Internal Hemorrhoid Anatomy
Internal hemorrhoids are normal vascular cushions located in the subepithelial space of the anal canal above the dentate line, consisting of connective tissue surrounding direct arteriovenous communications between terminal branches of the superior rectal arteries and the superior, middle, and inferior rectal veins. 1
Structural Components
Internal hemorrhoids have three distinct anatomical parts 2:
- Lining: Covered by columnar epithelium (mucosa) above the dentate line
- Stroma: Contains blood vessels with direct arteriovenous communications, smooth muscle, and supporting connective tissue
- Anchoring system: Connective tissue that secures the hemorrhoidal cushions to the internal anal sphincter and conjoined longitudinal muscle layer
Vascular Architecture
The vascular structure is unique and clinically significant 1, 3:
- Direct arteriovenous communications exist between arteries and veins without intervening capillaries in the functional portion
- This arteriovenous shunting explains why hemorrhoidal bleeding is characteristically bright red (arterial oxygen tension) rather than dark venous blood
- The submucosal arteriovenous plexus can dilate abnormally when disease develops
- Vessels are wrapped in elastic fibers that provide structural support
Muscular Support System
The smooth muscle component is critical to hemorrhoidal function 1:
- Anal subepithelial smooth muscle arises from the conjoined longitudinal muscle layer
- Passes through the internal anal sphincter
- Inserts into the subepithelial vascular space
- This muscle suspends and contributes to the bulk of the hemorrhoidal cushions
Normal Configuration and Function
Most people have 3 hemorrhoidal cushions, though the classic configuration occurs in only 19% of individuals 1:
- Traditional positions: right anterior, right posterior, and left lateral
- Actual anatomical variation is much more common than textbooks suggest
Physiological Role
Internal hemorrhoids serve essential functions 1:
- Contribute 15-20% of resting anal pressure
- Act as a conformable plug to ensure complete closure of the anal canal
- Essential for maintaining fecal continence
- Provide sensory feedback about anal canal contents
Pathophysiological Changes
When internal hemorrhoids become symptomatic, specific anatomical changes occur 1, 2:
- Deterioration of the anchoring connective tissue system with aging (observable from third decade of life)
- Abnormal swelling of the anal cushions
- Stretching of the suspensory muscles
- Dilation of the submucosal arteriovenous plexus
- Loss of elastic fiber support causes veins to become distended
- Hemorrhoids not only bulge but descend into the lumen
Common Pitfall
Do not confuse internal hemorrhoids with rectal varices from portal hypertension—these are anatomically distinct entities located in the anal canal, and patients with portal hypertension do not have increased incidence of hemorrhoids 1. Standard hemorrhoidal treatments should never be used for variceal bleeding.
Clinical Relevance
The anatomical location above the dentate line means internal hemorrhoids are covered by columnar epithelium with visceral innervation, explaining why uncomplicated internal hemorrhoids are typically painless (unlike external hemorrhoids below the dentate line with somatic pain receptors) 1, 4.