In biology, a lumen (from Latin lūmen, meaning "an opening"; plural lumina) is the inside space of a tubular structure, such as an artery or intestine. By extension, the term lumen is also used to describe the inside space of a cellular component or structure, such as the endoplasmic reticulum.
- The interior of a vessel, such as the central space in an artery or vein through which blood flows.
- The interior of the gastrointestinal tract 
- The pathways of the bronchi in the lungs
- The interior of renal tubules and urinary collecting ducts
- The pathways of the female genital tract, starting with a single pathway of the vagina, splitting up in two lumina in the uterus, both of which continue through the fallopian tubes
- Within a cell, the inner membrane space of a thylakoid, endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria or microtubule.
Transluminal procedures are procedures occurring through lumina, including:
- Natural orifice transluminal endoscopic surgery in the lumina of, for example the stomach, vagina, bladder or colon
- Percutaneous transluminal angioplasty in the lumina of blood vessels
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