Chapter 17 The Urinary System

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

KIDNEYS 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

KIDNEYS Functions Excretes toxins and nitrogenous wastes Regulates levels of many chemicals in blood Maintains water balance Helps 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 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

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

URINARY BLADDER Structure (Figure 17-7) Functions 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

URETHRA Structure Functions 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

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

MICTURITION Urinary retention—urine produced but not voided Urinary suppression—no urine produced but bladder is normal Incontinence—urine is voided involuntarily 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