Blood contains numerous biconcave cells called , contained in a featureless matrix called

  Blood contains numerous biconcave cells called erythrocyte , contained in a featureless matrix called plasma

Explanation:

Blood is a liquid contain in the body of man and animals that transport the necessary nutrients and oxygen to the cell and then takes away metabolic waste from the cell. There are numerous cells in the body which are the Red blood cells and also the red blood cell.

This red blood cell is also called erythrocyte which is the numerous biconcave cells. This white blood cells and erythrocytes are   all in the blood plasma. Both the erythrocytes  (i.e red blood cell) and the white blood cell are all contained in a featureless matrix called the plasma.

This plasma is made up of blood, water, carbon dioxide, dissolved proteins,  and hormones.