Makindo Medical Notes

Related Subjects: | Abdominal Anatomy and Physiology | Brainstem Anatomy | Thalamic Anatomy

Anatomy and Physiology of the Abdomen

The abdomen is the region of the body located between the thorax (chest) and the pelvis, housing many vital organs and structures involved in digestion, metabolism, and other essential physiological processes. It is bounded superiorly by the diaphragm, inferiorly by the pelvic brim, and laterally by the abdominal wall muscles. Understanding the anatomy and physiology of the abdomen is crucial for clinical examination, diagnosis, and treatment of various medical conditions.

Quadrants of the Abdomen

The abdomen is commonly divided into four quadrants for clinical assessment:

• Right Upper Quadrant (RUQ):
  • Liver (right lobe)
  • Gallbladder
  • Duodenum
  • Head of the pancreas
  • Right kidney and adrenal gland
  • Part of the ascending and transverse colon
• Right Lower Quadrant (RLQ):
  • Cecum and appendix
  • Ascending colon
  • Right ovary and fallopian tube (in females)
  • Right ureter
  • Small intestine (ileum)
• Left Upper Quadrant (LUQ):
  • Stomach
  • Spleen
  • Left lobe of the liver
  • Body and tail of the pancreas
  • Left kidney and adrenal gland
  • Part of the transverse and descending colon
  • Small intestine (jejunum)
• Left Lower Quadrant (LLQ):
  • Descending colon
  • Sigmoid colon
  • Left ovary and fallopian tube (in females)
  • Left ureter
  • Small intestine (ileum)

Regions of the Abdomen

For a more detailed assessment, the abdomen is also divided into nine regions:

• Epigastric Region:
  • Located above the umbilical region; contains the stomach, part of the liver, pancreas, and duodenum.
• Umbilical Region:
  • Surrounds the umbilicus (navel); contains parts of the small intestine and transverse colon.
• Hypogastric (Suprapubic) Region:
  • Located below the umbilical region; contains the bladder, sigmoid colon, and reproductive organs.
• Right Hypochondriac Region:
  • Contains the liver (right lobe), gallbladder, and right kidney.
• Left Hypochondriac Region:
  • Contains the spleen, left kidney, and part of the stomach.
• Right Lumbar Region:
  • Contains the ascending colon, part of the small intestine, and right kidney.
• Left Lumbar Region:
  • Contains the descending colon and part of the small intestine.
• Right Iliac (Inguinal) Region:
  • Contains the cecum, appendix, and part of the small intestine.
• Left Iliac (Inguinal) Region:
  • Contains the sigmoid colon and part of the small intestine.

Muscles of the Abdomen

• Rectus Abdominis: Vertical muscles on either side of the midline (linea alba), responsible for flexing the trunk and maintaining posture.
• External Oblique: Superficial lateral muscles that assist in trunk rotation and lateral flexion; fibers run downward and medially.
• Internal Oblique: Located beneath the external obliques; fibers run upward and medially, aiding in trunk rotation and lateral flexion.
• Transversus Abdominis: The deepest abdominal muscle; fibers run horizontally, providing core stability and compressing abdominal contents.

Vascular Supply

• Abdominal Aorta: The major blood vessel supplying the abdomen; branches into several arteries that supply abdominal organs.
• Celiac Trunk: Gives rise to the left gastric, splenic, and common hepatic arteries, supplying the liver, stomach, spleen, and pancreas.
• Superior Mesenteric Artery: Supplies the small intestine and the proximal part of the large intestine (cecum, ascending colon, and part of the transverse colon).
• Inferior Mesenteric Artery: Supplies the distal part of the large intestine (descending colon, sigmoid colon, and rectum).

Innervation

• The abdominal organs receive innervation from the autonomic nervous system:
  • Sympathetic Fibers: Arise from the thoracic and lumbar spinal cord segments; inhibit gastrointestinal motility and secretion.
  • Parasympathetic Fibers: Provided by the vagus nerve (cranial nerve X) and pelvic splanchnic nerves; stimulate gastrointestinal motility and secretion.
• The celiac plexus supplies sympathetic innervation to the foregut, while the superior and inferior mesenteric plexuses supply the midgut and hindgut, respectively.

Omentum

• Definition: The omentum is a fold of peritoneum extending from the stomach to adjacent organs in the abdominal cavity.
• Types:
  • Greater Omentum:
    • Extends from the greater curvature of the stomach and hangs over the intestines like an apron.
    • Consists of four layers of peritoneum containing fat, blood vessels, lymphatics, and immune cells (macrophages).
    • Functions include fat storage, immune response, and protection of abdominal organs by isolating infections.
  • Lesser Omentum:
    • Connects the lesser curvature of the stomach and the proximal part of the duodenum to the liver.
    • Divided into the hepatogastric ligament (stomach to liver) and hepatoduodenal ligament (duodenum to liver).
    • Contains important structures, including the hepatic artery, portal vein, and bile duct (collectively known as the portal triad).
• Clinical Significance:
  • The greater omentum is often referred to as the "abdominal policeman" due to its role in immune responses and infection containment.
  • Omental adhesions can occur after surgery or inflammation, potentially leading to complications like bowel obstruction.
  • Omental infarction, a rare condition where blood supply to the omentum is compromised, can cause acute abdominal pain and may require surgical intervention.

Peritoneum

• The peritoneum is a serous membrane lining the abdominal cavity and covering the abdominal organs.
• It consists of two main layers:
  • Visceral Peritoneum:
    • Covers the external surfaces of most abdominal organs, including the stomach, intestines, liver, and spleen.
    • Provides lubrication to reduce friction between organs and aids in organ movement.
    • Contains blood vessels, lymphatics, and nerves supplying the abdominal organs.
  • Parietal Peritoneum:
    • Lines the abdominal wall, extending from the diaphragm to the pelvic cavity.
    • Supplies sensory innervation, making it sensitive to pain, pressure, and temperature.
    • Forms folds (ligaments) that anchor organs to the abdominal wall and contain blood vessels and nerves.
• Clinical Relevance:
  • Peritonitis: Inflammation of the peritoneum, often due to infection (e.g., appendicitis) or perforation of abdominal organs; causes severe abdominal pain and requires immediate medical attention.
  • Ascites: Accumulation of fluid in the peritoneal cavity, resulting from liver disease, heart failure, or malignancy; may necessitate diagnostic and therapeutic paracentesis.
  • Understanding the peritoneum's anatomy is crucial during surgical procedures to minimize complications and ensure proper healing.
  • The omentum and peritoneum play vital roles in the anatomy and function of the abdominal cavity, contributing to protection, support, and immune defense.
  • A thorough understanding of these structures is essential for clinicians in diagnosing and managing abdominal conditions effectively.

Gastrointestinal Anatomy and Physiology

Oesophagus

• Approximately 25 cm in length, connecting the pharynx to the stomach.
• Lined with stratified squamous epithelium to protect against abrasion from food particles.
• Muscular layers:
  • Outer longitudinal muscle layer.
  • Inner circular muscle layer.
• Muscle composition varies along its length:
  • Upper third: Striated (skeletal) muscle, under voluntary control.
  • Middle third: Combination of striated and smooth muscle.
  • Lower third: Smooth muscle, under involuntary control.
• Protected from acid reflux by:
  • Lower Oesophageal Sphincter (LES): A physiological sphincter that prevents backflow of stomach contents.
  • Salivary and Oesophageal Bicarbonate Secretion: Neutralizes any acid that refluxes into the oesophagus.
  • Gravity: Assists in keeping gastric contents in the stomach when upright.
  • "Pinchcock" Effect of Diaphragm: The diaphragmatic crura exert pressure on the oesophagus during respiration, enhancing sphincter function.

Stomach

• Located approximately 50 cm from the teeth along the digestive tract.
• Lined with simple columnar epithelium; the transition from squamous epithelium occurs at the oesophagogastric (OG) junction.
• Secretes about 2–3 liters of gastric juices daily.
• Gastric glands contain specialized cells:
  • Parietal Cells: Secrete hydrochloric acid (HCl) using proton pumps and intrinsic factor necessary for vitamin B₁₂ absorption.
  • Chief Cells: Produce pepsinogen, the inactive precursor of the enzyme pepsin, which digests proteins.
  • Surface Mucous Cells: Secrete mucus and bicarbonate (HCO₃^-) to protect the stomach lining from acidic gastric juices.
• Innervation:
  • Vagal Nerve (Parasympathetic): Provides motor and secretory signals to stimulate digestion.
  • Meissner's and Auerbach's Plexuses (Enteric Nervous System): Coordinate muscular contractions and regulate gastric motility.
  • Sympathetic Nervous System: Modulates gastric functions, generally inhibiting digestive activity and decreasing blood flow.

Small Intestine

• Approximately 6 meters in length, divided into three sections:
  • Duodenum: Receives chyme from the stomach and digestive enzymes from the pancreas and bile from the liver; primary site for chemical digestion.
  • Jejunum: Primary site for nutrient absorption; has a thick wall and rich blood supply.
  • Ileum: Absorbs vitamin B₁₂, bile salts, and any remaining nutrients; contains Peyer's patches for immune surveillance.
• Lined with simple columnar epithelium featuring villi and microvilli to increase surface area for absorption.
• Contains specialized structures:
  • Crypts of Lieberkühn: Glands secreting intestinal juices rich in enzymes and providing stem cells for epithelial renewal.
  • Peyer's Patches: Aggregated lymphoid nodules important for immune function, particularly in the ileum.
• Functions:
  • Completes digestion of proteins, carbohydrates, and lipids through enzymes from the pancreas and brush border enzymes.
  • Absorbs nutrients, electrolytes, and water into the bloodstream.
  • Secretes hormones like cholecystokinin (CCK) and secretin to regulate digestive processes.
• Motility regulated by:
  • Segmental Contractions: Mix chyme with digestive juices and enhance contact with absorptive surfaces.
  • Peristalsis: Propels contents toward the large intestine in a coordinated manner.
  • Enteric Nervous System: Coordinates local reflexes and motility patterns independently of central nervous input.

Large Intestine

• Approximately 1.5 meters in length, comprising:
  • Cecum: Receives material from the ileum; contains the appendix, which plays a role in immune function.
  • Colon: Divided into ascending, transverse, descending, and sigmoid sections; responsible for water absorption and feces formation.
  • Rectum and Anal Canal: Store and expel feces during defecation.
• Lined with simple columnar epithelium transitioning to stratified squamous epithelium near the anal canal.
• Functions:
  • Absorbs remaining water and electrolytes from indigestible food matter.
  • Forms and stores feces until defecation.
  • Hosts gut microbiota that ferment undigested carbohydrates and synthesize vitamins (e.g., vitamin K and certain B vitamins).
• Distinctive features:
  • Haustra: Pouch-like segments created by the arrangement of muscle layers, aiding in water absorption and feces formation.
  • Taeniae Coli: Three bands of longitudinal smooth muscle that contract to produce haustra and facilitate movement.
  • Epiploic Appendages: Fat-filled pouches hanging from the colon; their function is not entirely understood.
• Motility:
  • Haustral Churning: Mixes contents to facilitate water absorption and formation of feces.
  • Mass Movements: Powerful peristaltic contractions moving feces toward the rectum, typically occurring a few times a day.
  • Defecation Reflex: Triggered by rectal wall stretching, involving relaxation of the internal anal sphincter (involuntary) and voluntary control over the external anal sphincter.
• Regulation:
  • Enteric Nervous System: Manages local reflexes and controls motility and secretion.
  • Autonomic Nervous System:
    • Parasympathetic Stimulation: Increases motility and secretion, promoting digestion.
    • Sympathetic Stimulation: Decreases motility and secretion, inhibiting digestion.
  • Hormonal Influences: Gastrointestinal hormones like gastrin and motilin can modulate activity.

Clinical Considerations

• Understanding abdominal anatomy is critical for diagnosing and managing conditions such as appendicitis, cholecystitis, diverticulitis, and abdominal hernias.
• Physical examination techniques, including palpation, percussion, and auscultation, rely on knowledge of abdominal organ locations and their functions.
• Imaging studies, such as ultrasound, CT scans, and MRI, often require an understanding of normal anatomical relationships to identify abnormalities effectively.
• Knowledge of vascular supply is essential in surgical planning to prevent inadvertent injury to major blood vessels.
• Innervation patterns are important when considering pain referral and the effects of nerve damage or blockages.

Conclusion

The anatomy of the abdomen is complex, involving multiple organ systems that play essential roles in digestion, metabolism, immunity, and homeostasis. A thorough understanding of this anatomy and physiology is vital for healthcare professionals in both clinical and surgical settings. It enables accurate diagnosis, effective treatment planning, and the ability to manage complications that may arise during medical interventions.