Liver, Gallbladder, and Pancreas: Anatomy and Physiology Study Guide

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Liver, Gallbladder, and Pancreas

Overview of Accessory Digestive Organs

The liver, gallbladder, and pancreas are essential accessory organs that support the digestive functions of the small intestine. Each organ has specialized roles in digestion, absorption, and metabolic regulation.

Liver: Produces bile, a fat emulsifier necessary for digestion.
Gallbladder: Stores and releases bile as needed.
Pancreas: Supplies digestive enzymes and bicarbonate to neutralize stomach acid.

The Pancreas

Gross Anatomy and Location

The pancreas is a retroperitoneal organ located deep to the greater curvature of the stomach. Its head is encircled by the duodenum, and its tail abuts the spleen.

Head: Located within the C-shaped curve of the duodenum.
Body and Tail: Extend toward the spleen.

Anatomy of the Pancreas: gross anatomy and cellular organization Gross anatomy of the pancreas

Cellular Organization

The pancreas contains both exocrine and endocrine cells:

Exocrine cells (acini): Produce digestive enzymes and secrete them into ducts.
Endocrine cells (islets): Secrete hormones such as insulin and glucagon.

Diagram of cellular organization of the pancreas Light micrograph of the cellular organization of the pancreas

Exocrine Function and Pancreatic Juice

The pancreas produces 1200–1500 mL of pancreatic juice daily, which is a watery, alkaline solution (pH 8) containing:

Electrolytes: Primarily bicarbonate (HCO3-).
Digestive enzymes:
- Proteases: Digest proteins; secreted in inactive form.
- Amylase: Digests starch.
- Lipase: Digests triglycerides.
- Nucleases: Digest DNA and RNA.

Structure of the enzyme-producing tissue of the pancreas

Activation of Pancreatic Proteases

Proteases are secreted in inactive forms to prevent self-digestion. Activation occurs in the duodenum:

Enteropeptidase: Activates trypsinogen to trypsin.
Trypsin: Activates more trypsinogen, procarboxypeptidase, and chymotrypsinogen.

Activation of pancreatic proteases in the small intestine

Endocrine Function

The endocrine pancreas consists of islet cells that secrete:

Insulin: Lowers blood glucose.
Glucagon: Raises blood glucose.

The Liver

Gross Anatomy

The liver is the largest internal organ (~3 lbs), consisting of four primary lobes: right, left, caudate, and quadrate.

Falciform ligament: Separates right and left lobes; suspends liver from diaphragm and anterior abdominal wall.
Round ligament (ligamentum teres): Remnant of fetal umbilical vein.

Gross anatomy of the liver: lobes and ligaments Transverse section through the superior abdomen showing liver anatomy Anterior surface of the liver showing lobes and ligaments Posterior surface of the liver showing lobes and gallbladder

Blood Flow Through the Hepatic Portal System

The hepatic portal system delivers nutrient-rich blood from the digestive tract to the liver for processing and detoxification. Blood flow through the hepatic portal system

Microscopic Anatomy: Liver Lobules

Liver lobules are hexagonal units composed of plates of hepatocytes. Each lobule contains:

Central vein: Located in the longitudinal axis.
Portal triad: At each corner, consisting of a branch of the hepatic artery, hepatic portal vein, and bile duct.

Histology of the liver: lobule structure and portal triad Diagram of liver structure showing relationships among lobules Diagram of liver structure and cellular components Single liver lobule and its cellular components

Hepatocyte Functions

Hepatocytes are highly metabolic cells with abundant rough and smooth ER, Golgi apparatus, peroxisomes, and mitochondria. Their functions include:

Bile production: ~900 mL per day.
Metabolic regulation: Processing nutrients, storing glucose as glycogen, synthesizing plasma proteins.
Vitamin storage: Fat-soluble vitamins (D, E, A, K).
Detoxification: Converting ammonia to urea.

Microscopic anatomy of the liver: hepatocytes and bile canaliculi

Bile: Composition and Enterohepatic Circulation

Bile is a yellow-green, alkaline solution containing:

Bile salts: Cholesterol derivatives for fat emulsification and absorption.
Bilirubin: Pigment from heme breakdown; converted to stercobilin in intestine (brown color of feces).
Cholesterol, triglycerides, phospholipids, electrolytes.

Enterohepatic Circulation

Bile salts are recycled via the enterohepatic circulation:

Reabsorbed in the ileum.
Returned to the liver via hepatic portal blood.
Re-secreted in newly formed bile.
About 95% of bile salts are recycled.

Enterohepatic circulation of bile salts

The Gallbladder

Anatomy and Function

The gallbladder is a thin-walled muscular sac on the ventral surface of the liver. Its functions include:

Storage and concentration of bile: Absorbs water and ions.
Release of bile: Muscular contractions expel bile via the cystic duct into the bile duct.

Anatomy and physiology of the gallbladder and bile ducts Relationship of the liver, gallbladder, and pancreas to the duodenum Inferior surface of the liver showing gallbladder and bile ducts Sectional view of duodenal ampulla and related structures

Gallbladder Physiology

Bile is secreted continuously by the liver (~1 L/day).
Bile becomes more concentrated in the gallbladder.
Release of cholecystokinin (CCK) triggers contraction of the gallbladder and relaxation of the hepatopancreatic sphincter, ejecting bile into the duodenum.
Bile salts emulsify lipids in the digestive tract.

Radiograph of hepatic ducts, gallbladder, and bile duct Physiology of the gallbladder: bile secretion and emulsification

Homeostatic Imbalance: Gallstones and Jaundice

Gallstones (biliary calculi): Caused by excess cholesterol or insufficient bile salts; can obstruct bile flow and cause pain.
Obstructive jaundice: Blockage leads to accumulation of bile salts and pigments in blood, resulting in yellow skin.
Treatment: Includes drugs, ultrasound (lithotripsy), laser vaporization, or surgery.

Small Intestine

Gross Anatomy and Subdivisions

The small intestine is the major organ of digestion and absorption, measuring 2–4 meters in length.

Duodenum: ~25 cm, curves around the pancreas.
Jejunum: ~2.5 m, attached by mesentery.
Ileum: ~3.6 m, joins the large intestine at the ileocecal valve.

Regions of the small intestine Segments of the small intestine in the abdominopelvic cavity Gross anatomy of the jejunum

Microscopic Anatomy: Structural Modifications for Absorption

The small intestine has several modifications to increase surface area for absorption:

Circular folds: Permanent folds (~1 cm deep) that slow chyme movement.
Villi: Fingerlike projections (~1 mm high) with capillary beds and lacteals for nutrient absorption.
Microvilli: Cytoplasmic extensions forming the brush border, containing enzymes for final digestion.

Circular folds and gross anatomy of the jejunum Structural modifications of the small intestine for absorption Histology of the intestinal wall: villi and glands Layers of the small intestine and organization of the intestinal wall Cellular components of an intestinal gland Circular folds and villi in the small intestine Microvilli and cellular components of the villus Internal structures in a single villus A villus in sectional view

Intestinal Juice

The small intestine secretes 1–2 liters of intestinal juice daily, which is slightly alkaline and isotonic with blood plasma.

Stimulus: Distension or irritation of mucosa, hypertonic or acidic chyme.
Composition: Mostly water and mucus (from duodenal glands and goblet cells).

Digestive Processes in the Small Intestine

Chyme from the stomach contains partially digested carbohydrates and proteins, and undigested fats.
Digestion and absorption of all nutrients and most water occur over 3–6 hours.
Enzymes and substances (bile, bicarbonate, digestive enzymes) are imported from the liver and pancreas.
Brush border enzymes on enterocytes perform final digestion of water-soluble nutrients.

Microvilli of the small intestine forming the brush border

Ileocecal Valve Control

The ileocecal sphincter regulates the passage of chyme into the large intestine:

Gastroileal reflex: Enhances segmentation in the ileum.
Gastrin: Increases ileum motility.
Backward pressure: Closes valve flaps to prevent regurgitation.

Major Duodenal Hormones

Hormonal Regulation of Digestion

Several hormones secreted by the duodenum regulate digestive processes:

Gastrin: Increases stomach motility and gastric acid production.
Secretin: Stimulates pancreatic buffer secretion, increases bile production, reduces gastric motility.
Gastric Inhibitory Peptide (GIP): Inhibits gastric activity, stimulates insulin release, promotes lipid synthesis and glucose use.
Cholecystokinin (CCK): Stimulates pancreatic enzyme secretion, gallbladder contraction, and relaxation of hepatopancreatic sphincter; inhibits gastric activity and reduces hunger.
Vasoactive Intestinal Peptide (VIP): Stimulates intestinal gland secretion, dilates capillaries, inhibits stomach acid production.
Enterocrinin: Stimulates alkaline mucus production.

Secretion and effects of major duodenal hormones

Summary Table: Functions of Liver, Gallbladder, and Pancreas

Organ	Main Function	Key Products
Liver	Bile production, metabolic regulation, detoxification	Bile, plasma proteins, glycogen
Gallbladder	Storage and concentration of bile	Bile (concentrated)
Pancreas	Production of digestive enzymes and hormones	Pancreatic juice (enzymes, bicarbonate), insulin, glucagon

Additional info:

Pancreatic proteases are activated in the duodenum to prevent autodigestion.
Enterohepatic circulation conserves bile salts, reducing the need for new synthesis.
Structural modifications in the small intestine maximize nutrient absorption.