Digestive System Overview: The Liver's Role

Digestive System Overview

• The liver is the largest and most important metabolic organ in the body, acting as the body's major biochemical factory.
• It plays a crucial role in the digestive system by secreting bile salts.

Liver Functions

• Digestive function: Secretion of bile salts.
• Metabolic function: Metabolic processing of nutrients, detoxification of body wastes, hormones, and foreign compounds; synthesis of plasma proteins; storage of glycogen, fats, iron, copper, and vitamins; activation of vitamin D; removal of bacteria and worn-out red blood cells. It also excretes cholesterol and bilirubin.

Bile

• Actively secreted by the liver and diverted to the gallbladder between meals.
• Stored and concentrated in the gallbladder.
• Consists of bile salts, cholesterol, lecithin, and bilirubin.
• After a meal, bile enters the duodenum.
• Bile salts are cholesterol derivatives that convert large fat globules into a liquid emulsion.
• Most of the bile salts are reabsorbed into the blood after participating in fat digestion.

Liver Cell Types

• Hepatocytes: Liver cells with highly developed organelles.
• Kupffer cells: Macrophages involved in immunity and removal of bacteria and worn-out red blood cells.

Liver Blood Flow

• Portal Vein: Carries approximately 2/3 of the liver blood supply.
• Hepatic Artery: Carries approximately 1/3 of the liver blood flow and supplies the liver with oxygen (≈3/4 of liver oxygen supply).
• Portal vein drains: Stomach, Small Intestine, Large Intestine, Pancreas, and Spleen
• Facilitates liver function by enabling the flow of blood along rows of hepatocytes. Blood from the portal vein passes along the rows of hepatocytes into the central vein then the hepatic vein (systemic circulation), to the IVC, and then to the R Atrium.
• Liver histology: Hepatocyte plates are 2 layers with bile canaliculus in-between.

Liver Histology: Hepatic Lobules

• Functional units of the liver.
• Composed of hepatocyte plates arranged radially around a central vein, within hepatic sinusoids.

Peribiliary Glands

• Found within the liver, they are serous/mucinous secreting glands that regulate enzyme secretion.

Small Intestine Overview

• The small intestine is the site where most digestion and absorption take place.
• It has three segments: duodenum, jejunum, and ileum.

Small Intestine Motility

• Segmentation: The primary motility method in the small intestine involving ring-like contractions along the length of the intestine. Contracted segments relax and previously relaxed segments contract, mixing chyme with digestive juices.
• Migrating Motility Complex (MMC): Sweeps the small bowel clean between meals. Active during fasting, it repeats roughly every 1.5 hours and involves three phases of contractions: Phase I-Quiet phase (40-60 mins); Phase II-Some contractions (variable intervals); and Phase III-Intense peristaltic contractions from the upper stomach to the terminal ileum, lasting 5-10 mins.

Ileocecal Valve

• A sphincter that prevents bacterial contamination of the small intestine from colonic bacteria. It relaxes with distention on the ileal side and contracts with pressure on the caecal side.

Small Intestine Secretion

• The small intestine secretes a watery solution containing salts and mucus, but not digestive enzymes.
• This watery secretion helps with absorption.
• The small intestine synthesizes enzymes (enteropeptidase, disaccharidases, aminopeptidases) that act within the brush border membrane of epithelial cells.

Small Intestine Digestion

• Pancreatic enzymes continue carbohydrate and protein digestion within the lumen.
• Fat digestion takes place entirely within the small intestine lumen by pancreatic lipase, yielding monoglycerides and fatty acids.
• Proteins are broken down to small peptides and some amino acids, and carbohydrates are digested to simple sugars (monosaccharides).

Small Intestine Digestion-Absorption

• The length of the small bowel, circular folds, villi, and microvilli increase its surface area for digestion and absorption.

Small Intestine: Brush-Border Enzymes

• Brush-border enzymes located on the microvilli complete the digestion of carbohydrates and proteins.
• Membrane-bound enzymes like enteropeptidase, maltase, sucrase-isomaltase, and lactase play key roles.
• For example, lactase digests lactose into glucose and galactose.

Starch Digestion

• Yields two products: maltose and a-limit dextrins.

Imaging Techniques

• Barium contrast studies: X-ray images using barium contrast.
• Magnetic Resonance Enterography (MRI): Uses MRI to image the small bowel.
• Wireless Capsule Endoscopy: A capsule with a camera is swallowed to produce images of the interior of the small bowel.

Small Bowel Jejunal folds

• Jejunal folds have a "stack of coins" appearance in images.
• Superficial ulceration can be a manifestation of Crohn's disease or NSAID mucosal damage.

Explore the essential functions of the liver in the digestive system. Learn about bile secretion, metabolic processing of nutrients, and the liver's role in detoxification and storage. This quiz covers key concepts related to liver functions and digestion.