Download Urine formation

Urine formation
Yousaf Khan
Renal Dialysis Lecturer
Urine formation
Urine formation involves glomerular filtration,
tubular reabsorption, and tubular secretion.
 Glomerular Filtration
- Urine formation begins when the fluid portion
of the blood is filtered by the glomerulus and
enters the glomerular capsule as glomerular
filtrate.

Glomerular filtration
1. Blood moves from the afferent arteriole into the
glomerulus.
 Blood backs up in the glomerulus causing high pressure:
◦ The efferent arterioles (leaving the glomerulus) are
smaller than the afferent arterioles (entering the
glomerulus).
◦ Therefore, the blood backs up into the glomerulus
and pushes against the walls of the glomerulus
capillaries.
Glomerular filtration
2. The high blood pressure moves any substance
small enough to fit through the walls of the
glomerulus into Bowman’s capsule
 What moves through: water plus many solutes
(vitamins, amino acids, glucose, Na+, Cl-, K+,
bicarbonate ions, phosphates, urea, uric acid)
 What is left behind: Red blood cells and large
proteins are left behind to enter the efferent
arteriole.
Filtration Rate
• The factors that affect the filtration rate
are filtration pressure, glomerular plasma
osmotic pressure, and hydrostatic
pressure in the glomerular capsule.
180 liters of blood are filtered daily
 Glomerular filtration is a passive, non-selective
process
 Fluids and solutes are forced through a
membrane by hydrostatic (water) pressure,
provided by the push of the heart. It is passive
(no energy required)

Glomerulus
capillaries
Afferent arteriole
(Larger)
H2O
vitamins
Uric acid
Blood flow
Amino acids
urea
glucose
Electrolytes / ions
Efferent arteriole
(smaller)
Bowman’s
capsule
Glomerular
filtration
Tubular Reabsorption
Step 2- Tubular Reabsorption:
 Main goal: for blood to reclaim vital
nutrients that left the blood during
glomerular filtration.
 What returns to the blood in this step:
water, all nutrients and most ions.
Urine formation
As the efferent
arteriole leaves the
renal corpsucle
branches into the
peritubular
capillaries.The
capillaries sit up
against the nephron
tubules.
Water and solutes
leave the Bowmans
capsule and enter the
proximal convoluted
tubule.
The urine filtrate
flows through the
nephron tubule.
 Nephrons are
permeable because
of: 1000’s of
microvilli that line
the inside and
increase the surface
area
1.
The urine filtrate flows through the
nephron
tubule.
 This process is passive.
 Some substances are left in the urine
filtrate- urea, uric acid, creatinine, and
some of the water and few other wastes
 These substances stay inside the nephron
tubule.
1.
2. The tubule cells then go through active
transport and uses ATP to pump Na+ into the
interstitial space between the tubule and the
capillary.
 This causes the interstitial space to become
hypertonic and this pulls water out of the
tubule cells, along with valuable solutes
 This is called obligatory water reabsorption.
The water is “obliged” to follow the Na+
into the interstitial fluid.
3. Now, the water and solutes are in the
fluid around the capillary.
 The large proteins inside the capillary
make the blood a hypertonic solution, so
water and solutes are drawn inside the
capillary.
Active transport
Passive transport
blood
Na
H2O + nutrients
Loop of Henle
Most (70%) of the reabsorption of filtrate occurs
in the proximal convoluted tubule. The way
nephrons control how concentrated or dilute
urine is depends on how much of the other
20% of the filtrate is reabsorbed. This takes
place in the Loop of Henle, distal convoluted
tubules, and collecting tubules.
Urine concentration at the loop of
Henle




Main goal: removal of water and some ions
to concentrate the urine
takes place in the juxtamedullary nephrons
which specialize in water homeostasis.
In the descending portion of the loop of
Henle, the tubule is impermeable (not open)
to solutes but permeable to water.
The interstitial fluid (fluid in between the
tubule and the peritubular capillaries) is very
concentrated with Na+ (salty) and becomes
more concentrated as the loop descends
into the renal medulla.
STEPS IN THE URINE
CONCENTRATION PROCESS:
Since water is in a lower concentration in the tubule,
it exits the loop of henle via osmosis and into the
interstitial space and then into the peritubular
capillaries.
 So, the filtrate in the Loop of Henle becomes more
concentrated, as 10% more of the original filtrate
volume returns to the blood.

STEPS IN THE URINE
CONCENTRATION PROCESS:
The amount of water that moves out of the
descending Loop of Henle is controlled by the
ascending loop.
 If the body wants more water to move out of the
tubule, the ascending loop makes the area more
hypertonic.
 If the body wants to keep water in the tubule it
makes the interstitial space less hypertonic.
2.
STEPS IN THE URINE
CONCENTRATION PROCESS:
In the ascending portion of the loop of
Henle the tubule changes permeability.
 It is now impermeable to water.
 The cells in this area of the loop have
many Na+ and Cl- pumps, and these ions
get pumped out of the ascending loop of
Henle, so the filtrate becomes a bit more
dilute again.
2.
Active transport
Passive transport
blood
cortex
H2O
H2O
Outer medulla
NaCl
NaCl
H2O
NaCl
H2O
NaCl
H2O
H2O
H2O
H2O
Loop of Henle
Tubular Secretion
Main goals:
 To filter out any waste that may have re-entered
the blood stream during tubular reabsorption.
 To maintain blood pH homeostasis
 This step requires active transport.

So waste moves from the: peritubular capillary
 interstial space distal convoluted tubule
Urine formation
Common substances that must be filtered in this
step are:
1. Wastes that moved back into the blood
passively by accident.
2. Artificial materials that didn’t go into the
Bowman’s capsule during glomerular filtration
(penicillin and pesticides)
3. Excess ions the body needs to get rid of to
adjust the blood pH.
How it Adjusts the blood pH:
Blood will pump out excess H+ and K+ ions.
These H+ ions are what give urine its acid
quality.
This takes place along the distal convoluted
tubule.
Tubular secretion:
Distal Convoluted Tubule
Tubule cell
Nephron
tubule
URINE FILTRATE
H2O, urea, creatinine, few other wastes
To collecting
tubule
NEPHRON TUBULE
capillary
creatinine
Capillary cell
pesticide
rbc
blood H+
urea
pennicillin
Active transport
Passive transport
blood
Distal convoluted tubule
cortex
H2O
H2O
Outer medulla
NaCl
NaCl
H2O
NaCl
H2O
NaCl
H2O
H2O
H2O
H2O
Loop of Henle
Wastes
H+ ions
Step 5: Final Urine concentration and
regulation of electrolytes at the Collecting
tubule
Completes 2 main goals here:
1. Adjusts blood pH and salt homeostasis
2. Maintains water homeostasis.
Adjusting blood pH and salt homeostasis
continues:

K+, H+ and HCO3 - ions that were collected in
the distal convoluted tubule can be either stay
in the urine for excretion or reabsorbed by
the blood if the body needs them.
Water Homeostasis
• When the urine enters the collecting tubules they are
normally almost totally impermeable to water.
• At this point, the urine is still fairly dilute, and very
light yellow in color.
• If the body is well hydrated, nothing more happens in
the collecting tubules and they remain impermeable
to water and salts.
• The water and salts are then excreted.
Concentrated Urine
If the individual needs to conserve water due to
dehydration, ADH, anti-diuretic hormone is
released by the posterior pituitary gland.
 This is a urine concentrating hormone. It
causes the collecting tubules to become
permeable to water (opens water transport
proteins)
 Water moves into the interstitial space and
back to the capillaries, therefore, less water
stays in the collecting tubules and the urine
becomes more concentrated.
Thank You