Chapter 25Urinary system

Lecture 16

Part 2: Regulation of Urine Osmolarity and Volume

Marieb’s HumanAnatomy and

PhysiologyNinth Edition

Marieb Hoehn

2

Where have we been; we are we going?

Renal corpuscle – provides the raw materials to the nephron for processing

PCT – reclaims those substances the body can use

DCT – gets rid of those substances the body doesn’t want or need; reabsorb some more Na+, Ca2+

Collecting duct – provides the OPTION of reclaiming H2O or letting it pass out of the body

300 mOsm/L

*Note osmolarity of fluid in PCT

Obligatory H2O reabsorption…

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Overview of Facultative H2O Reabsorption

• Note that outflow of water from collecting duct is dependent upon the osmotic gradient in the medulla

Under influence of ADH

(Facultative water reabsorption)

Urea

Figure from: Hole’s Human A&P, 12th edition, 2010

Increasin

g concen

tration →

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The Loop of Henle (Nephron Loop)

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Crucial renal function is to keep the body fluids at about 300 mOsm (osmolarity of blood plasma) by varying the concentration of urine

The mechanism shown is called the “countercurrent multiplier”

SO HOW DOES THIS HELP?

Increasin

g concen

tration →

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The Countercurrent Multiplier

• Recall that all movement of H2O occurs passively by osmosis

• We would like some mechanism to concentrate urine– Excrete more H2O when body fluids are tending to become

hypotonic (more dilute)– Excrete less H2O when body fluids are tending to become

hypertonic (more concentrated)• Utilizes two factors

– Hypertonicity of the peritubular fluid, established by the countercurrent multiplier

– Variable permeability of the collecting ducts to H2O depending upon levels of ADH

• We will use the phrase, “Water follows salt (solute) when it can”

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Influence of ADH on H2O Reabsorption• Note that amount of water pulled out of collecting duct is dependent upon the osmotic gradient in the medulla that was established by the countercurrent multiplier.

Under influence of ADH

(Facultative water reabsorption)

Urea

Figure from: Hole’s Human A&P, 12th edition, 2010

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Vasa Recta of Juxtamedullary Nephrons

Recall that the vasa recta is present in juxtamedullary nephron loops (which give the kidneys the ability to produce a concentrated urine)

The vasa recta functions to

1. Deliver blood to medullary cells

2. Return reabsorbed solutes and water in the medulla to the general circulation without disrupting the medullary concentration gradient

Figure from: Hole’s Human A&P, 12th edition, 2010

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Urea and Uric Acid Excretion

Urea• product of amino acid catabolism (deamination)• plasma concentration reflects the amount or protein in diet• enters renal tubules through glomerular filtration• 50% reabsorbed• rest is excreted

Uric Acid• product of nucleic acid metabolism• enters renal tubules through glomerular filtration• 100% of filtered uric acid is reabsorbed• 10% secreted and excreted

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Summary of Events in the Nephron/Collecting Duct

1. Filtrate produced

2. Reabsorption of 65% of filtrate

3. Obligatory water reabsorption

4. Reabsorption of Na+ and Cl- by active transport

5,6. Facultative reabsorption of water

7. Absorption of solutes and water by vasa recta

(Aldosterone)

(Aldosterone)

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Diuretics

• Osmotic diuretics, e.g., mannitol, glucose• Drugs that block Na+/Cl- transport in PCT and DCT,

e.g., hydrochlorothiazide• High-ceiling/loop diuretics that reduce gradient along

nephron loop, e.g., furosemide (Lasix)• Aldosterone-blocking agents, e.g., spironolactone (K+

sparing), natriuretic peptides• ACE inhibitors, e.g., Captopril• Drugs with diuretic side-effects, e.g., alcohol (how?),

caffeine

A diuretic promotes the loss of water in the urine

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Review• Regulation of urine concentration and volume

– Results from a combination of

• Countercurrent multiplier in loop of Henle

• Responsiveness of the DCT and collecting ducts to ADH and aldosterone

– Is critical to homeostasis

• Urine composition – Is variable

– Depends upon both diet and activity

– Consists of mostly water plus

• Creatinine

• Urea

• Uric acid

• Traces of amino acids

• Electrolytes