b. Aldosterone. The aldosterone is secreted by adrenal cortex, that is
situated above each kidney. Aldosterone increases sodium reabsorption in the distal
tubules and collecting ducts. This leads to an increase in sodium reabsorption and of
concentration of the urine.
The ureters are tubes that connect the kidneys to the urinary bladder. The
smooth muscle walls of the ureters produce a peristalsis (wave-like movement) that
moves the urine along drop by drop.
6-8. URINARY BLADDER
a. The urinary bladder is a muscular organ for storing the urine. Near the
inferior posterior corners of the urinary bladder are openings where the ureters empty
into the bladder. Also at the inferior aspect of the urinary bladder is the exit, the
beginning of the urethra. The triangular area, between the openings of the ureters and
the urethra, is called the trigone, or base of the urinary bladder.
The urinary bladder wall is stretchable to accommodate varying volumes of
c. Nerve endings called stretch receptors are found in the wall of the urinary
bladder. Usually, the pressure within the urinary bladder is low. However, as the
volume of the enclosed urine approaches the bladder's capacity, stretching of the wall
stimulates the stretch receptors. The cycle of events controlling urination (voiding or
emptying of the urinary bladder) is known as the voiding reflex.
The urethra is a tube that conducts the urine from the urinary bladder to the
outside of the body. It begins at the anterior base of the urinary bladder.
a. Urethral Sphincters. The urethral sphincters are circular muscle masses
that control the passage of the urine through the urethra. There are two urethral
sphincters: an internal urethral sphincter and an external urethral sphincter.
(1) The internal urethral sphincter is located in the floor of the urinary
bladder. It is made of smooth muscle tissue. Nerves of the autonomic nervous system
(2) The external urethral sphincter is inferior around the urethra in the area
of the pelvic floor. It is made up of striated muscle tissue. It is controlled by the
peripheral nervous system.