Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News & Views is delighted to present this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

A single-celled organism is like a microscopic island of life, in that it can get what it needs to live from the surrounding water in which it is suspended. The chemicals it needs to survive are present in the water that bathes its plasma membrane. However, the human body is surrounded by air, not water, and consists of trillions of cells, most of which are not in direct contact with its environment.

Yet, in general, the body needs many of the same chemicals to survive as does a single-celled organism. Rather than a microscopic island of life, the body is like a huge landmass where each cell is separated from the environment by other cells. Therefore, unlike a single-celled organism, human cells are unable to get what they need directly from the environment. How does the body solve this problem?

The human body is made up of many different organ systems that together bring in the chemicals it needs and delivers them to its cells. In other words, the different organ systems do for the body what a single-celled organism can do for itself. This means that each human cell needs the other cells in the body to survive. Some cells work to bring in oxygen, others work to bring in water, salt, sugar, and other important chemicals, and still others work to deliver these chemicals to where they are needed. How is this accomplished? Let’s begin at the beginning.

Each person comes into being when the sperm of his father joins with the egg of his mother to form a new one-celled human being, a zygote. Half of the DNA in the nucleus of the zygote comes from the father and half comes from the mother. The DNA in the nucleus of each of the body’s trillions of cells is the same. Within a few days, the zygote doubles several times, going from one, to two, to four, to eight, and then to sixteen or more cells, becoming an embryo.

The embryo migrates and then nestles into the lining of the mother’s uterus, becoming connected to her body through the placenta and the umbilical cord. At this stage the new human being is unable to live outside of its mother’s uterus because, unlike the single-celled organism, it cannot provide itself with the chemicals it needs to live. This placental connection allows the developing embryo to receive what it needs in the way of important chemicals. Over the following weeks the cells of the embryo grow and develop into the different organ systems of the body. It is then called a fetus.

In the embryo, all of the cells look alike and all they do is grow and multiply. But as these cells develop into organs and tissues, they start to look very different from each other. This process of development is called differentiation. There are over two hundred different types of cells in the body which together make up its different organ systems and tissues. Each cell has been programmed by this process of differentiation to use only what it needs from the DNA in its nucleus to perform its job within the body. It takes about nine months before the fetus has developed enough to safely live outside the uterus. Here are a few examples of some of the organ systems we have and what they do to keep our cells, and us, alive.

When we breathe in and out, the respiratory system works to bring oxygen into the body and release carbon dioxide. When we drink and eat, the gastrointestinal system works to bring water, salt, sugar, and other important chemicals into the body. The nervous system tells us when to breathe, drink, and eat, and makes it possible for us to do so. It also makes us aware of our surroundings, allows us to think, and gives us the ability to be physically active. The bones give support and protection to the organs and provide a solid framework for the muscles, so we can move about and handle things. The renal system (kidneys) takes care of waste products from metabolism and balances the body’s water and chemical content. The skin provides a barrier to the outside world while performing numerous other tasks. The endocrine system helps the body control many aspects of its metabolism. And the reproductive system allows for new human life.

Finally, there is one more very important system that must be mentioned. After all, unlike the single-celled organism that lives in water and can get what it needs directly from its surroundings, our body has trillions of cells all packed together that cannot. Each of our cells needs to have many different chemicals to survive. The body must transport these chemicals to the cells in the tissues where they are needed. The blood that travels within the cardiovascular system performs the job of transport.

With these numerous organs and tissues, the body is able to follow the rules by taking control. But just as a human cell can die if it doesn’t have enough energy or loses control of its chemical content or volume, so too, the human body can die if any of these organ systems does not meet certain functional parameters. For when it comes to combating the laws of nature, real numbers have real consequences for life and death. So how do we die? That’s the question we’ll consider next time.

Image by Ewan Munro from London, UK (Baby HeadUploaded by tm) [CC BY-SA 2.0], via Wikimedia Commons.