Chapter 20a

Integrative Physiology II:

Fluid and Electrolyte Balance

About this Chapter

• Fluid and electrolyte homeostasis• Water balance• Sodium balance and ECF volume• Potassium balance• Behavioral mechanism in salt and water

balance• Integrated control of volume and osmolarity• Acid-base balance

Na+ and water ECF volume and osmolarity

K+ Cardiac and muscle function

Ca2+ Exocytosis, muscle contractions, and other functions

H+ and HCO3– pH balance

Body must maintain mass balance

Excretion routes: kidney and lungs

Fluid and Electrolyte Homeostasis

Fluid and Electrolyte Homeostasis

• The body’s integrated responses to changes in blood volume and blood pressure

Figure 20-1a

Blood volume

Blood pressure

Volume receptors in atria andcarotid and aortic baroreceptors

Cardiovascularsystem

Cardiac output,vasoconstriction

Thirst causeswater intake

ECF and ICFvolume

Behavior Kidneys

Conserve H2Oto minimize

further volumeloss

Bloodpressure

trigger homeostatic reflexes

Stimulus ReceptorEffectorTissue responseSystemic response

(a)

KEY

Blood volume

Blood pressure

Volume receptors in atria,endocrine cells in atria, and

carotid and aortic baroreceptors

Cardiac output,vasodilation

Excrete saltsand H2O in urine

KidneysCardiovascularsystem

ECF and ICFvolume

Bloodpressure

trigger homeostatic reflexes

Stimulus

Receptor

Effector

Tissue response

Systemic response(b)

KEY

Fluid and Electrolyte Homeostasis

Figure 20-1b

Urine

Lungs

Feces

Skin

Water lossWater gain

Totals

1.5 L/day

0.1 L/day

Food and drink

Metabolism

2.2 L/day

2.5 L/day 2.5 L/day

0.3 L/day

Insensiblewater loss0.9 L/day

Intake 2.2 L/day

Metabolic production0.3 L/day

Output2.5 L/day+ – = 0

Water Balance

Figure 20-2

Water Balance

• The kidneys conserve volume but cannot replace lost volume

Figure 20-3

Volume gain Volume loss

GFR canbe adjusted.

Glomerularfiltration rate(GFR)

Ifvolumefallstoo low,GFRstops.

Regulated H2Oreabsorption

Volumeloss in

the urine

Body fluidvolume

Kidneysconservevolume.

Kidneysrecyclefluid.

Urine Concentration

• Osmolarity changes as filtrate flows through the nephron

Figure 20-450–1200 mOsMurine excreted

300 mOsM300 mOsM

600 mOsM

900 mOsM

1200 mOsM1200

300 100

Distaltubule

Proximaltubule

Collectingduct

Loopof

Henle

CORTEX

MEDULLA

Permeability towater and solutesis regulated byhormones.

Variable reabsorptionof water and solutes

Ionsreabsorbedbut nowaterOnly water

reabsorbed

Isosmotic fluid leaving theproximal tubule becomesprogressively more concentratedin the descending limb.

Removal of solute in the thickascending limb createshyposmotic fluid.

Hormones control distal nephronpermeability to water and solutes.

Urine osmolarity depends onreabsorption in the collectingduct.

1

23

4

1

2

3

4

Urine Concentration

Figure 20-4, step 1

50–1200 mOsMurine excreted

300 mOsM300 mOsM

600 mOsM

900 mOsM

1200 mOsM1200

300 100

Distaltubule

Proximaltubule

Loopof

Henle

CORTEX

MEDULLA

Permeability towater and solutesis regulated byhormones.

Variable reabsorptionof water and solutes

Ionsreabsorbedbut nowaterOnly water

reabsorbed

Isosmotic fluid leaving theproximal tubule becomesprogressively more concentratedin the descending limb.

1

1

Collectingduct

Water Reabsorption

• Vasopressin makes the collecting duct permeable to water

Figure 20-5a

Water Reabsorption

Figure 20-5b

Collecting duct cell

Secondmessengersignal

Collectingduct

lumen

Medullaryinterstitial

fluid

Vasarecta

Filtrate300 mOsM

cAMP

Exocytosisof vesicles

Vasopressin receptor

700 mOsM

600 mOsM

Aquaporin-2water pores

600 mOsM

Cross section ofkidney tubule

Vasopressin

Vasopressin binds tomembrane receptor.

Receptor activates cAMPsecond messenger system.

Water is absorbed byosmosis into the blood.

Storage vesicles

Cell inserts AQP2 waterpores into apical membrane.

H2O H2O

H2O

H2O

1

2

3

41

2

3

4

Water Reabsorption

• Vasopressin causes insertion of water pores into the apical membrane

Figure 20-6

Water Reabsorption

Figure 20-6, steps 1–4

Collecting duct cell

Secondmessengersignal

Collectingduct

lumen

Medullaryinterstitial

fluid

Vasarecta

Filtrate300 mOsM

cAMP

Exocytosisof vesicles

Vasopressin receptor

700 mOsM

600 mOsM

Aquaporin-2water pores

600 mOsM

Cross section ofkidney tubule

Vasopressin

Vasopressin binds tomembrane receptor.

Receptor activates cAMPsecond messenger system.

Water is absorbed byosmosis into the blood.

Storage vesicles

Cell inserts AQP2 waterpores into apical membrane.

H2O H2O

H2O

H2O

1

2

3

41

2

3

4

Factors Affecting Vasopressin Release

Figure 20-7

Water Balance

• The effect of plasma osmolarity on vasopressin secretion by the posterior pituitary

Figure 20-8

Countercurrent Heat Exchanger

Figure 20-9

Warmblood

(a) (b)

Limb

Warmblood

Warmblood

Coldblood

Heat lostto externalenvironment

H2O =

Cl– =

KEY

Na+ =

K+ =

Blood in thevasa recta

Filtrate entering thedescending limb

The ascending limb pumpsout Na+, K+, and Cl–

100mOsM

300mOsM

300mOsM

300mOsM

1200mOsM

1200mOsM

600 600 600

900900

500 500

600

900

900

1200

Loop of Henle

(a)

Vasa recta

Water Balance

• Countercurrent exchange in the medulla of the kidney

Figure 20-10a

Ion reabsorption

• Active reabsorption of ions in the thick ascending limb creates a dilute filtrate in the lumen

Figure 20-10b

(b)

KEY1200 mOsmenteringascendingloop of Henle

Saltreabsorption

Water cannotfollow solute

Cells of ascendingloop of Henle

Interstitialfluid

100 mOsmleavingthe loop

H2O =

Cl– = Na+ =

K+ =

1

2

3

4

Fluid and Electrolyte Balance

• Vasa recta removes water• Close anatomical association of the loop of

Henle and the vasa recta• Urea increases the osmolarity of the

medullary interstitium

Sodium Balance

• Homeostatic responses to salt ingestion

Figure 20-11

Sodium Balance

Figure 20-12

Newchannels

P cell of distal nephron

Translation andprotein synthesis

Proteins modulateexisting channels and pumps

New pumps

K+

Na+

K+ secreted

Na+ reabsorbed

Lumenof distaltubule

Interstitialfluid

Blood

Aldosterone

Aldosteronereceptor

K+

Na+

Na+

Aldosterone combines witha cytoplasmic receptor.

Hormone-receptor complexinitiates transcription inthe nucleus.

New protein channels andpumps are made.

Aldosterone-inducedproteins modify existingproteins.

Result is increased Na+

reabsorption andK+ secretion.

K+

ATP

ATP

1

2

3

4

5

12

3

4

5

Sodium Balance

• The renin-angiotensin-aldosterone system (RAAS)

Figure 20-13

Bloodpressure

Angiotensinogenin the plasma

ANG I in plasma

Granular cells

(kidney)Renin

Liver

constantlyproduces

produce

containsBlood vesselendothelium

ACE(enzyme)

Volumeand maintain

osmolarity

HypothalamusCardiovascularcontrol center

in medullaoblongata

Adrenal cortex

Bloodpressure

Arterioles

Vasoconstrict Cardiovascularresponse

Na+ reabsorptionThirstVasopressin

Aldosterone

ANG II inplasma

Sodium Balance

• Decreased blood pressure stimulates renin secretion

Figure 20-14

Bloodpressure

Cardio-vascularcontrolcenter

Paracrines Granular cells ofafferent arteriole

Macula densaof distal tubule

Sympatheticactivity

NaCltransport

Reninsecretion

GFR

across

direct effect

Increasedblood volume

causes increasedatrial stretch

Myocardialcells

stretch andrelease

Natriuretic peptides

Kidney Adrenalcortex

MedullaoblongataHypothalamus

Lessvasopressin

Decreasedrenin Less aldosterone

Decreasedblood pressure

IncreasedGFR

NaCl andH2O excretion

Sodium Balance

• Natriuretic peptides promote salt and water excretion

Figure 20-15