Very small blood vessels constituting the end of an artery or vein where the exchange of vital substances takes place.

the capillaries.

The blood capillaries are blood vessels with very small caliber. They form the blood distribution and collection network in cells. They are the communication between branches originating from the arteries and with the smaller veins. It is on the walls of the capillaries that occurs the exchange of water, oxygen, CO2,  other nutrients and chemical residues.

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.