Page 441 - Anatomy and Physiology of Farm Animals, 8th Edition

426 / Anatomy and Physiology of Farm Animals

the pelvic urethra is surrounded by a true accommodate a gradual increase in uri-
nary volume until the stretch becomes
sphincter, the striated skeletal m. urethralis,
VetBooks.ir over which the animal exercises voluntary great enough to stimulate reflex centers in
the spinal cord, which in turn relax the
control. The m. urethralis is innervated by
the pudendal nerve. smooth muscle sphincter in the neck of
The pelvis, ureter, bladder, and urethra the bladder and contract the muscle wall
are all lined with transitional epithelium. of the urinary bladder by way of sacral
This epithelial lining is useful in these parasympathetic nerves. However, reflex
areas, where considerable distension of the control of the bladder can be overridden to
lumen may occur. When these organs are a certain extent by voluntary suppression
empty, the lumen is small, the walls or activation of the reflex and voluntary
are thick, and the lining epithelial cells are control of the skeletal muscle m. urethralis
piled deeply to form many layers. However, surrounding the proximal urethra.
when the organs are distended, the lumen
is enlarged, the walls are thinner, and a
transition to a much lower stratification of Overview of Function
the lining occurs (Fig. 23‐5). The urethra, and Histology of the Kidneys
like the urinary bladder, is lined with tran-
sitional epithelium; this changes to the The urinary system is responsible for
typical stratified squamous epithelium of maintaining the relatively constant inter-
mucous membranes at or near the urethral nal environment of the body fluids. This is
orifice at the tip of the penis of male ani- accomplished by the formation and excre-
mals or at its junction with the vestibule of tion of urine of an appropriate volume and
the vagina in the female animal. composition. Urine formation occurs in
the kidneys, and by adjusting the volume
and composition of urine in response to
Micturition changes in dietary intake or metabolism,
the kidneys regulate the body balance of
Micturition is the term for expulsion of water, various electrolytes, acids, and
urine from the bladder. It normally is a bases. The kidneys also excrete metabolic
reflex activity stimulated by stretch recep- waste products in the urine, including the
tors in the bladder wall that respond to nitrogenous waste, urea, and a by‐product
distension of the urinary bladder created of skeletal muscle metabolism, creatinine.
by the constant inflow of urine by way Signs of kidney diseases include imbal-
of the ureters. The bladder relaxes to ances of water, electrolytes, acids, and

(A) (B)

Figure 23-5. Transitional epithelium. (A) Bladder full, wall distended. (B) Bladder empty, wall relaxed.