Portal Vein

Veins are the blood vessels that help to carry blood towards the guts. Most of the veins carry deoxygenated blood from the tissues to the heart, except some of the veins are the pulmonary and umbilical veins. Both of these veins carry oxygenated blood to the guts from the heart. Thus in contrast to these veins, other arteries carry blood far away from the guts. Let us define a hepatic portal vein.

The portal vein is known as the hepatic portal vein (HPV), it is a type of blood vessel that carries blood from the alimentary canal, gallbladder, pancreas, and spleen and transfers to the liver. This blood contains nutrients and toxins that are extracted from the digested contents. Approximately 75% of total liver blood flow is through the hepatic portal vein, where the rest of the blood comes from the arteria hepatica proper. In the hepatic veins, the blood leaves the liver to the guts.

The hepatic portal vein is not a true vein, because it transports blood to the capillary beds within the liver and indirectly to the heart. For the hepatic portal vein branches, the portal vein is the important component; it is one of only two portal venous systems that are present in the body along with the hypophyseal vascular system which is being the opposite to the system.

Portal Vein Anatomy

Measuring approximately eight cm (three inches) in adults, the hepatic portal vein is found within the right upper quadrant of the abdomen, which originates behind the neck of the pancreas. The principal hepatic portal vein tributaries to the portal vein are the lienal vein, along with blood from the stomach, the gastrocolic omentum it is a curtain of membrane and fat that hangs down over the intestines, the pancreas, the massive intestine, and therefore the spleen. The superior vena mesenterica, with the blood from the tiny intestine and a part of the massive intestine, the pyloric veins, with blood from the stomach, and therefore the cystic veins, with blood from the gallbladder.

In most individuals, portal vein formation is found by the union of the superior mesenteric vein and the splenic vein. For this reason, the hepatic portal vein is sometimes also known as the splenic-mesenteric confluence. Also the hepatic portal vein sometimes directly communicates with the inferior vena mesenterica, although this is often highly variable. Other tributaries of the hepatic portal vein include the cystic and the right and left gastric veins, and also para-umbilical vein and prepyloric vein.

Before reaching the liver, the hepatic portal vein divides into the right portal vein and the left portal vein. It further forms smaller venous branches and ultimately forms portal venules. Each portal venule is found present along the side of the hepatic arteriole and thus these two vessels combined form the vascular components of the portal triad. These vessels go into the hepatic sinusoids that are ultimately empty in order to supply the blood to the liver.

1. Portacaval Anastomosis: The portal venous system has several anastomoses that are present within the systemic venous system. In the case of malignant hypertension, these anastomosis may become dilated, engorged, or varicose and thus subsequently ruptures.

2. Accessory Hepatic Portal Veins: Accessory hepatic portal veins are those veins without joining the hepatic portal vein they drain directly into the liver. These include the paraumbilical veins also like veins of the ligament, lesser omentum, and the veins that are draining from the gallbladder wall.

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Venous Drainage of the Stomach

There are two venous systems that drain abdominal structures they are:

1. Portal Venous System: The tributaries of this venous system includes right and left gastric veins, paraumbilical veins, and cystic veins. Where the right and left gastric veins are responsible for the venous drainage of the stomach. 

2. Systemic Venous System: It consists of the inferior vena cava that is responsible for the drainage of the following structures they are, lumbar veins, renal veins, common iliac veins, right testicular or ovarian vein, right suprarenal vein, inferior phrenic veins, and hepatic veins.

The vascular system transports blood to the liver for processing, while the systemic venous system returns blood to the proper atrium of the guts. The vascular system carries blood that is rich in nutrients that are extracted from food to the liver for processing. The major vessel of the vascular system is the hepatic portal vein. It is the purpose of convergence for the venous drainage of the spleen, pancreas, gallbladder, and therefore the abdominal part of the alimentary canal. 

The hepatic portal vein is typically formed by the confluence of the superior mesenteric, splenic veins, inferior mesenteric, left gastric veins, right gastric veins, and therefore the venae pancreatica. The hepatic portal vein is made by the union of the vena lienalis and therefore the superior vena mesenterica, posterior to the neck of the pancreas, at the extent of L2.

As it ascends towards the liver, the hepatic portal vein passes posteriorly to the superior neighbourhood of the duodenum and thus the common bile duct. Before entering the liver, immediately the hepatic portal vein divides into the right branch and left branches where these veins then separately enter the parenchyma of the liver.

Portal Vein Function

The hepatic portal vein and hepatic arteries combined involve the formation of the dual blood supply of the liver. Approximately about 75% of hepatic blood flow is derived directly from the portal vein, while the remaining is derived from the hepatic arteries. Unlike most veins, the hepatic portal vein doesn't drain into the guts. This portal vein is just a part of a portal venous system but doesn’t involve the drainage into the guts that help to deliver blood into another capillary system called the hepatic sinusoids of the liver.

In blood is carried from the alimentary canal to the liver, the hepatic portal vein is involved in the perforation of two tasks: it supplies the liver with metabolic substrates and it ensures that substances ingested are first processed by the liver before reaching the circulation. This is related to two things they are: 

• First, possible toxins which will be ingested are often detoxified by the hepatocytes before they're released into the circulation.

• Second, the liver is the first organ to soak up nutrients just taken in by the intestines. When these nutrients are drained into the liver sinusoids, after that the blood from the liver is drained by the vena hepatica.

Clinical Significance - Hepatic Portal Vein

1. Portal Hypertension: Increased vital signs within the hepatic portal vein, called malignant hypertension, may be a major complication of the disease, most ordinarily cirrhosis. A dilated hepatic portal vein that has a diameter of greater than 13 or 15 mm, may be a sign of malignant hypertension, with a sensitivity estimated at 12.5% or 40%. The normal range of peak systolic velocity that is found on Doppler ultrasonography from the foremost hepatic portal vein ranges normally in between the 20 cm/s and 40 cm/s. A slow velocity of less than 16 cm/s in addition to dilatation in the MPV is diagnostic of portal hypertension. Clinical signs of portal hypertension include some chronic liver diseases such as esophageal varices, ascites, caput medusae, spider nevi, and palmar erythema.

2. Pulsatility: The pulsatility of the portal vein can be measured by doppler ultrasonography. The cirrhosis can cause an increased pulsatility, as well as the increased right atrial pressure which in turn causes the heart failure of the right portion of tricuspid regurgitation. Portal vein pulsatility can be quantified by pulsatility indices that are denoted by PI.

3. Infection: The infection that is caused to the portal vein is pylephlebitis, usually, this infection arises from an infectious intra-abdominal process such as diverticulitis.

4. Portal Venous Gas: Hepatic portal venous gas is a rare finding that is found on radiological exams. This gas is allowed to enter the portal venous system. It is most commonly caused by intestinal ischemia, but sometimes it is also associated with colon cancer.

Difference Between Hepatic Vein and Portal Vein

The easiest way to differentiate in between the hepatic and portal vein is to find the way where these veins are leading to:

• Most of the hepatic veins travel towards the diaphragm since they are involved in the emptying of the blood from the liver and moves into the IVC (Inferior vena cava). When they get closer or closer to the IVC these veins divide each other to form three larger veins such as left, middle, and right hepatic veins.

• With the help of porta hepatis, the portal veins enter the liver. The main portal vein is formed by the confluence of the three smaller veins such as SMV (Superior mesenteric vein), IMV (Inferior mesenteric vein), and spleen

Conclusion

The blood is carried to the liver from the gastrointestinal tract with the help of the venous system. Where most of the blood that is carried to the liver is sent through the portal vein. The substances that are absorbed in the small intestine are first travelled through the portal vein to the liver for the purpose of processing before entering the heart. But it does not include all the blood vessels that are present in the way instead it has only selected vessels to perform this function.