Final answer:
Anemia refers to a deficiency of either erythrocytes or hemoglobin in the blood, causing a lack of oxygen. It can lead to symptoms such as weakness and fatigue.
Step-by-step explanation:
Anemia refers to a deficiency of either erythrocytes or hemoglobin in the blood. Erythrocytes, also known as red blood cells (RBCs), are responsible for carrying oxygen to cells, while hemoglobin is the protein in RBCs that carries oxygen. When there is a deficiency of RBCs or hemoglobin, the blood is unable to carry enough oxygen, leading to symptoms such as weakness and fatigue.