Liver

Structure/Morphology

The liver is a convex dome shaped structure that fills much of the upper right portion in the abdomen.

Describing the structure of the liver is complicated two classifications, one using an external anatomic description and a more functional anatomic description (the Couinaud classification).

Externally, it’s made up of two major lobes, the right and left lobes, and two accessory lobes, the caudate and quadrate lobes. Anteriorly, the right and left lobes are separated by the falciform ligament which connects inferiorly to the ligamentum teres.

In the Couinaud classification, the liver consists of eight segments. These segments can be loosely associated with the external anatomic lobes.

- Segment I corresponds to the caudate lobe.

- Segments II and III are associated with the left lobe.

- Part of segment IV consists of the quadrate lobe.

- The remaining segments (V, VI, VII, and VIII) are part of the right lobe, although segment IV is largely part of the left-functional liver, but anatomically associated with the right lobe. However, segments are best defined by their vascular relationship.

Based on vascularization, the liver and its segments are divided into left and right by the middle hepatic vein. Those segments that are functionally part of the right liver drain via the right or middle hepatic veins into the inferior vena cava. The functional left liver drains via the left or middle hepatic vein to the inferior vena cava. This includes the quadrate lobe and portions of the right liver (segment IV). The caudate lobe drains into the inferior vena cava via its own smaller hepatic vein.

Segments V-VIII are supplied with blood by the right hepatic artery and portal vein, while segments II-IV are supplied by the left hepatic artery and portal vein. Segment I is supplied by both the left and right hepatic arteries and portal vein (Standring, 2016).

The basic unit of the liver is the roughly hexagonally arranged lobule which consists of portal triads surrounding hepatocytes and a central vein. Oxygenated blood from hepatic arteries and nutrient-laden blood from the portal vein flows through sinusoids where the blood is filtered as it passes to a central vein. Central veins send blood via progressively larger hepatic veins to the inferior vena cava. Hepatocytes in the lobules produce bile. Bile travels in the opposite direction to blood towards the biliary ducts of the portal triad. Biliary ducts from many lobules coalesce eventually into the right and left hepatic ducts and to the common hepatic duct, sending bile to the gallbladder for storage or to the duodenum during digestion.

The liver is encapsulated by a connective tissue called the fibrous capsule of the liver. This connective tissue penetrates the liver tissue, down to the level of the liver lobule.

Anatomical Relations

The liver is a large wedge-shaped structure found in the abdomen. It fills much of the upper right and middle abdomen, sitting just below the diaphragm. The liver projects to the left midclavicular line, becoming much smaller and ending in a point, forming a triangular appearance from the anterior view.

Inferior to the liver sits the gallbladder between the right lobe and the quadrate lobe. The inferior vena cava runs along the posterior margin of the liver between the right lobe and caudate lobe. Posterior and inferior to the right lobe sits the right kidney. Posterior and inferior to the left lobe sits the stomach with the pylorus below the quadrate lobe. The transverse colon runs along the anterior portion of the inferior surface of the liver.

The anterior, superior, and posterior surfaces of the right liver are dome shaped and lay up against the diaphragm. On the left, it tappers to a point reaching the midclavicular line inferior to the diaphragm.

Depending on age, body position, breathing, and contents of the stomach, the exact position of the liver can change dramatically, particularly along the rostral caudal axis. The size of the liver also varies according to body weight and sex (typically larger in males).

The liver is covered with visceral peritoneum, except at the bare area. Here, the peritoneum covering the diaphragm reflects down onto the liver as the coronary ligament leaving a portion of the liver without a peritoneal covering. The surface of the bare area is instead connected to the diaphragm by loose connective tissue.

Contiguous with the coronary ligament is the falciform ligament. This tissue separates left and right anatomic lobes of the liver and attaches to the anterior abdominal wall.

The lesser omentum runs from the posterior region of the left lobe, behind the bare area inferiorly and to the right, ending at the porta hepatis. It envelops the portal triad, marking the free edge of the lesser omentum (Standring, 2016).

Function

The liver performs many functions including metabolic regulation, synthesis of bile, proteins, hormones, and amino acids, breakdown and detoxification of the blood, and storage of essential nutrients and energy supplies.

Bile, which functions to help break down fats in the small intestine is produced by liver hepatocytes. Bile flows through canaliculi into bile ducts and out of the liver to be stored in the gallbladder or secreted into the duodenum.

The liver regulates glycogen levels by converting glucose to glycogen, storing glycogen, and converting glycogen back to glucose depending on metabolic needs. Gluconeogenesis also occurs where hepatocytes create glucose from sources other than glycogen.

The liver produces lipoproteins and cholesterol, synthesizes amino acids and proteins (e.g., clotting factors and albumin), regulates red blood cell production, and synthesizes several hormones (e.g., insulin-like growth factor 1 and angiotensinogen).

The liver breaks down insulin, bilirubin to form bile, ammonia to urea, and many commonly used drugs.

Besides storage of glycogen, the liver stores vitamins such as Vitamin A, Vitamin D, Vitamin B12, Vitamin K, and essential minerals such as iron (Sircar, 2016).

Arterial Supply

The proper hepatic artery, a branch of the celiac trunk, supplies all oxygenated blood to the liver. At the porta hepatis, the hepatic artery splits into right and left branches. Roughly speaking, the right hepatic artery supplies most of the right lobe of the liver and the caudate lobe. Similarly, the left hepatic artery supplies the quadrate lobe and the left lobe of the liver. Variation in vascular supply to the caudate lobe, quadrate lobe, and segment IV are common. Because the anatomic lobes arn’t related directly to the vascular supply, the territories served by the right and left hepatic arteries are better discussed in the context of liver segments.

As the vasculature penetrates the liver, the hepatic arteries bifurcate until they terminate at the level of the lobule. Here hepatic arterial blood flows through the sinusoids to the hepatic veins.

Portal Venous System

The portal venous system brings partially deoxygenated and nutrient rich blood from the digestive canal to the liver alongside the hepatic arterial supply. Together with the biliary system carrying bile in the opposite direction, these three components of the portal triad bifurcate alongside each other such that each lobule in the liver is typically surrounded by terminal portal triads. Thus, in the porta hepatis, the portal vein splits to form left and right portal veins. These supply roughly the left and right portions of the liver, with the supply to the caudate lobe, the quadrate lobe, and segment IV being variable.

As with the hepatic arteries, the portal veins bifurcate until they terminate at the level of the lobule where portal blood drains through sinusoids to the hepatic veins.

Venous Drainage

The venous drainage of the liver refers to the blood flowing from the hepatic veins into the inferior vena cava, not the portal vein which sends blood to the liver.

At the level of the lobule, blood flows from the sinusoids and into central hepatic veins. The sinusoids merge and grow until three major veins have formed.

The right hepatic vein drains the blood from segments V, VI, VII, and VIII into the inferior vena cava. The left hepatic vein drains blood from segments II, III, and IV. The middle hepatic vein drains blood from segments IV, V, and VIII. Segment I, the caudate lobe, typically drains via its own small hepatic vein directly into the inferior vena cava.

Innervation

Innervation of the liver includes the autonomic nervous system (sympathetic and parasympathetic) and extrinsic sensory innervation (visceral afferents) (Standring, 2016).

The vagus nerve (CN X) provides parasympathetic innervation to the liver via both the anterior and posterior vagal trunks and the hepatic plexus.

The liver receives sympathetic innervation from the hepatic plexus. The hepatic plexus receives postganglionic sympathetic fibers from the celiac and superior mesenteric plexuses (Standring, 2016).

Autonomic innervation is largely related to vascular and sinusoid constriction (sympathetic) or relaxation (parasympathetic).

Visceral afferent nerves from the liver ascend likely along the vagus nerve or the thoracic splanchnic nerves. Visceral afferents relay information relating to homeostatic conditions to the central nervous system (Jensen, Alpini and Glaser, 2013).

Lymphatic Drainage

Lymphatic drainage of the liver occurs in several different directions, largely dependent on anterior versus posterior and superficial versus deep origin.

The superficial lymph vessels on the anterior surface drain towards the porta hepatis to the hepatic lymph nodes, and on to the celiac lymph nodes and cisterna chyli. Superficial lymph vessels on the posterior surface of the liver drain to two separate regions. They either drain towards the bare area of the liver and adjacent phrenic nodes, or into deeper tissues to follow hepatic veins and the inferior vena cava to the thorax and the posterior mediastinal nodes. From there, lymph drains into the thoracic lymphatic duct.

Lymph originating deep within the liver drains to perisinusoidal spaces that parallel the intralobular portal triads. The deep lymphatics from the anterior and inferior regions of the liver will drain towards the porta hepatis, the hepatic lymph nodes, the celiac lymph nodes, and the cisterna chyli. Lymph from deep tissues in the superior and posterior liver drains instead along hepatic veins to the inferior vena cava, posterior mediastinal nodes, and the thoracic lymphatic duct.

Additional routes of drainage include:

- lymph from the left posterior liver can drain to left gastric lymph nodes;

- lymph from near the falciform ligament drains to parasternal lymph nodes;

- lymph from around the round ligament of the liver drains towards nodes of the umbilicus and anterior abdominal wall (Moore, Dalley and Agur, 2013).

List of Clinical Correlates

- Cirrhosis

- Hepatitis

- Fatty liver disease

- Cancer

- Jaundice

- Polycystic liver disease

- Portal hypertension

- Budd-Chiari syndrome

- Hepatic encephalopathy

- Hemochromatosis

References

Jensen, K. J., Alpini, G. and Glaser, S. (2013) 'Hepatic nervous system and neurobiology of the liver', Comprehensive Physiology, 3(2), pp. 655-665.

Sircar, S. (2016) Principles of Medical Physiology. 2nd edn.: Thieme.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series 41 edn.: Elsevier Limited.