretion of H+ against a large

concentration gradient, as in the

cortical collecting tubule

Regulation of Tubular Reabsorption

Glomerulotubular Balance

= The tubules adjust the reabsorption rate according to the

tubular load

Realised by changes in physical forces in the tubule and

surrounding renal interstitium

Can be demonstrated in completely isolated kidneys

Helps prevent overloading of the distal tubular segments

when GFR increases

Peritubular Capillary Physical Forces

Reabsorption = Kf x Net reabsorptive force

Peritubular capillary reabsorbtion depends on two factors directly

influenced by renal hemodynamic changes:

The hydrostatic pressure in the peritubular capillaries, Pc:

influenced by the arteryal pressure and the afferent and

efferent arterioles resistance

The colloid osmotic pressure in the peritubular capillaries, pc:

determined by the systemic plasma colloid osmotic pressure

and the filtration fraction

Renal Interstitial Hydrostatic and Colloid Osmotic Pressures

Changes in peritubular capillary physical forces changes of the

physical forces in the renal interstitium surrounding the tubules

influence tubular reabsorption:

Forces that increase peritubular capillary reabsorption increase

reabsorption from the renal tubules

Hemodynamic changes that inhibit peritubular capillary

reabsorption inhibit tubular reabsorption of water and solutes

Humoral and Nervous Influences on Tubular Reabsorbtion and Secretion

Angiotensin II

Stimulates aldosterone secretion, which in turn increases

sodium reabsorption

Constricts the efferent arterioles raises filtration fraction in

the glomerulus

increased colloid osmotic pressure in the peritubular

capillaries, with consecutive raise of tubular reabsorption of

sodium and water

Directly stimulates sodium reabsorption in the proximal

tubules, the loops of Henle, the distal tubules, and the

collecting tubules

Sympathetic Nervous System Activation

Decreases sodium excretion by

Increasing sodium reabsorbtion in the proximal tubule and the

ascending limb of the loop of Henle

Incresing the renin release

Constricting afferent and efferent arterioles

Use of Clearance Methods to Quantify Kidney Function The renal clearance of a substance is the volume of plasma that is

completely cleared of that substance by the kidneys per unit time

The clearence of

inulin and

creatinine can be

used to estimate

the GFR:

PAH clearence can be used to estimate the Renal Blood

Flow:

PAH – para-aminohippuric acid

Urine Concentration and Dilution

Urine Concentration and Dilution

The body water is controlled by:

Fluid intake, which is regulated by factors that determine thirst

Renal excretion of water, controlled by factors that influence

glomerular filtration and tubular reabsorption

The renal ability to preserve the hydro-electrolitic homeostasis is

based on:

The mechanisms that cause the kidneys to eliminate excess

water by excreting a dilute urine

The mechanisms that cause the kidneys to conserve water by

excreting a concentrated urine

The renal feedback mechanisms that control the extracellular

fluid sodium concentration and osmolarity

Formation of a Dilute Urine

When there is a large excess of water in the body, the kidney can

excrete as much as 20 L/day of dilute urine, with a concentration as

low as 50 mOsm/L

How?

By continuing to reabsorb solutes while failing to reabsorb water in

the distal parts of the nephron, where water reabsorbtion is ADH-

dependent (late distal tubule and the collecting ducts)

Proximal tubule: Tubular fluid remains isosmotic

Descending loop of Henle: Water is reabsorbed by osmosis until

the tubular fluid reaches equilibrium with the interstitial fluid of the

renal medulla, which is very hypertonic

Ascending loop of Henle: Tubular fluid becomes dilute (100

mOsm/L) by the time the fluid enters the early distal tubular segment

- regardless of whether ADH is present or absent

Distal and collecting tubules: Tubular fluid is further diluted (50

mOsm/L) in the absence of ADH; the failure to reabsorb water

and the continued reabsorption of solutes lead to a large volume

of dilute urine

When there is a water deficit in the body, the kidney forms a

concentrated urine

How?

By continuing to excrete solutes while increasing water

reabsorption and decreasing the volume of urine formed

Formation of a Concentrated Urine

Obligatory Urine Volume

= The minimal volume of urine that must be excreted in order to

eliminate the solutes in excess from the body

Depends on:

the average amount of solutes that must be eliminated:

about 600 mOsm of solute/day for a healthy 70-kg human

the maximal urine concentration capacity of the kidney:

1200 mOsm/L to 1400 mOsm/L

Obligatory urine volume:

The minimal loss of volume in the urine contributes to

dehydration, along with water loss from the skin, respiratory tract,

and gastrointestinal tract, when water is not available to drink

The basic requirements for forming a concentrated urine are:

A high level of ADH, which increases the permeability of the

distal tubules and collecting ducts to water

A high osmolarity of the renal medullary interstitial fluid,

which provides the osmotic gradient necessary for water

reabsorption to occur in the presence of high levels of ADH

Formation of a Concentrated Urine

The osmolarity of interstitial fluid in almost all parts of the body is

about 300 mOsm/L

The osmolarity of the interstitial fluid in the medulla of the kidney is

increasing progressively to about 1200 to 1400 mOsm/L towards the

papilla

How?

The renal medullary interstitium accumulates solutes in great excess

of water

The Countercurrent Mechanism

Responsible for making the renal medullary interstitial fluid

hyperosmotic

Depends on:

The special anatomical arrangement of the loops of Henle (LH)

25% of the nephrons are juxtamedullary, with long LH

descending deep into the renal medulla

The special anatomical arrangement of vasa recta

They parallel the LH of juxtamedullary nephrons

The collecting ducts which carry urine through the hyperosmotic

renal medulla also play a role in the countercurrent mechanism

Major factors that contribute to the buildup of solute concentration

into the renal medulla:

Active transport of sodium ions and co-transport of potassium,

chloride, and other ions out of the thick portion of the ascending

limb of the loop of Henle into the medullary interstitium

Active transport of ions from the collecting ducts into the medullary

interstitium

Facilitated diffusion of large amounts of urea from the inner

medullary collecting ducts into the medullary interstitium

Diffusion of only small amounts of water from the medullary tubules

into the medullary interstitium, far less than the reabsorption of

solutes into the medullary interstitium

The characteristics of the loop of Henle cause solutes to be trapped

in the renal medulla:

Sodium, potassium, chloride, and other ions are transported from

TAL into the interstitium; the concentration gradient created between

the lumen and the interstitium cannot exceed 200 mOsm

TAL is impermeable to water solutes are added in excess to

water to the renal medullary interstitium

Sodium chloride is passively reabsorbed from the thAL, impermeable

to water as well solutes are added further to the high solute

concentration of the renal medullary interstitium

The descending limb of Henle’s loop is permeable to water the

tubular fluid osmolarity gradually rises as it flows toward the tip of the

loop of Henle

The repetitive reabsorption of sodium chloride from the thick ascending loop

of Henle and continuous inflow of new sodium chloride from the proximal

tubule into the loop of Henle is called the countercurrent multiplier

Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium

Role of Distal Tubule and Collecting Ducts In the presence of high concentration of ADH, the late distal tube and

cortical collecting tubule become highly permeable to water

The water from the cortex interstitium is swept away by the rapidly

flowing peritubular capillaries

The collecting ducts become permeable to water as well in the

presence of high levels of ADH the fluid at the end of the

collecting ducts has the same osmolarity as the interstitial fluid of the

renal medulla - about 1200 mOsm/L

The water is removed from the medullary interstitium through vasa

recta

Urea Contributes to Hyperosmotic Renal Medullary Interstitium Urea contribution = 40 – 50 % (500 - 600 mOsm/L) when the kidney

is forming a maximally concentrated urine

How?

Along the tubule, the concentration of urea increases:

Proximal Tubule:

Is less permeable for ureea than for water

40 – 50 % of urea present in the filtrate is reabsorbed, but urea

concentration rises due to intense water reabsorbtion

Thin Loop of Henle:

The concentration of urea continues to rise because of

water reabsorption

secretion of urea into the thin loop of Henle from the medullary

interstitium

Thick Loop of Henle, Distal Tubule and Cortical Colecting Tubule:

Impermeable to urea

With high ADH plasma levels, large amounts of water are

reabsorbed urea becomes more concentrated

Inner Medullary Collecting Duct:

More water is reabsorbed, urea concentrates further

The concentration gradient leads to urea diffusion from the IMCD

into the medullar interstitium; this diffusion is facilitated by specific

urea transporters (eg. UT-AI, activated by ADH)

Urea Recirculation A moderate share of the urea that moves into medullary interstitium

diffuses into the thin loop of Henle, passes again through the

ascending LH, DT, CCT, and back down into the MCD

Urea can recirculate several times before it is excreted, contributing

to a higher concentration of urea

This mechanism for concentrating urea before it is excreted is

essential for keeping water into the body when water is in short

supply

When there is excess water in the body (low ADH), the IMCD

permeability for water and urea decreases and more urea is

excreted in the urine

Importance of the Vasa Recta Vasa recta preserve

hyperosmolarity of the renal

medulla because

The blood flow is low: 2 – 5 %

of the total renal blood flow,

sufficient to supply the

metabolic needs of the tissues

and low enough to minimize

solute loss from the medullary

interstitium

They serve as countercurrent

exchangers, minimizing

washout of solutes from the

medullary interstitium

The Vasa Recta’s Countercurrent Exchange Mechanism Plasma flowing down the descending limb of the vasa recta

becomes more hyperosmotic because of diffusion of water out of

the blood and diffusion of solutes from the renal interstitial fluid

into the blood

In the ascending limb of the vasa recta, solutes diffuse back into

the interstitial fluid and water diffuses back into the vasa recta

Certain vasodilators or large increases in arterial pressure can

markedly increase renal medullary blood flow, “washing out” some of

the solutes from the renal medulla and reducing maximum urine

concentrating ability

Changes in osmolarity of the tubular fluid in the presence of high

levels of antidiuretic hormone (ADH) and in the absence of ADH

Osmoreceptor-ADH Feedback System

Cardiovascular Reflex Stimulation of ADH Release In addition to increased osmolarity, two other stimuli increase

ADH secretion:

(1) decreased arterial pressure

(2) decreased blood volume

Afferent stimuli are carried by the vagus and glossopharyngeal

nerves from high-pressure regions of the circulation (the aortic

arch and carotid sinus) and low-pressure regions (cardiac atria) up

to the tractus solitarius

Projections from these area influence the hypothalamic nuclei that

control ADH synthesis and secretion

ADH is more sensitive to small

changes in osmolarity than to

similar changes in blood volume:

An increase of plasma osmolarity

of 1% is sufficient to increase ADH

levels

A decrease in blood volume of

more than 10% increases

significantly the ADH levels

The day-to-day regulation of ADH

secretion is effected by changes in

plasma osmolarity

With severe decreases in blood

volume, the cardiovascular reflexes

play a major role in stimulating ADH

secretion

Stimuli for ADH Secretion; Thirst Control