To begin with, something upsetting has happened to bring you to the point of tears. This in itself is stressful. Stress and tension break down the immune system and can lead to illness over time. So, when you cry, and I mean really sob, your body is releasing stress, sadness, perhaps anger, and all around tension. It isn't just the crying that is tiring, it is the emotional drainage. Did you know it takes more muscles to frown than to smile?
The cells in the body belong to four groups; muscle, nervous, epithelial (skin), and connective. The largest group with the most cells is connective tissue.