Most of us may not know that the kidneys also play a role in regulating blood sugar levels in our body. This unawareness can be caused by, among others, the lack of information on this matter or the limited time to find out, because it is generally assumed that the function of the kidneys is only to remove toxins or water no longer needed by the body. In fact, kidney's functions are more diverse. The functions include filtration, absorption, secretion, and excretion. In clinical practice, kidney functions are described as regulating blood pressure, removing toxins and electrolyte waste, regulating the body's acid-base levels, producing hormones (such as erythropoietin, vitamin D), and regulating water and sodium balance. In addition, the kidneys also play a major role in regulating blood sugar levels through a certain mechanism. Therefore, it is natural that these various functions are less known. Unfortunately, unawareness about these various functions can lead to fatalities, moreover the characteristics of chronic kidney disease generally do not exhibit an early alarm. This is what makes chronic kidney disease called a silent killer.

Physiologically, when our blood sugar is above 190 mg/dl, the kidneys will automatically and actively dispose of excess blood sugar into the urine. If a person's urine is tested with Fehling A and Fehling B solutions and the results are positive, this means that the person has diabetes mellitus. The number is the threshold value of blood sugar in the kidney system from the body that was originally normal. If the blood sugar condition remains higher than the normal value, pathophysiologically the threshold value will continue to shift towards 260 mg/dl. However, if blood sugar levels remain high, the threshold value will shift towards 260 mg/dl, which can lead to wrong urine sugar level results (glucosuria). Furthermore, if a person is undergoing certain medications, for example, currently taking SGLT-2 (Sodium Glucose Transport-2 Inhibitors) drugs, the threshold value, which is originally 260 mg/dl, will shift to 160 mg/dl. This is a general illustration of kidneys' role in regulating blood sugar.

Dear readers, it is essential to pay attention to this topic as it is incredibly interesting and beneficial to understand. Knowing about this topic is not only beneficial for ourselves but also for our loved ones, particularly if we want to prioritize our well-being and that of others in our lives. In our society today it is estimated that more than 10% suffer from diabetes. This means that, out of every 100 adults, there are more than 10 people who suffer from diabetes. This figure is very high and in the end will be very burdensome, especially for health expenditure budget. Moreover, those people are also at very high risk of various attacks such as stroke, heart disease, and blindness, both due to the damage to the retina and to cataracts. Diabetes also attacks other organs of the body, including the kidneys. As an illustration of the malignancy of diabetes to the kidney, out of 100 people who undergo dialysis at a service point, around 40 people have already had diabetes. Not surprisingly, diabetes occupies the first position, surpassing hypertension, in attacking the kidneys. 

Our kidneys are remarkable organs that not only consist of Bowman's capsule components and tubules, but are also predominantly made up of blood vessels and capillaries. These blood vessels, including afferent arterioles, glomerular capillaries, efferent arterioles, peritubular capillaries, and vasa recta, also play a crucial role in regulating blood sugar levels in our body. When sugar is present in the blood, it is transported along the blood vessels. Typically, most of the blood sugar is returned to circulation, while any excess is removed from the circulation and combined into a substance known as ultrafiltrate. The ultrafiltrate then collects in a space called Bowman's capsule before draining into the proximal tubule of the kidney. This pathway is the place where urine products are processed into urine, and sugar is regulated to be either reabsorbed into circulation or excreted into the urine along with other toxins and excess substances such as water, electrolytes, peptides, and the toxin urea.

Public in general should know the anatomy and physiology of the kidney, especially medical students who must fully comprehend the anatomic-physiological relationship regarding blood sugar regulation in the kidney. Students must understand the course of blood from the renal artery to the afferent arteriole and to the glomerular capillaries and finally to the efferent arteriole. The efferent arteriole is unique as it is the only site where capillaries flow into arterioles (which usually lead directly to venules). In summary: blood flows from the afferent arteriole to the glomerulus, then to the efferent arteriole. If the afferent arterioles before the glomerular capillaries constrict, blood flow into the glomerulus will decrease, so that the glomerular filtration rate (GFR) also decreases. If the efferent arterioles after the glomerular capillaries constrict, peritubular blood flow decreases, and the GFR increases. This is the kidney's mechanism for maintaining a constant GFR to maintain proper kidney function. If the kidneys work too hard, the system will decompensate and the kidneys will be damaged as the homeostatic system in the kidneys works very hard without stopping.

As an illustration, when there is reduced oxygen and nutrients in the kidneys or a decrease in sodium load as detected in the macula densa of the distal tubule, a signal will be sent the juxtaglomerular system, the prostaglandin and the Renin Angiotensin Aldosterone System (RAAS). Prostaglandin causes vasodilation in the afferent arteriole, while the RAAS system, especially angiotensin-2, causes vasoconstriction of the efferent arteriole. This results in hyperfiltration by the glomerulus (intraglomerular hypertension) in response to decreased perfusion or sodium load. This basic concept must be understood by medical students.

Physiologically, the body filters around 180 grams of sugar every day, and all of the sugar is absorbed by the proximal tubules so that no sugar is present in the urine, which can be confirmed with urine test using Fehling A, Fehling B system. Actually, the proximal tubule's capacity cannot be separated from the presence of the S1, S2, and S3 segments of its receptors. As much as 90% of the sugar is absorbed by S1 and S2 segments through their SGLT2 (sodium-glucose linked transporter 2) receptor to prevent the wasting o tthe sugar into the urine. This mechanism has inspired the experts to invent an efficacious drug by exploring thoroughly the superiority of SGLT2 receptor. This effort turned out to be successful and now the drug is already on the market with high effectiveness.

The sugar that is absorbed back into blood circulation through the transporters or protein ligands is processed in such a way that the glucose binds with sodium and finally re-enters the blood. This process does not require ATP energy, but it is still regarded as active transport because it uses Na+ gradient that enters the proximal tubule. The absorption is not 100%. The remaining 10% is unabsorbed, but the body provides the S3 segment to absorb the remaining 10% through a receptor called the SGLT-1.

This natural mechanism proves that the experts can just follow what has been outlined by the Creator of the Nature. Undoubtedly, the existence of this new drug for diabetes (type-2) further proves that the role of the kidneys greatly contributes to blood sugar metabolism regulation. It is remarkable how such a small organ, the size of our fist, plays such an amazing role. Therefore, it is essential to take care of and love our kidneys. []

Djoko Santoso
Professor, Faculty of Medicine, Universitas Airlangga
Chairman of Health Department, 
Indonesian Council of Ulama, East Java

Translated from:
Peran Ginjal pada Homeostasis Gula Darah Tubuh
by Djoko Santoso
https://s.id/PeranGinjalpadaHomeostasisGulaDarah