Dear Peter, in the earlier issues my friends heart and stomach spoke with you and I patiently waited for my turn. I am located at the waist level to the right of the spine and my twin left kidney on the left, on whose behalf I’m also speaking now. We are truly vital organs to you. Before you say “each one of you claims that it’s vital,” let me talk a bit first, and then you decide.

Dear Peter, in your body, which is built like a well-functioning factory, I am the most essential sanitary device. As the heart pumps blood to take food and oxygen to all your organs, you obtain the energy you need. But do you ever think about the disposal after so splendid activities are carried out in your body? If you burn a stove, you need to dispose of the smoke and the ashes if you want to keep it working. Likewise, as you burn your calorie intake, you dispose of the smoke (carbon dioxide) with your lungs and the waste with nitrate-poisonous after a certain level of intensity-thanks to my quietly and perfectly working filters. That is, I am an organ which saves your life by removing the poisonous substances in your blood. I do not only filter your blood, but also play a role at controlling the balances of the sensitive levels of water, sugar, amino acids and different minerals in your body. In short, I am a tiny but strategic laboratory.

The acid-base balance in your body and the amount of water and different salts are significant values which concern all of your bodily activities. When their balance is upset, different troubles arise in different units of your bodily mechanism. I am such a blessing, which works so sensitively to adjust the levels of water and mineral salts in your body while you don’t even realize it.

A watery environment is needed for the thousands of biochemical activities taking place in your body. In addition, activities like the contracting of the muscles and transmission of electrical stimuli between your neurons are realized with the presence of a pinch of mineral salt you don’t even give a thought about. Sometimes you sweat due to running or hot weather. The white stains on your shirt are the salts you lose after the water evaporates. When you suffer from diarrhea, you lose salt again, for the salt in the nutrients are thrown out without being absorbed. Especially when little children are concerned, this loss is of vital importance. The transfer between the blood and the liquid in the tissues is mainly realized through the concentration differences which are kept in a certain balance. If your body holds too much water, your tissues swell. You particularly feel it when you press your finger over the flesh near your shins.

Dear Peter, I will not list all of my duties in detail in order not to confuse you. However, let me tell you one more. Since not everyone knows this duty of mine, they just see me as an organ of liquid disposal. But I also take part in controlling the blood production! Surprised? Well, I also have the duty of secreting the hormone which stimulates blood production in the bones. I must always stay alert and maintain this balance in the best way; if you start losing blood, for instance, I must increase the hormone and accelerate blood production.

Yes Peter, I’ve told you about a few of my basic duties, but haven’t told you about how wonderfully designed I am. I am a bean-shaped organ and a single kidney like me weighs 130-160 grams on average. We are approximately 10 grams lighter in female bodies. I am surrounded by a soft but protective membrane. I need 35 grams of daily oxygen supply to survive and I use 13 % of your total body energy.

As an army is made up of individual soldiers, I am like a complex army, and a single soldier of mine is called a “nephron,” which does the real job. Thus, you can see me as a body of nephrons. Millions of these nephrons are brought together to make up one kidney.

A single nephron is a thin tubular structure with closed ends, and its length is about 3-4 cm. So the total length of my nephrons is about 50 km. The cup-like sac at the beginning of a nephron is named the “Bowman’s capsule.” The main artery bringing blood to the kidneys branch into smaller units, and one road leads to every nephron. The knot of capillaries (glomerulus) inside this double-walled capsule is more complicated than any road map you might have seen. The total length of the capillaries is nearly 25 km. The unwelcome substances in the blood are passed to the capsule thanks to blood pressure, and they proceed through the tubule. The total surface area of my tubules is about 20 m2. Within five minutes the whole of your blood passes through us. That is to say, an average of 1.2 liters of blood per minute, and 1800 liters a day are filtered by my nephrons, leaving the toxic substances in me. As this amount of blood (nearly 400 times the normal amount in your body) pass through my tubules and return to the veins they leave behind an average of 180 liters of liquid in me. In this case, you could be supposed to throw out 180 liters of urine a day. However, if you really did that, you would neither be able to find a sufficient supply of water nor salt. Fortunately, Providence granted you the mechanism to absorb back nearly 178.5 liters of this filtered substance. This way, the thickened urine throws out the toxic nitrogen-containing byproducts together with a little amount of water. Therefore, I give you back the substances you need with an amount of 1.5 liters of liquid disposal a day. This reabsorbing is realized in what you call the Henle loop. The cells in the walls of my tubules have neither intellect, nor consciousness, nor any knowledge of physiology; in spite of this, they work as if they were perfectly aware of their duty to adjust the amounts and types the substances to be kept or released. To give you an idea, you can think about the huge dialysis machines your engineers design to fulfill the job my tiny tubules do. You decide which one of us is the perfect work of engineering.

A rich network of veins surround the Heinle loop and the reabsorbed substances are released into the bloodstream. The drops to be disposed of are collected and then passed to your bladder. When the liquid in your bladder amounts to a certain value it tightens the bladder walls. The muscles blocking the way out give way and the urine is disposed of. However, know that the bladder has the capacity to expand as large as 1.5 liters when you cannot find the proper time and place.

Peter, most people take for granted the blessings they enjoy. If you visit the nephrology service in a hospital and talk to the patients waiting to be dialyzed, you understand what I mean better. Do not forget that many people who suffer from kidney failure need that huge machine to filter the whole of their blood and they pray for a kidney suitable for transplantation to be found at once.

Incidentally, thinking of chronic kidney failure recalled various reasons which ruin me: long lasting infections, long-term use of certain medicine, different chemicals like ethylene and mercury, heavy loss of blood, high blood pressure, serious burns, and wrong blood transfers etc. In such cases I can receive irreparable damage.

Another issue which troubles me is the formation of kidney stones. The stones which form owing to failures in different metabolic processes really hurt. When excessive decrease of liquids or increases of salts in the body upset my sensitive balance, some dissolved substances remain, begin to collect, and form stones. These stones hinder urine flow and might cause infections. You may drink water abundantly to prevent these stones. Most importantly do not wait too much before going to the toilet. If you excuse me, I also strongly recommend you to urinate in sitting position; this helps emptying your bladder completely and reduces the risk of kidney stones.

I do my job properly until I lose 90 % of my working capacity. When a considerable part of me loses its capacity, the remaining good part boosts its activity to make up for the loss. When one of our twins are taken out with an operation, the other one does not complain at all; it grows a bit bigger and keeps working.

As the nature of honey depends on the nectars bees collect, the ingredients of the urine I produce depend on what’s in your body. Therefore, a urine analysis tells much in the case of illness. For example, I normally do not release valuable substances like glucose and protein in your blood into the urine, but return them to the bloodstream. As my friends cannot fulfill this function in diabetic patients, their urine analyses reveal glucose. As for medicines, I throw them out right away since they are alien substances to me.

Peter! You are young and healthy but be careful and do not get cold around the waist, otherwise I might trouble you. There’s a lot to tell you Peter, but I do not want to confuse you. Like any other organ, I do not like being taken for granted and I just wish for you to appreciate what a blessing I am. Let me note that my perfect cooperation with the rest of your body is another wonder in itself. Anyway, the urea in your blood is increasing, so I must go help my twin now. Goodbye Peter!

Irfan Yilmaz is a professor of biology at Dokuz Eylul University, Izmir.