The efferent arterioles are
blood vessel
Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...
s that are part of the
urinary tract of
organisms
An organism is any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been pr ...
. Efferent (from Latin ex + ferre) means "outgoing", in this case meaning carrying blood out away from the glomerulus. The efferent arterioles form a convergence of the
capillaries
A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the in ...
of the
glomerulus, and carry blood away from the glomerulus that has already been filtered. They play an important role in maintaining the
glomerular filtration rate
Renal functions include maintaining an acid–base balance; regulating fluid balance; regulating sodium, potassium, and other electrolytes; clearance (medicine), clearing toxins; absorption of glucose, amino acids, and other small molecules; reg ...
despite fluctuations in
blood pressure
Blood pressure (BP) is the pressure of Circulatory system, circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term ...
.
In the
mammalian kidney
The mammalian kidneys are a pair of excretory organs of the urinary system of mammals, being functioning Kidney (vertebrates), kidneys in postnatal-to-adult individuals (i. e. Kidney (vertebrates)#Metanephros, metanephric kidneys). The kidneys in ...
, 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
The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. In the adult, it forms a continuous smooth outer zone with a number of projections ( cortical columns) that extend down between the pyramids. I ...
and
renal medulla
The renal medulla (Latin: ''medulla renis'' 'marrow of the kidney') is the innermost part of the kidney. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which ...
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
Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...
when circumstances require. 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
Angiotensin is a peptide hormone that causes vasoconstriction and an increase in blood pressure. It is part of the renin–angiotensin system, which regulates blood pressure. Angiotensin also stimulates the release of aldosterone from the ...
levels are increased due to activation of the
renin–angiotensin–aldosterone system, most of the arteries in the body experience
vasoconstriction
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vesse ...
, 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. However, in a state of very high angiotensin II for a prolonged period of time, the colloid oncotic pressure of the capillaries will increase, counteracting the increased hydrostatic pressure from the efferent constriction. This will decrease the glomerular filtration rate, depending on the level of oncotic increase in the capillaries, resulting in a decreased filtration fraction.
See also
*
Afferent arteriole
Additional images
File:Corpuscule-malpighi.JPG, Malpighian corpuscle.
File:Glomerular Physiology.png, Glomerulus.
File:Renal corpuscle-en.svg, Renal corpuscle
External links
*
* - "Mammal, renal vasculature (EM, Low)"
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Kidney anatomy