Kidney: anatomy and functions

The kidneys filter blood, remove excess water and waste products, and form urine, maintaining a stable internal environment. This affects blood pressure, fluid volume, electrolyte concentrations, and acid-base balance, so a "kidney problem" often manifests not as pain, but rather as changes in test results and overall well-being. [1]

The kidney's function is structured as a series of "fine-tuning" processes: primary filtration occurs in the glomeruli, followed by tubules that return essential substances to the blood and remove excess. As a result, the body loses not "everything," but precisely what needs to be eliminated, while maintaining sodium, potassium, glucose, water, and other important molecules within the required limits. [2]

The kidney is also an endocrine organ: it participates in the regulation of blood pressure through the renin-angiotensin system, influences red blood cell formation through erythropoietin, and is involved in the activation of vitamin D, which is important for bones and calcium and phosphorus metabolism. These functions explain why anemia and mineral metabolism disorders develop in chronic kidney disease. [3]

It is important to distinguish between "kidney disease" and "reduced estimated glomerular filtration rate": a decrease in the rate can be associated with age, dehydration, medications, or an acute condition, while a diagnosis of chronic kidney disease requires persistent changes and additional signs of kidney tissue damage. Current guidelines emphasize the need to assess risk based on a combination of filtration and albuminuria, rather than a single number. [4]

Table 1. The main functions of the kidneys and the consequences of failure [5]

Function	What exactly does the kidney do?	What may occur if there is a violation?
Removal of metabolic products	Removes urea, creatinine and other metabolites	Nausea, weakness, itching, loss of appetite, increased creatinine
Fluid balance	Regulates the volume of water in the body	Edema, shortness of breath when overloaded, dehydration when lost
Electrolytes	Supports sodium, potassium, bicarbonate	Convulsions, arrhythmia, weakness, acidosis
Pressure	Effect via renin and sodium	Persistent hypertension, risk of cardiovascular complications
Hematopoiesis and bones	Erythropoietin and vitamin D activation	Anemia, brittle bones, phosphorus and calcium disorders

How the kidney is structured: the framework, vessels, and urine drainage pathways

The kidney is a paired, bean-shaped organ located retroperitoneally on either side of the spine. The kidney is supplied by the renal artery and nerves, and the renal vein and ureter branch off through the hilum, where the lymphatic vessels also pass.

The cortex and medulla are distinguished within the kidney: the cortex contains the glomeruli and a significant portion of the tubules, while the medulla forms pyramids through which urine moves to the papillae and then to the minor calyces. This "flow architecture" helps concentrate urine and conserve water.

Urine then travels through the minor calyces, major calyces, renal pelvis, and ureter. Obstructions and stagnation are possible in this area, so with a stone or ureteral stenosis, pain and dilation of the renal pelvis may be more pronounced than with "silent" chronic kidney disease.

Externally, the kidney is protected by a capsule and surrounded by fatty tissue and fascial structures that stabilize the organ and cushion movement. With sudden weight loss, this support weakens, and some people may experience increased kidney mobility and discomfort, especially during exercise.

Table 2. Parts of the kidney and their role “in 1 line”

Structure	Where is it located?	Leading role
Cortex	Outer layer	Blood filtration in the glomeruli, start of urine processing
Medulla and pyramids	Inner layer	Concentration of urine, direction of flow to the papillae
nipples	The tops of the pyramids	Drainage of urine into the cups
Calyx and pelvis	Cavity system	Collection of urine and transfer to the ureter
Gates	Medial edge	Entrance of vessels and nerves, exit of the ureter

Nephron: How urine is formed

Each kidney consists of approximately 1,000,000 nephrons—microscopic "filtering units." A nephron includes the glomerulus and tubular system, where the blood is first converted into a filtrate and then finely tuned into the final urine. [11]

Filtration begins in the glomerulus: fluid containing dissolved substances passes through the filtration barrier into the capsule, while large proteins and blood cells are normally retained. Therefore, the appearance of significant albuminuria or blood in the urine often signals damage to the filtration barrier or inflammation. [12]

The tubules then return to the blood what the body needs to conserve and secrete what needs to be eliminated. The proximal tubule actively reabsorbs water, sodium, and glucose, the loop of Henle helps create a gradient for urine concentration, and the distal sections and collecting ducts "fine-tune" sodium, potassium, and water under the influence of hormones. [13]

This logic explains why some conditions cause edema and high blood pressure, while others cause dehydration and electrolyte imbalances, and why some medications act "locally" in different segments of the nephron. This is why a kidney assessment is always a combination of symptoms, blood tests, urine tests, and context, rather than a single indicator. [14]

Table 3. Segments of the nephron and what happens in each [15]

Segment	Main processes	Clinical "hint"
Glomerulus and capsule	Plasma filtration	Protein or blood in the urine often indicates glomerular filtration rate.
Proximal tubule	Massive reabsorption of water and salts	Here, a lot is “lost” in tubulopathies and toxic damage.
Loop of Henle	Creating conditions for urine concentration	The disorders affect the ability to concentrate urine
Distal tubule	Fine-tuning sodium and potassium	Important for potassium control and acid-base balance
Collecting tubules	Final adjustment of water and sodium	It strongly depends on hormonal signals and water regime.

How Kidney Health Is Measured: Filtration and Albuminuria

Blood creatinine and estimated glomerular filtration rate (EGFR), as well as urine albumin and albumin-to-creatinine ratio (ACR), are most commonly used for screening and monitoring. These indicators complement each other: filtration reflects the "power" of purification, while albuminuria reflects damage to the filtration barrier and the risk of complications. [16]

Chronic kidney disease is a persistent, usually at least 3 months old, impairment of kidney structure or function with clinical consequences. Current guidelines emphasize a risk-based approach: staging is determined not only by filtration but also by albuminuria, as the combination of low filtration and high albuminuria signifies a significantly higher risk of progression. [17]

Filtration categories are described by ranges: from normal or high to severely reduced and renal failure. However, values of 60-89 ml per minute per 1.73 m2 do not, by themselves, confirm chronic kidney disease unless there are other signs of damage. [18]

Albuminuria categories are typically divided into 3 levels: less than 30 mg/g, 30-300 mg/g, and more than 300 mg/g. Guidelines also note that it is important for people with chronic kidney disease to have filtration and albuminuria assessed at least annually, and more frequently as clinically necessary in those at higher risk.[19]

Table 4. Filtration categories in chronic kidney disease [20]

Category	Estimated glomerular filtration rate, ml per minute per 1.73 m2	Meaning
G1	90 and above	Normal or high
G2	60-89	Mild decrease, diagnosis depends on other signs
G3a	45-59	Mild to moderate reduction
G3b	30-44	Moderate or severe decrease
G4	15-29	A sharp decrease
G5	below 15	Renal failure

Table 5. Albuminuria categories and what they mean [21]

Category	Albumin to creatinine ratio, mg/g	Interpretation
A1	less than 30	Norm or minimum increase
A2	30-300	Moderate increase, damage marker
A3	more than 300	Marked increase, high risk of progression

Common kidney problems and how they manifest themselves

The most common causes of chronic kidney disease in adults are diabetes mellitus and hypertension. Symptoms may be absent for a long time, with the first "clues" appearing in tests: increased albumin in the urine, decreased filtration, and changes in urine sediment. [22]

Acute kidney injury is a rapid deterioration in kidney function, usually due to severe illness, dehydration, decreased blood flow, infection, toxic exposure, or urinary tract obstruction. Symptoms can be nonspecific, including decreased urine output, nausea, lethargy, and increasing swelling, so early laboratory diagnosis is important. [23]

Urinary tract infections involving the kidney often cause fever, flank pain, chills, and urinary changes, while stones cause typical renal colic with radiating pain and often blood in the urine. These conditions are often clinically "noisy," but renal function may be preserved between attacks, so it is important to distinguish acute from chronic processes. [24]

A separate group includes glomerular diseases and systemic inflammatory processes, in which the leading symptoms are protein in the urine, blood in the urine, edema, and increased blood pressure. Here, it is especially important not to delay examination, as early identification of the cause changes the prognosis. [25]

Table 6. Symptoms and situations that require rapid kidney evaluation [26]

Sign	Possible cause	What is usually checked first?
A sharp decrease in urine, increasing swelling, shortness of breath	Acute kidney injury, fluid overload	Blood creatinine, electrolytes, urinalysis, ultrasound
Blood in the urine, especially visible to the eye	Stone, tumor, inflammation, glomerular lesion	Urinalysis, ultrasound, risk assessment and further imaging
Severe pain in the side in waves, nausea	Stone, obstruction	Ultrasound examination, if necessary, computed tomography
Protein in urine, persistent high blood pressure	Chronic kidney disease, glomerular lesion	Albumin to creatinine ratio, estimated filtration rate, pressure monitoring
Fever and pain in the side	Kidney infection	Urinalysis, culture as indicated, assessment of complications

Modern diagnostics and what is usually done next

A basic diagnostic test for suspected chronic kidney disease includes serum creatinine with estimated glomerular filtration rate, urine analysis, and albumin-to-creatinine ratio measurement. This approach allows for a simultaneous assessment of function and the extent of damage, as well as a rough stratification of the risk of progression. [27]

If changes are confirmed, the next step is to identify the cause and complications: monitoring blood pressure, assessing blood sugar, lipids, signs of anemia, and mineral metabolism disorders. Guidelines for the management of chronic kidney disease specifically emphasize the prevention of cardiovascular complications and the treatment of complications of the kidney disease itself. [28]

Instrumental methods are selected based on the clinical need: ultrasound helps assess kidney size, the presence of dilated renal pelvis, cysts, and obstruction, while more complex imaging techniques are used when clear indications are present. In the case of acute deterioration in function, it is important not to "treat the analysis," but to find a reversible cause, such as dehydration, drug overload, or obstruction. [29]

For monitoring, the frequency of monitoring depends on the risk: in a stable condition, annual assessment of filtration and albuminuria is often sufficient, while at a higher risk of progression, monitoring is performed more frequently. This is important because the dynamics of indicators and the rate of functional decline are sometimes more informative than a single measurement. [30]

The key preventive strategy for most people at risk or with early signs of kidney damage is monitoring blood pressure and glucose, stopping smoking, maintaining a healthy weight, and having regular blood tests if risk factors are present. These measures are highlighted in guidelines as the most practical and impact prognosis. [31]