Download kidney 2 - MBBS Students Club

Urine Formation results from
• Glomerular Filtration
• Tubular Reabsorption
• Tubular Secretion
Glomerular Filtration
• As blood flows through the glomerulus protein
free plasma filters through the glomerular
capillaries into Bowman’s capsule
• Normally about 20% of the plasma that enters
the glomerulus is filtered
• This process is known as glomerular
filtration (the first step in urine formation)
• 125 ml/min or 180 lit/day of glomerular filtrate
is formed
Tubular Reabsorption
• As the filtrate flows through the tubules
substances of value to the body are returned to
the peritubular capillary plasma
• This selective movement of substances from
inside the tubule (the tubular lumen) into the
blood is called tubular reabsorption
Tubular Secretion
• Tubular secretion is the selective transfer of
substances from the peritubular capillary blood
into the tubular lumen
• Urine formation begins with fitration of large
amounts of protein free fluid from the
glomerular capillaries into Bowman's capsule
• Most substances in the plasma except for
proteins are freely filtered
• As filtered fluid leaves Bowman's capsule and
passes through the tubules it is modified by
reabsorption of water and specific solutes back
into the blood or by secretion of other
substances from the peritubular capillaries into
the tubules
• Glomerular filtration, tubular reabsorption, and
tubular secretion is regulated according to the
needs of the body
• Tubular reabsorption is quantitatively more
important than tubular secretion
• Secretion plays an important role in
determining the amounts of potassium and
hydrogen ions and a few other substances that
are excreted in the urine
• The end products of metabolism such as urea,
creatinine, uric acid, and urates, are poorly
reabsorbed and are excreted in large amounts
in the urine
• Certain foreign substances and drugs are also
poorly reabsorbed but in addition are secreted
from the blood into the tubules so their
excretion rates are high
• Electrolytes such as sodium ions, chloride
ions, and bicarbonate ions are highly
reabsorbed so only small amounts appear in
the urine
• Nutritional substances such as amino acids and
glucose are completely reabsorbed from the
tubules and do not appear in the urine even
though large amounts are filtered by the
glomerular capillaries
Glomerular Capillary Membrane
• It has three major layers:
(1) the endothelium of the capillary
(2) a basement membrane
(3) a layer of epithelial cells (podocytes)
Despite three layers filters several hundred times
as much water and solutes as the usual capillary
membrane
Even with this high rate of filtration the
glomerular capillary membrane normally
prevents filtration of plasma proteins
The high filtration rate across the glomerular
capillary membrane is due to
• Fenestrae in the endothelial cell layer
• The basement membrane consists of a
meshwork of collagen and proteoglycan
fibrillae that have large spaces through which
large amounts of water and small solutes can
filter
• Layer of epithelial cells (podocytes) line the
outer surface of the glomerulus that are not
continuous(slit pores) but have long footlike
processes that encircle the outer surface of the
capillaries
Role of Negative Charges in Preventing
Filtration of Plasma Proteins
• Fenestrations are large but the endothelial cells
have negative charges that hinder the passage
of plasma proteins
• The basement membrane effectively prevents
filtration of plasma proteins because of strong
negative electrical charges associated with the
proteoglycans
• The foot processes are separated by gaps
called slit pores through which the glomerular
filtrate moves but epithelial cells also have
Minimal Change Nephropathy
• The negative charges on the basement
membrane are lost
• As a result of loss of negative charges on the
basement membranes some of the lowmolecular-weight proteins especially albumin
are filtered and appear in the urine
( proteinuria or albuminuria)