The red blood cell's journey starts in the bone marrow, where after 7 days of maturation, it is released into the bloodstream. The production of red blood cells is controlled by the kidney releasing a chemical called erythropoietin (EPO), which is triggered by an O2 deficiency in the Kidney.
Once the red blood cell has matured, it can start doing the job it is created for. The red blood cell exists to carry oxygen throughout the bloodstream and body, which it does through hemoglobin; hemoglobin carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues back to the lungs. Hemoglobin is a protein found within red blood cells that is crucial to the red blood cell, it can also carry up to 4 oxygen molecules at a time.
Along the way, a red blood cell encounters many other things in the blood. White blood cells, which fight infections, are another crucial part of your body functions. Platelets are present to help clotting if the skin gets cut and one starts bleeding, so that the bleeding eventually stops. Plasma is a yellow-ish liquid responsible for carrying nutrients, hormones, proteins, etc. through the body. Hopefully it won't, but a red blood cell could also encounter blood-borne viruses such as HIV, AIDS, or hepatitis B.