Download urine formation

KIDNEY FUNCTIONS
URINE FORMATION
Learning Outcomes:
• Describe the function of each section of
the nephron
• Describe the process of urine formation
• Describe how the kidneys maintain
blood pH
Urine Composition
•
•
•
•
•
Water
Nitrogenous wastes (urea etc.)
Salts
May contain H+ and other ions
May contain drugs, vitamins and other
substances
Urine Formation
3 major steps:
1. Glomerular filtration - in the glomerular
capsule
2. Tubular reabsorption - in the proximal
tubule and loop of Henle
3. Tubular secretion - distal tubule
• Different parts of the nephron have
different permeability characteristics
• Substances are filtered out of the blood
by the nephron and excreted in urine
• Some substances are reabsorbed back
into the blood from the nephron
Pressure Filtration
• Blood enters the
glomerulus from the
afferent arteriole
• Blood pressure forces
small molecules out of the
capillaries into the
capsule
• Substances that leave the blood:
Water, salts, nutrients (glucose, amino
acids), nitrogenous wastes (urea)
• Filtrate resembles plasma
Selective Reabsorption
• At the proximal tubule and loop of Henle
• Some substances must be reabsorbed
back into the blood
• Examples:
• Water, nutrients (glucose, amino acids),
salts
• Na+ ions, glucose, amino acids - move
back into the peritubular capillaries by
active transport (why active transport?)
• Cl- ions follow passively
• Water is reabsorbed by osmosis
Absorption of Water at the
Loop of Henle
• Function of the loop of
Henle is to remove as
much water as possible
from the filtrate and
return it to the blood
• Descending limb is
permeable to water but
not salt
• Ascending limb is
permeable to salt but not
water
• Sodium is actively
pumped out of the
ascending limb
• Medulla is hypertonic to
the filtrate (contains high
[Na+])
• Water leaves the tubule
by osmosis and enters
the medulla, then the
capillaries
Fig. 16.7
Fig. 16.6
Tubular Secretion
• At the distal tubule
• Excess H+ ions, drugs,
and other substances
move from the blood into
the nephron by active
transport
• Ions secreted or kept in
the blood as needed, to
maintain correct pH
Reabsorption of Water
• More water is reabsorbed into
the blood from the collecting
duct
• Collecting duct is in the
hypertonic medulla so water is
drawn out by osmosis
Fig. 16.6
• Urine is now ready to be excreted from
the body
• Collects in the renal pelvis before going
to the bladder
Path of Blood
1.
2.
3.
4.
5.
6.
Renal artery & arterioles
Afferent arteriole
Glomerulus
Efferent arteriole
Peritubular capillary network
Venules to renal vein
Questions
• How is the composition of blood in the renal
artery different from that in the renal vein?
• What is the importance of the hypertonic
condition of the renal medulla?
• Why is glucose transported actively instead of
passively at the proximal tubule?
Fig. 16.6
Fig. 16.6