For example, moving to a high altitude (low oxygen)
causes rapid, shallow breathing. The rapid breathing reduc-
es CO2
concentration and shifts the bicarbonate system to
the left, using up some of the H+, thereby causing alkalosis
(see Figure 15.14). Within a few days, the kidneys excrete
the excess HCO3-, and blood pH returns to normal. If your
body cannot compensate for large swings in pH, medical
treatment may be required (see
W h a t a H e a lth P rovider Sees).
pH buffer system is important in regulat-
ing the pH of the cytosol?
does a decrease in the rate and depth of
breathing change blood pH?
do the kidneys respond to an increase in
blood pH?
is metabolic acidosis, and how does the
body compensate for it?
he Urinary System Plays a Vital Role in
Maintaining Homeostasis 438
• The kidneys regulate the composition of the blood. The
basic unit of the kidney, the nephron, is composed of a renal
corpuscle and a renal tubule. The tubule is divided into four
segments: the proximal convoluted tubule (PCT), nephron
loop, distal convoluted tubule (DCT), and collecting duct.
• Blood plasma is filtered in the renal corpuscle. As the filtrate
passes through the renal tubule, water and various solutes
(sodium, potassium, glucose, amino acids, chloride) are reab-
sorbed in different segments. As shown, reabsorption returns
materials to the blood and occurs via various passive and ac-
tive transport mechanisms on the membranes of tubule cells.
F u n c tio n a l p r o c e s s e s in t h e n e p h r o n o f t h e k id n e y •
F ig u r e 1 5 .4 •
Renal tubule and
collecting duct
• As the filtrate passes through the tubule, various substances
(such as H+, urea, and drugs) are secreted into the filtrate.
Secretion eliminates extra waste materials from the blood.
What remains of the filtrate at the end of the nephron be-
comes urine.
The Urinary Bladder Stores Urine and Expels
It from the Body 450
• Urine passes from the kidneys through the ureters to the
urinary bladder. As shown, the urinary bladder is a hollow
organ made of mucosa and smooth muscle. The urinary
bladder can stretch to store urine.
T h e u r e te r s , u rin a ry b la d d e r , a n d u r e th r a • F ig u r e 1 5 .7
Urinary bladder
• The micturition reflex involves the urinary bladder, para-
sympathetic nervous system, and spinal cord; it causes the
bladder to expel urine (micturition). The flow of urine into
the urethra can be suppressed for a limited amount of time
by nerve impulses from the cerebral cortex.
• Urine passes from the urinary bladder to the outside
through the urethra. In females, the urethra is short and
separate from the reproductive system. In males, the ure-
thra is longer, separated into segments, and shared with the
reproductive system; both urine and semen pass through
the male urethra.
Summary 463
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