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Chapter 17The Urinary System

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KIDNEYS• Location—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)• Cortex—outer layer of kidney substance• Medulla—inner portion of kidney• Pyramids—triangular divisions of medulla• Papilla—narrow, innermost end of pyramid• Pelvis—expansion of upper end of ureter; lies inside kidney• Calyces—divisions of renal pelvis

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KIDNEYS (cont.)• Microscopic structure—nephrons are microscopic units of

kidneys; consist of (Figure 17-3):• Renal corpuscle

• Bowman’s capsule—the cup-shaped top• Glomerulus—network of blood capillaries surrounded

by Bowman’s capsule

• Renal 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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KIDNEYS (cont.)

• Functions• Excretes toxins and nitrogenous wastes• Regulates levels of many chemicals in blood• Maintains water balance• Helps regulate blood pressure via secretion

of renin

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FORMATION OF URINE (Figure 17-5)

• Occurs by a series of three processes that take place in successive parts of nephron• Filtration—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

• Reabsorption—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

• Secretion—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

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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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URINARY BLADDER

• Structure (Figure 17-7)• Elastic muscular organ, capable of great expansion• Lined with mucous membrane arranged

in rugae, as is stomach mucosa• Functions

• Storage of urine before voiding• Voiding

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URETHRA

• Structure• Narrow tube from urinary bladder to exterior• Lined with mucous membrane• Opening of urethra to the exterior called urinary meatus

• Functions• Passage of urine from bladder to exterior

of the body• Passage of male reproductive fluid (semen)

from the body

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MICTURITION• Passage of urine from body (also called urination or voiding)• Regulatory sphincters

• Internal urethral sphincter (involuntary)• External urethral sphincter (voluntary)

• Bladder wall permits storage of urine with little increase in pressure

• Emptying reflex• Initiated by stretch reflex in bladder wall• Bladder wall contracts• Internal sphincter relaxes• External sphincter relaxes, and urination occurs

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MICTURITION (cont.)• 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• May be caused by spinal injury or stroke• Retention of urine may cause cystitis

• Cystitis—bladder infection• Overactive bladder—need for frequent urination

• Called interstitial cystitis• Amounts voided are small• Extreme urgency and pain are common