URINARY SYSTEM
PG. 179
Purpose of Urinary System
 Excretion of nitrogenous liquid wastes, salts, and water
in the form of urine
 Retain water and salt
 Kidneys purify many times their weight in fluid each day
 Regulate pH and saltiness (osmolality) of blood
 Make hormones and convert vitamins into active forms
 Includes: Two kidneys (form the urine), two ureters, one
bladder, and one urethra
Urinary System Structures
Urinary System Structures
Kidneys Location and Size
 Located on either side of spinal column, high in the lumbar (lower back)
region
 Located retroperitoneal (behind the peritoneum)
 Two lowest ribs offer some protection to kidneys from physical blows
 Each is about 11 cm long, 6 cm wide, and 3 cm thick
 Weighs about 1/3 lb (150 grams)
 Liver pushes right kidney slightly lower than left
 Convex lateral edge and concave medial surface
 Indentation where nerves and blood vessels enter is called renal hilum
 Cushioned by some fat
 Small adrenal gland, an endocrine system organ, sits on top of each kidney
Anatomy of Kidney
 Frontal section separates it into “front half” and “back half”
 Light-colored outer part of kidney called renal cortex
 Dark-colored inner part of kidney called renal medulla
 Divided into renal pyramids where the base of each pyramid
faces outward to the cortex and the rounded tip of the
pyramids called papilla face towards the center of kidney
 Pyramids separated by renal columns, inward extensions of
cortex-like tissue
 Renal pelvis is deepest part of kidney
 Urine produced in cortex and medulla seeps into renal
pelvis which drains into the ureters leading to the bladder
The Kidney
The Kidney
Nerve and Blood Supply
 Kidneys make up 0.5% of total body weight, but
receive 20-25% of blood supply pumped by heart
 Renal nerve fibers from sympathetic division of
autonomic nervous system form irregular mesh on
outside of renal artery
 Renal artery, vein, nerves, and ureter all connect to
kidney at the hilum
The Nephron
 Basic working unit of each kidney is the nephron (~1 million in each
kidney)
 Each has its own blood supply and creates urine
 Two types of nephrons: cortical nephrons and juxtamedullary
nephrons (produce highly concentrated urine)
 Two main parts of each nephron: renal corpuscle, and renal tubule
 Renal corpuscle has capillaries called glomerulus and a surrounding,
cuplike glomerular (Bowman’s) capsule
 Blood enters glomerulus through afferent arteriole, passes through ball of
capillaries, and exits via efferent arteriole
 Renal tubule has three main parts: proximal convoluted tubule (PCT),
nephron loop, and distal convoluted tubule
Blood Flow Through Kidney
 Blood enters via renal artery and reaches the afferent
arteriole at entrance of glomerulus
 Blood flows through glomerular capillaries where a significant
fraction of blood plasma enters glomerular capsule space
 Blood exits glomerulus, passes through efferent arteriole, and
enters a second set of capillaries
 As blood passes through second set of capillaries, it reabsorbs
most of the fluid (but not waste) that is lost in the glomerulus
 The blood, now largely free of waste, collects into venules
which merge to form larger veins and leave the kidneys via
renal veins
The Nephron
Bowman’s Capsule
PG. 181
Urine Formation
 Three processes involved: filtration, reabsorption, and
secretion
 Filtration occurs at renal corpuscle and is the
movement of water and small solutes from capillaries
 Total amount of water filtered in a unit of time is called
the glomerular filtration rate (GFR) and is about 125 mL
per min. in males and 105 mL per min. in females
 Driving forces for glomerular filtration is hydrostatic
pressure (“regular” pressure) and osmotic pressure
(created by presence of dissolved substances in water)
Urine Formation CONT.
 Reabsorption mainly occurs in the proximal
convoluted tubule…the rest occurs in the distal
convoluted tubule and collecting duct
 Most of the water and “good” dissolved substances
are reabsorbed into the blood in the capillaries
surrounding the tubule
 Secretion is an active process that constantly
moves wastes still in the blood into the renal tubule
that will eventually be eliminated from body as
urine
Urine Formation in the Nephron
Hormonal Regulation of Urine
 By the time filtrate reaches the distal convoluted
tubule, about 80% of the water and 90% of the
sodium and chloride have been reabsorbed
 Reabsorption in proximal convoluted tubule and
nephron loop is constant
 Reabsorption in distal convoluted tubule and
collecting duct is controlled by hormones
Hormonal Regulation CONT.
 Aldosterone: causes a decrease in volume and
sodium content of urine, and an increase in
potassium content
 Atrial Natriuretic Peptide (ANP): urine volume and
sodium excretion increases
 Antidiuretic Hormone (ADH): reabsorption of
water causing concentrated urine
Urine Storage
 Urinary system excretes waste intermittently and need a place
to store urine until the time and place for elimination are
appropriate
 The renal pelvis of the kidney is connected by a ureter to the
posterior wall of the bladder
 The urinary bladder is a hollow muscular organ that stores urine
 Sits on the floor of pelvic cavity and its superior surface is
directly covered by the peritoneum
 In men, positioned directly in front of rectum and above
prostate gland
 In women, positioned in front of vagina and uterus
 Moderately full bladder can hold up to 500 mL of urine,
extremely full bladder can hold up to 1,000 mL of urine
Urine Storage CONT.
 Urethra is a thin tube that connects to the
bladder neck
 Prostate gland contains tiny ducts that deliver its
secretions into prostatic urethra in men
 Total length of male urethra is about 20 cm long
 Total length of female urethra only 2-3 cm long
 Surrounded by the external urethral sphincter as it
passes through the urogenital diaphragm
 External opening is called external urethral orifice
Urine Excretion
 Release of urine from bladder is called urination, voiding, or
micturition
1. As bladder fills, it stretches and neurons send impulses to
sacral portion of spinal cord.
2. Interneurons in spinal cord signal motor neurons in sacral
cord that bladder is stretched
3. Sacral cord neurons send impulses back out to bladder
4. Impulses sent to bladder through parasympathetic nerve
fibers cause detrusor to contract and internal urethral
sphincter to relax
Urine Excretion CONT.
5. Spinal cord send signal to brain “telling” it the bladder is
stretched
6. Sensory message from spinal cord activates micturition reflex
center in brain stem
7. Detrusor muscle contracts, and internal urethral sphincter
relaxes. At the same time, person becomes consciously aware
the bladder is full.
8. From sacral spinal cord, the nerve impulses travel to external
urethral sphincter signaling it to relax. As a result, the pathway
for micturition becomes fully open, the detrusor contracts, and
the urine is expelled
Male versus Female
Pathway of Urine Formation
PG. 183
Effects of Aging
 Kidneys shrink
 Decreased renal blood flow
 Kidney compromised in removing waste
products
 Decreased glomerular filtration rate
– Drug dosages have to be adjusted
 Glucose reabsorption also decreases
– Hyperglycemia
 Loss of muscle tone in the urinary bladder
 Urinary incontinence
Assessing Renal Function
 Amount, appearance, smell, and chemical content of urine
reveal clues to abnormalities in kidney function
 Because kidneys regulate composition of blood, analysis of
blood is essential for evaluation of kidney function
 Physical characteristics of urine: urine is normally clear and
yellow so cloudy urine or a color other than yellow may signal an
issue; pH of urine can vary from 4.5 to 8.0 under normal
conditions
 Chemical composition of urine: urine is about 95% water with
urea being the most abundant solute but also typically contains
potassium, chloride, sodium and other ions; presence of red or
white blood cells, protein or glucose is abnormal
Diabetes
 Diabetes mellitus: characterized by the production of large
amounts of urine that contain glucose
 Develops when the body fails to produce insulin or when the body’s
cells fail to respond to insulin
 Patients need to choose calories carefully to avoid large peaks and
valleys in blood glucose
 Type I is an autoimmune disease, Type II is less well understood
 Both involve in inability to metabolize glucose properly after
carbohydrate digestion
 Most common cause of chronic kidney disease and renal failure
 Diabetes Insipidus: characterized by production of large amounts
of urine that is highly diluted
 Caused by failure of pituitary gland to produce normal amounts of ADH
 Patient is always thirsty due to constant loss of fluid
Diabetes
Chronic Kidney Disease
 Defined by evidence of kidney damage
 Develops slowly
 Diabetes mellitus most common cause followed by
hypertension (high blood pressure)
 Renal failure is the most severe stage of chronic kidney
disease
 Kidneys unable to perform task of maintaining homeostasis
 Waste products accumulate in blood, levels of pH and
various ions are not controlled
 Patient must receive renal dialysis or kidney transplant in
order to survive
Renal Dialysis
 Removal of wastes from the blood by artificial means
 Two forms: hemodialysis and peritoneal dialysis
 Both remove water, urea, and some sodium from body
 In hemodialysis, blood is withdrawn from an artery, pumped through
a dialyzer (large machine positioned next to patient) and then
returned to patient through vein
 Dialyzer acts as artificial kidney, but can only remove urea and some
wastes from blood
 In peritoneal dialysis, dialysis fluid is added to abdominopelvic cavity
through surgically implanted port in abdomen
 As dialysis solution sits in abdomen for an hour or so, it absorbs water and
wastes from capillaries in mesentery
 Peritoneum itself serves as dialysis membrane
Dialysis
Kidney Stones
 Kidney stone is a solid, crystalline mass that forms in the urine
 Made of calcium-containing compounds, but they can also be
composed of magnesium or uric acid
 Usually form in renal pelvis, but they may form in ureter or bladder
 Stones smaller than 5 mm in diameter may pass from body without
difficulty
 If a larger stone develops and gets lodged in the ureter, intense
pain occurs
 Lithotripsy is the use of intense, ultrasonic sound waves to break up
a kidney stone into smaller pieces to pass in urine
Kidney Stones
Urinary Tract Infections
 Usually caused by bacteria that enter the urethra at its outside
opening
 If the bacteria reach the bladder, the resulting infection can
lead to cystitis (inflammation of bladder epithelium)
 If bacteria travel up the ureters to kidneys, the resulting
infection is called pyelonephritis which causes pain in kidney
region of back
 More common in women than men due to the much shorter
urethra
 Symptoms include pain during urination, increased urinary
frequency, fever, and sometimes cloudy or dark urine
Kidney Transplants
 Done in cases of prolonged chronic debilitating diseases and renal
failure involving both kidneys
 Usually the patient has been on dialysis for a long period of time
waiting for a compatible organ
 Two types of transplants: one from a living donor (usually a family
member is the best match) or an unrelated donor who has died
 Major concern is rejection of kidney by the recipient
–
Medications taken daily to prevent rejection
 Allows for better quality of life than dialysis, increased energy levels
and less restricted diet
Kidney Transplants