PSL201Y1 Lecture Notes - Arteriole, Negative Feedback, Macula Densa
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Filtration is the first step in urine formation. Filtration involves transfer
of fluid from the blood into the renal tubule. Filtration is a relatively
non specific process. It creates a filtrate much like that of the
interstitial fluid or plasma except the plasma does not have plasma
proteins. Therefore filtrate under normal circumstances consist of
water and dissolved solutes. Filtration will not allow plasma proteins
and blood cells from being filtered. Therefore, at the end of filtration
we have a protein free plasma called the glomerular filtrate which is
filtered at a phenomenal rate of 125 mL/min or 180L per day.
There are two key players in filtration.
Structure of Glomerulus
There are three layers through which substances need to pass through
in the process of filtration. Fluid leaving the capillaries need to pass
through capillary pores or fenestra then through the basement
membrane and finally through the filtration slits. These three layers
form the filtration barrier or glonerular membrane.
Glomerular capillary wall
The glomerular capillary wall is made up of endothelial cells which are
perforated by pores called the fenestra. These fenestra are large
enough to allow fluid and small solutes to pass through them but are
small enough to exclude blood cells and plasma proteins from passing
through them. Glomerular fenestra are very permeable, more than the
fenestra in skeletal muscle capillaries.
The basement membrane is acellular and separates the capillary
endothelium from the epithelial lining of the Bowman’s capsule. The
basement membrane is composed of collagen which provides the
structural strength and glycoproteins which discourage protein
filtration because of their negative charge.
These epithelial cells have specialized extensions of food processes
thus giving them the name podocytes. As fluid moves out of the
glomerular capillaries, it passes through the gaps in the podocytes
called slit pores. The size of filtration slits can be varied and are
regulated by slit diaphragms.
To accomplish glomerular filtration, a force must be present to drive
the fluid forward. No active mechanism or local energy is spent or is
involved in moving the fluid forward.
The forces that drive the movement of fluid in and out of the
capillaries are called the Starling forces. There are four starling forces
involved in glomerular filtration.
Glomerular capillary hydraulic pressure (Pgc)
The pressure is the fluid pressure that is exerted by blood within the
glomerular capillaries. These are ultimately dependent on the
contraction of the heart or the force with which the heart contracts and
the resistance to blood or the blood flow that is offered by the afferent
and efferent arterioles. The Glomerular capillary hydraulic pressure is
the major force that is responsible for filtration. This is about 60 mm