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Chapter 25 Control of Body Temperature (Ch.
20) and Water Balance=Urinary System
 Homeostasis is the maintenance of steady internal
conditions despite fluctuations in the external
environment.
 Examples of homeostasis include
– thermoregulation—the maintenance of internal
temperature within narrow limits (Ch. 20 and discussion
endotherms),
– osmoregulation—the control of the gain and loss of
water and solutes, and
– excretion—the disposal of nitrogen-containing wastes.
© 2012 Pearson Education, Inc.
25.6 The urinary system plays several major
roles in homeostasis
 The urinary system
– forms and excretes urine and
– regulates water and solutes in body fluids.
 In humans, the kidneys are the main processing
centers of the urinary system.
© 2012 Pearson Education, Inc.
25.6 The urinary system plays several major
roles in homeostasis
 Nephrons
– are the functional units of the kidneys,
– extract a fluid filtrate from the blood, and
– refine the filtrate to produce urine.
 Urine is
– drained from the kidneys by ureters,
– stored in the urinary bladder, and
– expelled through the urethra.
© 2012 Pearson Education, Inc.
Figure 25.6
Renal cortex
Renal medulla
Aorta
Inferior
vena cava
Renal artery (red)
and vein (blue)
Ureter
Kidney
Renal pelvis
Urinary bladder
Urethra
The urinary system
Bowman’s
capsule
Glomerulus
Arteriole
from renal
artery
1
Ureter
Proximal tubule
Capillaries
3
Arteriole
from
glomerulus
Distal
tubule
Collecting
duct
From
another
nephron
Branch of
renal vein
The kidney
Bowman’s
capsule
Tubule
Renal cortex
Branch of
renal artery
Branch of
renal vein
Collecting
duct
Renal medulla
2
Loop of Henle
with capillary
network
Detailed structure of a nephron
To
renal
pelvis
Orientation of a nephron within the kidney
25.6 Overview: The key processes of the urinary
system are filtration, reabsorption,
secretion, and excretion
 Filtration
– Blood pressure forces water and many small molecules
through a capillary wall of glomeruli surrounded by
Bowman’s capsule into the start of the kidney tubule.
 Reabsorption
– In the tubules filtrate is refined,
– reclaims valuable solutes (such as glucose, salt, and
amino acids) from the filtrate, and
– returns these to the blood.
© 2012 Pearson Education, Inc.
Figure 25.6
Key
Filtration
Reabsorption
Secretion
From
renal
artery
Excretion
Bowman’s
capsule
Filtration
Reabsorption
(Removal from filtrate)
Secretion
(Addition to filtrate)
Excretion
Nephron tubule
Filtrate
H2O, other small molecules
Urine
Interstitial fluid
Capillary
To renal vein
25.6 Overview: The key processes of the urinary
system are filtration, reabsorption,
secretion, and excretion
 Substances in the blood are transported into the
filtrate by the process of secretion.
 By excretion the final product, urine, is excreted
via the ureters, urinary bladder, and urethra.
© 2012 Pearson Education, Inc.
Figure 25.6
Key
Filtration
Reabsorption
Secretion
From
renal
artery
Excretion
Bowman’s
capsule
Filtration
Reabsorption
(Removal from filtrate)
Secretion
(Addition to filtrate)
Excretion
Nephron tubule
Filtrate
H2O, other small molecules
Urine
Interstitial fluid
Capillary
To renal vein