The Urinary System
KIDNEYS
ØLocation—under
back muscles, behind parietal peritoneum, just above waistline; right kidney
usually a little lower than left (Figure 17-1)
˜Internal
structure (Figure 17-2)
ØCortex—outer
layer of kidney substance
ØMedulla—inner
portion of kidney
ØPyramids—triangular
divisions of medulla
ØPapilla—narrow,
innermost end of pyramid
ØPelvis—expansion
of upper end of ureter; lies inside kidney
ØCalyces—divisions
of renal pelvis
˜Microscopic
structure—nephrons are microscopic units of kidneys; consist of (Figure 17-3):
ØRenal
corpuscle
•Bowman’s capsule—the cup-shaped top
•Glomerulus—network of blood
capillaries surrounded
by Bowman’s capsule
by Bowman’s capsule
ØRenal
tubule
•Proximal convoluted tubule—first
segment
•Loop of Henle—extension
of proximal tubule; consists
of descending limb, loop, and ascending limb
of descending limb, loop, and ascending limb
•Distal convoluted tubule—extension
of ascending limb
of loop of Henle
of loop of Henle
•Collecting tubule—straight
extension of distal tubule
˜
˜Functions
ØExcretes
toxins and nitrogenous wastes
ØRegulates
levels of many chemicals in blood
ØMaintains
water balance
ØHelps
regulate blood pressure via secretion
of renin
of renin
FORMATION
OF URINE
(Figure 17-5)
(Figure 17-5)
˜Occurs
by a series of three processes that take place in successive parts of nephron
ØFiltration—goes
on continually in renal corpuscles; glomerular blood pressure causes water and
dissolved substances to filter out of glomeruli into Bowman’s capsule; normal
glomerular filtration rate 125 mL per minute
ØReabsorption—movement
of substances out of renal tubules into blood in peritubular
capillaries; water, nutrients, and ions are reabsorbed; water is reabsorbed by
osmosis from proximal tubules
ØSecretion—movement
of substances into urine in the distal and collecting tubules from blood in peritubular
capillaries; hydrogen ions, potassium ions, and certain drugs
are
secreted by active transport; ammonia is secreted
by diffusion
by diffusion
˜Control
of urine volume—mainly by posterior pituitary hormone’s ADH, which decreases it
URETERS
˜Structure
(Figure 17-6)—narrow, long tubes with expanded upper end (renal pelvis) located
inside kidney and lined with mucous membrane
˜Function—drain
urine from renal pelvis to urinary bladder
•
˜Structure
(Figure 17-7)
ØElastic
muscular organ, capable of great expansion
ØLined
with mucous membrane arranged
in rugae, as is stomach mucosa
in rugae, as is stomach mucosa
˜Functions
ØStorage
of urine before voiding
ØVoiding
URETHRA
˜Structure
ØNarrow
tube from urinary bladder to exterior
ØLined
with mucous membrane
ØOpening
of urethra to the exterior called urinary
meatus
˜Functions
ØPassage
of urine from bladder to exterior
of the body
of the body
ØPassage
of male reproductive fluid (semen)
from the body
from the body
MICTURITION
˜Passage
of urine from body (also called urination
or voiding)
˜Regulatory
sphincters
ØInternal
urethral sphincter (involuntary)
ØExternal
urethral sphincter (voluntary)
˜Bladder
wall permits storage of urine with little increase in pressure
˜Emptying
reflex
ØInitiated
by stretch reflex in bladder wall
ØBladder
wall contracts
ØInternal
sphincter relaxes
ØExternal
sphincter relaxes, and urination occurs
˜Urinary
retention—urine produced but not voided
˜Urinary
suppression—no urine produced but bladder is normal
˜Urinary
incontinence—urine is voided involuntarily; a common bladder control problem in
elderly people
ØMay
be caused by spinal injury or stroke
ØRetention
of urine may cause cystitis
˜Cystitis—bladder
infection
˜Overactive
bladder—need for frequent urination
ØCalled
interstitial cystitis
ØAmounts
voided are small
ØExtreme
urgency and pain are common
