Answer: The surface of an epithelial cell which lies closest to its blood supply is called as the Basal surface.
Epithelial cell is primarily composed of two surface-
1) Apical surface- It is exposed to the environment and lies towards the lumen or internal organ cavity.
2) Basal/ Basolateral surface- It is not exposed to the environment. It lies interiorly and attached to the basement membrane via integrin.
Epithelial cells do not have own blood supply. For obtaining nutrients, they are dependent on the blood capillaries which are present in the nearby tissues. For accomplishing this, the basal surface of epithelial cell plays an essential role. The nutrients from blood enters basal surface via basement membrane which is a selectively permeable membrane.
Thus, basal surface of epithelial cells lies closest to the blood supply.