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Anatomy and Function of the Small and Large Intestine

Small Intestine Structure and Connections

The small intestine is connected to the posterior abdominal wall by a supportive structure known as the mesentery. This mesentery specifically connects the jejunum and ileum. The insertion line on the posterior abdominal wall is called Radix mesenterii, which runs horizontally across from left to right, with the following anatomical relationships:

  • Left ureter
  • Inferior mesenteric vein
  • Duodenum
  • Abdominal aorta
  • Inferior vena cava
  • Right ureter
  • Right genital vessels

Radix mesenterii also serves to divide the inframesocolic space into two triangles:

  • Right mesenterico-colic triangle
  • Left mesenterico-colic triangle

Small Intestine Mucosal Features

Plicae Circulares (Kerckring Folds)

These are prominent mucosal folds present in the small intestine, more developed in the jejunum and less pronounced in the ileum. Their main function is to increase the surface area for absorption by approximately 400%.

Villi Intestinalis

The small intestine also features villi intestinalis, which are tiny, finger-like projections that further increase the surface area by an additional 600%. These villi contain various structures, including epithelium, capillaries for nutrient absorption, and nerve fibers for sensory functions.

Tunicas of the Small Intestine
  • Tunica Mucosa: This comprises columnar epithelium, which is lined by absorptive cells that help in nutrient uptake. The surface area is further augmented by small intestinal glands referred to as glandulae Lieberkühn. It also contains Noduli lymphoidei solitarii and Noduli lymphoidei aggregatii (Peyer’s plaques), which are components of the immune system.
  • Tunica Submucosa: This layer consists of alveolar connective tissue and houses the submucosal plexus (Meissner's plexus), which is crucial for innervation.
  • Tunica Muscularis: It has two layers: an inner circular layer and an outer longitudinal layer, containing the myenteric plexus (Auerbach's plexus) for motility regulation.
  • Tunica Serosa: This outermost layer is a peritoneal covering that excludes the areas of mesenteric insertion.
Vascular Supply and Nerve Supply
  • Arterial Supply: Blood vessels originate from the superior mesenteric artery, which supplies 10-16 jejunal and ileal branches.
  • Venous Drainage: The veins follow the same pathway as the arteries and drain into the superior mesenteric vein.
  • Lymphatic Drainage: This occurs through Noduli lymphoidei solitarii and Peyer’s plaques, eventually draining into the trunci intestinalis.
  • Nervous Supply: The sympathetic innervation comes from the plexus coeliacus, while the parasympathetic fibers originate from the vagal trunks, terminating in the myenteric plexus and submucosal plexus.

Large Intestine Structure

General Anatomy

The large intestine consists of several sections:

  • Caecum
  • Colon Ascendens (ascending colon)
  • Colon Transversum (transverse colon)
  • Colon Descendens (descending colon)
  • Colon Sigmoideum (sigmoid colon)
  • Rectum
  • Canalis Analis (anal canal)
Muscular Structure Features
  • The large intestine is characterized by the presence of longitudinal muscles arranged into three distinct bands, known as taeniae. These include:
  • Taenia libera (on the anterior surface)
  • Taenia mesocolica
  • Taenia omentalis
    These bands begin at the appendix in the caecum and disappear at the rectum.
  • The colon also has appendices epiploicae (fatty appendages) and haustra coli (sac-like pouches that give the colon a segmented appearance).
Tunicas of the Large Intestine
  • Tunica Mucosa: This lining is flat with no villi or circular folds and contains columnar cells in the intestinal glands (glandulae intestinalis Lieberkühn) which secrete mucus.
  • Tunica Submucosa: Contains blood vessels, lymphatics, and nerves.
  • Tunica Muscularis: The outer longitudinal layer is arranged into taenia, while the inner circular layer forms the haustra coli.
  • Tunica Serosa: The peritoneum covers most of the colon except for the mesocolic insertions and the posterior aspects of the ascending and descending colon, where it is replaced by tunica adventitia.
Special Features of the Caecum

The caecum is located in the right iliac fossa and has several important anatomical relationships and parts:

  • It lies over the iliopsoas muscle and the femoral nerve.
  • Anteriorly, it is adjacent to the abdominal wall, while medially it connects to the ileocecal junction and laterally to the iliac muscle and crest.
  • Superiorly, it connects with the ascending colon and inferiorly leads to the appendix, with potential contact with the urinary bladder, uterus, or ovary.
  • Notable recesses include recessus ileocaecalis superior and inferior as well as recessus retrocaecalis, separated by the plica caecalis vascularis and plica ileocaecalis.
Vermiform Appendix

The vermiform appendix is a small, tubular appendage that is attached to the caecum approximately 2-3 cm below the ileocecal junction. Its anatomical positions can vary:

  • Appendix retrocaecalis: Located behind the sacrum.
  • Appendix mesoceliacus: Medially oriented towards the umbilical region.
  • Appendix pelvinus: Positioned at the top of the appendix in the lesser pelvis.
  • Appendix subhepaticus: Located near the gallbladder and can be anterior or lateral to the caecum.
Points of Interest
  • McBurney’s Point: Found at the junction of the lateral third and medial two-thirds of the right spino-umbilical line, important for locating the appendix.
  • Lanz Point: At the junction between the right lateral third and left two-thirds along the bispinous line.
  • Lacobovici Triangle: Bordered by the spinoumbilical line, bispinous line, and pararectalis spine for anatomical delineation in clinical examinations.
Colon Segments and Features
Colon Ascendens
  • Located in the right lateral abdomen, it extends from the caecum to the right colic flexure.
  • The anterior aspect is covered by the abdominal wall (facies colica) while the posterior is related to the iliacus, quadratus lumborum, and right kidney.
Colon Transversum

This is the most mobile segment of the colon, dividing the abdominal cavity into the supramesocolic and inframesocolic spaces. It passes over several important structures:

  • Left renal pelvis
  • Pancreatic body
  • Duodenojejunal junction
  • Pancreatic head
  • Descending duodenum
  • Inferior vena cava
  • Right kidney
Flexura Coli Sinistra
  • Anteriorly related to the greater curvature of the stomach and diaphragm via the phrenicocolic ligament.
  • Laterally located near the spleen.
  • Posteriorly related to the left kidney.
Colon Descendens

Located in the left abdominal region, it lies over posterior abdominal muscles, and the left kidney. The sulcus paracolicus sinistra helps in fluid drainage and the spread of infections.

Colon Sigmoideum
  • Divided into an iliac segment (fixed) related to the iliopsoas muscle and surrounding vessels, and a pelvic segment (mobile) suspended by the mesocolon sigmoideum.
Rectum

The rectum features:

  • Ampulla Recti and Canalis Analis: Including
  • Flexura Sacralis: The anterior concavity part of the ampulla.
  • Flexura Perinealis: Posterior concavity in the anal canal.
  • The superior part of the ampulla is covered by peritoneum, while the rest lies in the pelvis subperitoneal space.
Features of the Anus
  • Plicae Transverae Recti (Houston folds): Notable folds within the rectum, including Plica Kohlrausch, which is the most prominent.
  • Columnae Anales Morgagni: Distinct folds at the junction of the ampulla and canalis analis.
  • Sinus Anales: Located between the folds, with Valvulae Anales at their lower ends.
  • Linea Pectinea: Marks the end of anal columns, indicating a transition from visceral to somatic innervation.
  • White Line of Hilton: A significant landmark separating the internal and external anal sphincters.
  • Linea Anocutanea: The boundary between the anal mucosa and perineal skin.
  • Pecten Analis: The canal situated between Hilton’s line and the Linea Pectinata, featuring the anal sphincters.

Vascular Supply of the Large Intestine

Arterial Supply
  • Arteria Mesenterica Superior: Supplies various parts of the large intestine.
    • A. Ileocolica: Supplies the caecum and appendix.
    • A. Colica Dextra: Supplies the ascending colon and right colic flexure.
    • A. Colica Media: Divides into right and left branches, with the right joining the A. colica dextra and the left joining the A. colica sinistra, a connection termed Riolano-Haller anastomosis.
  • Arteria Mesenterica Inferior: Supplies the descending colon and contributes to the Riolano-Haller arch.
    • A. Sigmoidea: Contains 2-5 branches supplying the sigmoid colon, with the last one connecting with A. Rectalis Superior at the Sudeck Point.
  • A. Rectalis Superior: The final branch of the inferior mesenteric artery, supplying the ampulla recti.
  • A. Iliaca Interna:
    • A. Rectalis Media: A direct branch from the internal iliac artery.
    • A. Rectalis Inferior: Originates from the A. Pudenda Interna, which is derived from the internal iliac artery.
Venous Drainage

The veins follow a nominal correspondence with the arteries, specifically:

  • Vena Mesenterica Superior: Drains the veins of the ileocolica, colica dextra, and colica media.
  • Vena Mesenterica Inferior: Drains the veins of the colica sinistra, vein sigmoideae, and rectalis superior.
  • Vena Iliaca Interna: Drains the rectalis media and inferior.
Portocaval Anastomosis

This important anastomosis occurs between the V. Rectalis Superior and V. Rectalis Media et Inferior and connects the systemic venous system.

Lymphatic Drainage

Lymphatic drainage includes multiple lymph nodes associated with the intestines:

  • Nodi Lymphoidei Ileocolici: Located near the ileocecal junction.
  • Nodi Lymphoidei Colici Dextrum et Medium: Draining the ascending and transverse colon respectively.
  • Nodi Lymphoidei Mesenteri Superiores: For midgut structures.
  • Nodi Lymphoidei Colici Sinistri: Associated with the descending colon.
  • Nodi Lymphoidei Sigmoidei: Draining the sigmoid colon.
  • Nodi Lymphoidei Mesenteri Inferiores: For hindgut segments.
Pelvic Lymph Nodes
  • Nodi Lymphoidei Sacrales
  • Nodi Lymphoidei Pararectalis
  • Nodi Lymphoidei Iliaci Interni et Communis

Nervous Supply of the Intestinal Tract

Sympathetic Innervation

The sympathetic nervous system supplies the intestines from several plexi:

  • Plexus Mesentericus Superior et Inferior serves the colon.
  • Plexus Aorticus and Plexus Hypogastricus Inferior are involved in supplying the rectum.
  • The sympathetic response inhibits peristalsis, contracts sphincters, and reduces secretions.
Parasympathetic Innervation
  • The vagus nerve supplies the caecum, ascending colon, and the right two-thirds of the transverse colon.
  • The pelvic splanchnic nerves supply the left one-third of the transverse colon, descending colon, sigmoid colon, and rectum.
Cannon-Böhm Point

This anatomical landmark denotes the transition zone between the vagus and pelvic splanchnic nerve supply. It marks the vascular transition between the Superior Mesenteric Artery (SMA) and the Inferior Mesenteric Artery (IMA), as well as the embryological transition between midgut and hindgut.