!Seminario de Fisiología Renal

REGULACIÓN DEL BALANCE DE H2O Y MECANISMOS DE DILUCIÓN Y CONCENTRACIÓN URINARIA

Dr. Ernesto Castro Aguilar Julio 2014

TRASTORNOS DEL METABOLISMO DEL AGUA

PORCENTAJE DE AGUA

FUNCTIONS OF BODY WATERWithout water, the body is unable to maintain life. Five func-tions of water that the body needs to maintain a healthystate are stated in Table U1-2.

2 ● Unit I Fluids and Their Influence on the Body

Embryo Newborn Adult Older adult

__________ __________ __________ __________97% 70–80% 60% 45–55%

FIGURE U1-1 Percentages of body fluid per body weight.

Table U1-1 Percentage of Body Fluids in BodyFluid Compartments

Intracellular fluid (ICF) compartment 40%

Extracellular fluid (ECF) compartment 20%

Interstitial fluid 15%Intravascular fluid 5%

____Total 60%

113 2123 2

Table U1-2 Functions of Body Water

• Transportation of nutrients, electrolytes, and oxygen to the cells• Excretion of waste products• Regulation of body temperature• Lubrication of joints and membranes• Medium for food digestion

Mujeres : 50%

DISTRIBUCIÓN DEL AGUA CORPORAL TOTAL

Líquido intracelular (LIC): 2/3 partes

Líquido extracelular (LEC): 1/3 partes

Líquido intersticial (LI): 75% del LEC

Líquido intravascular (LIV): 25% del LEC

DISTRIBUCIÓN DE AGUA CORPORAL TOTAL

Masculino 75 Kg

ACT 60% peso= 45L

LIC = 30 L

LEC = 15 L

LI= 11.25 L

LIV= 3.75L

• Femenina 60 Kg

• ACT = 30 L

• LIC = 20 L

• LEC = 10L

• LI= 7.5L

• LIV 2.5L

FUNCIONES DEL AGUA

Tranporte de nutrientes, electrolitos y oxígeno a las células

Excreción productos de desecho

Regulación temperatura corporal

Lubricación de articulaciones y membranas

Medio para la digestión de alimentos

BALANCE HÍDRICO

When body water is insufficient and the kidneys are func-tioning normally, urine volume diminishes and the individualbecomes thirsty. Therefore, the person drinks more water tocorrect the fluid deficit. When there is an excessive amountof water intake, the urine output increases proportionately.

Sources of fluid intake include liquids, foods, and prod-ucts of the oxidation of food process. The average intakeand output of fluid per day is 1800–2600 mL. Body fluidsare lost daily through the urine, feces, lungs, and skin.Body water loss through the skin, which is not measurable,is called insensible perspiration. Appropriately 300–500 mLof fluid is lost daily through processes such as sweat glandactivity. Table U1-3 lists the daily body fluid intake andlosses. Definitions related to fluid functions and movementare presented in the accompanying box.

Unit I Fluids and Their Influence on the Body ● 3

Table U1-3 Daily Body Fluid Intake and Losses

Fluid Intake Fluid Losses

Liquid 1000–1200 mL Urine 1000–1500 mLFood 800–1000 mL Feces 100 mLOxidation 200–300 mL Lungs 400–500 mL

Skin 300–500 mLTotal 2000–2500 mL 1800–2600 mL

Definitions Related to Fluid Function and MovementMembrane. A layer of tissue covering a surface or organ

or separating spaces.Permeability. The capability of a substance, molecule,

or ion to diffuse through a membrane.Semipermeable membrane. An artificial membrane

such as a cellophane membrane.Selectively permeable membrane. Permeability of the

human membranes.Solvent. A liquid with a substance in solution.Solute. A substance dissolved in a solution.Osmosis. The passage of a solvent through a mem-

brane from a solution of lesser solute concentration toone of greater solute concentration.

MOVIMIENTO DE AGUA

Difunde libremente entre membranas en respuesta a gradiente de concentración de solutos.

Cantidad de agua dependerá de la cantidad de soluto en ese compartimento

Soluto principal en LEC : sodio y en el LIC : potasio.

Na+-K+ - ATPasa.

Movimiento de agua entre líquido intersticial e intravascular

Determinado por las fuerzas de Starling

Presión hidrostática capilar es mayor que presión oncótica

Se produce ultrafiltrado

Reabsorción por linfáticos

Fluid flows only when there is a difference in pressure atthe two ends of the system. The difference in pressurebetween two points is known as the pressure gradient. Ifthe pressure at one end is 32 mm Hg and at the other end is26 mm Hg, the pressure gradient is 6 mm Hg. The plasma inthe capillaries has hydrostatic pressure and colloid osmoticpressure. The tissue fluids have hydrostatic pressure andcolloid osmotic pressure. The difference in pressure betweenthe plasma colloid osmotic pressure and the tissue colloid os-motic pressure is known as the colloid osmotic pressure gra-dient; likewise, the difference in pressure between the plasmahydrostatic pressure and the tissue hydrostatic pressure isknown as the hydrostatic pressure gradient. Figure U1-2 de-scribes the fluid flow based upon the pressures in the in-travascular and interstitial spaces.

Because the plasma hydrostatic pressure (18 mm Hg) inthe arteriolar end of the capillary is higher than the tissuehydrostatic pressure ( mm Hg) in the tissue spaces, fluidmoves out of the capillary and into the tissue spaces. Theplasma colloid osmotic pressure (28 mm Hg) in the venularend of the capillary is higher than the tissue colloid osmoticpressure (4 mm Hg) in the tissue spaces, causing fluids to

!6

Unit I Fluids and Their Influence on the Body ● 5

Intravascular FluidPlasma hydrostatic pressure (18 mm Hg)Plasma colloid osmotic pressure (28 mm Hg)

Interstitial FluidTissue hydrostatic pressure (–6 mm Hg)Tissue colloid osmotic pressure (4 mm Hg)

Arteriole End:Movement of fluid is fromblood stream into tissue space

Venous End:Movement of fluid is fromtissue space into blood stream

Capillary

Tissue space

FIGURE U1-2 Pressures in the intravascular and interstitial fluid.

OSMOLARIDAD

Concentración de solutos en un determinado solvente

Adición de solutos aumenta osmolaridad.

Osmolaridad sérica calculada:

2(Na+) + NU/2.8 + glucosa/18

NU difunde libremente por membranas y glucosa es internalizada

No ejercen gradiente osmótico.

TONICIDAD

Es la osmolaridad efectiva

Concentración de solutos en plasma que ejercen gradiente osmótico

Producen movimiento de agua : (edema o deshidratación celular).

ORTOSTATISMO Y EDEMA

Exceso de agua en LEC se reflejará en compartimento con mayor cantidad de agua (intersticio): edema

Deficiencia de agua en LEC se reflejará en compartimento con menor cantidad de agua (intravascular): ortostatismo.

SODIO CORPORAL TOTAL Y CONCENTRACIÓN DE SODIO

Sodio corporal total: determina la cantidad de líquido extracelular.

Concentración de sodio: relación entre soluto y solvente.

Determina la tonicidad (movimiento de agua entre membranas)

Determina cantidad de agua intracelular

REGULACIÓN DE OSMOLARIDAD

Cuando el aporte de agua es poco o hay pérdidas hipotónicas (sudor) se conserva agua al producir orina concentrada

Producción orina <0.5mL/min (<1L/d), Uosm 1200 mOsm/kgH2O

Cuando el aporte de agua es abundante

Flujo urinario 10mL/min (14L/d) , Uosm 75-100mOsm/KgH2O

REGULACIÓN DE BALANCE DE AGUA

Osmoreceptores en hipotálamo controlan la secreción de AVP en respuesta a cambios en la tonicidad.

En estado normal las pérdidas son iguales a las ingestas

Mantener osmolaridad entre 285-290 mOsm/Kg

VASOPRESINA

Nonapéptido cíclico sintetizado en neuronas de núcleos supraóptico y paraventricular.

Transportada a neurohipófisis

Liberada en respuesta a aumento en plasma de osmolaridad y disminución de PA. Vida medía 15 min.

Osmoreceptores en hipotálamo.

3 receptores: V1a, V1b y V2.

Receptor V1a: presentes en músculo liso y acoplados a vía de fosfoinositol.

Causa aumento de calcio intracelular, resultado en contracción.

V1b: a nivel hipofisis anterior donde regulan liberación de ACTH por vasopresina

RECEPTORES V2

Se encuentran en membrana basolateral de células principales en túbulo distal y todo el túbulo colector.

Acopladas a proteína G y generación de AMPc que lleva a la inserción de aquaporinas en membrana apical de un segmento previamente impermeable.

C H A P T E R 2 Renal Physiology 25

nephrons; it is largely responsible for the permanently high water permeability of these segments. Aquaporin 3 is constitutively expressed in the basolateral membrane of CNT cells and cortical and outer medullary principal cells, and aquaporin 4 is constitu-tively expressed in the basolateral membrane of outer medullary principal cells and inner medullary collecting duct cells; but it is aquaporin 2 that is responsible for the variable water permeabil-ity of the late distal tubule and collecting duct. Acute vasopressin release causes shuttling of aquaporin 2 from intracellular vesicles to the apical membrane, while chronically raised vasopressin levels increase aquaporin 2 expression. The apical insertion of aquaporin 2 allows reabsorption of water, driven by the high interstitial osmolality achieved and maintained by the counter-current system. Vasopressin also contributes to the effectiveness of this system by stimulating Na+ reabsorption in the TAL (although this effect may be functionally significant only in rodents16) and urea reabsorption through the UT-A1 and UT-A3 transporters in the inner medullary collecting duct. In the rare autosomal recessive and even rarer autosomal dominant forms of nephrogenic diabetes insipidus, aquaporin 2 is abnormal or fails to translocate to the apical membrane.15

Aquaporin 2 dysfunction also appears to underlie the well-known urinary concentrating defect associated with hypercalce-mia. Increased intraluminal Ca2+ concentrations, acting through an apically located calcium-sensing receptor, interfere with the insertion of aquaporin 2 channels in the apical membrane of the medullary collecting duct.17 In addition, stimulation of a calcium receptor in the basolateral membrane of the TAL inhibits solute transport in this nephron segment (through inhibition of the apical NKCC-2 and potassium channels), thereby reducing the medullary osmotic gradient.18

INTEGRATED CONTROL OF RENAL FUNCTION

One of the major functions of the kidneys is the regulation of blood volume, through the regulation of effective circulating volume, an unmeasurable, conceptual volume that reflects the degree of fullness of the vasculature. This is achieved largely by control of the sodium content of the body. The mechanisms involved in the regulation of effective circulating volume are discussed in detail in Chapter 7. Some of the more important mediator systems are introduced here.

Renal Interstitial Hydrostatic Pressure and Nitric OxideAcute increases in arterial blood pressure lead to natriuresis (pres-sure natriuresis). Because autoregulation is not perfect, part of this response is mediated by increases in RBF and GFR (see Fig. 2.3), but the main cause is reduced tubular reabsorption, which appears to result largely from an increase in renal interstitial hydrostatic pressure (RIHP). An elevated RIHP could reduce net reabsorption in the proximal tubule by increasing paracellular backflux through the tight junctions of the tubular wall (see Fig. 2.11). The increase in RIHP is thought to be dependent on intrarenally produced nitric oxide.19 Moreover, increased nitric oxide production in macula densa cells (which contain the neu-ronal [type I] isoform of nitric oxide synthase [nNOS]) blunts the sensitivity of TGF, thereby allowing increased NaCl delivery to the distal nephron without incurring a TGF-mediated decrease in GFR.20

Another renal action of nitric oxide results from the presence of inducible (type II) nitric oxide synthase in glomerular

adaptations are overwhelmed, with subsequent apoptotic and necrotic cell death.

VASOPRESSIN (ANTIDIURETIC HORMONE) AND WATER REABSORPTION

Vasopressin, or antidiuretic hormone, is a nonapeptide synthe-sized in specialized neurons of the supraoptic and paraventricular nuclei. It is transported from these nuclei to the posterior pitu-itary and released in response to increases in plasma osmolality and decreases in blood pressure. Osmoreceptors are found in the hypothalamus, and there is also input to this region from arterial baroreceptors and atrial stretch receptors. The actions of vaso-pressin are mediated by three receptor subtypes: V1a, V1b, and V2 receptors. V1a receptors are found in vascular smooth muscle and are coupled to the phosphoinositol pathway; they cause an increase in intracellular Ca2+, resulting in contraction. V1a recep-tors have also been identified in the apical membrane of several nephron segments, although their role is not yet clear. V1b recep-tors are found in the anterior pituitary, where vasopressin modu-lates adrenocorticotropic hormone release. V2 receptors are found in the basolateral membrane of principal cells in the late distal tubule and the whole length of the collecting duct; they are coupled by a Gs protein to cyclic adenosine monophosphate generation, which ultimately leads to the insertion of water chan-nels (aquaporins) into the apical membrane of this otherwise water-impermeable segment (Fig. 2.15). In the X-linked form of nephrogenic diabetes insipidus (the most common hereditary cause), the V2 receptor is defective.14

Several aquaporins have been identified in the kidney.15 Aqua-porin 1 is found in apical and basolateral membranes of all proximal tubules and of thin descending limbs of long-looped

Figure 2.15 Mechanism of action of vasopressin (antidiuretic hormone). The hormone binds to V2 receptors on the basolateral mem-brane of collecting duct principal cells and increases intracellular cyclic adenosine monophosphate (cAMP) production, causing, through interme-diate reactions involving protein kinase A, insertion of preformed water channels (aquaporin 2 [AQP2]) into the apical membrane. The water permeability of the basolateral membrane, which contains aquaporins 3 and 4, is permanently high. Therefore, vasopressin secretion allows tran-scellular movement of water from lumen to interstitium. AC, adenylate cyclase.

Action of Vasopressin

InterstitiumLumen Cells

H2O

H2O

Vesiclescontaining

AQP2

AQP2

H2O

H2O

Protein kinase AATP

cAMP

AC

H2O

V2 receptor

Vasopressin

AQP4AQP3

AQP 1: membranas apicales y basolaterales de túbulos proximales y en asa descendente fina

AQP 3: expresión constitutiva en membrana basolateral de celulas corticales y tubulo conector.

AQP4: expresada en membrana basolateral de de células principales medulares externas.

AQP-2

Responsable de permeabilidad variable de agua en túbulo distal y colector.

Inserción apical de AQP2 permite reabsorción de agua

Producida por alta osmolaridad intersticial alcanzada y mantenida por sistema contracorriente. .

C H A P T E R 8 Disorders of Water Metabolism 101

drinking water and eating, and water generated from metabo-lism. In general, overall intake and output come into balance at a plasma osmolality of 288 mOsm/kg.

QUANTITATION OF RENAL WATER EXCRETION

Urine volume can be considered as having two components. The osmolar clearance (Cosm) is the volume needed to excrete solutes at the concentration of solutes in plasma. The free water clear-ance (Cwater) is the volume of water that has been added to (posi-tive Cwater) or subtracted from (negative Cwater) isotonic urine (Cosm) to create either hypotonic or hypertonic urine.

Urine volume flow (V) comprises the isotonic portion of urine (Cosm) plus the free water clearance (Cwater).

V C C= +osm water

and, therefore,

C V Cwater osm= −

The term Cosm relates urine osmolality to plasma osmolality Posm by

CU V

Posmosm

osm= ×⎛

⎝⎜⎞⎠⎟

Therefore,

C V

U VP

V UP

waterosm

osm

osm

osm

= − ×⎛⎝⎜

⎞⎠⎟

= −⎛⎝⎜

⎞⎠⎟1

Figure 8.1 Mechanisms maintaining plasma osmolality. The response of thirst, vasopressin levels, and urinary osmolality to changes in serum osmolality. (Modified from reference 2.)

Response to Changes in Serum OsmolalityS

erum

vas

opre

ssin

(ng

/l)U

rinary osmolality (m

Osm

/l)

Serum osmolality (mOsm/l)280 284 288 290 294 296

1.0 300150

50070090011001200

2.0

3.0

4.0

5.0

Urine osmolalityThirstVasopressin

Thirst

Maximally effectivevasopressin levels

Figure 8.2 Cellular mechanism of vasopressin action. Vasopressin binds to V2 receptors on the basolateral membrane and activates G proteins that initiate a cascade resulting in aquaporin 2 (AQP2) insertion in the luminal membrane. This then allows water uptake into the cell. ATP, adenosine triphosphate; AVP, arginine vasopressin; cAMP, cyclic adenosine monophosphate; PKA, protein kinase A; VAMP2, vesicle-associated membrane protein 2. (Modified from reference 3.)

Recycling vesicle

VAMP2

AQP2

AQP4

H2O

AQP2

AQP3

PKA

ATP

AVPG!s

G!s

cAMP

Endocyticretrieval

Actinfilamentmotor

Luminal BasolateralMicrotubule

Actinfilament

Exocyticinsertion

Syntaxin 4

Microtubulemotor

Ade

nyly

l cyc

lase

V2

rece

ptor

Gi

Gi

+

+

Cellular Mechanism of Vasopressin Action

threshold for vasopressin release (see Fig. 8.1). It closely approxi-mates the level at which maximal concentration of urine is achieved. Hypovolemia, hypotension, and angiotensin II (ANG II) are also stimuli for thirst. Between the limits imposed by the osmotic thresholds for thirst and vasopressin release, plasma osmolality may be regulated more precisely by small, osmoregu-lated adjustments in urine flow and water intake. The exact level at which balance occurs depends on various factors, for example, insensible losses through skin and lungs, the gains incurred from

Hoorn/Zietse

Nephron Physiol 2008;108:p46–p59p48

cAMP and protein kinase A (PKA), which triggers the phosphorylation of many proteins including AQP2 [10] . However, several other signaling pathways have been im-plicated in the V2R-AQP2 cascade, involving for example PKA-independent pathways [12] , calcium-calmodulin [13] , Rho, soluble N - ethylmaleimide-sensitive factor at-tachment receptors (SNARE) proteins, cAMP responsive element-binding protein (CREB) and extracellular sig-nal-regulated kinase (ERK) [13] .

Hyponatremia: Non-Osmotic Vasopressin Release and Its Escape The presence of hyponatremia nearly always implies

that vasopressin is released non-osmotically, thereby pre-venting the excretion of electrolyte-free water, and dilut-ing the serum sodium concentration. For example, An-derson et al. [14] showed that non-osmotic vasopressin secretion was present in 97% of hyponatremic patients studied. Causes of non-osmotic vasopressin release in-

Fig. 1. The pathophysiology of vasopressin during hyponatremia. In the majority of patients, hyponatremia develops because vaso-pressin is secreted non-osmotically resulting in renal water reab-sorption. The figure shows the four mechanisms that can cause this effect: (1) non-osmotic vasopressin release by the posterior pituitary induced by specific stimuli from the paraventricular or supraoptic nuclei; (2) ectopic vasopressin production; (3) factors

that may enhance the renal effects of vasopressin, and, finally, (4) a vasopressin-like effect caused by an activating mutation of the vasopressin-2 receptor. In addition, details of the hypothalamo-neurohypophyseal system and the intracellular signaling cascade in the renal collecting duct principal cell are shown. The illustra-tion of the hypothalamo-neurohypophyseal system was adapted from Patel and Balk [74], with kind permission.

MECANISMO DE ACCIÓN

C H A P T E R 8 Disorders of Water Metabolism 101

drinking water and eating, and water generated from metabo-lism. In general, overall intake and output come into balance at a plasma osmolality of 288 mOsm/kg.

QUANTITATION OF RENAL WATER EXCRETION

Urine volume can be considered as having two components. The osmolar clearance (Cosm) is the volume needed to excrete solutes at the concentration of solutes in plasma. The free water clear-ance (Cwater) is the volume of water that has been added to (posi-tive Cwater) or subtracted from (negative Cwater) isotonic urine (Cosm) to create either hypotonic or hypertonic urine.

Urine volume flow (V) comprises the isotonic portion of urine (Cosm) plus the free water clearance (Cwater).

V C C= +osm water

and, therefore,

C V Cwater osm= −

The term Cosm relates urine osmolality to plasma osmolality Posm by

CU V

Posmosm

osm= ×⎛

⎝⎜⎞⎠⎟

Therefore,

C V

U VP

V UP

waterosm

osm

osm

osm

= − ×⎛⎝⎜

⎞⎠⎟

= −⎛⎝⎜

⎞⎠⎟1

Figure 8.1 Mechanisms maintaining plasma osmolality. The response of thirst, vasopressin levels, and urinary osmolality to changes in serum osmolality. (Modified from reference 2.)

Response to Changes in Serum Osmolality

Ser

um v

asop

ress

in (

ng/l)

Urinary osm

olality (mO

sm/l)

Serum osmolality (mOsm/l)280 284 288 290 294 296

1.0 300150

50070090011001200

2.0

3.0

4.0

5.0

Urine osmolalityThirstVasopressin

Thirst

Maximally effectivevasopressin levels

Figure 8.2 Cellular mechanism of vasopressin action. Vasopressin binds to V2 receptors on the basolateral membrane and activates G proteins that initiate a cascade resulting in aquaporin 2 (AQP2) insertion in the luminal membrane. This then allows water uptake into the cell. ATP, adenosine triphosphate; AVP, arginine vasopressin; cAMP, cyclic adenosine monophosphate; PKA, protein kinase A; VAMP2, vesicle-associated membrane protein 2. (Modified from reference 3.)

Recycling vesicle

VAMP2

AQP2

AQP4

H2O

AQP2

AQP3

PKA

ATP

AVPG!s

G!s

cAMP

Endocyticretrieval

Actinfilamentmotor

Luminal BasolateralMicrotubule

Actinfilament

Exocyticinsertion

Syntaxin 4

Microtubulemotor

Ade

nyly

l cyc

lase

V2

rece

ptor

Gi

Gi

+

+

Cellular Mechanism of Vasopressin Action

threshold for vasopressin release (see Fig. 8.1). It closely approxi-mates the level at which maximal concentration of urine is achieved. Hypovolemia, hypotension, and angiotensin II (ANG II) are also stimuli for thirst. Between the limits imposed by the osmotic thresholds for thirst and vasopressin release, plasma osmolality may be regulated more precisely by small, osmoregu-lated adjustments in urine flow and water intake. The exact level at which balance occurs depends on various factors, for example, insensible losses through skin and lungs, the gains incurred from

ABSORCIÓN TUBULAR DE AGUA

REABSORCIÓN TUBULAR

En cada punto a lo largo de la nefrona la presión osmótica del fluido tubular es menor que la del espacio intersticial.

Tasa de reabsorción está determinada por la magnitud del gradiente y la permeabilidad de agua del segmento.

Permeabilidad en ducto colector es variable

HIPERTONICIDAD MEDULAR

Para permitir la absorción de agua por medio osmótico la concentración en el intersticio medular debe ser superior a la presente en el lumen del ducto colector.

La generación de este ambiente extracelular se logra a través de la multiplicación contracorriente producto de la disposición de las porciones descendiente y ascendente del asa de Henle.

SISTEMA CONTRACORRIENTE

Asa de Henle se encarga de la generación y mantenimiento de gradiente intersticial osmótico

Requiere consumo de energía y la presencia de diferencias en las características de la membrana de ambas porciones.

Aumenta desde corteza renal (290 mOsm/kg) hasta médula (1200 mOsm/Kg).

C H A P T E R 2 Renal Physiology 23

descending limb of superficial nephrons is relatively impermeable to water.10) Both the thin ascending limb (found only in deep nephrons) and the TAL are essentially impermeable to water; however, Na+ is reabsorbed—passively in the thin ascending limb but actively in the TAL. Active Na+ reabsorption in the TAL is again driven by the basolateral sodium pump, which maintains a low intracellular Na+ concentration, allowing Na+ entry from the lumen through the Na+-2Cl−-K+ cotransporter (NKCC-2) and, to a much lesser extent, the Na+-H+ exchanger (Fig. 2.12). The apical NKCC-2 is unique to this nephron segment and is the site of action of loop diuretics like furosemide and bumetanide. Na+ exits the cell through the sodium pump, and Cl− and K+ exit through basolateral ion channels and a K+-Cl− cotransporter. K+ also re-enters the lumen (recycles) through apical membrane potassium channels. Re-entry of K+ into the tubular lumen is necessary for normal operation of the Na+-2Cl−-K+ cotransporter, presumably because the availability of K+ is a limiting factor for the transporter (the K+ concentration in tubular fluid being much lower than that of Na+ and Cl−). K+ recycling is also partly respon-sible for generating the lumen-positive potential difference found in this segment. This potential difference drives additional Na+ reabsorption through the paracellular pathway; for each Na+ reabsorbed transcellularly, another one is reabsorbed paracellu-larly (see Fig. 2.12).11 Other cations (K+, Ca2+, Mg2+) are also reabsorbed by this route. The reabsorption of NaCl along the TAL in the absence of significant water reabsorption means that the tubular fluid leaving this segment is hypotonic; hence the name diluting segment.

The U-shaped, countercurrent arrangement of the loop of Henle, the differences in permeability of the descending and ascending limbs to Na+ and water, and active Na+ reabsorption in the TAL are the basis of countercurrent multiplication and gen-eration of the medullary osmotic gradient (Fig. 2.13). Fluid

Figure 2.12 Transport mechanisms in the thick ascending limb of Henle. The major cellular entry mechanism is the Na+-K+-2Cl− cotrans-porter. The transepithelial potential difference drives paracellular trans-port of Na+, K+, Ca2+, and Mg2+.

Transport Mechanisms inthe Thick Ascending Limb

Lumen Cells of thick ascending limb Interstitial fluid

K+K+

K+

Na+

Na+

Na+

Na+

K+

Ca2+

Mg2+

H+

K+

K+

Cl−

Cl−

2Cl−

Paracellulardiffusion

Lumen-positivepotential difference

+

Figure 2.13 Countercurrent multiplication by the loop of Henle. The nephron drawn represents a deep (long-looped) nephron. Figures represent approximate osmolali-ties (mOsm/kg). Osmotic equilibration occurs in the thin descending limb, whereas NaCl is reabsorbed in the water-impermeable ascending limb; hypotonic fluid is delivered to the distal tubule. In the absence of vasopressin, this fluid remains hypotonic during its passage through the distal tubule and collecting duct, despite the large osmotic gradi-ent favoring water reabsorption. A large volume of dilute urine is therefore formed. During maximal vasopressin secretion, water is reabsorbed down the osmotic gradient, so that tubular fluid becomes isotonic in the cortical collect-ing duct and hypertonic in the medullary collecting duct. A small volume of concentrated urine is formed.

Countercurrent Multiplication

Cortex

Outermedulla

Innermedulla

Papilla 1200

900

600

290

1200

900

600

290

700

400

100

1200

900

600

290

NaCl NaCl NaCl

NaClNaCl

NaCl

H2O

H2O

H2O

NaCl

H2O

outer and inner medulla, whereas those of deep nephrons (long-looped nephrons) penetrate the inner medulla to varying degrees. The anatomic loops of Henle reabsorb approximately 40% of filtered Na+ (mostly in the pars recta and TAL) and approximately 25% of filtered water (in the pars recta and in the thin descending limbs of deep nephrons). (Recent evidence suggests that the thin

GENERACIÓN DE GRADIENTE OSMÓTICO

1. Transporte activo de NaCl a través de porción ascendente genera diferencia osmótica entre fluido tubular e intersticio local

2. Baja permeabilidad al agua en porción ascendente previene la disipación del gradiente.

3. Alta permeabilidad al agua en porción descendente permite el equilibrio con el intersticio.

Flujo proveniente de túbulo proximal es isotónico.

Se encuentra con hipertonicidad a nivel intersticial medular ( producto de reabsorción de NaCl en asa ascendente).

Flujo en asa descendente busca encontrar equilibrio osmótico (entrada de solutos o salida de agua por ósmosis).

Estos eventos sumado a reabsorción continua de NaCL en asa ascendente resulta en un aumento progresivo de osmolaridad desde unión corticomedular hasta puntas de papilas.

Al final se entrega una fluido hipotónico (100 mOsm/kg) a nivel de túbulo distal .

Fuerza impulsadora de multiplicación contracorriente está dada por la reabsorción activa de Na+ en el asa gruesa ascendente.

Mecanismo alterado por diuréticos de asa y de ahí su fuerte poder diurético al no poder generar un gradiente osmótico.

MULTIPLICACIÓN CONTRACORRIENTE

ROL DE LA UREA

Asas delgadas son permeables a urea (ascendente > descendente)

Segmentos distales (TAL ) son impermeables a urea.

Asociado a reabsorción de agua en ducto colector por ADH lleva al desarrollo de alta concentración de urea en el lumen.

Transportadores de urea sensibles a vasopresina (UT-A1 y UT-A3) a nivel de segmento terminal de ducto colector medular interno.

Urea es reabsorbida de forma pasiva al intersticio medular

Resultado de reciclaje de urea es el de añadir urea a intersticio medular interno, aumentando la osmolaridad intersticial.

Urea en ductos colectores no tiene efecto osmótico

Por otro lado la elevada concentración de urea en intersticio medular debe aumentar la reabsorción de agua.

Vasopresina estimula la reabsorción de sodio en TAL y de urea por UT-A1 y UT-A3 en ducto colector medular interno.