Maintenance of Electrolyte and Fluid

Balance

A.A.J.Rajaratne

The loop of Henle:

In the loop of Henle about 20% of filtered Na+, Cl-, and K+ are reabsorbed.

Ca++, Mg++, and HCO3- are also

reabsorbed. About 17% of filtered water is

reabsorbed in Henle's loop. Water is reabsorbed from the

descending limb; the ascending limb is impermeable to water.

The loop of Henle - contd:

1. Transport processes in thick ascending limb.

Na+ enters the TAL epithelial cells across the luminal membrane via the Na/K/2Cl transporter, which transports Cl- and K+ into the cell against their electrochemical potential gradients.

The energy of the Na+ gradient is used for this process.

There is also a luminal Na/H exchanger, which exports H+ into the lumen and causes reabsorption of HCO3

-.K+, Cl-, and HCO3

- are transported across the basolateral membrane by other transport proteins.

The luminal fluid in the TAL is electrically positive to the extracellular fluid of the basolateral membrane.

This powers the reabsorption of Na+, K+, Ca++, and Mg++, partly via transcellular and partly via paracellular pathways.

3 Na+

2 K+Na+

2Cl-

K+

Transport processes in the thick ascending limb

K+

H+

Na+

K+

Cl-

H2O+CO2H2CO3

H+ + HCO3 HCO3-

+

K+, Ca2+, Na+, Mg2+

C

A

Significant absorption of solute occurs in the TAL, but water cannot follow due to the impermeability of the TAL to water.

Thus the osmolarity of TAL fluid falls below isotonicity, reaching less than 150 milliosmolar.

The extrusion of Na+ and Cl- by the TAL contributes to an osmotic gradient in the medullary interstitium.

The osmotic pressure is highest near the renal papillae and lowest near the corticomedullary junction.

Since the descending limb is permeable to water (the ascending limb is not), water is reabsorbed from the descending limb as it descends through the osmotic gradient.

The distal tubule and the collecting tubule

Reabsorb about 10% of filtered Na+ and Cl-

Secrete K+ and H+.

Reabsorb a variable amount of water.

The first part of the distal tubule reabsorbs Na+, Cl-. and Ca2+.

Since this segment of the distal tubule is impermeable to water, the luminal fluid becomes still more dilute, approaching 100 mOsm.

The luminal membrane has electrogenic Na+ channels (blocked by amiloride and triamterine diuretics) and a coupled Na+/Cl- transporter (blocked by thiazide diuretics).

3 Na+

2 K+

Cl-

H2O

Na+

Cl-

Na+

Thiazides

Amiloride

Transport mechanisms in Distal Tubule

Last part of distal tubule and collecting tubule:

Contain two types of epithelial cells: 1. Principal cells 2. Intercalated cells

a. Principal cells in the last part of the distal tubule and in the cortical collecting tubule reabsorb Na+, Cl-, and water and usually secrete K+.

3 Na+

2 K+

K+

Na+

K+

Transport processes in Principal Cells

Intercalated cells Secrete H+ and reabsorb K+ and HCO3

-.

The H+ and HCO3- is derived from

CO2 produced by cellular metabolism. Carbonic anhydrase catalyzes formation of carbonic acid which dissociates into H+ and HCO3. H+ is extruded across the luminal membrane by an H+-ATPase. HCO3

- is absorbed into the blood across the basolateral membrane.

2 K+

3 Na+

H+

HCO3-

Cl-

Transport mechanisms in the Intercalated cells

Collecting ducts have two portions,1. Cortical portion2. Medullary portion

The water permeability is increased by antidiuretic hormone (ADH), also known as vasopressin. ADH from the posterior pituitary increases the permeability to H2O by causing the rapid insertion of aquaporin-2 water channels to the luminal surface of principal cells. When vasopressin is absent, the collecting duct epithelium is relatively impermeable to water.

Control of ECF Control of ECF osmolality and osmolality and

volumevolume

MAIN DIFFERENCES BETWEEN ICF AND ECF

• More Na+ in ECF

• More K+ in ICF

• More Cl- in ECF

• More PO4, HCO3, and Pr- in ICF

These differences are maintained by transport processes in the cell membrane

Na+ K+

Total intracellular 9.0 89.6

Total extracellular 91.0 10.4

Plasma 11.2 0.4

Interstitial fluid 29.0 1.0

Connective tissue 11.7 0.4

Bone 36.5 7.6

Transcellular 2.6 1.0

Distribution of Na+ and K+ in the body

ECF volume

20% of body weight

14 L (in a 70 kg man)

3.5 L plasma; 10.5 L interstitial fluid

Measured by using inulin, mannitol or sucrose

Osmolar concentration of plasma:

290 mosm/L - 142 mEq/L

0.9% saline is isotonic

270 mosm/L is contributed by Na+, Cl- and HCO3

-

Plasma proteins contribute less than 2 mosm/L (28 mm Hg oncotic pressure)

Ranges of salt and water intake and excretion:

a. Salt intake from 50 mg to 25 g/day

b. Water excretion from 400 ml to 25 l/day

Total body sodium is relatively constant.

Freely filtered

Reabsorbed but not secreted

Therefore,

Na+ excretion = Na+ filtered – Na+ reabsorbed

= (GFR X PNa) - Na+ reabsorbed

PNa is relatively constant

Therefore control is exerted by

GFR

Na+ reabsorption

Sensors:

1. Extrarenal baroreceptorsCarotid sinusesArteriesGreat veinsAtria

2. Renal juxtaglomerular apparatus

Efferents:1. Renal sympathetic nerves2. Macula densa renin

angiotensin II aldosterone

Control of GFR:

1.Angiotensin II efferent arteriolar constriction PGC

2.Renal sympathetic nerves Na+ adrenergic receptors Constriction of afferent and efferent arterioles PGC

Osmoreceptor -Osmoreceptor -ADH mechanismsADH mechanisms

Renal handling of NaCl and water:

NaCl & H2O are freely filterable at the glomerulus.

There is extensive tubular reabsorption but no tubular secretion.

Na+ reabsorption is driven by the basolateral Na+/K+-ATPase and is responsible for the major energy expenditure in kidney.

H2O permeability of the late DT:Water permeability of distal tubule

and initial collecting tubule, is also extremely low.

However under the influence of ADH it becomes highly water permeable.

Further removal of solute in the EARLY DT presents the LATE DT with markedly hypotonic urine containing even less Na+

Removal of Na+ continues in the LDT and collecting system, so that the final urine may contain virtually no Na+.

Anti-diuretic hormone:

ADH (antidiuretic hormone), vasopressin or arginine vasopressin (AVP) is the major regulator of urine osmolality and urine volume.

ADH is a nonapeptide produced by neurons in the supraoptic and paraventricular nuclei of the hypothalamus.

The axon terminals of these neurons reside in the posterior pituitary.

ADH is stored in these axon terminals.

When ADH is released from the posterior pituitary it causes the kidney to produce urine that is high in osmolality and low in volume.

In the absence of ADH the kidney tends to produce a large volume of urine with low osmolality.

Total solute excretion is relatively constant over a wide range of urine flow rates and osmolalities.

Control of ADH release:

1. Increased osmolality of ECF is a powerful stimulus for ADH release: a 1% change in osmolality induces significant increase in ADH release.Hypothalamic supra-optic and paraventricular nuclei respond to increased osmolality of ECF by producing ADH.As a result of this high sensitivity, responses to increased osmolality occur rapidly.

Control of ADH release:

2. Volume: In a volume-depleted individual, the release of ADH is more sensitive to increased osmolality.

In a volume-expanded state, ADH release is less sensitive to increases in osmolality.

3. Decreased blood pressure or blood volume also enhance ADH release, but not with such high sensitivity: 5 to 10% changes are required to alter ADH secretion.

Effects of ADH on the kidney:

ADH increases the water permeability of the epithelial cells of late distal tubules and the collecting tubules

May also increase NaCl absorption in the thick ascending limb of the loop of Henle.

ADH also increases the urea permeability of the inner medullary collecting tubules.

Action of ADH:

Binds to receptors in the basolateral membrane, causing increased cAMP.

This results in rapid insertion of aquaporin-2 protein channels into the luminal membrane of principal cells.

The water channel proteins are present in preformed intracellular vesicles, so this up regulation of water permeability can occur quickly.

The water channels can be rapidly re-internalized when ADH is no longer present.

Aquaporin-2

H2O

3 Na+

2 K+

ADH

Adenyl cyclasecAMP

Effect of ADH on collecting tubule cells

Summary:

osmolality

Stimulation of osmoreceptors in anterior hypothalamus

Supraoptic & paraventricular Nuclei

Posterior pituitary ADH

permeability of LDT, CCD, MCD to H2O

Summary of handling of Na+ by the kidney

Glomerular filtrate

26 000 mEq/Day

PCT 65% Active transport

Thick ascending loop

27% Active transport

LDCT 8% Aldosterone

Cortical collecting duct

Aldosterone

Thirst (conscious desire for water):

Under hypothalamic osmoreceptor control

Water intake is regulated by- increased plasma

osmolality- decreased ECF volume- psychological factors

Stimulus:

Intracellular dehydration due to increased osmolar concentration of ECF

Excessive K+ loss Low intracellular K+ in osmoreceptors

Mechanism is activated by

The arterial baroreceptor reflex BP

The volume receptors - low pressure receptors in atria; CVP

Angiotensin II

Increased Na+ in CSF

Hyp

Hypertonicity

Osmoreceptors

Hypovolaemia

BaroreceptorsAngiotensin II

Thirst

Other factors regulating water intake:

Psycho-social

Dryness of pharyngeal mucous membrane

? Gastrointestinal pharyngeal metering

Renin-angiotensin Renin-angiotensin –aldosterone –aldosterone

systemsystem

Renin:

Produced by

Juxtaglomerular cells – located in media of afferent arterioles

Lacis cells – junction between afferent and efferent arterioles

Factors affecting renin secretion:Stimulatory

Increased sympathetic activity via renal nervesIncreased circulating catecholaminesProstaglandins

InhibitoryIncreased Na+ and Cl- reabsorption in macula densaAngiotensin IIVasopressin

Renin

Angiotensinogen Angiotensin I

Angiotensin-converting enzyme

Angiotensin I Angiotensin II

Adrenal cortex Aldosterone

Actions of angiotensin II

Arteriolar vasoconstriction and rise in SBP and DBP

On adrenal cortex to produce aldosterone

Facilitates release of noradrenaline

Contraction of mesangeal cells - GFR

Brain - sensitivity of baroreflex

Actions of aldosterone:

Increased reabsorption of Na+ from urine, sweat, saliva and GIT – ECF volume expansion

Kidney Principal cells – increased amounts of Na+ are exchanged for K+ and H+

Salt appetiteSalt appetite

ECF Na+

Blood volume

Hypothalamic centers

Salt appetite

Potassium Potassium excretionexcretion

Renal handling of K+:

800 mEq/day enter the filtrate

100 mEq/day is secreted

PCT – reabsorption

DCT and CD – both reabsorption and secretion

Secretion is mainly by the Principal cells

3 Na+

2 K+

Na+

K+

Aldosterone

ENaC

Nucleus

ENaC = epithelial sodium channels

Control by Principal cells

1. Na:K pump

2. Electrical gradient from blood to lumen

3. Permeability of luminal cell membrane to K+

Stimulation Inhibition

ECF K+ Acidosis

Aldosterone

Urine flow rate

How the kidney How the kidney makes makes

concentrated and concentrated and dilute urinedilute urine

There is a gradient in osmolality in the medullary interstitium.

Cortico-medullary junction 300 mOsM.

Renal papilla 1200 mOsM in presence of ADH.

About half of this is due to NaCl and the other half to urea.

The vasa recta function as countercurrent exchangers, so that the blood flow does not wash out the gradients of NaCl and urea.

Effect of ADH:

In the presence of ADH, the collecting tubule is highly permeable to water.

As fluid in the collecting tubule descends through the medullary gradient, water is extracted, producing a low volume of urine that is high in osmolality (up to 1200 mOsM).

In the absence of ADH a large volume of dilute urine leaving the DCT (about 100 mOsM).

In the absence of ADH, the collecting tubule is highly impermeable to water. The dilute fluid does not lose water as it descends through the medullary interstitium.

Thus, volume does not decrease, nor does solute concentration increase, and a high volume of dilute urine is produced.

The Medullary Countercurrent System:

The maximal concentration of urine produced ~ 1200-1400 mosmol/l.

Urea, sulphate, phosphate and other waste ~ 600 mosmol/day obligatory water loss ~ (600 mosmol/day)/(1300 mosmol/l) ~ 0.46 l/day.

The countercurrent multiplier effect of the loop of Henle relies on the following:

1. ALH is not homogeneous, structurally or functionally

a. very thin from the bend until the outer medulla where the thick limb begins.

b. remains very impermeable to water over the entire length.

c. thick ALH actively transports NaCl. Relatively permeable to NaCl

Maximum transluminal gradient ~ 200 mosmol/l

Countercurrent multiplier effect relies on :

2. DLH (late PCT not included) a. does not actively transport Na + or Cl -

b. is highly permeable to water over its entire length

c. is relatively impermeable to ions

Countercurrent multiplier in loop of Henle

At each horizontal level, the medullary interstitium is concentrated by transport of solute from ALH.

As the DLH is freely water permeable, water passively leaves the tubule concentrating the luminal contents.

These two processes proceed at each horizontal level so that the final concentration of solute deep in the medullary interstitium is ~ 1200-1400 mosmol/l.

The gradient at each horizontal level across the ALH remains at only 200 mosmol/l.

Role of the vasa recta:

Form a countercurrent exchange system, having descending and ascending branches running in close proximity to the loop and each other.

Countercurrent exchange in the vasa recta prevents washout of too much NaCl and urea.

Role of the vasa recta:

As blood descends in the vasa recta, NaCl and urea enter the blood.

As blood rises in the ascending limbs of the vasa recta, NaCl and urea diffuse out of the blood and back into the interstitium and the osmolality decreases to about 350 mOsM.

Thus blood perfuses the medulla (flow is low), but relatively small amounts of NaCl and urea are removed from the medulla by the blood.

1. In the PT ~ 65% of filtered NaCl and water are reabsorbed but the urine remains iso-osmotic.

2. In the loop, water is reabsorbed from the DLH but a greater amount of NaCl is removed from the ALH, so that hypo-osmotic fluid enters the DT.

3. The early DT is always impermeable to water, therefore further dilutes the urine. This, with the ALH are referred to as "diluting segments”.

4. from the late DT, plasma ADH determines water permeability of the tubule.

i. low [ADH] Little water reabsorbed, therefore these segments production of large volume of dilute urine.

ii. high [ADH] By the end of the cortical CT's

the urine is again iso-osmotic, and is further concentrated through the medulla to 1400 mosmol/l (containing ~ no NaCl)

5. Although NaCl in the medullary interstitium is essential for the concentrating ability of the kidney, the final urine may contain virtually none, the excreted solute being urea, creatinine, urate, K+, etc.

6. The excretion of large quantities of Na+ is always accompanied by the excretion of large amounts of water.

7.However, the excretion of large amounts of water does not necessitate the excretion of Na+, decreased [ADH]pl not significantly altering Na+ transport.

H2O + NaCl

H2O

H2O

H2O

H2O

H2O

H2O + NaCl

H2O + NaCl

H2O + NaCl

H2O

H2O

H2O

H2O

H2O+NaCl

H2O+NaCl

NaClNaCl

NaCl

NaCl

NaCl

NaCl

ADH also needed to concentrate urine: how does it work?•Antidiuretic Hormone (ADH)/Arginine Vasopressin (AVP)

•Increases permeability of collecting ducts to H2O by inserting H2O channels (Aquaporins).

•Helps to make small amount of concentrated urine.

•Reabsorption of H2O increase urea conc. in tubule, increasing its recycling effect.•ADH allows rapid, graded control of urine conc. – v. sensitive.•ADH released in response to plasma osmolality and ECF volume – osmoreceptors and baroreceptors.

ADH (aka AVP)

Increased plasma osmolality stimulates osmoreceptors in the hypothalamus that trigger the release of ADH, which inhibits water excretion.

Increased osmolality stimulates a second group of osmoreceptors that trigger thirst, which promotes water intake.

Other factors also trigger ADH release e.g. decreased effective circulating volume, decreased BP, pregnancy, pain, morphine, nausea, congestive heart failure (CHF) (due to reduced ECV).

CHF may cause such retention of H2O = hyponatremia.

Hyperaldosteronism = hypernatremia. Due to chronic volume expansion, where osmoreceptors become less sensitive to ADH, reducing ADH inappropriately.

Renin-angiotensin-aldosterone axis

Principal factor controlling Ang II levels is renin release.

Decreased circulating volume stimulates renin release via: Decreased BP

(symp effects on JGA).

Decreased [NaCl] at macula densa (“NaCl sensor”)

Decreased renal perfusion pressure (“renal” baroreceptor)

Angiotensin II – important actions

Stimulation of aldosterone release from adrenal cortex.

Vasoconstriction of renal and other systemic vessels.

Enhanced tubuloglomerular feedback – makes macula densa more sensitive.

Enhance Na-H exchanger and Na channel function to promote Na reabsorption.

Renal hypertrophy. Stimulates thirst and ADH release by acting

upon hypothalamus.

Aldosterone

Aldosterone stimulates Na+ reabsorption and K+ excretion by the renal tubule.

Aldosterone exerts indirect negative feedback on RAAS by increasing ECV and by lowering plasma [K+].

Really important in conserving Na+ and water, but also really good at preventing massive swings in K+ levels.

Atrial Natriuretic Peptide (ANP)

ANP promotes natriuresis (loss of sodium). Atrial myocytes synthesise, store and release

ANP in response to stretch (low P volume sensor).

Major effect is renal vasodilatation. Increased blood flow = increased GFR.

Thus, more Na+ reaches macula densa. More Na+ excreted. May inhibit actions of renin, and generally

opposes effects of angiotensin II.

Feedback systems involved in osmolality control

Comparison of systems controlling effective circulating volume and osmolality

What is sensed?

Effective Circulating Volume Plasma Osmolality

SensorsCarotid sinus, aortic arch, renal

afferent arteriole, atriaHypothalamic

osmoreceptors

Efferent Pathways

RAAS, Symp NS, ADH, ANP ADH Thirst

EffectorShort term: heart, blood vessels

Long term: KidneyKidney

Brain: drinking

behaviour

What is affected?

Short term: Blood pressure

Long term: Na+ excretion

Renal water

excretionWater intake

Control of effective circulating volume

Feedback control of effective circulating volume.

A low effective circulating volume triggers 4 parallel effector pathways that act on the kidney.

Either changes haemodynamics or changes Na+ transport by renal tubule cells.

ECF volume receptors

“Central” vascular sensors Low pressure (very important)

Cardiac atria Pulmonary vasculature

High pressure (less important) Carotid sinus Aortic arch Juxtaglomerular apparatus (renal afferent

arteriole)Sensors in the CNS (less important) Sensors in the liver (less important)N.B. Regulation of ECF volume = Regulation of body

Na+. Thus, regulation of Na+ also dependent upon baroreceptors.