Every minute of every day, someone needs blood. That blood can only come from a volunteer donor, a person like you who makes the choice to donate. There is no substitute for your donation. When you make a blood donation, you join a very select group. Currently only 3 out of every 100 people in America donate blood. From its beginning, the American Red Cross has formed a community of service, of generous, strong and decent people bound by beliefs beyond themselves. The American Red Cross blood donor embodies this principle. Please join us in our mission to maintain a safe and stable blood supply by making your appointment to donate blood today. Source: “Give Blood.” RedCross.org. The American National Red Cross, n.d. Web. 27 Apr. 2011. Based on this excerpt, what is the author’s primary purpose for writing?