functions of the kidneys regulation of extracellular fluid volume and blood pressure regulation of...

Functions of the Kidneys

Regulation of extracellular fluid volume and blood pressure

Regulation of osmolarity--close to 300 mOsm

Maintenance of ion balance: Na+--regulates ECF volume

Homeostatic regulation of pH--kept in a narrow range

Excretion of nitrogenous and other water-soluble wastes Urea & uric acid

Creatinine from muscle metabolism

Urobilinogen (breakdown of hemoglubin)

Production of hormones Renin (sodium balance and blood pressure homeostasis

Anatomy: The Urinary System

Figure 19-1a

Figure 19-1b


Figure 19-1c


Figure 19-1i


Figure 19-1g–h


Figure 19-1d–e


Figure 19-1f


Figure 19-1j


Renal Summary

Summary of normal renal function: Substance Units Filtered Excreted % Absorbed Water liter 180 1.5 >99 Sodium mEq 25,000 150 >99 Potassium mEq 630 95 85 Chloride mEq 18,000 150 >99 Bicarbonate mEq 4,500 0 100 Glucose grams 180 0 100 Urea grams 58 23 60

Figure 19-2

Kidney Function

Filtration, reabsorption, secretion, and excretion

Efferentarteriole

Afferentarteriole

Glomerulus

Peritubular capillaries

Proximaltubule

Bowman’scapsule

Collectingduct

To renal vein

F

R

S

E

F

R

S

R R

R

S

R S

E

Loopof

Henle

To bladder andexternal environment

= Filtration: blood to lumen

= Reabsorption: lumen to blood

= Secretion: blood to lumen

= Excretion: lumen to external environment

KEY

Distaltubule

Figure 19-2 (1 of 4)

Kidney Function: Filtration

Efferentarteriole

Afferentarteriole

Glomerulus


Proximaltubule

Bowman’scapsule

Collectingduct

To renal vein

F

F

Loopof

Henle = Filtration: blood to lumen

KEY

Distaltubule


Kidney Function: Reabsorption

Efferentarteriole

Afferentarteriole

Glomerulus


Proximaltubule

Bowman’scapsule

Collectingduct

To renal vein

F

R

F

R

R R

R

R

Loopof



KEY

Distaltubule


Kidney Function: Secretion

Efferentarteriole

Afferentarteriole

Glomerulus


Proximaltubule

Bowman’scapsule

Collectingduct

To renal vein

F

R

S

F

R

S

R R

R

S

R S

Loopof




KEY

Distaltubule


Kidney Function: Excretion

Efferentarteriole

Afferentarteriole

Glomerulus


Proximaltubule

Bowman’scapsule

Collectingduct

To renal vein

F

R

S

E

F

R

S

R R

R

S

R S

E

Loopof

Henle

To bladder andexternal environment

= Filtration: blood to lumen



= Excretion: lumen to external environment

KEY

Distaltubule

Figure 19-3

Kidney Function

The urinary excretion of substance depends on its filtration, reabsorption, and secretion

Figure 19-4a

The Renal Corpuscle

Figure 19-4d

The Renal Corpuscle

Figure 19-4c

The Renal Corpuscle

Where does a drink of water go?

Drink watergutfeces

plasmakidneys

everywhere else75-80% ofcardiac output

nephrons

80% of renalplasma flow20% of renalplasma flow

urine

>99% offiltered fluid<1% offilteredfluid

20-25%of cardiacoutput

Where does a drink of water go?

Figure 19-5

Filtration Fraction

Kidney Function

Measuring glomerular filtration rate A. Compare the amount of material filtered at the glomerulus with the amount of material excreted in the urine:

1. You can measure the amount of material excreted per unit of time in the urine by measuring the volume of urine produced over a period of time and measuring the concentration of the material in the urine:

concentration X volume/min = amount/minute

2. It is much harder to measure directly the amount of material that was filtered, but it can be calculated.

3. The most common way to accomplish this calculation is to measure the renal clearance of inulin, which allows you to calculate the glomerular filtration rate (GFR).

a. Inulin (not INSULIN) is a fructopolysaccharide with a molecular weight of approximately 5000. A common source of this molecule is Jerusalem artichokes.

b. It is freely filtered. In other words, the concentration of inulin in the fluid within Bowman's capsule is identical to the concentration of inulin in plasma.

c. It is neither reabsorbed nor secreted in any portion of the nephron. All of the inulin that enters the nephron will be excreted in the urine. Any inulin that is not filtered into the tubule remains in the circulation.

e. Because of these properties, all of the inulin that is filtered into the nephron appears in the urine, and only the inulin that is filtered into the nephron appears in the urine. In other words,

or

where V = volume/time.

f. This measurement is called a "clearance," because it tells you the amount of plasma that was "cleared" of inulin. (You can't measure that volume directly because a lot of it - but you don't know how much - was reabsorbed while the fluid was moving along the nephrons.)

[ i n u l i n ]p l a s m a

× Vf i l t e r e d

= [ i n u l i n ]u r i n e

× Vu r i n e

Vf i l t e r e d

= G F R =

[ i n u l i n ]u r i n e

[ i n u l i n ]p l a s m a

× Vu r i n e

Measuring renal plasma flow It is difficult to measure total renal blood flow, but it can be calculated by measuring the renal clearance of

PAH, or any other substance that is entirely removed from the blood in the renal vasculature in a single pass through the kidney.

1. All molecules of PAH that enter the afferent arterioles are either filtered or secreted into the tubular fluid, so all PAH has been transferred into the tubular fluid before the blood leaves the kidney.

2. I.e.,

and

C. The ratio of the inulin clearance to the PAH clearance, then, tells you the fraction of the plasma entering the kidney that got filtered into the nephrons; this value is called the filtration fraction.

D. Normal values for humans are (from Guyton and Hall's Textbook of Medical Physiology):

1. Renal blood flow = 1200 ml/minute.

2. The normal hematocrit is about 45%, so renal plasma flow = 650 ml/minute. REMEMBER TO CORRECT FOR HEMATOCRIT if a question asks for plasma flow, rather than blood flow!

3. Normal GFR = 125 ml/minute. This value remains remarkably constant even when the renal blood flow varies.

[ P A H ]p l a s m a

× Vr e n a l p l a s m a

= [ P A H ]u r i n e

× Vu r i n e

R P F =

[ P A H ]u r i n e

[ P A H ]p l a s m a

× Vu r i n e

Forces that Influence Filtration

Hydrostatic pressure (blood pressure)

Colloid osmotic pressure

Fluid pressure created by fluid in Bowman’s capsule

Figure 19-6

Filtration

Filtration pressure in the renal corpuscle depends on hydrostatic pressure, colloid osmotic pressure, and fluid pressure

Figure 19-7

Filtration

Autoregulation of glomerular filtration rate takes place over a wide range of blood pressure

Figure 19-8a

Filtration

Resistance changes in renal arterioles after GFR and renal blood flow

Figure 19-8b

Filtration

Figure 19-8c

Filtration

GFR Regulation

Myogenic response Similar to autoregulation in other systemic arterioles

Tubuloglomerular feedback

Hormones and autonomic neurons By changing resistance in arterioles

By altering the filtration coefficient

Figure 19-9

Juxtaglomerular Apparatus

Figure 19-10, step 1

Tubuloglomerular Feedback

Afferentarteriole

Maculadensa

Efferent arteriole Bowman’s capsule GlomerulusDistal tubuleProximal

tubule

Collectingduct

Loopof

Henle

Granularcells

GFR increases.

1

1

Figure 19-10, steps 1–2


Afferentarteriole

Maculadensa


tubule

Collectingduct

Loopof

Henle

Granularcells

GFR increases.

Flow through tubule increases.

2

1

1

2

2



Afferentarteriole

Maculadensa


tubule

Collectingduct

Loopof

Henle

Granularcells

GFR increases.


Flow past macula densaincreases.

2

1

1

2

3

23



Afferentarteriole

Maculadensa


tubule

Collectingduct

Loopof

Henle

Granularcells

GFR increases.



Paracrine diffuses from macula densa to afferent arteriole.

2

1

1

2

3

42

3

4

Figure 19-10, steps 1–5 (1 of 4)


Afferentarteriole

Maculadensa


tubule

Collectingduct

Loopof

Henle

Granularcells

GFR increases.




Afferent arteriole constricts.

2

1

1

2

3

4

5

23

4

5



Afferentarteriole

Maculadensa


tubule

Collectingduct

Loopof

Henle

Granularcells

GFR increases.





Resistance in afferent arteriole increases.

2

1

1

2

3

4

5

23

4

5



Afferentarteriole

Maculadensa


tubule

Collectingduct

Loopof

Henle

Granularcells

GFR increases.






Hydrostatic pressurein glomerulus decreases.

2

1

1

2

3

4

5

23

4

5



Afferentarteriole

Maculadensa


tubule

Collectingduct

Loopof

Henle

Granularcells

GFR increases.






Hydrostatic pressurein glomerulus decreases.

GFR decreases.

2

1

1

2

3

4

5

23

4

5

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