The duodenum can be divided into four main “parts”, with each part being defined based on its spatial relationship to other organs/structures in this region.

The first part of the duodenum extends directly from the pylorus of the stomach.

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The first part of the duodenum extends posteriorly away from the midline of the abdomen, terminating at the right border of the Inferior Vena Cava (IVC).

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The very beginning of the first part of the duodenum is often referred to as the “free” part (or duodenal cap) because it has no attachments to the posterior abdominal wall.

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This makes the free part “or duodenal cap” very mobile. Internally, the duodenal cap can be characterised by the lack of circular folds (plicae circulares) which are present in other parts of the duodenum.

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The first part of the duodenum is anterior to (in front of) the following structures:
- Pancreas,
- Bile Duct &
- Portal Vein

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The first part of the duodenum is posterior to (behind) the following structures:
- Gall bladder &
- Edge of Liver

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