Liver | Applied Anatomy

Applied Anatomy of the Liver

Definition

The liver is the largest gland in the body and it is central to many metabolic functions. It is known as the “metabolic warehouse”.

This diagram of the portal circulation shows the central role of the liver in collecting, filtering, and metabolizing products from the stomach, pancreas, spleen, small bowel and large bowel. Image courtesy of LifeArt Lippincott, Williams & Wilkins All rights reserved

The liver is integral to the digestive system, producing both internal and external secretions. The external secretion, bile, aids in the digestive process, while internal secretions are responsible for the metabolism of both nitrogenous and carbohydrate materials absorbed from the intestine.

As the largest gland in the human body, the liver serves several important functions. It secretes bile in order to chemically alter toxic substance (e.g. converts ammonia to urea), converts glucose to glycogen, and can produce glucose from breaking down certain proteins. The liver also synthesizes triglycerides and cholesterol, breaks down fatty acids and produces plasma proteins necessary for the clotting of blood such as clotting factors I, III, V, VII, IX and XI. Nearly 30% of the blood pumped by the heart passes through the liver in one minute.

One of the unique structural features of the liver is its dual blood supply. It is supplied both by an artery (hepatic artery) and a vein (yes a vein!) – the portal vein. The portal vein (shown in the diagram below) drains the gastrointestinal tract of digested metabolic products and transports the nutrients to the liver for processing.

The Liver Long Ago

4000 – 5000 years ago, the sheep’s liver held godly powers over the Babylonian culture. The Babylonians, and many cultures thereafter, believed that since the liver was the largest organ, it certainly must be the organ of most importance.

Anatomy and Physiology of the Liver: Overview I

It has been said by the wise that “life depends on the liver!” Think twice on this statement. As we previously mentioned, the Babylonian civilization of some 4500 years ago looked upon the liver as a god. In modern biology and medicine we see the liver as the largest gland in the body and being central to many metabolic functions. It is a solid organ divided into two lobes, with the right lobe being larger than the left lobe.

The red arrow points to the middle hepatic vein which is one of the three main hepatic veins. To the right of the middle hepatic vein is the right lobe and to its left is the left lobe. By convention we are describing the right as seen in cross sectional imaging (i.e. right side of patient as we see it from below). Image courtesy of Ashley Davidoff, M.D,

The middle hepatic vein divides the liver into two lobes – right and left lobes. The middle hepatic vein runs parallel and superior to the long axis of the gallbladder.

The cross sectional image of the liver shows the middle hepatic vein which is highlighted in the following image. Image courtesy of Ashley Davidoff, M.D,

This middle hepatic vein runs superior and parallel to the long axis of the gallbladder which is shown in the following images. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Overview II

A good way of defining the left from the right lobes of the liver is to use the gallbladder as a line of demarcation. The gallbladder runs in the same axis as the middle hepatic vein and performs the same structural function in dividing the liver into right and left lobes.

The pear shaped gallbladder can be identified as the low density structure running in the same axis as the middle hepatic vein and performing the same structural function in dividing the liver into right and left lobes. Image courtesy of Ashley Davidoff, M.D.

A line drawn through the long axis of the gallbladder divides the liver into its two lobes. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Position – Normal Frontal View

The liver resides in the upper and right quadrants of the abdominal cavity, nearly occupying the entire right hypochondrium, the greater part of the epigastrium, and not uncommonly extending into the left hypochondrium as far as the mammillary line.

This plain film of the abdomen shows the position of the liver in the abdominal cavity. Note it lies just below the right hemidiaphragm and it occupies almost the entire right upper quadrant. The metallic object in the center of the abdomen is an umbilical ring. Image courtesy of Ashley Davidoff, M.D.

The thick blue line that traverses the green gallbladder represents the position of the middle hepatic vein that divides the liver into right and left lobes. The thinner line represents the position of the falciform ligament which divides the left lobe into a medial rightward segment (IV)and a lateral leftward segment. (II and III) Part of the left lobe usually lies toward the right side. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Position – Normal Transverse View

The left lobe is usually mostly on the right side of the body. Sometimes the lateral segments of the left lobe will cross the midline to lie leftward of the midline. The medial segment, segment IV, usually remains on the right side.

In this cross sectional image the liver is seen in relation to the J shaped stomach just medial, the spleen posteriorly and the colon laterally. It occupies almost the entire right upper quadrant. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Position – Abnormal

Although unusual, some day you may come across a patient with situs inversus, which can make you look twice. In these patients, the position of the organs are reversed, with the liver on the left!

This image shows a contrast enhanced CT of a patient with situs inversus where the position of the internal organs are reversed from right to left and left to right. Notice the liver on the right side of the image and the left side of the body. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Character – Normal

The liver has the consistency of a soft, solid, malleable, organ with a “jello” – like consistency. It is highly vascular giving it a reddish brown color. Due to its size and vascular nature the liver seemed to be at the center of life itself, and it thus attained godly powers over this culture. By virtue of its malleable and vascular character it is easily lacerated with a tendency to hemorrhagic complications.

It has a soft tissue density by CT, (25-40HU – non contrast), is more echogenic than the parenchyma of the kidney by ultrasound, and is marginally brighter than the spleen on T1 weighted images, and darker than the spleen on T2 weighted images.

The malleability of the organ results in easy structural deformity by neighboring structures. In this case the tendinous insertions of the diaphragm impinge on the soft liver and cause ridges on its surface. Even the hyperinflated lung anterolaterally is able to indent the liver. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Character – Normal CT

The following is a cross sectional, portal venous phase CT. In this study, the spleen is more dense than the liver. On a non-contrast CT the liver is slightly denser than the spleen due to the glycogen content of the liver, and the water nature of the spleen.

This cross sectional image shows the liver and spleen in the portal venous phase. The spleen is slightly denser than the liver. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Character – Normal Ultrasound

Image courtesy of Philips Medical Systems

Anatomy and Physiology of the Liver: Character – Normal MRI

This T1-weighted MRI of the liver shows the spleen to be darker than the liver, reflecting the higher water content of the spleen. Image courtesy of Ashley Davidoff, M.D.

This T2-weighted MRI of the liver shows the spleen to be much brighter than the liver reflecting the higher water content of the spleen. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Character – Abnormal – Trauma

The liver is commonly injured with blunt trauma producing a classic “bear claw” abnormality.

The normal homogeneous pattern of the liver has been replaced by lacerations that run along the axis of the blood vessels. Image courtesy of Ashley Davidoff, M.D.

To try and visualize this abnormality, sometimes it is easy to visualize a large bear claw.

Bear Claw Injury to the Liver from Trauma

Anatomy and Physiology of the Liver: Character – Abnormal – Masses

In the following image, a mass can be identified in the posterior aspect of the right lobe of the liver. Notice the difference in character between the mass and the rest of the liver.

The puddling of the contrast in the periphery in the early phases of imaging is characteristic of a benign hemangioma. These are rather soft lesions that conform to the more rigid shape of the liver capsule.
Image courtesy of Ashley Davidoff, M.D.

Additional abnormalities are seen in patients with masses in the liver. In this non-contrast cross sectional CT image the normal homogeneous soft tissue density of the liver is disturbed by a heterogeneous mass with a lower density periphery and central coarse calcification. This mass represents a metastatic carcinoma.

In contrast to the benign hemangioma, this aggressive lesion pushes the capsule and deforms the surface of the liver. This mass represents a calcified metastatic carcinoma.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Size – Normal

In the male, the liver weighs between 1.4 to 1.6 kg, and in the female, between 1.2 to 1.4 kg. Its greatest horizontal measurement is from 20 to 22.5 cm. Vertically, near the right surface, it measures about 15 to 17.5 cm., while its greatest antero-posterior diameter is on a level with the upper end of the right kidney, and measures 10 to 12.5 cm.

In this color overlay the liver is seen in relation to other upper abdominal organs. Note its dominance in the upper abdomen.
Image courtesy of Ashley Davidoff, M.D.

The linear measurements of the liver are not easy to apply in the clinical context. Sometimes the right lobe will reach all the way down to the iliac crest as a normal variant called a Riedel’s lobe. The size and shape of the left lobe is a better indicator of the size of the liver. Using planimetry and density conforming techniques, the volume and hence the size of the liver is more accurately assessed. This method is not yet automated and therefore only performed under special circumstances such as in the screening of candidates for liver transplantation. The normal volume of the liver is between 1200 and 1500 cc’s.

The left lobe has been manually outlined and shaded in this liver donor patient. This process has to be repeated at all levels for the volume of the left lobe to be determined. It is essential therefore to know and understand the boundaries of the liver lobes and segments to accurately outline and measure them.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Size – Abnormal – Acquired

A case of a larger than normal liver.

This image shows a large left lobe of the liver – compare its size to a normal sized left lobe below. Image courtesy of Ashley Davidoff, M.D.

A case of a smaller than normal liver.

This liver has suffered the consequences of cirrhosis, in which liver tissue is replaced by scar tissue resulting in an overall shrinkage of the liver. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Size – Abnormal – Congenital

This image shows a small lateral segment (II and III) of the left lobe. This condition is called congenital hypoplasia of the left lobe, implying that the person was born with this anatomic variant. It has no functional significance.

Congenital hypoplasia of the left lobe. Image courtesy of Ashley Davidoff, M.D.

In this patient, part of the anterior segment (V)of the liver is missing and the colon pokes its head through the missing right lobe segment. This condition is also congenital and has no clinical significance.

Congenital hypoplasia of part of the right lobe – segment V.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Shape – Normal – Variations

We reviewed the following image when we discussed the malleability of the liver, indicating that the tendons of the diaphragm were able to indent the soft liver.

Anatomy and Physiology of the Liver: Shape – Normal – Frontal View

The liver is pyramidal in shape with its base at the left and its apex toward the right. The external surface of the liver is smooth with the apex rounded over, similar to a large mushroom cap. (see diagram below)

The pyramidal and mushroom shape of the liver is exemplified by this drawing. The falciform ligament runs an oblique course between the medial and lateral segments of the liver. At its inferior edge is a rounded ligament called the ligamentum teres. It is the remnant of the umbilical vein that brought maternal and placental blood to the developing fetus and baby.
Image courtesy of LifeArt Lippincott, Williams & Wilkins All rights reserved

Anatomy and Physiology of the Liver: Shape – Normal – Transverse view

In cross sectional imaging, the surface of the liver is interrupted by normal notches and grooves. The most obvious are the grooves and fissures within the porta hepatis and the region of the entry point of the falciform ligament.

The anterior smooth surface of the liver is interrupted by the notch caused by the entrance of the falciform ligament. Opposite this notch is a corresponding “internal” notch. These notches separate the medial (IV) and lateral (II & III) segments of the left lobe. Other internal notches are the interlobar groove which separates the right and left lobes, (IV & V) and the notch which reflects the position of the right hepatic vein. This notch indicates the line of division between the anterior and posterior segments of the right lobe (in this case V & VI).
Image courtesy of Ashley Davidoff, M.D.

In the following image, the thick blue line runs to the interlobar fissure and corresponds to the border between the right and left lobes as well as the expected location of the middle hepatic vein and gallbladder axis. The thin blue line runs between the border of the anterior and posterior segments of the right lobe and corresponds to the expected location of the right hepatic vein.

Of special note: The yellow dot in the falciform ligament represents the obliterated umbilical vein which is a remnant of the embryologic left umbilical vein which transported blood from the mother to the developing fetus. The thick blue vein runs to the interlobar fissure and corresponds to the border between the right and left lobes, as well as the expected location of the middle hepatic vein and gallbladder axis. The thin blue line runs between the border of the anterior and posterior segments of the right lobe and corresponds to the expected location of the right hepatic vein.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Shape – Normal – Variations

We reviewed the following image when we discussed the malleability of the liver, indicating that the tendons of the diaphragm were able to indent the soft liver.

In this instance we see the example to show a variation of the normal smooth, and mushroom shaped surface. This lobular contour is a normal variant. Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Shape – Abnormal

Compare the previous normal variant with the abnormal lobular contour of this liver. There is a distinct lack of uniformity with coarse micronodular and macronodular change. This is an example of severe cirrhosis.

Micronodular disease

This image highlights the abnormal area. Compare this image to the previous normal images to review the normal vs. abnormal contour of the liver.
Image courtesy of Ashley Davidoff, M.D.

The highlighted area in the image below reveals micronodular cirrhosis, a characteristic of alcoholic cirrhosis.

Anatomy and Physiology of the Liver: Segments of the Liver – Frontal View

There have been numerous methods of dividing and naming the parts of the liver. The earliest methods divided the liver into the left lobe, the quadrate lobe, the right lobe and the caudate lobe. Subsequently, the liver divisions were based on the venous anatomy. The right lobe was separated from the left by the middle hepatic vein. The falciform ligament divided the left lobe into the medial segment (closest to the middle hepatic vein) and the lateral segment. The right lobe was divided by the right hepatic vein into an anterior segment and a posterior segment. The next several diagrams outline the current segmental nomenclature which is still based on the distribution of the hepatic veins:

Segment I = caudate lobe
Segments II, III & IV = left lobe
Segments V, VI, VII & VIII = right lobe

This coronal image shows the division of the liver into right and left lobes, the division of the right lobe into its superior and inferior segments and the division of the left by the green falciform ligament into its segments.
Image courtesy of Ashley Davidoff, M.D.

The right lobe is divided at the level of the porta hepatis into superior and inferior parts. Segments VII and VIII are superior segments and V and VI are inferior. The left lobe is divided by the falciform ligament into leftward segments II and III and rightward segment IV. Segment IVa lies above the porta hepatis and segment IVb below.

Anatomy and Physiology of the Liver: Segments of the Liver – Transverse View – Superior Segments

Starting from a cross section of the liver just below the diaphragm, we can identify the superior segments of the liver.

The following cross sectional image is viewed through the superior aspect of the liver with the hepatic venous system in color overlay. The right hepatic vein divides the right lobe into a posterior segment VII and an anterior segment VIII. The left hepatic vein divides the left lobe in the expected location of the falciform ligament into a rightward sub segment IVa and leftward II and III.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Segments of the Liver – Transverse View – Inferior Segments

The following image reveals the cross sectional segmental pattern at the level of the porta hepatis.

Note that the falciform ligament anteriorly divides the left lobe.
Image courtesy of Ashley Davidoff, M.D.

This image demonstrates the royal blue IVC, posterior to the caudate lobe (I), the dark navy blue portal vein anteriorly, the green line and yellow dot indicating the position of the falciform ligament, the thick blue line of the middle hepatic vein and the thin blue line of the right hepatic vein. Since we are seeing the inferior aspect of the liver we are thus seeing segments V and VI of the right lobe, subsegment IVb and segments II and III of the left.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: The Caudate Lobe – Segment I

The caudate lobe (also known as segment I) is positioned medial to the right lobe and posterior to the left lobe. It is bounded inferiorly by the porta hepatis, posteriorly by the inferior vena cava, anteriorly by the portal vein and more superiorly and anteriorly by the ductus venosus. The caudate lobe is a rather curious structure, situated almost as an appendage to the liver. A unique characteristic is that its venous drainage flows directly and independently to the inferior vena cava below the diaphragm. As previously stated, it belongs neither to the left nor the right lobe, and it is considered as an “independent” segment.

This cross sectional image is taken through the more inferior aspect of the caudate lobe (I) demonstrating its snug position between the posterior large inferior vena cava, and anterior smaller portal vein.
Image courtesy of Ashley Davidoff, M.D.

The color overlay of the image outlines the transverse anatomy of the caudate lobe, again demonstrating its position between the royal blue inferior vena cava posteriorly, and the navy blue portal vein anteriorly.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: The Caudate Lobe – Have you seen the woodpecker?

At a slightly higher level, the caudate lobe curls around the IVC giving it a woodpecker appearance.
Image courtesy of Ashley Davidoff, M.D.

This image of the superior aspect of the caudate outlines (in red) its “woodpecker” shape. The yellow lines indicate the position of the ligamentum venosum.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: The Liver Lobule

What is Mickey doing in the bowels of the liver? You will have to read the next few pages to find out. This image shows the spoke wheel pattern of the liver lobule with the central vein, a tributary of the hepatic vein, at its center. Plates of liver cells form the spokes and between the spokes lie the sinusoids – the capillary network of the lobule. At the periphery of the lobule lie 4- 5 groups of portal triads. Each consists of a tributary of the portal vein, (navy) hepatic artery (red) and bile duct (green).
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: The Liver Lobule to Central Vein Connection

This is a venogram of a small branch of the hepatic vein revealing the smallest tributaries. The next image highlights these tributaries.
Image courtesy of Ashley Davidoff, M.D.

This color overlay shows the smallest venules which connect to the central vein of the lobule. The next image inserts a liver lobule to show the connection.
Image courtesy of Ashley Davidoff, M.D.

The central vein of the lobule collects the blood processed by the liver cells and delivers the blood to the venules.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: The Liver Lobule Conglomerate – Power in Numbers

There are thousands of lobules, each with a central vein and each delivering the “goods” to the venules. The venules collectively join to form the hepatic veins, and eventually, merge with the inferior vena cava (IVC). Destination? The heart, from where the metabolic products will be delivered to the rest of the body.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: The Porta Hepatis – Gateway to the Liver

A unique feature of the liver is that it is nourished by a dual blood supply: 75% via the portal vein, and the remainder from the hepatic artery. Blood is drained from the liver by the hepatic veins.

This cross sectional image is taken through the porta hepatis, revealing the posteriorly positioned portal vein and the anteriorly positioned hepatic artery and bile duct. An enlarged portal node is seen just medial to the portal vein.
Image courtesy of Ashley Davidoff, M.D.

The celiac axis coming off the aorta gives rise to the main hepatic artery which lies anterior to the portal vein. The bile duct (green) also lies anteriorly while the navy blue IVC and its tributaries lie posteriorly.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: The Porta Hepatis – Where is Mickey Mouse?

A close-up of the porta hepatis.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Hepatic Artery

The hepatic artery originates from the celiac axis, which is a short thick trunk extending from the the aorta just below the diaphragm. (the other two branches of the celiac are the left gastric and the splenic) The hepatic artery enters the liver at the porta, along with the portal vein and hepatic duct, and divides into two main branches that supply the right and left lobes respectively. The liver depends on the hepatic artery for only 25-30% of its blood flow.

This image shows the origin of the hepatic artery from the celiac axis, and the anterior position of the hepatic artery in relation to the portal vein as it enters the liver. Note also how relatively small the hepatic artery is in relation to the portal vein.
Image courtesy of Ashley Davidoff, M.D.

The celiac artery arises from the aorta between T12 and L1 and gives rise to the hepatic artery. The hepatic artery divides into two main branches each going to a lobe of the liver. Anatomic variations are common.
Image courtesy of Ashley Davidoff, M.D.

The arterial branch pattern will follow the segmental pattern previously described. In this image you can see that the right lobe branches into a superior group (VII and VIII), and an inferior group (V and VI). The left artery gives off a rightward branch to segment IV (of the left lobe), sometimes known as the middle hepatic artery, and two branches to the left (II and III).
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Portal Vein

The portal system includes all the veins, which drain the blood from the abdominal part of the digestive tube, including the pancreas, spleen, and gall-bladder. The blood and other products of the digestive process are conveyed to the liver by the portal vein. The blood enters the sinusoids, which are the unique capillary system of the liver. The end point of the sinusoid is the central venule. Blood is transferred into the hepatic veins, and eventually to the inferior vena cava (IVC). The portal vein is responsible for 75-80% of the blood flow to the liver. The hepatic artery transports oxygenated blood, while the portal vein transports metabolic products to the liver.

Here’s Mickey again! The main portal vein enters the porta hepatis with its companions, the hepatic artery and the common hepatic duct. Together they are known as the portal triad. The main portal vein lies posterior to the hepatic artery and bile duct at the porta hepatis.
Image courtesy of Ashley Davidoff, M.D.

The portal vein divides into a main left and right branch, each supplying a lobe of the liver.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Sinusoids of the Liver

Once in the liver, the portal vein branches into the main left and right branches and after several divisions, end in capillary-like vessels termed sinusoids.

The end point of the portal vein (and the hepatic artery) is the liver lobule and the capillary beds of the sinusoid.
Image courtesy of Ashley Davidoff, M.D.

At the sinusoids, metabolic exchange occurs with the liver cells -the hepatocytes. The products of digestion in the portal vein are exported into the liver cells and processed nutrients are transferred into hepatic venules. The end point of the sinusoids is the central hepatic venule. The venules from each lobule join to form segmental hepatic veins and eventually blood is transferred from the hepatic veins to the inferior vena cava.

This image shows the portal triad at the periphery of the lobule with the portal vein in navy blue. Both the hepatic artery and the portal vein will deliver blood into the sinusoid which will enable the liver cells to be bathed in their contents. Active exchange will occur between the sinusoidal blood and the liver cells. The end point is the central vein shown in royal blue.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Sinusoids to Central Veins – The End of the Blood Line

The hepatic veins originate from the sinusoids and convey blood from the liver to the vena cava. Hepatic veins tend to be open and solitary, allowing them to be distinguished from the branches of the portal vein, which are more or less collapsed and always accompanied by an artery and duct.

A branch of the hepatic vein terminating in a liver lobule.
Image courtesy of Ashley Davidoff, M.D.

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There are a multiplicity of lobules, each with a central vein and each delivering the “goods” to venules, which collectively join to form the hepatic veins and then into the IVC. Destination? The heart, from where it will be distributed to the body wide system.
Image courtesy of Ashley Davidoff, M.D.

The venogram shows a catheter in the middle hepatic vein filling the venules of a subsegment. The destination of the blood is the inferior vena cava, which is the structure filled with contrast above the diaphragm (red color overlay).
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Hepatic Veins to the IVC – Have you seen the rabbit?

This cross sectional ultrasound image of the confluence of the hepatic veins has been likened to the face of a “bunny”.
Image courtesy of Ashley Davidoff, M.D.

This coronal view of the abdominal cavity shows the confluence of the 3 major hepatic veins as they form the IVC at the diaphragm.

This coronal MRI of the abdominal cavity shows the confluence of the 3 major hepatic veins as they form the inferior vena cava (IVC) at the diaphragm.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Bile Ducts

Bile ducts accompany the hepatic artery and the portal vein throughout the liver. Bile, which is created in the hepatocytes, is secreted and collected after passing through the bile capillaries by the bile ducts, which join to form two large ducts that unite to form the hepatic duct. The bile is either carried to the gallbladder by the cystic duct or transported directly into the duodenum by the common bile duct.

The biliary branches follow the pattern as described for the hepatic artery and portal vein. Each lobe of the liver is supplied by a main branch – the right or the left, and then they each branch into the appropriate subsegments.
Image courtesy of Ashley Davidoff, M.D.

This color overlay shows the confluence of the right, left, and middle hepatic bile ducts. Usually the middle duct joins the left duct and as a common left trunk, they join the right. This case is a variant of the normal.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Bile Ducts, Liver Lobule, and Duodenum – What are the connections?

The liver cells are accompanied by bile canaliculi (small bile channels) which are situated between neighboring liver cells. These become confluent at the periphery of the liver lobule and form the bile ductule member of the portal triad.

The color overlay shows the connection between the bile ductule at the periphery of the sinusoid and the branches of the bile ducts.
Image courtesy of Ashley Davidoff, M.D.

The right and left ducts join to form the common bile duct, which usually joins with the pancreatic duct before entering the duodenum.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Bile Ducts – Abnormal – Obstruction

A large filling defect, representing a bile duct stone is noted in the distal common bile duct. Review the last image on the previous page to refresh the anatomy.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Lymphatic Drainage – Sinusoids to Porta

The liver represents the largest single source of lymph in the body, producing nearly 20% of overall total volume. Most hepatic lymphatics leave the liver at the porta hepatis and drain into hepatic nodes along the hepatic artery. Lymph is a clear fluid that is collected from the various organs and tissues of the body, flows through the lymphatic vessels, and eventually rejoins the venous circulation.

The lymphatic fluid is collected by small side channels called the spaces of Disse, which run along and in parallel with the sinusoids. These become confluent and form small lymphatic channels that surround the portal vessels – particularly the arterioles and portal vein.

This image, hopefully a familiar one, shows the schematic relationship of the ductules, in yellow, with the portal triad. The portal triad is therefore not really a triad when accompanied by lymphatics and nerves.
Image courtesy of Ashley Davidoff, M.D.

The lymphatics are too small to visualize and the expected distribution within the capsule and along the portal triads has been intimated in yellow. Portal lymph nodes are colored in as yellow “nodes” in the porta hepatis.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Lymphatic Drainage – Portal Lymph Nodes

Normal lymph nodes are not usually visualized. In this section of the liver at least two small nodes are seen in the porta. Can you see them? A second set is seen close to the crus of the diaphragm. The next image highlights these nodes.
Image courtesy of Ashley Davidoff, M.D.

In this section of the liver the two small nodes have been overlaid in yellow. A second set is seen close to the crus of the diaphragm.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: Lymphatic Drainage – Companion to the Hepatic Artery

This color overlay represents the hepatic artery in red and a schematic imposition of lymphatics in yellow. The lymphatics travel with the hepatic artery culminating in lymph nodes around the main hepatic artery near the porta and at its origin around the celiac axis.
Image courtesy of Ashley Davidoff, M.D.

Lymph nodes along the celiac axis and hepatic artery are not usually visualized and if they are seen in the normal patient, they are between 5 and 7 mm in dimension. This color overlay shows the lymph node distribution around the celiac axis and hepatic artery and the position of the lymphatic duct with the aorta and IVC in the retrocrural space.
Image courtesy of Ashley Davidoff, M.D.

Anatomy and Physiology of the Liver: The Capsule – Wrapping it up

The liver is fully invested by a serous and a fibrous coat. The serous coat derives from the peritoneum, and covers the greater part of the surface of the organ. The fibrous coat lies beneath the serous coat, and covers the entire surface of the organ. It is difficult to see, except in cases where the serous coat is deficient.

This image reveals the two layers of the liver capsule. The inner white layer is the fibrous component and closely adheres to the liver parenchyma. The outer serous coat is reflected at various locations of the liver including the bare area where only the fibrous coat is seen (posterolateral aspect behind the IVC). At the porta hepatis, the two coats form Glisson’s capsule, which surrounds the portal triads.
Image courtesy of Ashley Davidoff, M.D.

Embryology – Where did this all come from?

The transverse septum arises at an embryonic junctional site, where the ectoderm of the amnion meets the endoderm of the yolk sac. The junctional region internally is where the foregut meets the midgut. The mesenchymal structure of the transverse septum provides a support within which both blood vessels and the liver begin to form. The liver develops as the hepatic diverticulum from the endodermal lining of the most distal portion of the foregut during the 3rd and 4th weeks of gestation, and the liver grows rapidly from this point. The transverse septum then differentiates to form the hepatic diverticulum and the hepatic primordium. These two structures together will go on to form different components of the mature liver and gall bladder. The rapidly developing liver forms a visible surface bulge on the embryo directly under the heart bulge. Bile production begins in the 12th week, and excretion of bile into the duodenum by the 16th week. The liver is fully developed by the third trimester.

CT Imaging of the Liver: Overview

Dynamic bolus contrast-enhanced spiral CT can provide high quality images enabling the visualization of anatomic detail. CT provides good spatial and contrast resolution giving an excellent global view of the liver and abdominal cavity. With thinner sections, 3D reconstructions can be obtained providing in-depth information about the portal venous and arterial systems, as well as the liver parenchyma itself. Taking advantage of the liver’s dual blood supply, focal lesions, especially tumors, are well demonstrated with mutli-phase scanning techniques. Accurate segmental localization is key to surgical planning. Since the acquisition of images is operator independent and is not affected by patient habitus, CT can be more consistent and widely applied than other modalities. Normal liver density on CT (25-35 HU) is equal or slightly greater than spleen with or without contrast.

MR Imaging of the Liver: Overview

Although once limited by motion artifacts, the role of MRI in liver imaging has been increasing due to faster imaging techniques. MRI provides great detail about the character of liver tissue, specifically in identifying abnormalities in the soft tissue structure. MRI also allows for image acquisition in the axial as well as coronal and sagittal planes, which can provide better image quality without 3D reconstruction. MRI also provides good contrast in tissues with water and fat. However, MRI does not provide optimal visualization of calcification. Spatial resolution of MRI in the abdomen is generally inferior to CT due to motion and the nature of the abdominal tissues. This is not the case with the brain and the musculoskeletal system where MRI shows superior spatial resolution. On T1-weighted images, a normal liver is of slightly higher signal intensity than the spleen. On T2-weighted images, the liver is slightly less than the spleen.

T1-weighted in the coronal projection showing normal liver, spleen, and kidneys.

T2-weighted in the coronal projection showing normal liver and spleen.

US Imaging of the Liver: Overview

Ultrasound is a good screening technique for patients who have abdominal symptoms and suspected focal or diffuse liver disease. It also has the advantage of easy identification of dilated bile ducts, cysts, or tumors in the liver. Visualization of the gallbladder and porta hepatis with ultrasound is generally superior to other modalities. Accuracy, however, is decreased in obese patients and is operator dependent. In addition, ultrasound allows only certain portions of the liver to be visualized at any given moment and in any given plane due to a limited scanning field. On the other hand, a significant advantage is its real time capability. By using color-flow and spectral doppler techniques, hepatic vessels and tumor vascularity can be assessed dynamically. More recently, contrast agents have been used for improved contrast resolution and characterization of disease. US is also the study of choice for non-invasive imaging of the bile ducts. The normal liver is homogeneous and slightly hyperechoic to the kidney and hypoechoic to the spleen.

Gray scale ultrasound of the normal kidney and liver.
Image courtesy of Philips Healthcare

Doppler ultrasound of the portal vein (red) showing normal hepatopetal flow
Image courtesy of Philips Healthcare

3D reconstruction of a Doppler ultrasound of the normal portal venous system
Images courtesy of Philips Healthcare

Conclusion: Overview

As the largest gland in the body, and second only to the skin as the largest organ in the body, the liver is a fairly easy target for all imaging modalities. It is remarkable, however, how the strength of each modality plays a role in defining anatomy and pathology in any given patient. It is also a continuing source of fascination to identify the differences between each liver, since no two organs are exactly the same. The variations in size, shape, position and character, as well as the multifaceted manifestations of disease keep us interested, focused, and stimulated. The technology continues to advance. By adding contrast and harmonic technology to US studies, multidetector scanning to CT, and advancing the speed and complexity of pulse sequences to MRI, the resolution of structures continue to unfold and improve. Additional imaging modalities, such as positron emission tomography (PET), may also lead to improved characterization, particularly in oncological imaging.