EFFERENT ARTERIOLE

The efferent arterioles are blood vessels that are part of the urinary tract of organisms. The efferent arterioles form from a convergence of the capillaries of the glomerulus. They play an important role in maintaining the glomerular filtration rate despite fluctuations in blood pressure. In the mammalian kidney they follow two markedly different courses, depending on
the location of the glomeruli from which
they arise. In the mammalian kidney about 15% of
glomeruli lie close to the boundary
between the renal cortex and renal medulla and are known as juxtamedullary glomeruli. The rest are simply undifferentiated cortical glomeruli. In undifferentiated cortical glomeruli The efferent arterioles of the
undifferentiated cortical glomeruli are the
most complex. Promptly on leaving the
glomerulus they break up into capillaries
and become part of a rich plexus of vessels surrounding the cortical portions of the
renal tubules. In juxtamedullary glomeruli The efferent arterioles of the
juxtamedullary glomeruli are much
different. They do break up, but they form
bundles of vessels (arteriolae recti) that cross the outer zone of the medulla to
perfuse the inner zone. Vessels returning from the inner medulla
(venulae recti) intersperse themselves in a highly regular fashion among the
descending arteriolae recti to form a well
organized rete mirabile. This rete is responsible for the osmotic
isolation of the inner medulla from the rest
of the kidney and so permits the excretion
of a hypertonic urine when circumstances require. Interestingly, since the rete also
isolates the inner medulla from gaseous
exchange, any metabolism in this area is anaerobic, and red cells, which would serve no purpose there, are ordinarily
shunted from the arteriolae recti by an
unknown mechanism into the capillary
plexus surrounding the tubules of the outer
zone of the medulla. Blood in this plexus and returning from the
inner medulla finds its way to the renal
vein and the general circulation by
pathways similar to those providing
drainage for the rest of the cortex. Regulation of glomerular filtration rate When angiotensin II levels are increased due to activation of the renin-angiotensin- aldosterone system, most of the arteries in the body experience vasoconstriction, in order to maintain adequate blood pressure.
However, this reduces blood flow to the
kidneys. To compensate, the efferent
arterioles constrict to a greater degree
than the other arteries, in response to
increased levels of angiotensin II. Pressure in glomerular capillaries is therefore
maintained and glomerular filtration rate
remains adequate.