What is the anatomy of hemorrhoids?

Answer

Hemorrhoids are not varicosities; they are clusters of vascular tissue (eg, arterioles, venules, arteriolar-venular connections), smooth muscle (eg, Treitz muscle), and connective tissue lined by the normal epithelium of the anal canal. Hemorrhoids are present in utero and persist through normal adult life. Evidence indicates that hemorrhoidal bleeding is arterial and not venous. This evidence is supported by the bright red color and arterial pH of the blood.

Hemorrhoids are classified by their anatomic origin within the anal canal and by their position relative to the dentate line; thus, they are categorized into internal and external hemorrhoids (see the following image).

Hemorrhoids. Anatomy of external hemorrhoid. Image courtesy of MedicineNet, Inc.

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External hemorrhoids develop from ectoderm and are covered by squamous epithelium, whereas internal hemorrhoids are derived from embryonic endoderm and lined with the columnar epithelium of anal mucosa. Similarly, external hemorrhoids are innervated by cutaneous nerves that supply the perianal area. These nerves include the pudendal nerve and the sacral plexus. Internal hemorrhoids are not supplied by somatic sensory nerves and therefore cannot cause pain. At the level of the dentate line, internal hemorrhoids are anchored to the underlying muscle by the mucosal suspensory ligament.

Hemorrhoidal venous cushions are a normal part of the human anorectum and arise from subepithelial connective tissue within the anal canal. Internal hemorrhoids have three main cushions, which are situated in the left lateral, right posterior (most common), and right anterior areas of the anal canal. However, this combination is found in only 19% of patients; hemorrhoids can be found at any position within the rectum. Minor tufts can be found between the major cushions.

Present in utero, these cushions surround and support distal anastomoses between the superior rectal arteries and the superior, middle, and inferior rectal veins. They also contain a subepithelial smooth muscle layer, contributing to the bulk of the cushions. Normal hemorrhoidal tissue accounts for approximately 15-20% of resting anal pressure and provides important sensory information, enabling the differentiation between solid, liquid, and gas.

External hemorrhoidal veins are found circumferentially under the anoderm; they can cause trouble anywhere around the circumference of the anus.

Venous drainage of hemorrhoidal tissue mirrors embryologic origin. Internal hemorrhoids drain through the superior rectal vein into the portal system. External hemorrhoids drain through the inferior rectal vein into the inferior vena cava. Rich anastomoses exist between these two and the middle rectal vein, connecting the portal and systemic circulations.

Mixed hemorrhoids are confluent internal and external hemorrhoids.

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Hemorrhoids. Anatomy of external hemorrhoid. Image courtesy of MedicineNet, Inc.
Hemorrhoids. Thrombosed hemorrhoid. This hemorrhoid was treated by incision and removal of a clot.

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Author

Kyle R Perry, MD Emergency Physician, The Queen's Medical Center; Volunteer Faculty, University of Hawaii, John A Burns School of Medicine

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

William G Gossman, MD, FAAEM Associate Clinical Professor of Emergency Medicine, Creighton University School of Medicine; Chairman, Department of Emergency Medicine, Creighton University Medical Center

William G Gossman, MD, FAAEM is a member of the following medical societies: American Academy of Emergency Medicine

Disclosure: Nothing to disclose.

Chief Editor

John Geibel, MD, DSc, MSc, AGAF Vice Chair and Professor, Department of Surgery, Section of Gastrointestinal Medicine, Professor, Department of Cellular and Molecular Physiology, Yale University School of Medicine; Director of Surgical Research, Department of Surgery, Yale-New Haven Hospital; American Gastroenterological Association Fellow

John Geibel, MD, DSc, MSc, AGAF is a member of the following medical societies: American Gastroenterological Association, American Physiological Society, American Society of Nephrology, Association for Academic Surgery, International Society of Nephrology, New York Academy of Sciences, Society for Surgery of the Alimentary Tract

Disclosure: Nothing to disclose.

Additional Contributors

Brian J Daley, MD, MBA, FACS, FCCP, CNSC Professor and Program Director, Department of Surgery, Chief, Division of Trauma and Critical Care, University of Tennessee Health Science Center College of Medicine

Brian J Daley, MD, MBA, FACS, FCCP, CNSC is a member of the following medical societies: American Association for the Surgery of Trauma, Eastern Association for the Surgery of Trauma, Southern Surgical Association, American College of Chest Physicians, American College of Surgeons, American Medical Association, Association for Academic Surgery, Association for Surgical Education, Shock Society, Society of Critical Care Medicine, Southeastern Surgical Congress, Tennessee Medical Association

Disclosure: Nothing to disclose.

Scott C Thornton, MD Associate Clinical Professor of Surgery, Yale University School of Medicine; Director, Colorectal Teaching, Bridgeport Hospital; Private Practice, Park Avenue Surgical Associates

Scott C Thornton, MD is a member of the following medical societies: American Society of Colon and Rectal Surgeons

Disclosure: Nothing to disclose.

Adam J Rosh, MD Assistant Professor, Program Director, Emergency Medicine Residency, Department of Emergency Medicine, Detroit Receiving Hospital, Wayne State University School of Medicine

Adam J Rosh, MD is a member of the following medical societies: American Academy of Emergency Medicine, American College of Emergency Physicians, Society for Academic Emergency Medicine

Disclosure: Nothing to disclose.

Acknowledgements

The authors and editors of Medscape Reference gratefully acknowledge the contributions of previous authors David R Gurley, MD, Richard Sinert, DO, and Pilar Guerrero, MD,to the development and writing of a source article.

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