urine formation..mechanism,acidification,urinary buffers by dr.tasnim

DR.TASNIM ARA JHILKY

MD Part II

Phase A

DEPARTMENT OF BIOCHEMISTRY

SIR SALIMULLAH MEDICAL COLLEGE MIDFORD,DHAKA

Afferent arteriole

• Glomerulus: fenestrated

capillaries

Efferent arteriole

• Peritubular capillaries

(PCT and DCT) involved in

reabsorption

• Vasa recta (medullary loop

of Henle) involved in

forming concentrated urine

Figure 25.5a

Process concerned with urine formation

Glomerular filtration.. Tubular reabsorption.. Tubular secretion..

Urine formation begins with the ultrafilteration of plasma from glomerular capillaries into bowman s capsule to make filterate.

Therefore urinary excretion is equal to:

Glomerular filteration-tubular reabsorption+tubular secretion

Cortical

radiate

artery

Afferent arteriole

Glomerular capillaries

Efferent arteriole

Glomerular capsule

Rest of renal tubule

containing filtrate

Peritubular

capillary

To cortical radiate vein

Urine

Glomerular filtration

Tubular reabsorption

Tubular secretion

Three major

renal processes:

Pore size. Glomerular capillary – 8 nm.podocyte filtration slits – 25 nm.4 nm -- freely filtered.4-8 nm -- inversely proportional to diameter> 8 nm – not filtered.

Electrical charge.Negatively charged – It contain proteoglycan & sialic

acid .so cationic & neutral particals – more permeability

Filterate turns into urine with followings

changes…

Reduction of volume

Acidification of filterate,

rise of osmolarity

Complete reabsorption of some subs.

Addition of new subs by tubular secretion

Reabsorption is defined as movement of a substance from

the tubular fluid to the blood, and this process occurs either

via the tubular cells ”the transcellular route” or between the

cells” the paracellular route .

Tubular secretion is defined as movement of a substance

from the blood into the tubular fluid.

The reabsorption and secretion that occur via the

transcellular route are largely the result of secondary active

transport of solutes by the tubular cells.

Paracellular reabsorption occurs as a result of

concentration or electrical gradients that favor movement of

solutes out of the tubular fluid.

Proximal Convoluted Tubule (PCT);

Reabsorption

.

• PCT is the most active in reabsorption

• All glucose, lactate, & amino acids vitamin

• Most Na+, H2O, HCO3- , CL-

• and K+

–60-70% Na+ and H2O

–90% HCO3-

–50% CL-

–70% K+

– Ca,mg,phosphate,urea

– Ammoniagenesis from glutamin

Production of new HCO3–

Loop of Henle: Reabsorption

• Descending limb: – H2O 20% reabsorbed by osmosis,not

permeable for solute.so fluid become concentrated.so osmolarity increase in lumen

Loop of Henle: ReabsorptionAscending limb:

Na+25% Cl-, K+ active transport

Ca2+, Mg2+ passive transport

H2O : impermeable

Distal Convoluted Tubule: Reabsorption

Early tubule:

• Na+ Cl- : symporter mediated

• Ca2+ : PTH mediated

• H2O : impermeable

Distal Convoluted Tubule & Collecting

Tubule reabsorption

Principal cells:

• Na+ : Aldosterone mediated,Increase Na-k+ ATPase,causesmore Na channel in luminal mem.then reabsorption by Na-k atpase into cell.&k+ secretion into cell.

• Ca2+ : PTH mediated

• H2O : ADH mediated,aquaporin channel,absence of ADH no water channel in luminal site..so no reabsorption of water

Intercalated cells:acid secreting cell.secrets H+ in lumen &HCO3- in blood

• H2CO3

Late Distal Convoluted Tubule &

Collecting tubule

Principal cells:

• Na+ : Aldosterone mediated

• Ca2+ : PTH mediated

• H2O : ADH mediated &

concentrated urine

Intercalated cells

• H2CO3

Mechanism of Action of Aldosterone

Medullary Collecting Duct:

Reabsorption

H2O : ADH dependent,Actively secrete H+,HCO3-IN blood

Urea: this cell has special transporter for urea reabsorption LH to CD urea not permeable.but here it is permeable so goes to medullary interstisium ,taken up by LH.Then recyclc.

Tubular Secretion• PCT

• Secretion of H+(80 to 90%)

• Secretion of NH4+

• Na-H counter transport

• DCT

• Secretion of H+5%

• ATPase driven proton pump.

• Tubules also secrete substances into the filtrate.

• H+, K+, NH4+, creatinine

• Important functions:– Disposes of substances not in original filtrate (certain

drugs and toxins)

– Bile salts, oxalate, urate and catecholamines

– Disposes of excess K

In PCT: H+, K+, NH3,bile salt, oxalate,

urate, PAH.

In DL of loop of henle: Na+ is secreted by

passive diffusion.

In DCT: NH3 is secreted by diffusion. H+,

K+, is secreted by exchange pump.

In CT: secretion of K+ & NH3 with the

help of aldosterone

Formation of dilute

urine depends on

decreased secretion

of ADH from pituitary.

Kidneys continue to

absorb solute; while

fail to absorb the

water.

Achieved by

continuing to secrete

the solutes; while

increasing the water

reabsorption.

This requires:

• High level of ADH

• Highly osmolar renal

medullary interstitium

ADH increases the permeability of the distal tubules & collecting ducts to water.

Highly osmolar renal medullary interstitium provides osmotic gradient for water reabsorption in presence of ADH.

There is a progressively

increasing osmolar gradient

in medulla.

This gradient is due to:

• LOH acting as Countercurrent

Multiplier

• Vasa Recta acting as

Countercurrent Exchanger

• Urea cycling also contributes

to the medullary osmolarity.

LOH act as countercurrent multiplier to produce the medullary osmotic gradient.

AL pumps out NaCl into the interstitium & is capable of producing an osmotic gradient of iin tubule & interstitium.

The countercurrent flow in LOH, with differing permeability of DL & AL is capable of multiplying this effect to produce an osmotic gradient.

Vasa recta prevents the wash

down of medullary

concentration gradient while

absorbing excess solutes &

water from interstitium.

It does not contribute to the

production of medullary

concentration gradient but

helps to preserve it.

Low blood flow (5-10% of

total) to the medulla also helps

in this.

It is the minimal volume of urine that must be excreted each day to get rid the body of the products of metabolism & ingested ions.

It depends upon the maximal concentrating ability of the kidney.

Total solutes to be excreted each day in 70 kg man = 600 mosmol

Maximum conc. ability of human kidney = 1200 mosmol/l

OUV = 600/1200 = 0.5 L/day

Inappropriate secretion of ADH:

• ↓ ADH: Central Diabetes Insipidus

• ↑ ADH: SIADH (Syndrome of Inappropriate

secretion of ADH)

Impairment of countercurrent mechanism:

• High flow rate: osmotic diuresis

Inability of tubules to respond to ADH:

• Nephrogenic Diabetes Insipidus

Urinary buffers.

To prevent sudden & and large swings in

Ph buffers are necessary.

Buffers have two major characters…

consist of either a weak acid &conjugate

base or weak base &conjugate acid

Resist pH changes.

1. Phosphate buffer

2. Bicarbonate buffer

3. Ammonia buffer

bicarbonate/carbonic acid

major plasma buffer

phosphate: H2PO4- / HPO42-

major urine buffer

ammonium: NH3 / NH4+

also used to buffer the urine

About 70mmol nonvolatile acid formed in the form of HCl,H2SO4,H3PO4 Lactic acid . In normal individual.immediately they production ,acid are buffered by body fluid buffers ,mainly HCO3-buffer system.

At the level of DCT &CD ,again tubular cell generate H+&simultaneous production of HCO3-.thenH+ secreted into lumen by proton pump.HCO3- goes blood.In filterate of DCT &CD no HCo3-

Phosphate and NH3 buffer are available.H+ combines with conjugate base of urinary buffers. To make NH4+& H2PO4- excess H+ excreted of non volatile acids.

PHOSPHATE BUFFER SYSTEM Dihydrogen phosphate ion (H2PO4

-)

• Weak acid

Monohydrogen phosphate ion (HPO42-)

• Weak base

H2PO4- H+ + HPO4

2-

More important in buffering kidney filtrate than in tissue

Buffering of hydrogen ion secretion by

ammonia (NH3) in the collecting tubule

Lowest attainable pH 4.5

Urinary con.rises to 40 micromole/L which

is 1000 times greater than intracellular H+

con.of 40mmo/Lat ph 7.4 in CD.

Against this huge cell to lumen gradient of

H+ at intracellular ph7.4 &, urine Ph

4.5.Proton pump of CD fails to secrate H+

any more to lower urine PH further.

Since the limiting pH of urine is 4.5 (lowest possible urinary ph )1L urine excrete only 40micromol/L H+.

To excrete 70mmol non volatile acid as freeH+,daily urine volume would have to be about 1750L.which is impossible.

Therefore urinary buffers take up the secreted H+ to form NH4+ and H2PO4.

This allowed con.secretion of H+ and its excretion as NH4+ and H2PO4 without disturbing urinary pH

It is conversion of alkaline filtrate to acidic

urine. It is done by separate mechanism

HCO3 reabsorption /reclamation

mechanism

It is concerned with complete reabsorption

of filtered HCO3- mainly from pct(80-90%)

& partly from ALH(10-20%).

Here secreted H+ from tubular cell is

titrated in tubular fluid by filtered HCO3-

So net loss or gain of HCO3- to blood.

It does not create high luminal H+ con.

It creates a very low PH gradient by

decresing filterate PH from 7.4 to 6.8.

It important becauses it saves huge

amount (4500mmol)HCO3- per day.

Production of new HCO3–

Role of NH3 and phosphact buffers

H+ secretion into the lumen & new HCO3-generation followed by its addition to blood

Secreted H+ is titrated in tubular fluid by urinary buffers with net loss of H+ from blood as NH4+,H2PO4- and net gain of HCO3- to blood to raise serum HCO3-back to normal.

It creates high luminal H+ con.

It creates large Ph gradient by decreasing

ph from 6.8 to 4.5

So, urinary acidification in fact occur ic CD

&DCT.

By this mechanism there is net gain of HCO3- to

the blood which is used during initial buffering of

nonvolatile acids.

There is also net excretion of H+ from the body in

the form of NaH2PO4, NH4Cl.

Phosphate & Ammonia buffer are involved here.

• It occurs in DCT & CD by ATP drive proton pump

If metabolic acids accumulate, the body PH will

fall

All non-volatile acid is excreted through kidney

That’s why urine become acidic

• The presence of acidic substances (NH4+CL &

NaH2PO4) finally make the urine acidic & this

acidification in fact starts from DCT

To maintain stable Ph at 7.40

Physiologic pH necessary to prevent

engyme inactivation and denaturiation.

proper cellular function.

Clinical consequences of dysregulation of

acid base balance

Poor vascular tone

Myocardial failure

Risk of arrthymia

Musle weakness

Electrolyte abnormalities

Delirium/coma etc

Most acid comes from CHO and fat metabolism

• 15 to 20 mol of CO2 daily(volatile)

.Incomplete metabolism of glucose and fatty acids to

lactic acid & ketoacids.

• Metabolism of sulfur-containing amino acids (cysteine,

methionine) to sulfuric acid

• Hydrolysis of dietary phospholipid & nucleic acid into

phosphoric acid.

Most base comes from metabolism of anionic

amino acids (glutamate and aspartate) and from

oxidation and consumption of organic anions

such as lactate and citrate, which produce HCO3−

If [H+]↑→ Resp. centre stimulated

→hyperventilation→↑volatile acid

excretion.

If [H+]↓→hypoventilation→↑volatile acid

retention

Secretion of H+

Reabsorption of filtered HCO3-

Production of new HCO3-

Kidney plays an important role in maintenance of

acid-base balance by excreting H+ ions and

retaining bicarbonate ions.

Buffer system of the body fluid-

1st line of defense

• immediately combines with acid / bases to prevent

marked change of [H+]

• works within a fraction of second to minimize the

changes

• do not eliminate H+ from the body but only keep them

tied up until balanced re-established

Respiratory system-

2nd line of defense• respiratory centre regulate the removal of CO2 &

therefore H2CO3 from the body.

• works within a few minutes.

Kidney system-

3rd line of defense• can excrete either acidic or alkaline urine to maintain

ECF [H+]

• works over a period of hours to several days but it is the

most powerful acid –base regulatory systems.