In
human anatomy, the hepatic portal system is the system of
vein
Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...
s comprising the
hepatic portal vein
The portal vein or hepatic portal vein (HPV) is a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver. This blood contains nutrients and toxins extracted from digested contents. Approx ...
and its tributaries. It is also called the portal venous system (although it is not the only example of a
portal venous system
In the circulatory system of animals, a portal venous system occurs when a capillary bed pools into another capillary bed through veins, without first going through the heart. Both capillary beds and the blood vessels that connect them are con ...
) and splanchnic veins, which is ''not''
synonymous with ''hepatic portal system'' and is imprecise (as it means ''
viscera
In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...
l veins'' and not necessarily the ''veins of the
abdominal
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso ...
viscera'').
[Splanchnic circulation. Online Medical Dictionary. URL]
http://cancerweb.ncl.ac.uk/cgi-bin/omd?splanchnic+circulation
Accessed on: October 22, 2008.
Structure
Large veins that are considered part of the ''portal venous system'' are the:
*
Hepatic portal vein
The portal vein or hepatic portal vein (HPV) is a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver. This blood contains nutrients and toxins extracted from digested contents. Approx ...
*
Splenic vein
*
Superior mesenteric vein
*
Inferior mesenteric vein
In human anatomy, the inferior mesenteric vein (IMV) is a blood vessel that drains blood from the large intestine. It usually terminates when reaching the splenic vein, which goes on to form the portal vein with the superior mesenteric vein (SMV) ...
The
superior mesenteric vein and the
splenic vein come together to form the actual
hepatic portal vein
The portal vein or hepatic portal vein (HPV) is a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver. This blood contains nutrients and toxins extracted from digested contents. Approx ...
. The
inferior mesenteric vein
In human anatomy, the inferior mesenteric vein (IMV) is a blood vessel that drains blood from the large intestine. It usually terminates when reaching the splenic vein, which goes on to form the portal vein with the superior mesenteric vein (SMV) ...
connects in the majority of people on the
splenic vein, but in some people, it is known to connect on the
portal vein or the
superior mesenteric vein.
Roughly, the portal venous system corresponds to areas supplied by the
celiac trunk
The celiac () artery (also spelled ''coeliac''), also known as the celiac trunk or truncus coeliacus, is the first major branch of the abdominal aorta. It is about 1.25 cm in length. Branching from the aorta at thoracic vertebra 12 (T12) i ...
, the
superior mesenteric artery
In human anatomy, the superior mesenteric artery (SMA) is an artery which arises from the anterior surface of the abdominal aorta, just inferior to the origin of the celiac trunk, and supplies blood to the intestine from the lower part of the du ...
, and the
inferior mesenteric artery.
Function
The portal venous system is responsible for directing blood from parts of the
gastrointestinal tract
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
to the
liver
The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
. Substances absorbed in the small intestine travel first to the liver for processing before continuing to the heart. Not all of the
gastrointestinal tract
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
is part of this system. The system extends from about the lower portion of the
esophagus
The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to ...
to the upper part of the
anal canal
The anal canal is the part that connects the rectum to the anus, located below the level of the pelvic diaphragm. It is located within the anal triangle of the perineum, between the right and left ischioanal fossa. As the final functional seg ...
. It also includes venous drainage from the
spleen
The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes . ,
pancreas
The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an end ...
and
visceral
In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...
fat.
The benefit of first-pass metabolism, whereby substances absorbed from food in the gut pass through the liver before entering the systemic circulation, is to use the liver as a shield (a first line of defense) between (a) the food, its toxins (whatever they may be), and its metabolic intermediates/
metabolite
In biochemistry, a metabolite is an intermediate or end product of metabolism.
The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...
s (such as ammonia) and (b) the rest of the body's tissues, including the brain. The necessity of such a system is demonstrated by what happens when the system breaks down, as seen when advanced hepatic fibrosis in
cirrhosis
Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, and end-stage liver disease, is the impaired liver function caused by the formation of scar tissue known as fibrosis due to damage caused by liver disease. Damage causes tissue rep ...
leads to
hepatic encephalopathy
Hepatic encephalopathy (HE) is an altered level of consciousness as a result of liver failure. Its onset may be gradual or sudden. Other symptoms may include movement problems, changes in mood, or changes in personality. In the advanced stages ...
in the brain owing to the blood being loaded with ammonia and other substances not conducive to brain function.
Blood flow to the liver is unique in that it receives both oxygenated and (partially) deoxygenated blood. As a result, the
partial gas pressure of oxygen (pO
2) and perfusion pressure of portal blood are lower than in other organs of the body. Blood passes from branches of the portal vein through cavities between "plates" of
hepatocytes
A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass.
These cells are involved in:
* Protein synthesis
* Protein storage
* Transformation of carbohydrates
* Synthesis of cholesterol, ...
called
sinusoids
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
. Blood also flows from branches of the
hepatic artery
The common hepatic artery is a short blood vessel that supplies oxygenated blood to the liver, pylorus of the stomach, duodenum, pancreas, and gallbladder.
It arises from the celiac artery and has the following branches:
Additional images
...
and mixes in the sinusoids to supply the hepatocytes with oxygen. This mixture
percolates through the sinusoids and collects in a central vein which drains into the
hepatic vein
In human anatomy, the hepatic veins are the veins that drain venous blood from the liver into the inferior vena cava (as opposed to the hepatic portal vein which conveys blood from the gastrointestinal organs to the liver). There are usually thre ...
. The hepatic vein subsequently drains into the
inferior vena cava
The inferior vena cava is a large vein that carries the deoxygenated blood from the lower and middle body into the right atrium of the heart. It is formed by the joining of the right and the left common iliac veins, usually at the level of the ...
.
The
hepatic artery
The common hepatic artery is a short blood vessel that supplies oxygenated blood to the liver, pylorus of the stomach, duodenum, pancreas, and gallbladder.
It arises from the celiac artery and has the following branches:
Additional images
...
provides 30 to 40% of the oxygen to the liver, while only accounting for 25% of the total liver blood flow. The rest comes from the partially deoxygenated blood from the
portal vein. The liver consumes about 20% of the total body oxygen when at rest. That is why the total liver blood flow is quite high, at about 1 litre a minute and up to two litres a minute. That is on average one fourth of the average
cardiac output at rest.
Clinical significance
Portal hypertension is a condition in which the blood pressure of the portal venous system is too high. It is often the result of
cirrhosis
Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, and end-stage liver disease, is the impaired liver function caused by the formation of scar tissue known as fibrosis due to damage caused by liver disease. Damage causes tissue rep ...
of the liver.
Drug metabolism
Many drugs that are absorbed through the
GI tract
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...
are substantially metabolized by the liver before reaching general circulation. This is known as the
first pass effect. As a consequence, certain drugs can only be taken via certain routes. For example,
nitroglycerin cannot be swallowed because the liver would deactivate the medication, but it can be taken
under the tongue or transdermally (through the skin) and thus is absorbed in a way that bypasses the portal venous system. Inversely,
dextromethorphan
Dextromethorphan (DXM) is a medication most often used as a cough suppressant in over-the-counter cold and cough medicines. It is sold in syrup, tablet, spray, and lozenge forms. In 2022, the FDA approved a formulation of it combined with bu ...
, a cough suppressor, is best taken orally because it needs to be metabolised by the liver into
dextrorphan
Dextrorphan (DXO) is a psychoactive drug of the morphinan class which acts as an antitussive or cough suppressant and dissociative hallucinogen. It is the dextrorotatory enantiomer of racemorphan; the levorotatory enantiomer is levorphanol. D ...
in order to be effective. This latter principle is that of most
prodrugs.
The use of
suppositories is a way to partially bypass the
portal vein: the upper 1/3 of the
rectum
The rectum is the final straight portion of the large intestine in humans and some other mammals, and the Gastrointestinal tract, gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the s ...
is drained into the
portal vein while the lower 2/3 are drained into the
internal iliac vein that goes directly in the
inferior vena cava
The inferior vena cava is a large vein that carries the deoxygenated blood from the lower and middle body into the right atrium of the heart. It is formed by the joining of the right and the left common iliac veins, usually at the level of the ...
(thus bypassing the liver).
See also
*
Systemic venous system
*
Pulmonary venous system
References
{{Veins of the torso
External links
Hepatic portal systemVIRTUAL Liver, an interactive liver anatomy tool (Contains rich media; Flash player and Javascript required)
Veins
Abdomen