The Urinary System
rev 4/11
• Excretion refers to the processes that remove
wastes and excess materials from the body.
• Urinary system has primary responsibility for
– homeostasis of water and most solutes in blood
and other body fluids.
• Urine is essentially water and solutes.
• Water is the most abundant molecule in our
bodies. We excrete about 2 ½ liters of water
per day, 1.5 liters per hour in urine.
Urinary System
1
– Regulates nitrogenous and other solute waste
(nitrogen from protein metabolism made into urea as
waste)
– Others: sodium, chloride, potassium, calcium,
hydrogen ions, creatinine (waste product from muscle
metabolism) toxic chemicals waste produced from
liver and cellular metabolism, drugs, vitamins
– Water is lost in urine, evaporation, feces.
The urinary system is composed of the organs that
produce, transport, store and excrete urine:
– 2 kidneys, 2 ureters, one bladder and one urethra
• The bladder stores urine (600–1000 ml)
Urinary System
2
• The Kidneys
– are bean shaped organs about the size of a
clenched fist
• Renal artery and renal vein connect each kidney to
the aorta and inferior vena cava
– are located in the abdominal cavity*, lateral to
the 2nd lumbar vertebrae and close to our
back.
Urinary System
3
Kidney’s Role in Homeostasis
Besides making urine, the kidney:
• Maintains water balance: adjusts blood volume
and blood pressure
• Maintains acid–base balance and blood pH
• Regulates red blood cell production (via the
hormone erythropoietin)
• Activates an inactive form of vitamin D
Urinary System
4
Gross anatomy of the kidneys:
• the capsule is the protective outer covering
of the kidney
• the interior of the kidney has 2 areas: the
cortex and the medulla
• the cortex is the outer portion. It has most of
the capillary blood flow, and contains the
nephrons (the functional unit of the kidney).
Urinary System
5
• The kidney medulla is composed of cone
shaped areas called pyramids.
– The pyramids appear striped because they
are formed of parallel bundles of
microscopic urine collecting tubules,
many loops of Henle and the collecting
ducts.
• The tips of the pyramids are called the
papillae. These insert into the opening
of the calyx.
• Filtered urine is transported from the
calyx to the kidney pelvis.
• The pelvis empties into the ureters.
Urinary System
6
• The smooth muscle of the ureters create
peristaltic waves which move the urine into the
bladder
– The internal urethral sphincter and the
external urethral sphincter prevent the
bladder from emptying.
– During urination, urine passes into the
urethra.
Urinary System
7
• The nephron is divided into several different
areas:
• a glomerular or Bowman’s capsule
which surrounds the glomerulus
• a proximal convoluted tubule
• a loop of Henle, composed of a
descending tubule, a loop, and an
ascending tubule
• a distal convoluted tubule
• a collecting duct
• blood vessels
Urinary System
8
How a Nephron works to produce urine and
also reclaim substances the body still
needs:
• The glomerular capsule surrounds the
glomerulus which is a network of
capillaries.
• A tubule exits from the back of the glomerular
capsule and continues as a long, thin tube
with 4 areas:
the proximal convoluted tubule
The loop of Henle which is composed of
Urinary System
9
• the descending limb
• the actual loop
• the ascending limb
• the distal convoluted tubule
• the collecting duct –
• The collecting duct leads into the renal
pelvis
Urinary System
10
Blood vessels which supply the tubule
• The renal artery supplies the kidney and
branches many times.
• Each nephron is supplied by a single afferent
arteriole which enters a Bowman’s capsule and
then divides to become the network of capillaries
that makes up the glomerulus.
• After filtering the blood, the capillaries rejoin to
become the efferent arteriole. This divides
again into the peritubular capillaries, another
capillary network that surrounds the proximal
and distal tubules in the cortex.
Urinary System
11
• The peritubular capillaries remove water, ions
and nutrients which are reabsorbed by the
proximal and distal tubules.
• Some efferent arterioles descend into the
medulla and divide into capillaries called the
vasa recta which supply the loop of Henle and
the collecting duct.
• After this, the efferent arterioles meet with
efferent venules, flowing ultimately into the renal
vein.
Urinary System
12
Formation of Urine: Glomerular Filtration
How do the kidneys select what to keep and what to
excrete in urine?
Urine formation involves 3 processes:
1. Glomerular filtration: filters fluid from glomerulus
into Bowman’s capsule
•
Glomerular filtration is caused by the high blood
pressure in the glomerulus, about twice as high as
other capillaries.
•
Rate of filtration:
– Resting rate is under local chemical control
– Stress causes sympathetic nervous system
to reduce blood flow to kidneys
Urinary System
13
Formation of Urine: Tubular Reabsorption
2. Tubular reabsorption: returns water and
needed solutes back to blood capillaries
– Process:
• Sodium moved by active transport from
tubule cells to interstitial fluid and diffuses
to capillaries
• Chloride passively accompanies sodium to
maintain a balanced electrical charge
• Water reabsorbed with salts
• Movement of sodium creates energy to
transport glucose and amino acids into
renal tubule then diffuses to the interstitial
fluid
Urinary System
14
Formation of Urine: Tubular Secretion
3. Tubular secretion: removes other substances
from blood into the tubule to be excreted; can
be performed either by passive diffusion or
active transport
– Purpose: removing or regulating chemical
levels in body; excretion of harmful chemicals
– Substances which can be secreted: penicillin,
cocaine, marijuana, pesticides, preservatives,
hydrogen ions, ammonium, potassium
Urinary System
15
Concentration or Dilution of Urine: ADH
The ability to dilute or concentrate urine depends
on a high concentration of solutes in the renal
medulla along with the ability to change the
collecting ducts’ permeability to water
• Dilute urine: excreting excess water
– If you drink a lot of liquids, they are absorbed
by the digestive system, enter your blood,
increasing blood volume and decreasing the
concentration of ions in your blood and body
fluids.
Urinary System
16
• Most of the water enters your cells
• To prevent osmotic swelling and damage to
cells, the kidneys reasorb less water and
produce dilute urine.
– Mechanism: cycling of NaCl and urea create a
concentration gradient in the medulla that
allows water to diffuse from the renal tubules
into the interstitial fluid and then into the blood
capillaries
Urinary System
17
Concentrated urine: conserving water
• If we drink too little liquid or sweat a lot, we may
lower our blood volume, decrease our blood
pressure, and risk dehydrating our cells. The
kidneys compensate by reabsorbing more water
and making a more concentrated urine.
– Mechanism: Countercurrent exchange
mechanism allows us to produce a more
concentrated urine
Urinary System
18
• Because of the hairpin turn in the loops of
Henle, fluid flows in opposite directions in
the 2 sides of the loop. This is called a
countercurrent flow.
–This allows a small difference in
composition between the tubular fluid
and the interstitial fluid to trigger
reabsorption of liquid.
– Increased ADH causes increased
permeability to the collecting tubules and
increased conservation of water
Urinary System
19
Urination
• Depends on a neural reflex: Micturition reflex
– Internal urethral sphincter
– External urethral sphincter
As the bladder fills, it starts to stretch.
• This sends a message to the spinal cord
• Spinal cord nerves begin an involuntary reflex
– Stretch receptor input goes to the brain
• The brain can voluntarily override the
micturation reflex
Urinary System
20
Disorders of the Urinary System
• Kidney stones
– Minerals in urine crystallize in the renal pelvis
and form kidney stones
• Most are excreted with no problems
• Can be removed surgically
Urinary System
21
• Urinary tract infections
– Refers to the presence of microorganisms in
urine or in any part of the urinary system
– Symptoms include swelling and redness
around the urethral opening, a burning
sensation or pain while urinating, difficulty
urinating, bed wetting, low back pain, visible
blood or pus in urine
– Most are caused by bacteria that make their
way up the urethra and can travel up the
ureters to the kidneys
– Most can be cured with antibiotics
Urinary System
22
• Acute and chronic renal failure:
– Kidney function impairment that is short term
(acute) or long term (chronic)
– Acute causes:
• Sustained decreases in BP, large stones in the
renal pelvis, infections, transfusion reactions,
burns, severe injuries and toxic drugs or chemicals
– Chronic (may also be called end state renal
disease—ESRD):
• Long term irreversible damage leading to a
reduction in functioning nephrons with failure of the
kidneys to function properly
Urinary System
23
– Therapies:
• Dialysis tries to duplicate the kidney function
– CAPD-continuous ambulatory peritoneal
dialysis (fluid left in abdomen for several hours)
– Hemodialysis: blood is circulated through an
artificial kidney machine
• Kidney transplant—originally, biggest challenge
is to find a good immunological match so the
person’s body will not reject the foreign kidney
– Now not enough people have offered to donate
their organs after death
Urinary System
24