These days, most homes are equipped with more than one bathroom. Despite the enormous variety of baths, in size, shape, and color, that make every bathroom unique, there is one commodity that is common to every single bathroom. The object in question is small, unassuming, and costs little, but we couldn’t manage without it. Its name? The toothbrush.

The toothbrush lives in its own little stand, sometimes poking its head furtively out of the cup. When new, its bristles are proud and long, each one standing straight and strong. Gradually, as it ages, its head bends and slowly the bristles fall. But young or old, the ubiquitous toothbrush is everybody’s friend. You want your smile to be bright and white – so use the toothbrush regularly. Brush your teeth twice every day; in your mouth they are sure to stay.

Brushing one’s teeth is a simple procedure, requiring such little skill that little children can perform the task admirably. Yet, everyone understands that the appropriate tool for the job, our toothbrush, needs to be specially designed, manufactured and sold. What would you say about a particularly difficult cleaning job, cleaning not a solid surface, but cleaning a liquid? What would you say about an appliance that cleans our blood?

Complex Chemical Factories

Why should blood require cleaning? A car emits exhaust fumes.
A heating furnace gives off smoke and soot. These are waste products. The body, with its numerous chemical reactions, likewise produces wastes. Most body wastes are end products of chemical reactions that are carried out in cells, as the cells build up new substances and break down old ones. Living things are like vast, complex chemical factories. Hundreds of chemical changes and reactions occur every single second in each living cell. Bear in mind that the size of an individual cell is so small that several hundred of them could fit onto the period at the end of this sentence, that there are trillions of cells in a human body, that each and every one is producing waste, and that all this waste has to be collected and eliminated. You begin to realize the enormity of the task. Examples of waste include urea, containing broken-down proteins, and creatinine, produced when muscles contract. These wastes are collected by the blood, the body’s main transport fluid. They then need to be filtered. And that is no easy task.

Just consider the job at hand. All the blood in a person’s body has to be filtered many times each day. The filtering process must not permit red blood cells or large particles of essential blood proteins to pass through; otherwise, they might be lost to the body with rapid and calamitous results. In fact, 90 percent of the fluid that passes through the filters has to be reabsorbed. Essential vitamins, amino acids, glucose and hormones must be returned to the bloodstream.

Imagine you eat a large portion of kippered herring, or any particularly salty food. If the high salt content was not removed by the filtering system, there might be real danger. Salt holds water. If it were permitted to remain in the blood, excess fluid would start accumulating in the blood, and in the spaces between the cells. The consequences of this could be dire. Potassium is another chemical substance that needs strict control. It is obtained mainly from meat and fruit juices. If there is too little potassium, muscles begin to fail, particularly breathing muscles. But a pinch too much acts as a brake on the heart.

What type of machine can cope with such a variety of tasks?

The kidneys.

The Body’s
Master Chemist

The kidneys are two of the body’s most remarkable organs. Just 4.5 inches long (that’s smaller than a toothbrush), and weighing just six ounces, the kidney is the master chemist of the human body. The two kidneys sit on either side of the backbone, protected by the lower ribs. The diminutive maestro receives a plentiful blood supply, about 17.5 gallons every hour. This means that the body’s blood passes through them 400 times each day. The blood enters the kidneys through the wide renal artery, and very slightly less goes back along the renal vein. This slight decrease in volume means that the kidneys are performing their vital cleansing job. The kidney’s chief task is to filter the blood through microscopic filtering units called nephrons, and collect from it the wastes and unwanted substances to form urine. A single filtering unit in the kidneys would be sufficient reason to dance with joy. In fact, each single kidney contains approximately one million separate nephrons.

Every nephron is a wonder. Housed in the outer part of the kidneys, the microfilters receive blood. Because of the high pressure in the tiny capillaries that carry the blood (caused by the tortuous path through the capillaries that offers a resistance to the flow of blood), fluid is forced to filter out through the capillary walls, and is collected in an area called the bowman’s capsule. Microscopic droplets of this waste-laden fluid pass out of each of the million nephrons and feed into a tiny reservoir at the center of the kidney. This in turn connects with the bladder, and the bladder with the outside. (You might be interested to know that the bladder is made of a special type of tissue that is strong and stretchable, and is resistant to the corrosive chemical nature of urine.) Wavelike muscular action occurs every 10-30 seconds, pushing the fluid along the exit tubes. At night, the activity slows down to one-third of daytime levels, permitting undisturbed sleep.

Selective Reabsorption

If all the substances that were filtered by the million nephrons would pass out as urine, the body would be sadly depleted. The reason that it doesn’t happen is due to the remarkable workings of the kidneys. A series of various-sized pores in the nephron microfilters retain selected chemicals – according to the size, shape, and other features of their molecules – and allow other chemicals through. Red blood cells are large, and do not pass into the filtering system. All amino acids, glucose, arid much of the water and some of the salts are reabsorbed into a network of capillaries surrounding the tubes. This selective reabsorption prevents the loss of useful substances from the blood, and regulates its composition. The remaining liquid is now called urine, and contains only the waste products that the body does not need.

The kidneys are incredibly efficient. Every minute, about 0.24 pints
of water and chemicals pass from the blood into the tiny tubes.
Yet, so much water and useful substances are taken back from the tubes into the blood that only 0.002 pints of this is left as urine.

Certain things can increase the kidney’s activity. When you are cold, for example, the blood supply to the skin is reduced to preserve internal heat. (That is why when people are cold, they look pale.) This means an increased flow of blood to internal organs – including the kidneys. The more blood that flows through, the more urine collected. In times of heat, the system is reversed. The small blood vessels in your skin become wider, so that more blood flows near the surface and gives off its extra heat to the air. You look flushed. Less blood to the internal organs, and the kidneys, and less urine is gathered. In times of anger, blood pressure rises, and the kidneys receive an extra supply of blood for processing. Result – increased urine output. This is another good reason to remain calm.

Alcohol has the opposite effect. On the underside of the brain lies the pituitary gland, which produces a hormone (chemical messenger) that controls the kidney’s activity. Left to itself, the kidney might produce excess urine, resulting in dehydration. The hormone prevents this. Alcohol does not affect the kidneys directly, but it does dull the brain, retarding the production of the braking hormone. The kidneys, with the brakes removed, go into high gear, producing urine more rapidly. Caffeine in coffee has precisely the same effect.

“Many Openings and Cavities”

The twin master-chemists, the two kidneys, silently and efficiently perform their vital tasks. Not for nothing does the berachah recited after emptying one’s bladder speak about “many openings and cavities.” Untangled and stretched out, the tubules in each kidney extend to some 70 miles of wondrous construction, complex and super-efficient design, cleaning the blood and keeping the body healthy! The berachah is more than justified.

Everyone can see that a toothbrush is designed, the result of intelligent manufacture. Are the kidneys simpler, or more complicated? You don’t have to be too wise to recognize wisdom. You just have to be honest. All this points to an undeniable fact that the kidneys, along with the rest of the body, was created by a master Designer.

Tuvia Cohen, is a humorist, scientist, and an accomplished author. Adapted from an article that appeared in The Jewish World of Wonders.