FORMATION OF URINE
The formation of urine occurs in three separate steps.
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FILTRATION
The movement of fluids from the blood into the
Bowman's capsule of the nephron.
Blood moves from the afferent arteriole into the
glomerulus (a high-powered filter, acting at 65 mm
of Hg, ~1.5 times the normal pressure of a
capillary).
Dissolved solutes pass from the blood into the
Bowman's capsule along the pressure gradient.
Not all solutes make it through.
DO:
DON'T (too large)
Water
Plasma proteins
Sodium chloride
Glucose
Amino acids
protons
-Red blood cells
-Platelets
REABSORPTION



The transfer of essential solutes and
most water back into the surrounding
cells and the blood stream.
For every 120 mL of fluid (amount
filtered by kidneys each minute), 1 mL
of urine is formed, 119 mL of fluid and
solutes is reabsorbed.
Occurs throughout the nephron, mainly
in the proximal tubule and loop of
Henle
WATER REABSORPTION

Water is automatically
reabsorbed back into the cells
and bloodstream for 2 reasons:
 The blood that left the
glomerulus by the efferent
arteriole travels through the
peritubular capillaries. It
has a very high
SECRETION
The release of substances into the filtrate from
the blood and cells surrounding the nephron.
Occurs mainly in the distal tubules and in the
collecting duct, but some also occurs in the
proximal tubule.
Purpose:
to release any toxins and drugs that have not been
filtered
Maintain the electrolyte balance of the body (if
positive sodium ions are reabsorbed then positive
ions like potassium must be secreted to keep the
balance – even though reabsorbed intially).
Acid-base balance (usually it is an acid being
secreted, essentially a proton plus whatever it is
attached to).