Pylorus

The pylorus is the distal region of the stomach that acts as a junction between the stomach and the duodenum of the small intestine.^[3] It regulates the passage of partially digested food (chyme) from the stomach into the duodenum. The pylorus consists of two parts: the pyloric antrum, which connects to the body of the stomach, and the pyloric canal, which leads to the pyloric sphincter.

Location

The pylorus is located in the upper abdomen, in the epigastric region. It lies at the distal end of the stomach, just before the duodenum, and is positioned slightly to the right of the midline.

Anatomy

The pylorus is the distal portion of the stomach that serves as the transition point between the stomach and the duodenum.^[1] It has a specialized structure that facilitates its role in regulating the passage of stomach contents. Below is a detailed description of its anatomy:

Structure

The pylorus is divided into two distinct regions:

Pyloric Antrum

• The wider, proximal part of the pylorus.
• Located just distal to the body of the stomach.
• Functions as a reservoir and mixing area for chyme before it enters the pyloric canal.

Pyloric Canal

• The narrower, distal part of the pylorus.
• Connects the pyloric antrum to the duodenum.
• Houses the pyloric sphincter, a thickened ring of smooth muscle.

Pyloric Sphincter

• The pyloric sphincter is a circular muscle that regulates the flow of chyme from the stomach to the duodenum.
• It is a physiological and anatomical sphincter:
  • Anatomically, it appears as a thickened muscular ring.
  • Physiologically, it remains tonically contracted to control gastric emptying.^[8]
• The sphincter relaxes intermittently to allow small amounts of chyme to pass into the duodenum.

Layers of the Pylorus

The wall of the pylorus consists of the same four layers as the rest of the stomach, with some specialization:

Mucosa:

• Lined with simple columnar epithelium.
• Contains gastric pits and glands that secrete mucus, enzymes, and acid.
• The mucosa is thicker in the pyloric region due to increased glandular activity.

Submucosa:

Composed of connective tissue containing blood vessels, lymphatics, and nerves.

Muscularis Externa:

Composed of three layers of smooth muscle:

• Inner oblique layer
• Middle circular layer: Thickened in the pyloric sphincter.^[5]
• Outer longitudinal layer

Serosa:

The outermost layer, formed by the visceral peritoneum.

Vascular Supply

Arterial Supply:

The pylorus is supplied by:

• Right gastric artery (a branch of the common hepatic artery).
• Right gastroepiploic artery (a branch of the gastroduodenal artery).

Venous Drainage:

Blood drains into the right gastric vein and right gastroepiploic vein, which empty into the portal venous system.

Lymphatic Drainage

Lymphatic drainage from the pylorus flows to the gastric lymph nodes and then to the celiac lymph nodes.

Nervous Supply

Sympathetic Innervation:

Derived from the celiac plexus, which regulates blood flow and sphincter tone.

Parasympathetic Innervation:

Provided by the vagus nerve, which stimulates gastric motility and relaxation of the pyloric sphincter.

Anatomical Relations

• Anteriorly: Related to the liver (left lobe) and anterior abdominal wall.
• Posteriorly: Lies near the pancreas and the lesser sac.
• Inferiorly: Related to the first part of the duodenum.

Function

The pylorus plays a crucial role in regulating the passage of stomach contents into the duodenum, coordinating digestion, and preventing reflux.^[6] Its functions are facilitated by the pyloric sphincter and the specialized structure of the pyloric region. Below is a detailed explanation of its functions:

Regulation of Gastric Emptying

• The pylorus controls the rate at which chyme (partially digested food mixed with gastric juices) is delivered from the stomach to the duodenum.^[7]
• Pyloric Sphincter Activity:
  • The pyloric sphincter remains tonically contracted to prevent premature emptying of chyme.
  • It relaxes intermittently, allowing small amounts of chyme to pass into the duodenum in a regulated manner.
  • This ensures that the duodenum receives chyme at a pace compatible with its digestive and absorptive capacity.^[4]

Prevention of Duodenogastric Reflux

• The pyloric sphincter acts as a barrier to prevent the backflow of duodenal contents (bile, pancreatic enzymes, and intestinal juices) into the stomach.
• This protective function is essential to maintain the stomach’s acidic environment and prevent irritation or damage to the gastric mucosa.

Mixing and Grinding of Food

• The pylorus assists in the mechanical digestion of food:
  • Peristaltic contractions in the pyloric antrum churn and mix the stomach contents, breaking food particles into smaller pieces.
  • The smaller particles, combined with gastric juices, form a semi-liquid substance called chyme.
• The pyloric region selectively allows only adequately processed chyme to enter the duodenum, retaining larger particles in the stomach for further grinding.

Coordination with the Duodenum

• The pylorus works in close coordination with the duodenum to ensure efficient digestion:
  • Hormonal Regulation:
    • The release of hormones like cholecystokinin (CCK) and secretin from the duodenum influences the activity of the pyloric sphincter.
    • These hormones delay gastric emptying when the duodenum is full or when the chyme is too acidic or fatty.
  • Neural Feedback:
    • Signals from the vagus nerve and enteric nervous system regulate the opening and closing of the pyloric sphincter based on feedback from the duodenum.

Acid Neutralization

• By controlling the flow of chyme, the pylorus ensures that the acidic stomach contents mix gradually with alkaline secretions (bicarbonate from the pancreas and bile from the gallbladder) in the duodenum.^[2]
• This process prevents excessive acid from entering the small intestine at once, protecting its mucosa and optimizing enzyme activity.

Protection of the Small Intestine

• The pylorus prevents the premature entry of large, undigested food particles into the duodenum, ensuring that only properly processed chyme progresses in the digestive tract.
• This selective release minimizes the workload on the small intestine and promotes efficient digestion and absorption.

Gastric Pressure Regulation

The pylorus helps regulate intragastric pressure by controlling the release of chyme:

This function prevents overdistension of the stomach and contributes to overall gastric motility.

Clinical Significance

The pylorus plays a critical role in gastrointestinal health, and its dysfunction can lead to various clinical conditions:

• Pyloric Stenosis: A condition where the pyloric sphincter thickens, causing obstruction of gastric emptying. Common in infants, it presents with projectile vomiting, dehydration, and weight loss.
• Gastroparesis: Delayed gastric emptying due to impaired pyloric sphincter relaxation, often associated with diabetes or nerve damage.
• Duodenogastric Reflux: Dysfunction of the pyloric sphincter can allow bile and duodenal contents to reflux into the stomach, causing irritation and gastritis.
• Peptic Ulcers: Ulcers near the pylorus can disrupt its function, causing pain, obstruction, or bleeding.
• Pyloric Tumors: Malignancies or benign growths at the pylorus can lead to obstruction or altered gastric motility.