Week 4: the liver

We started by looking at varices (dilated veins) in the oesophagus and around the umbilicus (caput medusae). We also mentioned hemorrhoids. Read more about these varices with these links:
Patient UK – esophageal varices
New England Journal of Medicine – caput medusae
MedicineNet – hemorrhoids (piles)
Our aim for the session was to use anatomy to explain the occurrence of these varices in patients with liver disease.
Arterial blood is supplied to the liver by the hepatic artery proper, which branches from the common hepatic artery. These vessels arise from the first anterior branch from the aorta: the celiac trunk. Arterial blood has high levels of oxygen.
Venous blood is also supplied to the liver by the hepatic portal vein. This vein receives the splenic vein and the superior mesenteric vein. As the inferior mesenteric vein drains into the splenic vein, almost all the blood from the gastrointestinal (GI) tract is passed to the liver in this way. This venous blood is low in oxygen, but rich in nutrients absorbed across the walls of the GI tract.
How does all this blood get back to the heart? The liver is wrapped around the inferior vena cava (IVC), a very large vein taking blood back to the heart. The IVC is analogous to the aorta in that the aorta carries blood from the heart to the abdomen, pelvis and lower limbs, whereas the IVC carries blood back from the limbs, pelvis and abdomen to the heart. Blood within the liver is passed directly back to the IVC. In this way, blood passes from the portal “circulation” back to the systemic circulation.
How does this help us understand the varices we saw at the beginning? Well, other than the route through the liver there are a number of other links between the portal and systemic circulations. These links between blood vessels are termed anastomoses (singular: anastomosis). Normally a small flow of blood passes through them, but if a normal route becomes blocked or restricted they could pass a greater flow of blood to a tissue that would otherwise receive none.
If the liver becomes progressively more diseased it can develop cirrhosis, where connective tissue replaces healthy tissue. This can restrict the normal flow of blood through it. If a large volume of blood needs to get from the portal circulation back to the systemic circulation but cannot pass through the liver at the normal rate, how else can it get there? Through anastomoses, and in the case of the liver through portosystemic (or portocaval) anastomoses.
There’s a diagram on Instant Anatomy of these portosystemic anastomoses.
So in the oesophagus, portal blood that should go through the liver instead passes to the oesophageal branches of the gastric veins, through anastomoses to the azygos veins of the posterior thoracic wall, and back to the heart. These veins are required to pass a much greater pressure and volume of blood, and being thin walled will stretch, dilate and become varicose.
In the umbilical region remnants of a connection to one’s mother drain blood from a small area of tissue around the umbilicus to the portal vein and the liver. This area also drains blood via epigastric veins to the systemic circulation. If the flow of blood through the liver is impeded, blood may instead pass out to the veins of the umbilical region, across anastomoses to the epigastric veins and back to the heart. Again these normally small vessels become distended, enlarged, and visible.
Parts of the rectum and anal canal are drained by superior, middle and inferior rectal veins. The superior rectal vein drains blood to the inferior mesenteric vein, which passes to the splenic vein, then the portal vein and then the liver. The middle and inferior rectal veins drain blood through pelvic veins to the iliac veins and the inferior vena cava. So, if blood cannot pass easily through the liver it may pass instead through the anasomoses between the superior, middle and inferior rectal veins to get back to the heart. These veins can become distended and form hemorrhoids.
So that’s how liver disease can cause these varices. We’ll also look at the development of these blood vessels in embryology lectures, explaining how and why these anastomoses are formed.