Kaori Hayashi: The Role of Healthy Kidneys in the Body

What image comes to mind when you hear the word "kidney"? Through our daily life activities, we produce many waste products inside our bodies. The kidneys are organs that quietly and diligently process these products and excrete them by creating urine. Therefore, many people likely think that if the kidneys fail, it will be problematic because they will no longer be able to produce urine and excrete waste products from the body.

However, the kidneys are not just organs that produce urine and excrete waste.

In fact, their functions in supporting health are extremely diverse. While working in close coordination with various organs throughout the body, they play a major role in maintaining systemic homeostasis.

A single kidney contains about 1 million units called nephrons, which consist of small filtration devices about 200 micrometers in size called "glomeruli." These filter the blood sent from the heart, remove water and waste products, and finally excrete them from the body as 1 to 2 liters of urine. During this process of making urine, the kidneys simultaneously adjust the balance of electrolytes such as sodium and potassium, as well as the acid-base balance. For example, when you consume too much salt, the kidneys excrete sodium into the urine; conversely, when it is lacking, they return it to the body.

In fact, the kidneys also produce hormones. Examples include "renin," a hormone closely related to high blood pressure, and "erythropoietin," a hematopoietic (blood-producing) hormone. Additionally, the kidneys play a role in keeping bones strong by activating vitamin D. This is why anemia or bone abnormalities often occur when kidney function declines.

In recent years, the deep connection between the kidneys and other organs has attracted attention. When kidney function declines, heart disease, cerebrovascular disorders, and immune abnormalities become more likely to occur, and it has been pointed out that the risk of cognitive decline and depression may also increase. Conversely, high blood pressure, heart disease, and diabetes can cause the kidneys to deteriorate. In other words, the kidneys maintain the internal environment in cooperation with organs throughout the body.

Furthermore, the relationship between aging and kidney disease has recently come into focus. Since kidney function gradually declines with age, the number of cases of chronic kidney disease is increasing against the backdrop of an aging population. In Japan, it is now said that 20 million people—about 1 in 5—have chronic kidney disease.

Our research has also yielded results suggesting that kidney aging may affect various organs such as the heart, liver, and adipose tissue. It is believed that the kidney, acting as a hub for "multi-organ linkage," is deeply involved in the aging of the entire body.

What happens when the function of the kidneys, which play such multi-faceted roles, declines? The kidney is known as a "silent organ," and because symptoms often do not appear at all until function has declined significantly, regular health checkups are necessary for early detection. When kidney function drops to a level where maintaining life is difficult, "renal replacement therapy" (hemodialysis, peritoneal dialysis, kidney transplant, etc.) becomes necessary.

However, the need for renal replacement therapy is not the only problem when kidney function declines. In fact, even at a stage where symptoms of decreased kidney function have not yet appeared, the risk of developing cardiovascular complications increases about threefold, and it has been suggested that decreased kidney function is related to life prognosis. It has been reported that these risks to cardiovascular complications and life prognosis are also related to the annual rate of decline in kidney function. It is important to detect the decline in kidney function early and provide treatment to prevent further deterioration. Recently, the number of options for new therapeutic drugs that slow the rate of decline and protect the kidneys has been increasing.

As described, the kidneys do more than just produce urine; they function as a "keystone of the body" that affects blood pressure, electrolytes, bones, blood, immunity, cardiovascular health, and even systemic aging. Along with receiving regular health checkups, striving to improve conditions that affect kidney function—such as high blood pressure, diabetes, hyperuricemia, dyslipidemia, and obesity—will lead to protecting your kidneys.

If kidney function declines chronically, it is said to be difficult to restore it to its original state. However, the options for new medications are increasing, and we provide daily medical care to prevent the decline of kidney function as much as possible. Hoping for further progress in the treatment of kidney disease, we will continue to dedicate ourselves to the clinical care and research of kidneys every day.