Consider the term "washed in the blood": nothing in modern culture corresponds to the idea of blood as a cleansing agent. We use water, with soap or detergent, to clean. Blood is a soiling or staining agent, something we try to scrub off, not scrub with. What possible meaning could the hymn writer, and Bible writers before him, have intended?

The symbol of blood with its specific quality of cleansing appears throughout the Bible, from the earliest books to the latest....

I suggest a simple experiment if you truly wish to grasp the function of blood as a cleansing agent. Find a blood pressure test kit and wrap the cuff around your upper arm. When it is in position, have a friend pump it up to about 200 mm. of mercury, a sufficient pressure to stop the flow of blood in your arm. Initially your arm will feel an uncomfortable tightness beneath the cuff. Now comes the revealing part of the experiment: perform any easy task with your cuffed arm. Merely flex your fingers and make a fist about ten times in succession, or cut paper with scissors or drive a nail into wood with a hammer.

The first few movements will seem quite normal as the muscles obediently contract and relax. Then you will feel a slight weakness. Almost without warning a hot flash of pain will strike, after maybe ten movements. Your muscles will cramp. If you force yourself to continue the simple task, you will likely cry out in absolute agony. Finally, you cannot will yourself to continue; the pain overwhelms you.

When you release the tourniquet and air escapes from the cuff, blood will rush into your aching arm and a wonderful sense of relief will soothe your muscles. The pain is worth enduring just to experience that acute relief. Your muscles move freely, the pain vanishes, and life feels good again. Physiologically, you have just experienced the cleansing power of blood.

- Paul Brand, In His Image, from an excerpt, "Blood, Part 1: The Miracle of Cleansing,"
an article appearing in the February 18, 1983, issue of Christianity Today.