Gallbladder

In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow organ that stores and concentrates bile prior to its release into the small intestine. In humans, it is a pear-shaped structure located beneath the liver, although its size, shape, and position vary considerably across species and individuals. Bile stored in the gallbladder plays a crucial role in the digestion and absorption of dietary fats. Disorders of the gallbladder, particularly gallstones and inflammation, are common and clinically significant.

Anatomical Overview

The human gallbladder is a hollow, grey-blue organ situated in a shallow depression on the inferior surface of the right lobe of the liver. In adults, it typically measures about 7–10 cm in length and 3–4 cm in diameter when fully distended, with a storage capacity of approximately 30–60 mL of bile. It is shaped like a pear, with its narrow end opening into the cystic duct.
Anatomically, the gallbladder is divided into three main regions:

• Fundus: the rounded distal portion that projects towards the abdominal wall
• Body: the largest part, lying in a fossa on the liver surface
• Neck: the tapered proximal region that continues into the cystic duct

The gallbladder fossa is located beneath the junction of hepatic segments IVB and V. The cystic duct joins the common hepatic duct to form the common bile duct, which conveys bile into the duodenum. Near the junction of the neck and cystic duct is a mucosal outpouching known as Hartmann’s pouch, a clinically important site where gallstones frequently lodge.

Lymphatic and Vascular Relations

Lymphatic drainage of the gallbladder initially passes to the cystic lymph node, located between the cystic duct and the common hepatic duct. From there, lymph flows to hepatic lymph nodes and ultimately to the coeliac lymph nodes. The serosal surface of the gallbladder contains blood vessels and lymphatics, while the surface in contact with the liver is covered by connective tissue rather than peritoneum.

Microanatomy

The wall of the gallbladder consists of several distinct layers. The innermost layer is a simple columnar epithelium with a dense brush border of microvilli, resembling intestinal absorptive cells. This epithelial lining facilitates active absorption and concentration of bile. Beneath it lies a thin lamina propria of connective tissue.
Unlike most of the gastrointestinal tract, the gallbladder lacks a muscularis mucosae and submucosa. Instead, a single muscular layer of smooth muscle lies directly beneath the mucosa. The muscle fibres are arranged irregularly in longitudinal, oblique, and transverse directions and contract to expel bile during digestion.
A distinctive histological feature is the presence of Rokitansky–Aschoff sinuses, which are deep invaginations of the mucosa extending into or through the muscular layer. These are commonly associated with adenomyomatosis. Surrounding the muscle layer is connective and adipose tissue, and the outermost layer is either serosa or connective tissue, depending on whether the surface faces the peritoneal cavity or the liver.

Anatomical Variations

The gallbladder exhibits notable anatomical variation. Differences in size, shape, and position are common and usually asymptomatic. Rare anomalies include duplication or triplication of the gallbladder, gallbladders divided by septa, or complete absence of the organ. Positional variants include gallbladders located intrahepatically, on the left side, posterior to the liver, or suspended by a mesentery.
One relatively common and benign variation is the Phrygian cap, an innocuous fold in the fundus of the gallbladder resembling the shape of the Phrygian cap. Such variations generally do not impair gallbladder function but may complicate imaging or surgery.

Embryological Development

The gallbladder develops from an endodermal outpouching of the embryonic foregut. During early embryogenesis, the developing gastrointestinal tract arises from the primitive gut tube formed as the embryo envelops portions of the yolk sac. In the fourth week of development, a ventral outgrowth of the foregut known as the hepatic diverticulum appears and gives rise to the liver and biliary system.
A secondary outpouching, the cystic diverticulum, emerges from the hepatic diverticulum and eventually differentiates into the gallbladder and cystic duct. Concurrent rotation of the stomach and growth of surrounding organs establish the final anatomical position of the gallbladder beneath the liver.

Physiological Function

The primary function of the gallbladder is the storage and concentration of bile, a digestive fluid produced continuously by the liver. Bile flows from the liver through the hepatic ducts and cystic duct into the gallbladder, where it is stored between meals. At any given time, a significant proportion of the body’s bile reserve is held within the gallbladder.
When fatty food enters the duodenum, specialised I cells in the duodenum and jejunum secrete cholecystokinin (CCK). This hormone stimulates rhythmic contraction of the gallbladder and relaxation of the sphincter of Oddi, allowing bile to flow into the common bile duct and then into the duodenum. In the intestine, bile salts emulsify dietary fats, increasing their surface area and facilitating enzymatic digestion and absorption.
During storage, bile is concentrated approximately 3–10 times by active transport of sodium and chloride ions across the gallbladder epithelium. This creates an osmotic gradient that draws water and other electrolytes out of the bile. Bile also serves as a route for excretion of bilirubin, a breakdown product of haemoglobin.

Clinical Significance

Gallstones are the most common gallbladder disorder and form when bile becomes supersaturated, most often with cholesterol or bilirubin. Many gallstones remain asymptomatic, but when symptoms occur, they typically involve severe, colicky pain in the upper right quadrant of the abdomen. Complications include cholecystitis, jaundice, and pancreatitis if stones obstruct the biliary or pancreatic ducts.
Gallstones are most commonly diagnosed using ultrasonography. Management may involve observation, medical dissolution therapy with agents such as ursodeoxycholic acid, or mechanical fragmentation using lithotripsy. Recurrent or complicated gallstone disease is frequently treated by cholecystectomy, the surgical removal of the gallbladder.
Cholecystitis, or inflammation of the gallbladder, is most often caused by obstruction of the cystic duct by gallstones but may also result from infection, ischaemia, or autoimmune processes. Acute cholecystitis presents with pain, fever, and tenderness, while chronic forms may lead to fibrosis and loss of gallbladder function.