Which mechanism describes nutrient transport into the nucleus pulposus?

• A. Imbibition
• B. Diffusion
• C. Osmosis
• D. Filtration

Answer: (A)

Nucleus pulposus gets its nutrients without its own blood supply, so nutrient delivery relies on the surrounding fluid moving into the disc matrix. The matrix is rich in proteoglycans, which draw in water and create an osmotic swelling pressure. This water uptake by the matrix is called imbibition, and it brings in dissolved nutrients from the endplates so they can diffuse through the tissue to resident cells. While diffusion describes how solutes move once inside, imbibition explains the initial uptake of water and dissolved nutrients into the avascular nucleus pulposus. Filtration and osmosis describe other fluid movements but don’t best capture this combined process of water uptake into the disc matrix that enables nutrient delivery.