And secretion occur in the renal tubules glomerular

and secretion occur in the renal tubules.
Glomerular Filtration
Filtration in the glomerulus takes place across the very porous membrane that lies between the
capillaries and the interior of the glomerular capsule. This is mechanical filtration, not energy-
requiring, in which fluids and solutes (such as water, glucose, amino acids, and nitrogenous
wastes) are forced through the membrane by the high hydrostatic blood pressure in the capillary
bed. The capillary pores prevent passage of blood cells and most blood proteins are also
prevented from moving across the filter. The loss of all of the water from the blood plasma is
prevented by retaining the plasma proteins in the capillaries since they maintain the osmotic
pressure of the glomerular blood.

The normal glomerular filtration rate (GFR) is 120-125 ml/min or 180 L/day due to the huge
surface area of glomerular capillaries, the large degree of filtration membrane permeability and
the moderate net filtration pressure. The GFR is increased by an increase in the arterial (and
glomerular) blood pressure in the kidneys and is decreased by an increase in glomerular osmotic
pressure most often caused by dehydration. Maintenance of a relatively constant GFR is
important for adequate reabsorption of water and other needed substances from the filtrate and
filtration of wastes since if flow is very rapid, needed substances cannot be adequately
reabsorbed and if flow is too slow nearly all of it is reabsorbed, including most of the wastes that
are normally disposed of.
There are three mechanisms that regulate renal flow and thereby regulate the GFR: renal
autoregulation, nervous system control, and hormone control.
Under normal conditions, the GFR is controlled by regulation of the diameter of the afferent and
the efferent arterioles allowing the kidney, through its autoregulatory system, to determine its
own rate of blood flow. By means of this autoregulatory system, the kidney can maintain a fairly
constant GFR despite variations in the arterial blood pressure of the rest of the body.
Occasionally it becomes necessary to divert blood to the heart, brain and skeletal muscles and
during these times the renal autoregulatory system may be superseded by nervous system
controls. In this event, a narrowing of the afferent arterioles is caused by sympathetic nerve
fibers and release of epinephrine by the adrenal medulla with a subsequent decrease renal flow
and the GFR.
A third mechanism, one that is hormonal in nature, controls the renal flow. This is the renin-
angiotensin mechanism in which the enzyme renin is released by the juxtaglomerular cells in
response to a drop in the body's blood pressure or direct stimulation of the JG cells by the renal
sympathetic nerves. Renin causes constriction of the afferent and efferent arterioles, a rise in the
body's blood pressure and also stimulates the release of aldosterone thereby causing the renal
tubules to reclaim more sodium ions (and subsequently water) from the filtrate. All of these