Maxicam-2-Pair-Single-Tier-Dumbbell-Rack is made from sturdy, powder coated 2" X 3" metal rectangular pipe and is hollow inside.
Hollow tubes is an redundant expression. Tubes must be hollow, by definition. However tubes can be filled with something so they are not "empty" (they still will be hollow).
For the same amount of metal, hollow tubes are much stronger than solid tubes.
Stalagmites
A capillary is hollow. It is the smallest of the blood vessels, the tubes that carry blood.
Microtubules consists of hollow tubes which provide support of the cell.
IPS tubes are bus tubes where the current is carried through AL / Cu tubes (Hollow inside) and ACSR conductor are conductors basically with no any hollow part inside. Bus tubes given hollow portion inside taking in consideration greater skin effect to the current they carry -Anand Girgaonkar
microtubules
cytoskeleton
Xylem are just made up from cell walls left from cells, making hollow tubes. The walls of these tubes are hydrophilic, so water attaches readily to them, helping it to climb on its way up the plant.
astral ray
Cilia.
microtubules