Transcript Urine Formation

Urine Formation
Chapter 15
Urine Formation
• Occurs in nephron
• Molecules exchanged
between blood vessels
(glomerulus and
peritubular) and renal
tubule
• 3 steps
– Pressure Filtration
– Selective Reabsorption
– Tubular Secretion
Pressure Filtration
• Occurs inside the Bowman’s capsule
• High blood pressure in the GLOMERULUS
forces SMALL MOLECULES [such as
nitrogenous wastes, *H2O, *nutrients, *ions
(salts)] into BOWMAN'S CAPSULE.
• The AFFERENT ARTERIOLE supplies the
glomerulus with blood.
Pressure Filtration
• Large, non-filterable molecules are unable
to pass (i.e. blood cells, platelets,
proteins).
• These remain in the blood and leave the
glomerulus via the EFFERENT
ARTERIOLE. (Efferent – “E” for “E”xit.)
• The small, filterable molecules that are
forced into Bowman's capsule form
FILTRATE
K+
Glucose
HCO3-
+
Na
NaCl
H2O
I
n
c
r
e
a
s
I
n
g
S
a
l
t
I
n
e
s
s
Cortex
+
H
+
K
NH3
H2O
+
H
NaCl
H2O
H2O
Outer
Medulla
NaCl
H2O
Urea
Active Transport
Passive Transport
Inner
Medulla
Selective Reabsorption
• If the kidneys only did
pressure filtration, we
would quickly die from
water and nutrient loss.
• Once the original filtrate
is made, the next task of
the kidneys is to
reabsorb molecules in
the filtrate that the body
cannot afford to lose.
(e.g. water, nutrients,
some salts)
Selective Reabsorption
• Takes place along renal tubule
– Proximal convoluted tubule
– Loop of Henle
– Distal convoluted tubule
Reabsorption and Secretion at
the Proximal Convoluted Tubule
• CARRIER MOLECULES determine what is
reabsorbed and what passes through the tubule;
– This is done by ACTIVE transport
– except for the water, which is reabsorbed by
OSMOSIS.
• The molecules that are reabsorbed move from the
proximal convoluted tubule to the peritubular
capillary network. Most of the glomerular filtrate
gets reabsorbed!
Reabsorption and Secretion at
the Proximal Convoluted Tubule
• WHAT GETS REABSORBED?: most H2O,
nutrients (glucose, amino acids, vitamin C,
potassium ions…) some salts (NaCl)
• A balanced salt concentration in the blood must
be maintained. The process of selective
reabsorption ensures this by actively
reabsorbing sodium ions while chloride ions
follow passively.
• WHAT DOES NOT GET REABSORBED and
therefore SECRETED?: some H2O, wastes,
excess salts.
• Non-reabsorbed material continues through
Loop of Henle.
K+
Glucose
HCO3-
+
Na
NaCl
H2O
I
n
c
r
e
a
s
I
n
g
S
a
l
t
I
n
e
s
s
Cortex
+
H
+
K
NH3
H2O
+
H
NaCl
H2O
H2O
Outer
Medulla
NaCl
H2O
Urea
Active Transport
Passive Transport
Inner
Medulla
Reabsorption and Secretion at the
Loop of Henle and the Distal
Convoluted Tubule
• Filtrate now enters the LOOP OF HENLE and,
eventually, the distal convoluted tubule
• Primary role of Loop of Henle and distal
convoluted tubule is REABSORPTION OF
WATER.
– Over 99% of the water in original filtrate is reabsorbed
by the nephron during urine formation.
– Much of this reabsorption is done by OSMOSIS at the
Loop of Henle.
• This CONCENTRATES THE URINE
Reabsorption and Secretion at
the Loop of Henle and the Distal
Convoluted Tubule
• Also, the loop of Henle secretes NaCl into
the surrounding tissue (the renal medulla)
to ensure that the neighbouring tissue is
hypertonic to the filtrate. By doing this, the
loop of Henle creates an osmotic gradient.
• Na+ ions are actively reabsorbed as their
uptake is associated with water retention
(remember, the body does not want to
lose or waste water!)
K+
Glucose
HCO3-
+
Na
NaCl
H2O
I
n
c
r
e
a
s
I
n
g
S
a
l
t
I
n
e
s
s
Cortex
+
H
+
K
NH3
H2O
+
H
NaCl
H2O
H2O
Outer
Medulla
NaCl
H2O
Urea
Active Transport
Passive Transport
Inner
Medulla
Tubular Secretion
• Although urine formation occurs primarily by selective
reabsorption, a supporting mechanism, called tubular
secretion, is also involved.
• This is an ACTIVE PROCESS by which other nonfilterable wastes (i.e. those wastes that cannot be added
to the filtrate at Bowman’s capsule) can be added to the
tubular fluid so that these wastes will also be excreted in
the urine.
• Tubular secretion occurs along the distal convoluted
tubule
• Actively secreted substances include some chemicals
(e.g. penicillin, histamine) H+ ions, NH3.
TUBULAR SECRETION
• Fluid now enters the COLLECTING DUCT
• H2O PASSIVELY DIFFUSES OUT OF
COLLECTING DUCT AND STAYS IN THE BODY
• Also, both H+ and K+ ions are secreted INTO the
filtrate exchanging it with Na+. This ensures that
water is reabsorbed as well.
• Tubular excretion is important in maintaining the
pH of blood
• The tubular fluid, which we can now
“OFFICIALLY” call URINE passes from the
collecting duct into the pelvis of kidney, and
enters the ureter for transport to the bladder.
K+
Glucose
HCO3-
+
Na
NaCl
H2O
I
n
c
r
e
a
s
I
n
g
S
a
l
t
I
n
e
s
s
Cortex
+
H
+
K
NH3
H2O
+
H
NaCl
H2O
H2O
Outer
Medulla
NaCl
H2O
Urea
Active Transport
Passive Transport
Inner
Medulla
Urine Formation
• http://health.howstuffworks.com/adam200032.htm
Characteristics of Urine
• Clear to yellow in color
– Due to urochrome (a pigment resulting
from destruction of hemoglobin)
• Sterile and slightly aromatic when
formed
– If allowed to stand, takes on ammonia
odor caused by action of bacteria on urine
solutes
• Slightly acidic but pH can vary from
4.5 to 8.0
Urine
• Water plus solutes
– therefore, more dense
than water
– Specific gravity
compares how much
heavier urine is than
distilled water
• Pure water – specific
gravity of 1.0
• Urine – specific gravity
of 1.001 (diluted) to
1.035 (concentrated)
Urine
• Substance found in
urine:
– Nitrogeneous wastes
– Water
– Ions like sodium and
potassium
Urine
• Substances NOT found in urine and their
presence can indicate a medical condition:
– Glucose
– Blood proteins
– Blood
– Pus
– Bile
Micturition
•
•
Act of emptying
bladder
Web link Video Clip
Micturition
• Steps:
1.Bladder collects about 200 ml of urine
2.Stretching of bladder activates stretch
receptors
3.Impulses from receptors sent to spinal cord
and cause bladder to contract
Micturition
4. As contractions become stronger, stored
urine is force past internal sphincter into
upper part of urethra. At this point, the urge
to void is felt
5. The external sphincter is controlled
voluntarily, so it can be kept closed
Micturition
6. When one chooses
to not relax the
external sphincter,
the contractions will
stop and urine will
continue to
accumulate in the
bladder
7. After 200-300 ml
more have been
collected, micturition
occurs whether one
wills it or not
Urethra Sphincters
• Internal urethral sphincter
• Thickening of smooth muscle at the bladder urethral junction
• Involuntary, keeps urethra closed when urine not being passed
• External urethra sphincter
• Made of skeletal muscle
• Voluntary control
Incontinence
• Unable to voluntarily
control the external
sphincter
• Common in
– children under 2
– older children who
sleep very soundly
– Pregnancy due to
pressure
– Stroke or spinal injury
patients
Urinary Retention
• Unable to expel urine
• Often occurs after surgery in which anesthesia has
been given, because it takes time for the smooth
muscles to regain activity
• A slender drainage tube called a catheter can be
inserted through the urethra to drain the urine and
prevent bladder trauma