Urine Formation: Tubular Processing of the Glomerular

Filtrate

Jethro Macallan, MD

Flow of urine pix

Urinary Excretion = Glomerular Filtration – Tubular

Reabsorption + Tubular secretion

Filtration = Glomerular filtration rate x Plasma

concentration

Tubular Reabsorption

Very large compared to urinary excretionHighly selective Completely reabsorbed: glucose and

amino acids Variable reabsorption: Na, Cl, HCO3 Poorly reabsorbed: urea and creatinine

Filtration, Reabsorption, and Excretion Rates of Different Substances by the Kidneys

Amount Filtered

Amount Reabsorbed

Amount excreted

% of Filtered Load Reabsorbed

Glucose (g/day)

180 180 0 100

Bicarbonate (mEq/day)

4,320 4,318 2 >99.9

Sodium (mEq/day)

25,560 25,410 150 99.4

Chloride (mEq/day)

19,440 19,260 180 99.1

Potassium (mEq/day)

756 664 92 87.8

Urea (g/day) 46.8 23.4 23.4 50

Creatinine (g/day)

1.8 0 1.8 0

Table 41-2 Composition of urine

Substance Concentration

Na+ 50-130 mEq/L

K+ 20-70 mEq/L

NH4+ 30-50 mEq/L

Ca++ 5-12 mEq/L

Mg++ 2-18 mEq/L

Cl- 50-130 mEq/L

Pi 20-40 mEq/L

Urea 200-400 mM

Creatinine 6-20 mM

pH 5.0-7.0

Osmolality 500-800 mOsm/kg H2O

Glucose 0

Amino acids 0

Protein 0

Blood 0

Ketones 0

Leukocytes 0

Bilirubin 0

Tubular Reabsorption

Involves both active and passive mechanisms

Transcellular route Paracellular route Ultrafiltration (bulk flow)

Tubular Reabsorption

Active Transport Against an electrochemical gradient Requires ATP 2 types:

1. Primary

2. Secondary

Tubular Reabsorption

Osmosis Water diffusion from a region of low solute

concentration to a region of high solute concentration

Primary Active Transport

Can move solutes against an electrochemical gradient

Energy comes from the hydrolysis of ATP Includes the following:

1. Na-K ATPase

2. Hydrogen ATPase

3. H-K ATPase

4. Ca ATPase

Primary Active Transport

Na-K ATPase Transports Na ions into the interstitium Maintains: 1) low intracellular Na and 2)

high intracellular K End result: -70mV inside the cell

Primary Active Transport

Na- K ATPase Involves 3 steps:

1. Na diffuses across the apical membrane into the cell membrane

2. Na is transported by the Na-K ATPase pump

3. Na, water and other substances are reabsorbed to the peritubular capillaries by ultrafiltration

Na- K pump pix

Secondary Active Transport

2 or more substances interact with a carrier molecule and are transported together across the membrane

Does not require energy directly from ATP and from high energy phosphate sources

Can either be co-transport and counter-transport

Glucose and amino acid co-transport

Na-H countertransport

Secondary Active Transport

Pinocytosis Reabsorbs large molecules eg. Proteins Requires energy

Secondary Active Transport

Transport maximum Limit to transport of solute (reabsorption

and/or secretion) Saturation of specific transport systems Tubular load exceeds the capacity of the

carrier proteins Can be increased by hormones, eg.

aldosterone

Secondary Active Transport

Glucose Transport System Found in the proximal tubule All filtered glucose is reabsorbed Transport maximum: 375mg/min – all

nephrons have reached their maximal capacity to reabsorb glucose

Filtered load of glucose: 125mg/min

Secondary Active Transport

Threshold for glucose Appearance of glucose in the urine Occurs before the transport maximum is

reached Reason: not all nephrons have the same

transport maximum for glucose

Transport Maximum of Substances Actively Reabsorbed

Substance Transport Maximum

Glucose 375mg/minPhosphate 0.10 mM/minSulfate 0.06 mM/min

Amino acids 1.50 mM/min

Urate 15 mg/min

Lactate 75 mg/minPlasma protein 30 mg/min

Transport Maximum of Substances Actively Secreted

Substance Transport Maximum

Creatinine 16 mg/min

PAH 80mg/min

Actively Transported but No Transport Maximums

Substances that are passively reabsorbed do NOT demonstrate a transport maximum

Gradient-Time Transport Also seen in some actively transported

substances, eg. Na reabsorption Determined by:1. Electrochemical gradient for diffusion2. Permeability of the membrane for the

substance3. Time a substance takes inside the fluid

Water Reabsorption

Occurs through osmosisOsmotic flow of water occurs through the

tight junctionsSeen in the proximal tubule – high

permeability for water and most ions

Water Reabsorption

Solvent Drag Solutes carried when water moves across

the tight junctions by osmosis Changes in Na reabsorption influence

reabsorption of water and solutes

Water Reabsorption

Proximal Tubule – highLoop of Henle – lowLast part of the tubules – high or low

Passive Diffusion of Cl, urea and other solutes

Cl ion diffusion Caused by electric potentials brought

about by Na Na goes out leaving the inside negatively

charged compared to the interstitial fluid Diffuses through the paracellular pathway Can be reabsorbed by secondary active

transport, eg. Na-Cl co-transport

Na+ reabsorption

H2O reabsorption

Lumen negative potential

Luminal Cl- concentration

Luminal urea concentration

Passive Cl- reabsorption

Passive urea reabsorption

Passive Diffusion of Cl, urea and other solutes

Urea Passively reabsorbed from the tubule Increases as water is reabsorbed from the

tubules Urea transporters- In the inner medullary collecting duct- ½ that is filtered is reabsorbed

Passive Diffusion of Cl, urea and other solutes

Creatinine Larger than urea All that is filtered is excreted in the urine

10 minute break

Reabsorption and Secretion Along the Different Parts of

the Nephron

Proximal Tubule

Reabsorbs about 65% of the filtered load of Na and water

Highly metabolic and have large numbers of mitochondria

Extensive brush border on the luminal side Rapid Na reabsorption Loaded with protein carrier molecules- Co-transport: glucose and amino acids- Counter-transport: hydrogen ions

Proximal Tubule

Na-K ATPase pump Major force for the reabsorption of NaCl

and water 1st half of the tubule: co-transport with aa,

glucose 2nd half of the tubule: co-transport with Cl

Proximal Tubule

Solute concentration along the Proximal tubule

[Na] remains constant Glucose, amino acids and HCO3 are

avidly reabsorbed [creatinine] increases

Proximal Tubule

Secretion of Organic acids and bases eg. Bile salts, oxalate, urate,

cathecholamines Secretion + filtration + no reabsorption =

rapid excretion Penicillin and salicylates: rapidly excreted

by the kidneys PAH: rapidly excreted

- Used to estimate renal plasma flow

Loop of Henle

Divided into 3 segments:

1. Thin descending segment

2. Thin ascending segment No brush borders Few mitochondria Minimal levels of metabolic activity

3. Thick ascending segment

Loop of Henle

Descending Segment Highly permeable to water Moderately permeable to most solutes~Na

and urea Function: simple diffusion through its walls Reabsorbs almost 20% of the filtered

water Does not reabsorb Ca, Mg, HCO3

Loop of Henle

Ascending Limb Both thin and thick Impermeable to water – important in

concentrating the urine Na-K 2Cl co-transport- Mediates movement of Na across the

luminal membrane

Loop of Henle

Thick Ascending Limb High metabolic activity Active reabsorption of Na, Cl and K ~25% Also reabsorbs Ca, Mg and HCO3 Na-K ATPase pump- Low intracellular [Na] Site of action of loop diuretics (furosemide,

ethacrynic acid, bumetanide)

Loop of Henle

Thick Ascending Limb Na-H counter-transport- Mediates Na reabsorption- Secretes hydrogen ions Impermeable to water

Early Distal Tubule

Forms part of the juxtaglomerular complex – provides feedback control of GFR and blood flow

Reabsorbs Na (5%), K and Cl Impermeable to water and urea Na-Cl co-transport Moves Na and Cl from the lumen into the cell Na-K ATPase Pumps Na out of the cell across the basolateral

membrane Site of action of thiazide diuretics

Late Distal Tubule and Cortical Collecting tubule

Composed of 2 distinct cell types:

1. Principal Cells Reabsorb Na and water from the lumen Secrete K ions to the lumen

2. Intercalated Cells Reabsorb K ions Secrete hydrogen ions into the lumen

Late Distal Tubule and Cortical Collecting tubule

Principal Cells Depends on the activity of the Na-K

ATPase pump Na reabsorption and K secretion Primary sites of K sparing diuretics

1. Aldosterone antagonists: spironolactone, eplerenone

2. Na channel blockers: amiloride, triamterene

Late Distal Tubule and Cortical Collecting tubule

Intercalated Cells Mediated by a hydrogen ATPase

mechanism Secrete hydrogen ions and reabsorbs K Plays a key role in acid base regulation of

body fluids

Late Distal Tubule and Cortical Collecting tubule

Functional characteristics:

1. Completely impermeable to urea

2. Reabsorb Na ions – controlled by aldosterone

3. Intercalated cells – acid-base regulation

4. ADH/ vasopressin – controls permeability to water

Medullary Collecting Duct

Absorbs <10% of filtered Na and waterFinal site of processing of urine Important role in final urine output of water

and solutesEpithelial cells are cuboidal in shapeRelatively few mitochondria

Medullary Collecting Duct

Special characteristics:

1. Permeability is controlled by ADH

2. Permeable to urea

3. Secretes hydrogen ions – role in acid-base balance

Medullary Collecting Duct

> 1.0 : more water is reabsorbed than solute; net secretion of the solute into the tubular fluid

< 1.0 : more solute has been reabsorbed than water

1.0 : concentration of the substance in the tubular fluid is the same as the concentration in plasma

Measurement of Water Reabsorption

Inulin Used to measure GFR Not secreted or reaborbed Changes in inulin reflect changes in the

amount of water in the tubular fluid

10 minute break

Regulation of Tubular Reabsorption

Glomerulotubular Balance

Most basic mechanism for controlling tubular reabsorption

Intrinsic ability of the tubules to increase their rate of reabsorption in response increased tubular load

Occurs independent of hormones

Glomerulotubular Balance

Total rate of reabsorption increases as the filtered load increases

Helps prevent the overloading of the distal tubular segments when GFR increases

Second line of defense: buffers spontaneous changes in GFR on urine output (first line: autoregulatory mechanisms~ tubuloglomerular feedback)

review autoregulation

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Normal rate of peritubular capillary reabsorption: 124 ml/min

Reabsorption = Kf x Net reabsorptive forces

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Starling Hypothesis Qf = k[ (Pc + πi) – (Pi + πp)]where:- Qf: fluid movement- k: filtration constant- Pc: capillary hydrostatic pressure- Pi: interstitial fluid hydrostatic pressure- πp: plasma oncotic pressure- πi: interstitial fluid oncotic pressure

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Net reabsorptive forces Peritubular hydrostatic pressure (Pc):

opposes reabsorption ~ 13mmHg Interstitial fluid hydrostatic pressure (Pif):

favors reabsorption ~ 6mmHg

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Net reabsorptive forces Peritubular colloid oncotic pressure (πc):

favors reabsorption ~ 32mmHg Interstitial fluid colloid oncotic pressure

(πif): opposes reabsorption ~ 15mmHg

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Hydrostatic pressure: Pc - Pif

13mmHg – 6 mmHg = 7mmHg: opposes reabsorption

Oncotic pressure: πc - πif

32mmHg – 15 mmHg = 17mmHg: favors reabsorption

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Net reabsorptive forces Net hydrostatic forces - Net oncotic forces 7mmHg – 17 mmHg = 10mmHg Favors net reabsorption

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Filtration Coefficient (Kf) Contributes to the high rate of fluid

reabsorption Review:

Reabsorption = Kf x Net reabsorptive forces

Normal Kf = 12.4 ml/min/mmHg

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Regulation of Peritubular Capillary Physical Forces

determined by Pc and πc

Pc - influenced by:

1. Arterial pressure- Increases cause an increase in Pc and a

decrease in reabsorption

2. Resistances of the afferent and efferent arterioles

- Increase in resistance causes an decrease in Pc and an increase in reabsorption

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Regulation of Peritubular Capillary Physical Forces

πc - is determined by: 1. Systemic plasma colloid oncotic pressure- Directly correlated to an increase in

[plasma protein] : reabsorption2. Filtration Fraction (GFR/ renal plasma

flow)- Directly correlated to reabsorption rate Kf is directly correlated to peritubular

capillary reabsorption rate

Factors That Can Influence Peritubular Capillary Reabsorption

↑PC →↓Reabsorption ↓RA →↑PC

↓RE →↑Ra

↑Arterial Pressure→↑PC

↑πC →↑Reabsorption ↑πA →↑πC

↑FF →↑πC

↑Kf →↑Reabsorption

Peritubular Capillary and Renal Interstitial Fluid Physical Forces

Renal Interstitial and Colloid Oncotic Pressures

Peritubular capillary reabsorption is directly correlated with tubular reabsorption of water and solutes

Effect of Arterial Pressure

Increased arterial pressure causes: Much larger increases in GFR A decrease in the filtered load of Na and

water that is reabsorbed in the tubules

Hormonal Control

AldosteroneAngiotensin IIADHANPParathyroid Hormone

Hormonal Control

Aldosterone Secreted by the zona glomerulosa of the

adrenal cortex Acts on the principal cells of the cortical

collecting tubule Stimulates Na reabsorption and K

secretion More important regulator of K than Na Addison’s disease – decreased

aldosterone Conn’s syndrome – increased aldosterone

Hormonal Control

Angiotensin II Most powerful Na retaining hormone Increases during times of: low BP,

hemorrhage and loss of water and Na Effects:1. Stimulates aldosterone secretion2. Constricts afferent arterioles3. Directly stimulates Na reabsorption

Hormonal Control

ADH Key role in the degree of

dilution/concentration of the urine Increases water permeability of the distal

tubule, collecting tubule and collecting duct

Binds to V2 receptors Stimulates AQP-2: form water channels

and permit rapid diffusion of water

Hormonal Control

ANP Secreted by cells of the cardiac atria Responds to plasma volume expansion Acts on the collecting duct Inhibits reabsorption of Na and water Increases urine excretion

Hormonal Control

Parathyroid Hormone Important in Ca regulation Increases tubular reabsorption of Ca Inhibits phosphate reabsorption Stimulates Mg reabsorption

Hormones That Regulate Tubular Reabsorption

Hormone Site of Action Effects

Aldosterone Collecting tubule and duct

↑ NaCl, H2O reabsorption, ↑K+

secretion

Angiotensin II Proximal tubule, thick ascending loop of Henle/distal tubule, collecting tubule

↑ NaCl, H2O reabsorption, ↑H+ secretion

Antidiuretic hormone

Distal tubule/ collecting tubule and duct

↑ H2O reabsorption

Atrial natriuretic peptide

Distal tubule/ collecting tubule and duct

↓ NaCl reabsorption

Parathyroid hormone

Proximal tubule, thick ascending loop of Hental/distal tubule

↓ PO4--- reabsorption,

↑ Ca- reabsorption

Sympathetic Nervous System

Constricts the renal arteriolesDecreases Na and water excretionReduces GFR Increases Na reabsorption Increases renin release and Angiotensin II

formation

10 minute break

Clearance Methods to Quantify Renal Function

Inulin

Polyssacharide molecule with MW = 5200Not produced in the bodyRequires IV infusionFreely filteredNot reabsorbed or secretedUsed to determine GFR ~ radioactive

iothalamate and creatinine

Inulin

Generalizations:

1. Filtered and not reabsorbed or secreted – clearance rate of a substance equals that of inulin

2. Reabsorbed - clearance rate of a substance is less than that of inulin

3. Secreted - clearance rate of a substance is greater than that of inulin

Creatinine

By-product of muscle metabolismCleared almost entirely by glomerular

filtrationCa also be used to assess GFRErrors: A small amount is secreted by the tubules:

excreted creatinine exceeds amount filtered

Error in measurin plasma creatinine: overestimate of plasma cratinine

Creatinine

Changes in GFR = [plasma creatinine]: inversely proportional to GFR

Creatinine excretion rate equals its production

Excretion = GFR x Pcreatinine

Table 40-1 Major hormones that influence GFR and RBF

Stimulus Effect on GFR Effect on RBF

Vasoconstrictors

Sympathetic nerves

↓ ECV ↓ ↓

Angiotensin II ↓ ECV, renin ↓ ↓

Endothelin Shear stress, angiotensin II, bradykinin, epinephrine

↓ ↓

Vasodilators

Prostaglandins (PGI2, PGE2)

↓ ECV, shear stress, angiotensin II

NC ↑

Nitric oxide Shear stress, acetylcholine, histamine, bradykinin, adenosine triphosphate

↑ ↑

Bradykinin Prostaglandin, ↓ angiotensin converting enzyme

↑ ↑

Renal Plasma Flow

Clearance rate of a substance that is completely cleared of plasma

No known substance that is completely cleared by kidneys

Para-aminohippuric acid 90% cleared from the plasma ~ extraction

ratio of PAH

Filtration Fraction

= GFR/ RPF= inulin clearance/ PAH clearance= 125/ 650= 0.19

Use of Clearance to Quantify Kidney Function

Term Equation Units

Clearance rate (Cs) ml/min

Glomerular filtration rate (GFR)  

Clearance ratio None

Effective renal plasma flow (ERPF)

ml/min

Renal plasma flow (RPF)

ml/min

Renal blood flow (RBF) ml/min

Excretion rate Excretion rate = Us x V mg/min, mmol/min, or mEq/min

Reabsorption rate Reabsorption rate = Filtered load – Excretion rate = (GFR x Ps) – (Us x V)

mg/min, mmol/min, or mEq/min

Secretion rate Secretion rate = Excretion rate – Filtered load mg/min, mmol/min, or mEq/min

Substance Clearance rate (ml/min)

Glucose 0

Sodium 0.9

Chloride 1.3

Potassium 12.0

Phosphate 25.0

Inulin 125.0

Creatinine 140.0

Summary

Table 41-3 NaCl transport along the nephron

Segment Percentage filtered reabsorbed

Mechanism of Na+ entry across the apical membrane

Major regulatory hormones

Proximal tubule 67% Na+-H+exchange, Na+-cotransport with amino acids and organic solutes, Na+/H+-Cl-/anion exchange

Angiotensin II

Norepinephrine

Epinephrine

Dopamine

Paracellular

Loop of Henle 25% 1 Na+-1K+-2Cl- symport

Aldosterone

Distal tubule ~4% NaCl symport Aldosterone

Late distal tubule and collecting duct

~3% Na+ channels Aldosterone

Atrial natriuretic peptide

Urodilatin

Table 41-4 Water transport along the nephron

Segment Percentage of filtered load reabsorbed

Mechanism of water

reabsorption

Hormones that regulate

water permeability

Proximal tubule 67% Passive None

Loop of Henle 15% DTL only; passive

None

Distal tubule 0% No water reabsorption

None

Late distal tubule and collecting duct

~8%-17% Passive ADH, ANP

Hormones That Regulate Tubular Reabsorption

Hormone Site of Action Effects

Aldosterone Collecting tubule and duct

↑ NaCl, H2O reabsorption, ↑K+

secretion

Angiotensin II Proximal tubule, thick ascending loop of Henle/distal tubule, collecting tubule

↑ NaCl, H2O reabsorption, ↑H+ secretion

Antidiuretic hormone

Distal tubule/ collecting tubule and duct

↑ H2O reabsorption

Atrial natriuretic peptide

Distal tubule/ collecting tubule and duct

↓ NaCl reabsorption

Parathyroid hormone

Proximal tubule, thick ascending loop of Hental/distal tubule

↓ PO4--- reabsorption,

↑ Ca- reabsorption

Thank You!!!