Chapter 25
The Urinary System
977
25
Clinical Evaluation of Kidney
Function
Define renal clearance and explain how this value
summarizes the way a substance is handled by the kidney.
Describe the normal physical and chemical properties of urine.
List several abnormal urine components, and name the
condition characterized by the presence of detectable
amounts of each.
Check Your Understanding
14.
Describe the special characteristics of the descending
and ascending limbs of the nephron loop that cause the
formation of the medullary osmotic gradient.
15.
Under what conditions is ADH released from the posterior
pituitary? What effect does ADH have on the collecting
ducts?
For answers, see Appendix H.
Active transport
Passive transport
Cortex
NaCI
NaCI
300
400
600
900
700
1200
100
100
100
300
300
600
900
1200
100
100
Urea
H
2
O
H
2
O
Osmolality of interstitial fluid (mOsm)
Outer
medulla
Inner
medulla
Large volume
of dilute urine
Small volume of
concentrated urine
Collecting
duct
DCT
Cortex
NaCI
NaCI
NaCI
NaCI
Urea
300
400
600
900
700
1200
300
300
400
600
900
1200
100
100
150
Urea
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
Osmolality of interstitial fluid (mOsm)
Outer
medulla
Inner
medulla
DCT
300
300
600
900
1200
Descending limb
of nephron loop
Descending limb
of nephron loop
Collecting duct
Osmolality of extracellular fluids
ADH release from posterior pituitary
Number of aquaporins (H
2
O channels) in collecting duct
H
2
O reabsorption from collecting duct
Large volume of dilute urine
Osmolality of extracellular fluids
ADH release from posterior pituitary
Number of aquaporins (H
2
O channels) in collecting duct
H
2
O reabsorption from collecting duct
Small volume of concentrated urine
Urea contributes to the
osmotic gradient. ADH
increases its recycling.
(a) If we were so overhydrated we had no ADH.
..
(b) If we were so dehydrated we had maximal ADH.
..
Figure 25.17
Mechanism for forming dilute or concentrated urine.
Under the control
of antidiuretic hormone (ADH), the collecting ducts fine-tune urine concentration using the
medullary osmotic gradient.
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