Final answer:
The hollow space inside a tube in biological systems is called the lumen, and in the context of the endoplasmic reticulum, it is also known as the cisternal space.
Step-by-step explanation:
The hollow space within a tube, like those found in biological systems, is often referred to as the lumen. This term is frequently used to describe the empty core of tubular structures such as blood vessels, the small intestine, or parts of cellular organelles like the endoplasmic reticulum (ER) and thylakoids.
In the context of the ER, the hollow portion of the ER tubules is additionally called the cisternal space. Surrounding this space, the ER membrane is a phospholipid bilayer that has various proteins embedded within it, and it is connected continuously to the nuclear envelope.
The inside of the hollow space within the tube is known as the lumen or cisternal space. In biology, this term is commonly used to describe the interior of various structures such as the endoplasmic reticulum (ER), small intestine, cochlea, thylakoids, or eye cavities.