Kidneys
The kidneys are located under back muscles, just above the waistline; the right kidney is usually
a little lower than left one
Functions
Excretes toxins and nitrogenous wastes
Regulates levels of many chemicals in the blood
Maintains water balance
Helps to regulate blood pressure
Formation of Urine
Urine formation occurs as a result of a series of three processes that take place in successive
parts of nephrons (the microscopic units that make up the interior of each kidney):
Filtration goes on continually in renal corpuscles; the normal filtration rate is ~125 ml per
minute
Reabsorption is the movement of substances out of renal tubules into blood; water, nutrients,
and ions are reabsorbed; water is reabsorbed by osmosis
Secretion is the movement of substances into urine
Ureters
Long, narrow tubes with expanded upper ends located inside the kidney, the ureters are lined
with mucous membrane and have a muscular layer
They drain urine from the kidneys to the urinary bladder
Urinary Bladder
Structure
The urinary bladder is an elastic, muscular organ, lined with mucous membrane and capable of
great expansion
Functions
It stores urine before voiding
Urethra
Structure
The urethra is a narrow, mucous membrane-lined tube that extends from the urinary bladder
to the exterior of the body
Functions
Passage of urine from the bladder
Passage of male reproductive fluid (semen)
Micturition is the passing of urine from the body (also called urination or voiding)
Regulatory sphincters
Internal urethral sphincter (involuntary)
External urethral sphincter (voluntary)
The bladder wall permits the storage of urine with little increase in pressure
Micturition
Emptying reflex
Initiated by a stretch reflex in the bladder wall: the bladder wall contracts, the internal
sphincter relaxes, the external sphincter relaxes, and urine flows
Enuresis: involuntary urination in young child
Urinary retention: urine produced but not voided
Urinary suppression: no urine produced but bladder is normal
Incontinence: urine is voided involuntarily
Renal and Urinary Disorders
Obstructive disorders interfere with normal urine flow, possibly causing urine to back up and
cause kidney damage
Renal calculi (kidney stones) may block ureters, causing intense pain called renal colic
Tumors: renal cell carcinoma (kidney cancer) and bladder cancer are often characterized by
hematuria (blood in the urine)
Urinary tract infections (UTIs) are caused by bacteria
Kidney failure (renal failure) occurs when the kidneys fail to function
Acute renal failure is an abrupt reduction in kidney function that is usually reversible
Chronic renal failure is a slow, progressive loss of nephrons caused by a variety of underlying
diseases
Early in this disorder, healthy nephrons often compensate for the loss of damaged nephrons
Loss of kidney function ultimately results in uremia and its life-threatening consequences
Complete kidney failure results in death unless a new kidney is transplanted or an artificial
kidney substitute is used (dialysis)