Venous Drainage of Small and Large Intestines
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| VEINS OF SMALL INTESTINE |
The veins that drain blood from the small and large intestines largely parallel the arteries that supply each organ and share the same names. However, because the veins eventually drain to the hepatic portal vein and liver, there are some notable departures from the arterial scheme. The superior mesenteric vein drains the midgut organs and receives blood from the inferior pancreaticoduodenal, jejunal and ileal, ileocolic, and right and middle colic veins. These veins run parallel to the concordantly named arteries as they leave the superior mesenteric artery, although blood is flowing in the opposite direction from blood in the arteries. Because of its position near the superior mesenteric vein and the fact that there is no celiac vein, the right gastroomental vein drains to the right side of the superior mesenteric vein shortly before the latter drains into the hepatic portal vein. The artery of the same name is a branch of the gastroduodenal artery, which branches from the common hepatic artery and celiac trunk.
The jejunal and ileal veins conform in number and
appearance of their arcades and straight arteries with those of their
respective arteries. Lying, as a rule, to the right of the arteries, the veins
from the small intestine extend from the duodenojejunal junction to the close
vicinity of the ileocecal junction, where the anterior and posterior
cecal veins and the appendicular veins unite to form the ileocolic
vein. The first or first two jejunal veins frequently receive, either via a
common trunk or as a separate vessel, an inferior pancreaticoduodenal vein running
alongside the corresponding arteries. The venous drainage of the first part of
the jejunum is often (again, in accord with the varying origins of the first
jejunal arteries) not into the superior mesenteric vein but rather into either
the anterior or posterior pancreaticoduodenal arcade. The venous
pancreaticoduodenal arcades are fashioned in the same way as the arcades of the
corresponding arteries. The posterior pancreaticoduodenal arcade, lying over
the arterial arcade, is covered by a thin layer of connective tissue, composed
of remnants of the fetal dorsal mesoduodenum, which may be readily seen during
surgery when one is mobilizing the duodenum and the head of the pancreas.
Around the head of the pancreas, two venous arcades are formed (similar to the situation
with the arteries), one anteriorly by the anterior superior and anterior
inferior pancreaticoduodenal veins and the other posteriorly by the posterior
superior and posterior inferior pancreaticoduodenal veins. Both
inferior pancreaticoduodenal veins (the anterior and posterior veins) empty
predominantly into the first and second jejunal veins (70%) or into the
superior mesenteric vein (30%), either separately or via a common trunk. Deviating from the
arterial arrangement, the posterior pancreaticoduodenal vein joins the portal
vein directly behind the head of the pancreas, its entry point lying shortly
ahead of that of the left gastric vein. The anterior superior
pancreaticoduodenal vein joins the right gastroepiploic vein, which,
after passing behind the first part of the duodenum, enters into the superior
mesenteric vein at the pancreatic notch, shortly before the latter empties into
the hepatic portal vein formed
by the union of the superior mesenteric and splenic veins.
Starting in the region of the terminal ileum, the superior
mesenteric vein first follows an oblique course and then a straight superior
course, lying to the right of and somewhat anterior to the accompanying artery.
In this way, both vessels describe a curve, with the convexity to the left.
Both also cross anterior to the third portion of the duodenum, a fact worth
remembering in cases of duodenal
obstruction due to compression by the vessels, perhaps caused by the excessive
weight of a neoplastic growth or weakness of the anterior abdominal wall.
The inferior mesenteric vein starts as a continuation of the
superior rectal vein, which brings blood from the rectum and superior part of
the anal canal. During its upward course, it receives venous blood from the rectosigmoid,
sigmoid, and left colic veins, which drain the sigmoid colon and
descending colon. All these tributaries follow closely the corresponding
arteries, lying mostly to their left. Their anastomosing and arcade formations
are the same as those described for the respective arteries. However, the main
trunk of the inferior mesenteric artery lies to the right, where it branches
from the abdominal aorta. Instead of paralleling it, the inferior mesenteric
vein continues superiorly as it receives blood from the left colic and upper
sigmoid veins, separating from the artery. The vein ascends anterior to the
left psoas muscles, just to the left of the fourth portion of the duodenum. It
continues behind the body of the pancreas to enter most frequently (in 38% of
observed cases) the splenic vein. The splenic vein, in turn, combines
with the superior mesenteric vein (at times, 3 to 3.5 cm from the union of the
inferior mesenteric and splenic veins). Sometimes (29%), the inferior
mesenteric vein enters the superior mesenteric vein, and at other times (32%),
it joins the superior mesenteric vein and the splenic vein at their junction.
In a few instances, a second inferior mesenteric vein has been found.
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| VEINS OF LARGE INTESTINE |
The splenic vein emerges from the hilus of the spleen as several
fanlike veins that converge into a single large vessel. It has an average
length of 15 cm and is never, unlike its accompanying artery, tortuous or
coiled. It demonstrates a great number of different divisional pat- terns,
which may include the short gastric veins and a superior polar splenic vein.
The portal vein and, especially, the variations of its
tributaries are extremely important when considering a portocaval shunt for
redirecting the portal blood flow in order to relieve or ameliorate the
consequences of portal hypertension. It seems, therefore, appropriate to
discuss in this volume variations that particularly involve the venous drainage
of the intestine in which the superior or inferior mesenteric veins
participate. The superior pancreaticoduodenal vein is sometimes (38%)
only a single vessel but more frequently (50%) is duplicated, with one branch
terminating in the portal vein and the other in the superior mesenteric vein.
The gastroomental veins,
corresponding to arteries of the same name, which are branches to the celiac
arterial trunk, usually terminate (in 83% of cases) in the superior mesenteric
vein but will, in some instances, join either the end of the splenic vein or
the first part of the portal vein. The left gastric vein, also
corresponding to an artery branching from the celiac trunk, can join the rest
of the portal circulation at the superior aspect of the union of the splenic
and superior mesenteric veins but has been found to join the portal vein
directly in some
instances, as well as the splenic vein distal to the just-mentioned junction.
The right gastric vein, which is usually a small vein, typically
terminates in the portal vein within 3 cm of its division into right and left
branches, but it may also enter the base of the superior mesenteric vein or,
far less frequently, the proximal segment of the right gastroomental vein or
the inferior pancreaticoduodenal vein.
The veins serving the rectum and anal canal are the unpaired superior
rectal veins, as well as the right and left middle rectal veins and
the right and left inferior rectal veins. These vessels follow
the same course as the arteries of the same name, but they return the blood
into two different systems. The superior rectal vein drains blood into the
portal system via the inferior mesenteric vein, whereas the middle and inferior
rectal veins drain into the internal iliac vein and then the common
iliac veins before entering the inferior vena cava. Blood in the inferior
rectal veins follows a similar course, first draining to the internal
pudendal vein before entering the internal iliac veins.
The contributions to the rectal veins begin in three venous
plexuses situated in the walls of the rectoanal canal. The lowest of these
plexuses, the external rectal plexus, lies in the perianal space, in the
subcutaneous tissue surrounding the lower anal canal near the external opening
of the anus. The internal rectal plexus is located in the submucosal
space of the rectum superior to the pectinate line. These two plexuses are
sometimes collectively referred to as the submucosal plexus or the superior
and inferior submucosal plexuses. The third venous plexus surrounds
the muscular wall of the rectum below its peritoneal reflection and is called
the perimuscular rectal plexus, though some authors refer to it as the
external rectal plexus, a term that leads to confusion with the first of the
three plexuses described above. The perimuscular rectal plexus withdraws blood
chiefly from the muscular wall of the rectum and evacuates the upper portion
into the superior rectal vein, although the chief route of drainage of the
perimuscular plexus is to the middle rectal veins.
The internal and external rectal plexuses serve the mucosal,
submucosal, and perianal tissues. The former encompasses the rectal
circumference completely, but the greatest aggregation of small and large veins
is in the rectal columns (of Morgagni). Dilatation of the internal rectal
plexus results in internal hemorrhoids, and dilatation of the external rectal
plexus or thrombosis of its vessels results in external hemorrhoids. The two
plexuses, the internal and external rectal plexuses, are separated by the
internal anal sphincter as well as the dense tissue of the pecten, but they
communicate with each other through these tissues by slender vessels; the
vessels increase in size and number with age and are also more voluminous in
the presence of hemorrhoids.
These connections between the external and internal rectal plexuses
as well as the perimuscular plexuses constitute anastomoses between the
inferior and superior veins and between the caval and portal venous systems.
The significance of this situation is enhanced by the fact that the inferior
and middle rectal veins and their collecting
vessels, the internal pudendal veins, have valves, whereas the superior rectal
vein is devoid of such valves, so that when the pressure in the portal vein
rises, perhaps owing to portal hypertension, the circulation in the superior
rectal vein may be reversed and portal blood may traverse the rectal plexuses
and be carried away by the inferior rectal vein. This shunts portal blood via
the internal iliac vein to the caval system. When this collateral venous
circulation develops, the increased blood volume and pressure in the vessels dilate them to the extent that
internal and/or external hemorrhoids may result.
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| VEINS OF RECTUM AND ANAL CANAL: FEMALE |
In the absence of portal hypertension, spasms of the anal sphincter
may also cause external hemorrhoids, because they may shut off the outflow of
blood to the inferior rectal veins. Internal hemorrhoids, on the other hand,
may develop when alterations (dilatation as well as constriction) occur within
the apertures of the rectal wall throug which branches of the internal rectal plexus pass.
