Intestinal Secretion

Introduction

Intestinal secretion plays a key role in digestion and absorption by supplying essential enzymes and supporting a large absorptive surface through microvilli. Gut bacteria aid nutrient processing and vitamin synthesis, while proper water balance prevents diarrhea.

Functional Anatomy

• The small intestine consists of the duodenum, jejunum, and ileum.
• It is the longest part of the gastrointestinal tract, measuring about 285 centimeters in adults.
• The duodenum is clearly defined, while the jejunum and ileum are not sharply demarcated.
• It is the principal site for digestion and absorption of nutrients.
• The intestinal secretion is called succus entericus. It contains enzymes required for digestion of carbohydrates, proteins, and lipids.
• Approximately 2 to 3 liters of isotonic fluid are secreted daily.

Intestinal Glands

• The intestinal wall has layers including mucosa, submucosa, muscle layers, and serosa.
• The mucosa is highly specialized with villi and glands for absorption and secretion.
• Crypts of Lieberkühn are tubular glands located at the base of villi.
• These glands secrete intestinal juice and contribute to epithelial cell renewal.
• The mucosal surface contains numerous villi, about 20 to 40 per square millimeter.
• Each villus is lined by columnar epithelial cells and contains capillaries and lymphatic vessels.
• The epithelial surface bears microvilli, forming the brush border.
• This structure significantly increases the absorptive surface area.
• Brunner glands in the duodenum secrete mucus-rich fluid for protection.
• The mucosa forms folds called plicae circulares, which are more prominent in the jejunum.
• These folds further enhance the surface area for absorption.
• The structural adaptations of the intestine ensure efficient digestion and maximal nutrient uptake.

Intestinal Mucosa

• The intestinal mucosa contains specialized structures that enhance absorption and protection. It includes lymphoid aggregates called Peyer patches, which contribute to immune defense.
• The mucosal surface forms folds known as valvulae conniventes, increasing absorptive area.
• Along with villi and microvilli, these structures increase surface area by nearly 600-fold.
• The mucosa contains various specialized cells:
  • Paneth cells are located in the deeper crypts and secrete defensins with antimicrobial action.
  • They also produce regulatory peptides that support intestinal function.

• Undifferentiated stem cells in the crypts continuously generate new epithelial cells. These cells migrate toward the villus tip and replace older cells.
• Enterocytes have a short lifespan of about 2 to 5 days. They are shed regularly, contributing to daily protein loss from the intestine.
• Goblet cells secrete mucins that form a protective mucus layer over the mucosa. This dynamic structure ensures efficient absorption, continuous renewal, and protection against pathogens.

Table 43.1: Actions of Intestinal enzymes.

Enzyme	Primary Action
Enterokinase	Activates trypsinogen to trypsin
Disaccharidases (maltase, sucrase, lactase)	Convert disaccharides into absorbable monosaccharides
α-dextrinase	Breaks dextrins into glucose units
Peptidases	Split peptides into amino acids
Nucleotidases	Degrade nucleotides into bases, sugars, and phosphates

Types of Cells in Villi

• Intestinal villi are finger-like projections that markedly increase the absorptive surface area of the mucosa.
• Each villus contains specialized epithelial cells adapted for absorption and secretion.
• Simple columnar cells are the predominant cells in villi. They possess a brush border formed by microvilli, which enhances nutrient absorption. These cells are responsible for uptake of carbohydrates, proteins, lipids, vitamins, and electrolytes.
• Goblet cells are interspersed between absorptive cells. They secrete mucins, which form a protective mucus layer over the mucosa. This mucus facilitates smooth passage of intestinal contents and protects the epithelium.
• Endocrine cells are scattered within the villi and throughout the gastrointestinal tract. These cells secrete regulatory peptides and amines that control digestion and secretion.
• Different types of endocrine cells include:
  • Enterochromaffin cells, which release serotonin and regulate intestinal motility.
  • Other specialized endocrine cells that produce hormones influencing digestion.

• Endocrine cells belong to a group characterized by uptake and processing of amine precursors. They play a key role in coordinating digestive activities. These cells are more abundant in the proximal small intestine, where absorption and secretion are maximal. Their number decreases toward the colon, where digestive activity is comparatively lower.
• The coordinated function of these cells ensures efficient digestion, absorption, and mucosal protection.

Intestinal Secretion

Composition

• Daily intestinal secretion ranges from 1 to 2 liters. It is isotonic, alkaline, and watery, with a pH of about 8.
• The fluid contains approximately 98.5 percent water and 1.5 percent solids.
• Solids include electrolytes such as sodium, potassium, calcium, magnesium, and anions like bicarbonate and chloride. It also contains digestive enzymes and mucus.
• Enterokinase is located on the brush border and activates pancreatic trypsinogen. This enzyme is released during normal epithelial cell turnover.

Mechanism and Regulation of Secretion

Mechanism

• Electrolytes are secreted through active transport mechanisms.
• Anions move along with cations to maintain electrical neutrality.
• Water follows passively due to osmotic gradients.
• Mucus is secreted by goblet cells, surface epithelial cells, and duodenal glands.
• Mucin forms a gel-like protective layer over the mucosa. This mucus protects against mechanical and chemical injury. It also facilitates smooth movement of intestinal contents and traps microorganisms.
• Secretion from duodenal glands is alkaline, helping neutralize acidic chyme.

Regulation of Secretion

• Intestinal secretion is regulated by hormonal and neural mechanisms.
• Vasoactive intestinal peptide increases fluid and electrolyte secretion.
• Excess secretion due to this hormone may cause diarrhea.
• Parasympathetic stimulation via the vagus nerve enhances secretion.
• Certain toxins can markedly increase intestinal fluid secretion, leading to fluid loss.

Experiment to Study Intestinal Secretion

• The Thiry–Vella loop is used to study intestinal secretion in experimental animals.
• A segment of intestine is isolated and exteriorized through the abdominal wall.

• Both ends open externally, allowing direct collection of secretions.
• Various stimuli are applied to assess changes in rate and composition of intestinal secretion.

Functions of Intestinal Secretion

• Intestinal secretion facilitates proper mixing of chyme, creating a favorable environment for digestion and absorption. It contains enzymes that aid in breakdown of carbohydrates, proteins, and lipids.
• Mucus protects the intestinal epithelium from mechanical and chemical injury. It also traps microorganisms and contributes to local immune defense through immunoglobulins.
• Alkaline secretion from Brunner glands neutralizes acidic chyme entering the duodenum. This prevents mucosal damage and maintains optimal pH for enzymatic activity.
• Intestinal fluid aids in smooth propulsion of chyme along the digestive tract.

Absorption of Water

• Approximately 8.5 liters of fluid enter the gastrointestinal tract daily.
• More than 99.5 percent of this fluid is reabsorbed.
• Only about 100 milliliters of water is excreted in feces.
• Around 75 percent of water absorption occurs in the distal small intestine.
• Efficient water absorption is essential for maintaining fluid and electrolyte balance.

Intestinal Bacteria

• The intestine normally contains a diverse population of commensal bacteria, forming the intestinal flora. These organisms are more abundant in the ileum and colon than in the upper intestine.
• Acidic conditions in the duodenum limit bacterial growth.
• Common organisms include Escherichia coli, Enterobacter species, and Bacteroides species. Other bacteria such as cocci and bacilli are also present.
• Bacteria are continuously lost in feces and replaced by ongoing growth.
• Loss of normal flora during diseases such as diarrhea can impair digestion and absorption.

Functions of Intestinal Flora

• Intestinal bacteria assist in digestion and absorption of nutrients, including minerals and water.
• They synthesize essential vitamins, especially vitamin K and certain vitamin B complex components.
• They produce short-chain fatty acids, which support growth and maintenance of intestinal mucosa.
• Some commensal bacteria exhibit anti-inflammatory effects by modulating immune responses.
• They promote release of regulatory cytokines and help maintain mucosal immunity.
• Bacteria convert bile salts into bile acids, which are reabsorbed and maintain bile acid balance.
• They contribute to the normal brown color of stool through metabolism of bile pigments.
• Pale stool may indicate absence of these transformations due to biliary obstruction.
• Bacterial activity produces gases that form normal flatus, which supports intestinal motility.
• Altered gas production may indicate imbalance in intestinal flora.
• They generate compounds such as indole and skatole, contributing to fecal odor.
• Intestinal bacteria influence lipid metabolism by affecting cholesterol absorption.

Applied Physiology

Malabsorption Syndrome

• Malabsorption syndrome refers to impaired digestion or absorption in the small intestine. It may also result from gastric, hepatic, or pancreatic dysfunction.
• Significant malabsorption usually occurs only when a large portion of the intestine is affected or removed, typically more than 50 percent.
• Common intestinal causes include celiac disease, tropical sprue, Crohn disease, Whipple disease, and radiation-induced injury.
• Early manifestation includes hypoproteinemia due to reduced amino acid absorption.
• Absorption of carbohydrates and lipids is also compromised.
• Defective lipid absorption leads to deficiency of fat-soluble vitamins such as vitamins A, D, E, and K.
• Excess fat in stool results in steatorrhea, characterized by bulky, pale, and foul-smelling feces.

Blind Loop Syndrome

• Blind loop syndrome occurs due to stagnation of intestinal contents. This promotes excessive growth of intestinal bacteria. It is commonly seen after surgical creation of a blind intestinal segment.
• Bacterial overgrowth interferes with nutrient absorption.
• Patients may develop macrocytic anemia due to vitamin B12 deficiency.
• Steatorrhea occurs because bacteria deconjugate bile salts, reducing fat digestion.
• Thus, both conditions highlight the importance of normal intestinal function and controlled bacterial growth.