In biology, a lumen (pl.: lumina) is the inside space of a tubular structure, such as an artery or intestine.^[1] It comes from Latin lumen 'an opening'.

It can refer to:

• the interior of a vessel, such as the central space in an artery, vein or capillary through which blood flows
• the interior of the gastrointestinal tract^[2]
• 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

In cell biology, a lumen is a membrane-defined space that is found inside several organelles, cellular components, or structures, including thylakoid, endoplasmic reticulum, Golgi apparatus, lysosome, mitochondrion, and microtubule.

Transluminal procedures are procedures occurring through lumina, including:^{[citation needed]}

• natural orifice transluminal endoscopic surgery in the lumina of, for example, the stomach, vagina, bladder, or colon
• procedures through the lumina of blood vessels, such as various interventional radiology procedures:
  • percutaneous transluminal angioplasty
  • percutaneous transluminal commissurotomy

• Foramen, any anatomical opening