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 is delighted to offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

If a unicellular organism is like a microscopic island that can get what it needs and get rid of what it doesn’t need through its surrounding water, the body, consisting of trillions of cells, is like a huge land mass that must move what it needs and doesn’t need in and out of its interior so it can survive. The cells of the body need the right amount of oxygen and carbon dioxide to work right, so the lungs, at the direction of the respiratory center in the brain, take care of that.

They also need to have the right amount of water, sugar, salt, and other nutrients, which is taken care of by the gastrointestinal system, at the direction of the nervous system. And to transport what it needs into and out of its continental mass of cells, the body uses the cardiovascular system to move blood containing these chemicals to where they need to go so they can be on- or off-loaded.

Evolutionary biologists are good at imagining how the molecular structures for life may have come about, as long as they don’t have to reckon with the objective standards those structures must meet to keep the cells and the body alive. To survive within the laws of nature, the cell must be able to control its volume and chemical content. So too, for the body to survive within the laws of nature, it must also control its volume and chemical content.

Stephen Meyer has described the complexity of the cell, the numerous systems within it, and what they must do for it to work properly. But each cell, no matter where it is located, is blind to the overall needs of the body. We have seen this in how the water content and blood levels of sodium, potassium, calcium, and nitrogen (protein) affect organ function and body survival. The common pathway the body uses to control all of these chemical parameters leads through the kidney.

The functioning unit in the kidney is the nephron, and there are about one million per kidney. The nephron filters fluid out of the blood by squeezing it through a specialized capillary system called the glomerulus. The kidneys filter about 7.5 liters of fluid, with its chemical content, out of circulation per hour. This fluid enters tubules, which wind their way through the tissue of the kidney on its way to becoming urine. As the fluid moves along the cells lining, the tubules reabsorb or secrete different chemicals to the degree that is necessary for body survival.

The body is always taking in different amounts of various chemicals through the gastrointestinal system, while simultaneously losing them through metabolism. Therefore, the ongoing chemical needs of the body are always in flux and the kidneys must constantly adjust to these changes by changing how much of a given chemical they keep or release from the body through the urine. We will look at the five vital chemicals mentioned above, water, sodium, potassium, calcium, and nitrogen, and explain how the body, through the kidneys, adjusts them to stay alive.

Evolutionary biologists may be good at describing how kidneys look and imagining how they evolved, but they never seem to mention how they work or what they would have had to do to keep the transitional organisms they belonged to alive.

Water is vital for life and is the commonest molecule in the body, making up sixty percent of its weight. Two-thirds of the body’s water is inside the cells and one third is outside, either between the cells or inside the blood. If the body loses one-quarter of its water (10 liters), it dies. Since the kidneys filter 7.5 liters of fluid per hour this means that if they didn’t take back any of it, the body would die in about ninety minutes.

Water can move freely in and out of the cell, so cell volume reflects the body’s water content. In general, if the body’s water content is below normal, then the volume of its cells will be below normal as well. The hypothalamus contains shrink-sensitive cells that can detect this drop in cell volume. These osmoreceptors react to worsening cell shrinkage by making more Anti-Diuretic Hormone (ADH) be released. ADH travels in the blood and attaches to specific receptors on certain tubules within the kidneys and tells them to bring more water back into the body from the urine in production. By using osmoreceptors in the brain, ADH, and its specific receptors on certain kidney tubules, the body is able to take of control of its water content.

Sodium is vital for life and dissolves in the body’s water as an Na⁺ ion. The fluid outside the cells contains about ninety percent of the body’s total sodium and is ten times more concentrated than the fluid inside the cells. Since water generally follows Na⁺ ions wherever they go in the body, this means that the relatively high concentration of sodium in the fluid outside the cells is responsible for not only its water content but also the blood volume. Much like water, if the body loses about one-quarter of its sodium, it dies. The amount of sodium in the blood is so high that the 7.5 liters of fluid the kidneys filter out per hour contains about one-half of the body’s total sodium. If the kidneys didn’t take back any of this filtered sodium, the body would die in about a half hour.

Since blood volume is dependent on water content and water content is dependent on sodium content, this means that the wall motion that takes place as blood flows into a blood vessel or chamber is a reflection of the body’s sodium content. One set of sensors, called mechanoreceptors, detect this wall motion within the kidneys, where blood enters to be filtered, and another is in the walls of the atria. The sensory cells in the kidneys release a hormone, called renin. The amount of renin released is inversely related to how much wall motion the sensors detect. The more the walls stretch, indicating more blood volume, the less renin is sent out, and the less the walls stretch, indicating less volume, the more renin is sent out. In contrast, the atrial cells send out a hormone, called Atrial Natriuretic Peptide (ANP), in an amount that is directly related to how much wall motion they detect. The more the walls stretch, indicating more blood volume, the more ANP is sent out, and vice versa.

Renin results in the formation of a hormone called angiotensin II which binds to specific receptors in the adrenal glands and tells them to release another hormone called aldosterone. Aldosterone travels to the kidneys and attaches to specific receptors on the cells lining some of its tubules. This tells them to bring more sodium back into the body. So, the less blood volume, the more renin, resulting in more angiotensin II and aldosterone, and more sodium the kidneys reabsorb. In contrast, the more blood volume, the more ANP attaches to specific receptorson the same tubules in the kidneys and tells them to release more sodium. In other words, the effects of renin and ANP counterbalance each other. By using mechanoreceptors in the kidneys and atria, renin and ANP and specific aldosterone and ANP receptors on certain kidney tubules, the body is able to take of control of its sodium content.

Potassium is also vital for life and dissolves in the body’s water as a K⁺ ion. The fluid inside the cells contains about ninety-eight percent of the body’s total potassium and is over thirty times more concentrated than the fluid outside the cells. The relatively low K+ ion level in the fluid outside the cells must be maintained within a very narrow range to make sure the difference between the electrical charge inside and outside the cell allows for proper heart, nerve, and muscle function. The relative amount of potassium in the blood is a lot lower than it is for sodium, and if the kidneys did not bring any of it back from the 7.5 liters of fluid it filters per hour, the body would die in about a day.

The body uses sensors in specialized cells within the adrenal glands to detect the ratio between the K⁺ and Na⁺ ion concentration in the blood. If the ratio rises, due to an increase in K⁺ ion concentration or a decrease in Na⁺ ion concentration, these cells send out more aldosterone. Conversely, if the ratio drops, due to a decrease in K⁺ ion concentration or an increase in Na⁺ ion concentration, it sends out less aldosterone.

Aldosterone travels in the blood and attaches to specific receptors on the cells lining certain tubules in the kidneys and tells them to release K⁺ ions out through the urine and bring Na⁺ ions back in. More aldosterone, due to an increase in the ratio between K+ ions and Na+ ions, makes more K+ ions leave the body and more Na⁺ ions come back in. Less aldosterone, due to a decrease in this ratio, makes less K⁺ ions leave the body and less Na⁺ ions come back in. By using receptors that detect the ratio between K+ and Na+ ions in the adrenals and aldosterone and its specific receptor on certain tubules in the kidneys, the body is able to take control of its potassium content.

Calcium is vital for life and the bones of the body house over ninety-nine percent of its content. However, the remaining one percent is just as important for survival. Calcium dissolves in the body’s water as Ca⁺⁺ ions and its concentration in the blood is about ten thousand times more than within the cell. Besides creating the skeleton, bone also acts as a reservoir for the calcium needs of the body, which include heart, nervous, glandular, muscle function, and clotting. The total content of calcium is over one thousand milligrams and if the kidneys did not bring back any of it from the 7.5 liters of fluid that it filters per hour, the body would lose its entire supply in about two months.

The cells of the four parathyroid glands have sensors that can detect the calcium level in the blood. In response to a drop in serum calcium, they release more parathormone (PTH). PTH travels in the blood and not only makes the bone release more Ca⁺⁺ ions into the circulation but tells the kidneys to activate Vitamin D so the gastrointestinal tract can absorb more calcium. It also attaches to specific receptors within the tubules and tells them to bring more calcium back into the body. By using calcium sensors, PTH, and its specific receptors in the kidneys, the body is able to take control of its calcium content.

Nitrogen is mainly present in the amino acids that make up the proteins of the body. Protein metabolism produces a highly toxic nitrogen-containing molecule called ammonia, which the liver converts into less toxic urea to be released from the body through the kidneys. The amount of fluid filtered by the glomeruli of the kidneys is called the Glomerular Filtration Rate (GFR) and is normally about 125 mL/min (7.5 liters/hr). The body’s ability to keep its blood level of nitrogen-containing substances under control is directly related to its kidney function.

Especially in people with long-standing hypertension and diabetes, worsening kidney function causes the level of urea and other nitrogen-containing substances to rise. In fact, when the GFR is less than ten percent of normal, severe weakness, nausea, and confusion are common symptoms. In addition, there is often retention of sodium and water. This results in fluid build-up in the lungs, which in turn can cause shortness of breath and high levels of potassium, both of which can lead to cardiopulmonary arrest. It is at this time that a person may be considered for dialysis, which artificially cleans the blood of urea and other nitrogen-containing substances and stabilizes its water, sodium, and potassium levels.

The kidney may not be as sophisticated as the brain or the liver, but it definitely has a lot of roles to play when it comes to human life. Each of the control systems mentioned above is irreducibly complex in that all of the parts must be present for it to do its job. And to get the job done right so the body can survive within the laws of nature requires a natural survival capacity — an inherent knowledge of what is required.

The word intelligence comes from the Latin words inter and lego which means to choose between, to choose one outcome from all possible outcomes. Most people would look at the complicated structure of the kidney and what it takes for the body to control its water content and blood levels of sodium, potassium, calcium, and nitrogen, and conclude that an intelligent agent, a mind, was at work here.

Funny thing about intelligence though; you must have it in order to detect it. One hopes, in the near future, students will learn the truth about how life really works, and not just how it looks, and with this knowledge see the inadequacy of neo-Darwinism as an explanation for biological origins.

Photo credit: © abhijith3747 — stock.adobe.com.