797
CHAPTER TWENTY
Urinary System
urea contributes to the reabsorption of water from the col-
lecting duct.
Uric acid is a product of the metabolism of certain
nucleic acid bases (the purines adenine and guanine). Active
transport completely reabsorbs the F
ltered uric acid. Some
uric acid, equal to approximately 10% of the amount F
ltered,
is excreted in the urine. This refl ects uric acid secretion into
the renal tubule.
PRACTICE
21
Describe a countercurrent mechanism.
22
How does the hypothalamus regulate urine concentration and
volume?
23
Explain how urea and uric acid are excreted.
Medullary
interstitial
fluid
Vasa
recta
Blood
flow
NaCl
NaCl
NaCl
NaCl
NaCl
Blood
flow
NaCl
NaCl
NaCl
Increasing
NaCl
concentration
FIGURE 20.26
A countercurrent mechanism in the vasa recta helps
maintain the NaCl concentration gradient in the medullary interstitial
f
uid (see F
g. 20.14).
TABLE
20.4
|
Functions of Nephron
Components
Part
Function
Renal Corpuscle
Glomerulus
±iltration o² water and dissolved substances ²rom
the plasma
Glomerular capsule
Receives the glomerular F
ltrate
Renal Tubule
Proximal convoluted
tubule
Reabsorption o² glucose; amino acids; creatine;
lactic, citric, uric, and ascorbic acids; phosphate,
sul²ate, calcium, potassium, and sodium ions by
active transport
Reabsorption o² proteins by endocytosis
Reabsorption o² water by osmosis
Reabsorption o² chloride ions and other negatively
charged ions by electrochemical attraction
Active secretion o² substances such as penicillin,
histamine, creatinine, and hydrogen ions
Descending limb o²
nephron loop
Reabsorption o² water by osmosis
Ascending limb o²
nephron loop
Reabsorption o² sodium, potassium, and chloride
ions by active transport
Distal convoluted
tubule
Reabsorption o² sodium ions by active transport
Reabsorption o² water by osmosis
Active secretion o² hydrogen ions
Secretion o² potassium ions both actively and by
electrochemical attraction
Collecting Duct
*
Reabsorption o² water by osmosis
*
Although the collecting duct is not anatomically part o² the nephron, it is ²unctionally linked in the pro-
cess o² urine ²ormation.
Excess uric acid may precipitate in the plasma and be deposited as
crystals in joints, causing the inf
ammation and extreme pain o² gout,
particularly in the digits and especially in the great toe. Gout has had
an interesting history. Hippocrates mentioned it. King Charles I o²
Spain gave up his vast empire in 1556 due to the pain²ul condition.
In 2006, Spanish researchers conF
rmed the diagnosis by detecting
uric acid deposits in the terminal joint o² a ²inger that, ²or reasons
unknown, had been preserved in a small box apart ²rom the rest o²
the king. Today, gout is treated with drugs that inhibit uric acid reab-
sorption or block an enzyme in the biosynthetic pathway ²or uric acid.
Limiting ²oods rich in uric acid, such as organ meats and sea²ood, and
drinking more to dilute urine can help. Gout is inherited, but an attack
may not occur until the person eats the o³
ending ²oods. The condi-
tion was once attributed to gluttony because only wealthy people
could a³
ord to eat meat.
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