Digestive System: Ingestion, Stomach, and Liver

Digestive System Overview

  • The main purpose of the digestive system is to process the food that an organism consumes.

  • This processing includes both mechanical and chemical processing of food.

Mechanical Digestion

  • Mechanical digestion begins in the mouth with the act of chewing (mastication).

  • Involves skeletal muscles of the mouth and pharynx to aid in swallowing (deglutition).

  • The swallowing process consists of three phases:

    • Voluntary Phase: Buccal Phase

    • Involuntary Phases: Pharyngeal and Esophageal Phases

  • The muscularis externa of the digestive tract has two layers of muscle tissue (three in the stomach) that contract radially and relax to push food in one direction, a process known as peristalsis.

  • Peristalsis is an involuntary propulsive process that moves food within the digestive tract.

Transport Function of the Esophagus

  • The esophagus is a 25-cm tube extending from the pharynx to the stomach, functioning as a passageway for food.

  • It does not participate in the digestion or absorption of nutrients but secretes mucus to lubricate food transport.

  • The esophagus is comprised of four layers:

    • Mucosa

    • Submucosa

    • Muscularis Externa

    • Adventitia

  • Most of the digestive tract features a muscularis externa made of smooth muscle; however, the upper third of the esophagus consists of skeletal muscle under voluntary control, with the middle part consisting of both types of muscle.

  • The movements of the digestive tract are also controlled by various sphincters, including the gastroesophageal sphincter, pyloric sphincter, and anal sphincters.

Ingestion and Saliva

  • Saliva contains a variety of solutes, including enzymes, that aid in the digestion process.

  • Upon ingestion, food is moistened by saliva.

  • Saliva is secreted by hundreds of minor salivary glands and three pairs of major salivary glands:

    • Parotid Glands

    • Submandibular Glands

    • Sublingual Glands

  • Enzymes in saliva, such as salivary amylase and lingual lipase, begin the breakdown of polysaccharides and fats, respectively.

  • Saliva contains antimicrobial agents, such as lysozyme and immunoglobulin A, that help break down bacterial cell walls.

  • Bicarbonate ions in saliva maintain optimal pH for salivary enzymes, and mucin helps form a gel-like coating that lubricates food.

Stomach Anatomy

  • The stomach is a muscular organ located in the left superior abdomen, functioning mainly in storage and mechanical digestion.

  • It is divided into four main parts:

    • Cardiac Region: Where food enters the stomach

    • Fundus: Superior region of the stomach

    • Body: Largest and central region

    • Pylorus: Funnel-shaped region leading to the small intestine

  • The gastroesophageal sphincter at the junction of the esophagus and stomach prevents reflux of acidic contents.

  • The stomach features gastric folds (rugae) of the mucosa that allow for stretching.

Storage and Mechanical Breakdown of Food

  • The stomach can stretch to hold up to 1 liter of food after a meal but can expand to nearly four times this amount.

  • Food is churned and mixed with gastric juices into a creamy paste known as chyme.

  • The pyloric sphincter regulates the passage of chyme into the small intestine.

Production of Digestive Enzymes in the Stomach

  • The gastric mucosa contains gastric glands that open into numerous gastric pits, and these contain four types of cells:

    • Mucous Cells: Secrete mucus to protect the stomach lining

    • Parietal Cells: Produce hydrochloric acid (HCl), which lowers pH to an average of 1-3 and secretes intrinsic factor for Vitamin B12 absorption

    • Chief Cells: Secrete pepsinogen, activated to pepsin for protein digestion, and gastric lipase for fat digestion

    • Endocrine Cells (G Cells): Release hormones like gastrin into the bloodstream, not contributing to gastric juices directly

Role of the Liver in Digestion

  • The liver is an accessory organ essential to the gastrointestinal system but not part of the alimentary canal.

  • Primary digestive function is bile synthesis.

  • Bile, a yellow-green solution containing bile salts, pigments (bilirubin), cholesterol, and electrolytes, mechanically emulsifies fats into smaller globules (micelles) that lipases can act on.

  • Bile enhances absorption of fat-soluble vitamins A, D, E, and K.

  • Bile is produced from cholesterol and stored in the gallbladder.

  • Upon food entering the small intestine, the hormone cholecystokinin (CCK) signals gallbladder contraction, releasing bile into the duodenum via the common bile duct.

Liver Functions in Metabolism

  • The liver regulates blood glucose concentration, averaging 100 mg/dL.

  • Blood enters the liver via the hepatic portal vein, connecting the digestive tract's blood supply.

  • Processes include:

    • Glycogenesis: Polymerization of glucose to form glycogen when blood sugar is high

    • Glycogenolysis: Breakdown of stored glycogen into glucose monomers when blood sugar is low

    • Gluconeogenesis: Production of glucose from non-carbohydrate sources, such as proteins and fats, during prolonged fasting

  • The liver also plays a vital role in detoxification:

    • Converts ammonia (a toxic byproduct of amino acid metabolism) to urea for excretion by the kidneys

    • Inactivates hormones circulating in the blood which are eliminated by the kidneys

    • Breaks down drugs and alcohol (exogenous compounds) for detoxification.