what is the lumen

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.