What is the tissue that lines blood capillaries and alveoli?

Answer 1

Endothelial cells (endothelium or endothelial tissue) line the walls of capillaries:

http://www3.umdnj.edu/histsweb/lab7/lab7capillaries.html

Epithelium (epithelial cells) lines the walls of alveoli:

http://classes.ansci.uiuc.edu/ansc438/Mamstructure/histology_4.html