General characteristics and role in the digestive system
The small intestine (intestinum tenue) is a key segment of the gastrointestinal tract located between the stomach and the large intestine. This is where the primary processes of nutrient hydrolysis and their subsequent absorption into the blood and lymph occur. Anatomically, this long organ is divided into three consecutive parts:
- Duodenum (duodenum) — the short initial segment.
- Jejunum (jejunum) — the middle segment.
- Ileum (ileum) — the distal segment.
The ileum completes the digestive process in the small intestine, after which the contents pass into the large intestine (intestinum crassum), where absorption is already significantly limited and relevant mainly for specific sustained-release dosage forms.
Topography of the mesenteric part
The jejunum and ileum together form the mesenteric part of the small intestine (pars mesenterica intestini tenuis). This massive section is located in the lower floor of the abdominal cavity, strictly below the transverse mesocolon. The mesenteric part originates from the duodenojejunal flexure (flexura duodenojejunalis), which is located to the left of the body of the 2nd lumbar vertebra. It ends in the right iliac fossa at the level of the body of the 4th lumbar vertebra.
Within the mesenteric part, the ileum accounts for the distal third (the proximal two-thirds are occupied by the jejunum). There is no distinct anatomical boundary between the jejunum and ileum. Unlike the jejunum, whose loops lie predominantly to the left and superiorly, the loops of the ileum occupy the lower and right regions of the abdominal cavity, including the right iliac fossa, and also descend into the lesser pelvis. The arrangement of the loops is highly mobile and subject to individual variation.
Macroscopic wall structure
As one moves from the proximal to the distal segments, the diameter of the mesenteric part gradually decreases. While in the proximal segment it is about 4.8–5.0 cm, in the distal segment corresponding to the ileum, it narrows to 2.7–3.0 cm.
The intestinal loops are located intraperitoneally, meaning they are covered by visceral peritoneum on all sides. The only exception is the narrow strip of the mesenteric attachment. Therefore, two borders are distinguished in the anatomy of the mesenteric part:
- Mesenteric border — the area where the intestine is fixed to its mesentery.
- Antimesenteric border — the free border facing directly into the peritoneal cavity.
Biopharmaceutics and absorption
The gastrointestinal tract is the main route of drug administration, and the anatomy of the small intestine determines the overall biopharmaceutics of enteral formulations. Unlike the oral cavity and esophagus (oesophagus), which serve as transit regions, and the stomach (gaster), where the absorption area is small (only passive diffusion of non-ionized forms of weak acids begins there due to the acidic environment of pH 1.5–2.0), the small intestine acts as the primary absorption organ.
The enormous absorption area in the ileum and jejunum is achieved through multilevel folding of the mucosa. This complex includes:
- Circular folds (plicae circulares).
- Intestinal villi (villi intestinales).
- Enterocyte microvilli (microvilli), forming the brush border.
This structure increases the total surface area up to 30–40 m², which is a key anatomical factor for high drug bioavailability. Venous drainage from the ileum collects into the portal vein (vena portae) and passes through the liver. This establishes the anatomical basis for the first-pass effect (first-pass effect), which can significantly reduce the bioavailability of many orally administered drugs.
To easily remember the arrangement of the mesenteric loops, use this association: Jejunum is Top (superior and left), Ileum is Low (inferior, right, and in the pelvis).
Class and exam questions
Where is the boundary between the jejunum and the ileum?
There is no distinct anatomical boundary between them. Macroscopically, one section smoothly transitions into the other, and it is generally accepted that the distal third of the mesenteric part is the ileum (ileum).
How is the ileum covered by the peritoneum?
It is located intraperitoneally. The visceral peritoneum covers it completely, leaving only a narrow strip at the mesenteric border for the attachment of the mesentery.
Why is the small intestine so important for pharmacology?
This is the main organ of absorption. Thanks to folds, villi, and microvilli, its surface area reaches 30–40 m², ensuring high bioavailability of oral medications.
What else to study on this topic
- Anatomy of the duodenum (duodenum) and its role in digestion.
- Topography and macroscopic structure of the jejunum (jejunum).
- First-pass hepatic effect (first-pass effect) and the portal vein system.
- Structure of the stomach wall and the influence of the acidic environment (pH 1.5–2.0) on drug absorption.
- Large intestine (intestinum crassum) and colon-specific drug delivery systems.
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