Chapter 17
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

ØRenal tubule

•Proximal convoluted tubule—first segment

•Loop of Henle—extension of proximal tubule; consists
of descending limb, loop, and ascending limb

•Distal convoluted tubule—extension of ascending limb
of loop of Henle

•Collecting tubule—straight extension of distal tubule

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˜Functions

ØExcretes toxins and nitrogenous wastes

ØRegulates levels of many chemicals in blood

ØMaintains water balance

ØHelps regulate blood pressure via secretion
of renin  

FORMATION OF URINE
(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

˜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

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˜Structure (Figure 17-7)

ØElastic muscular organ, capable of great expansion

ØLined with mucous membrane arranged
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

ØPassage of male reproductive fluid (semen)
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