2013B05 Outline the role of the kidneys in the regulation of body water.

 

List:

·         Intro

·         Glomerular filtration

·         Tubular reabsorption: location and hormonal control

 

Intro: kidney’s regulation of body water

Overall function

·  Control water content

·  Control water partitioning (ECF vs ICF)

·  Control osmolality

Summary of renal handling

·  Free glomerular filtration water (180L/day) and electrolytes

·  Selective tubular reabsorption

·  Urine output only ~2L/day and variably concentrated

Precise control because of

1.   Large volume of filtrate

2.   Hyperosmotic medullary interstitium

3.   Distinct hormonal systems for salt & water and for water alone

Kidney’s priorities

1.   Volume

2.   Tonicity

3.   pH

 

Glomerular filtration:

Description

·  Selective passage of water and small particles across filtration barrier

·  Normal 125mL/min (25% of renal plasma flow)

Control

·  Effective myogenic autoregulation between mAP 70 and 170

·  i.e. ↓afferent arteriolar pressure -> ↓stretch -> reflex relaxation

 

Tubular reabsorption: by location

 

Na+ reabsorption

H2O reabsorption

Proximal tubule

65%

65%

Thin descending loop of Henle

-

10%

Thick ascending loop of Henle

25%

-

Distal tubule

5%

-

Collecting duct

4-5%

5-24.7% (ADH!)

 

Tubular reabsorption: hormonal control

ANP

*↓ECF volume*

·  Source: right atrium

·  Stimulus: distension

·  Effect:

o Afferent arteriolar dilatation -> ↑GFR\

o Inhibit NCC in distal tubule NCC and ENaC in connecting tubule and collecting duct -> ↓Na+/H2O reabsorption (natriuresis)

ADH

*↑Total body water*

·  Synthesis: synthesized by hypothalamus, released by posterior pituitary

·  Stimulus:

o ↑Osmolality (sensitive to ∆2%)

o ↓Blood volume (sensitive to ∆10%)

o ↓mAP

o Angiotensin 2

o Stress

o Drugs

·  Effect:

o Insertion of aquaporins into apex of collecting ducts -> ↑free H2O reabsorption

o Insertion of ureaporins into apex of medullary collecting ducts -> ↑urea cycling -> ↑medullary interstitial osmolality -> ↑H2O reabsorptive capacity

Angiotensin 2

*↑ECF volume*

·  Stimuli for renin release: ↓afferent arteriolar pressure, ↓distal tubule NaCl flow, SNS stimulation

·  Effect:

o ↑Reabsorption of Na+/H2O from proximal tubule

o (Also stimulates ADH secretion)

·  Pathway:

               

 

Aldosterone

*↑ECF:ICF ratio*

·  Effect:

o Active in renal connecting tubule and collecting ducts, colon and sweat glands

o ↑Na+K+ATPase synthesis and activity -> ↑ENaC

o ↑ECF [Na+]

o ↑ECF osmolality -> indirect stimulation of ADH release

·  Pathway: as above

 

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