Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Anatomy and Physiology of the Digestive System
Liver

Structure. The Liver, jecor (Greek: hepar), is a parenchymatous organ located in the Abdominal cavity, predominantly in the right hypochondrium. Normally, its lower margin does not extend below the costal arch. It is the largest exocrine gland in The Human Body, weighing approximately 1.5–1.7 kg. The liver consists of two lobes, right and left, separated by the falciform ligament. The right lobe is 3–4 times larger than the left (Fig. 7.14).

The liver has two surfaces—diaphragmatic and visceral—as well as inferior and posterior borders. The diaphragmatic surface faces upward and is dome-shaped. It features the falciform and coronary ligaments, which anchor the organ to the Diaphragm. The falciform ligament lies in the sagittal plane, separating the right lobe from the left. Inferiorly, it merges with the round ligament of the liver, which is a remnant of the obliterated umbilical vein. The coronary ligament lies in the frontal plane, further securing the organ to the diaphragm.

The visceral surface faces downward and backward. It is in contact with The Stomach, duodenum, transverse colon, Gallbladder, and the right Kidney and Adrenal gland. The visceral surface features right and left longitudinal grooves, as well as a transverse groove. The right longitudinal groove houses the gallbladder and the INFERIOR VENA CAVA. The left groove contains the round ligament of The Liver and the ligamentum venosum (the obliterated ductus venosus, which in the fetus connects the umbilical vein to the inferior vena cava). The transverse groove is known as the porta hepatis (hepatic hilum). This is the region where the major Blood Vessels, nerves, and ducts enter and leave the organ. The structures of the porta hepatis include the portal vein, proper hepatic artery, nerves, common hepatic duct, and Lymphatic vessels. Blood is supplied to the liver by the portal vein (which collects blood from the gastrointestinal tract) and the proper hepatic artery (which delivers oxygen-rich blood). Venous blood drains from the liver via 3 to 5 hepatic Veins into the inferior vena cava, which courses through the hepatic parenchyma in the region of the right longitudinal groove.

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Fig. 7.14. Liver, gallbladder, duodenum, and Pancreas:

1 — coronary ligament; 2 — left lobe of the liver; 3 — round ligament of the liver; 4 — common hepatic duct; 5 — common Bile duct; 6 — tail of the pancreas; 7 — pancreatic duct; 8 — body of the pancreas; 9 — ascending part of the duodenum; 10 — HEAD of the pancreas; 11 — horizontal part of the duodenum; 12 — descending part of the duodenum; 13 — superior part of the duodenum; 14 — gallbladder; 15 — right lobe of the liver; 16 — falciform ligament

The diaphragmatic and visceral surfaces are joined by the inferior and posterior borders. The inferior border is sharp and, under certain conditions, can be palpated through the anterior abdominal wall. The posterior border is blunt and bears a depression corresponding to the prominence of THE Vertebral Column.

The liver is enclosed in a thin fibrous capsule known as Glisson's capsule. Externally, the organ is covered by the Peritoneum, except at the site of its fusion with the diaphragm.

The Structural and functional unit of the liver is the hepatic lobule, with a total count reaching 500,000. Each lobule is formed by liver Cells, or hepatocytes, arranged in rows that radiate outward as hepatic cords (plates). Branches of the hepatic artery and portal vein approach the lobule from the periphery. Arterial vessels deliver oxygenated blood to the hepatocytes, while blood flowing from the branches of the portal vein contains nutrients absorbed in the digestive tract. At the periphery of the lobule, the terminal branches of the hepatic artery and portal vein unite to form wide, specialized capillary networks known as sinusoids. A central vein lies in the center of the lobule, collecting blood that has passed through the sinusoids. All central veins converge to form 3 to 5 hepatic veins, through which blood drains into the inferior vena cava. This unique vascular architecture of the liver—where arterial and venous vessels merge within the sinusoids—is referred to as the "hepatic portal system" (or remarkable network). Its formula can be represented as follows:

where A is arteriole; V is venule; K is capillary (sinusoid); V_c is central vein.

Biliary tract and gallbladder. Hepatocytes produce a specialized secretion known as bile. Bile flows through bile canaliculi into interlobular bile ducts, and subsequently into the right and left hepatic ducts (corresponding to the right and left lobes of the liver). These ducts merge to form the common hepatic duct, which exits the liver (see Fig. 7.12). Outside of digestive periods, bile flows from this duct through the cystic duct into the gallbladder, vesica fellea (Greek: cholecystis). The gallbladder lies against the inferior (visceral) surface of the liver and has a capacity of 40–80 ml of fluid. Anatomically, the gallbladder is divided into a fundus, body, and neck, which continues into the cystic duct.

The wall of the gallbladder consists of three layers: mucosal, muscular, and adventitial (outer) coats. The outer coat is covered by the peritoneum, except on the side attached to the liver. Within the gallbladder, bile is concentrated, losing up to 80% of its Water. The union of the common hepatic duct and the cystic duct forms the common bile duct. When food enters the duodenum, this concentrated bile is expelled into the common bile duct via the contraction of the gallbladder's muscular layer.

During Digestion, bile flows directly into the common bile duct, bypassing the gallbladder. The gallbladder thus Functions to store, concentrate, and release bile.

Composition of bile. Daily bile production ranges from 0.5 to 1.0 L. Water accounts for 97.5% of bile, which also contains inorganic ions and organic substances, notably bile acids, Cholesterol, and pigments.

Bile has a brownish-yellow color and a pH of 7.8–8.6. This alkalinity helps neutralize the Hydrochloric acid entering the duodenum from the stomach with the chyme. Consequently, a mildly alkaline environment is established in the intestinal lumen, which is essential for normal digestion. This specific pH level provides optimal conditions for the enzymatic activity of both intestinal and pancreatic juices. The bile acids present in bile facilitate fat emulsification: they surround large clusters of dietary fats, lower surface tension, and break large fat globules down into microscopic droplets. Lipolytic (fat-digesting) Enzymes can only act on emulsified fats; therefore, bile is indispensable for the normal DIGESTION AND ABSORPTION of fats. Fat-soluble Vitamins are absorbed simultaneously with fats. Consequently, impaired fat emulsification and absorption lead to deficiencies in fat-soluble vitamins (A, D, E, K). Bile stimulates intestinal motility and activates Pancreatic and Intestinal enzymes. Most components of bile undergo reabsorption, returning via the bloodstream to the liver to form new portions of bile.

Bile is secreted continuously by hepatocytes, regardless of whether food is present in the intestinal lumen. However, food intake stimulates bile production within 5–10 minutes after a meal. Substances such as secretin and cholecystokinin stimulate bile secretion. Furthermore, cholecystokinin stimulates gallbladder motility and relaxes the sphincters that block bile entry into the duodenum. The parasympathetic Nervous system exerts an activating effect, whereas the sympathetic nervous system exerts an inhibitory one.

Functions of the liver. In addition to bile production, the liver performs a wide array of vital functions. First, nutrients absorbed in the intestine are transported to the liver via the portal vein. Here, dietary Amino Acids are used to synthesize the body's own Proteins. The liver is the primary site for the synthesis of Blood Plasma Proteins and clotting factors. Dietary glucose is stored in the liver as Glycogen, which is mobilized as needed, making the liver a key reservoir for glucose (glycogen), vitamins, and ions. In addition to hepatocytes, the liver contains specialized macrophage cells (Kupffer cells) capable of engulfing and destroying foreign substances and microorganisms. All toxins and poisons absorbed from the intestine are detoxified upon reaching the liver, losing their harmful properties; thus, the liver acts as a central detoxifying organ. The hepatic vasculature can hold up to 1 L of blood, meaning this largest gland of the human body also serves as a blood reservoir. Furthermore, the liver performs an excretory function, eliminating heavy metal salts and metabolic breakdown products of various drugs. The breakdown of Hemoglobin yields bilirubin, which undergoes chemical transformation in hepatocytes and is subsequently excreted in the bile. Bilirubin metabolites (such as stercobilin, which gives stool its characteristic color) function as bile pigments. In the fetus, the liver additionally serves a hematopoietic (blood-forming) function.

Without liver function, a human cannot survive even 24 hours, because toxins continuously arriving from the intestine would suppress all body systems.

Thus, the liver performs the following primary functions:

1) bile production;

2) detoxification of harmful substances;

3) Participation in the METABOLISM of various chemical compounds;

4) excretion of Metabolic waste products and heavy metal salts;

5) storage of glucose in the form of glycogen;

6) storage of vitamins and mineral salts;

7) blood storage;

8) synthesis of blood proteins, including certain proteins of the Blood Coagulation SYSTEM.