Kidney Functions: Filtration and Mineral Balance
The kidneys remove waste and regulate fluid, mineral, and acid-base balance. They also help control blood pressure, red blood cell production, and bone health, so function cannot be assessed by urine volume alone.
Kongpat and sinjang are two names for the same organ: the kidney.
The kidneys filter the blood, then reabsorb needed substances to produce urine.
The kidneys regulate blood acidity as well as fluid and mineral balance.
The kidneys help secrete a hormone that promotes red blood cell production and activate vitamin D.
Kidney health is evaluated using both blood filtration function and albumin in the urine.
The kidneys filter waste products from the blood and excrete them in urine. They also regulate water and mineral concentrations to maintain balance within the body. They are involved in blood pressure regulation, red blood cell production, and vitamin D activation. Therefore, passing urine normally does not necessarily mean that the kidneys are healthy.
The diagnostic values for chronic kidney disease in this article follow the KDIGO clinical practice guideline published in March 2024.
Are kongpat and sinjang the same organ?
Kongpat and sinjang refer to the same organ: the kidney. Most people have a pair located below the ribs on either side of the spine. Urine produced by the kidneys travels through the ureters and collects in the bladder.
The nephron is the kidney’s basic functional unit. It consists of the glomerulus, which performs filtration, and the tubules, which adjust the composition of the filtered fluid. These two structures work together to regulate the condition of the blood. NIDDK explanation of kidney function
Major roles of the kidneys
The kidneys both excrete waste products and regulate the body’s internal environment. The process of maintaining conditions within the body in a certain range is called homeostasis. Urine production is a result of this regulation.
Role | What the kidneys do | Problems that may occur when function declines
Waste excretion | Excrete waste products from metabolism in urine | Waste accumulation
Water regulation | Regulate how much water is retained and how much is excreted | Fluid overload and edema
Mineral regulation | Regulate the excretion of sodium, potassium, phosphorus, and other minerals | Electrolyte and mineral imbalances
Blood pressure regulation | Help regulate water and sodium and secrete renin | Difficulty controlling blood pressure
Support for red blood cell production | Secrete erythropoietin, a blood-forming hormone | Anemia
Maintenance of bone health | Activate vitamin D | Mineral and bone metabolism disorders
Acid-base regulation | Help excrete acids produced by the body | Metabolic acidosis
These problems do not occur simultaneously in every person with kidney disease. When abnormalities develop depends on the cause and the degree of functional decline. The kidneys’ excretory and endocrine functions are described in IQWiG’s explanation of the kidneys.
How the kidneys filter blood and produce urine
The kidneys produce final urine by further adjusting substances filtered from the blood. Not everything that is filtered is discarded. Most of the water and nutrients the body needs return to the blood.
· Filtration occurs in the glomerulus. Water and small substances are filtered out of the blood vessels.
· Reabsorption occurs in the tubules. Substances needed by the body return to the blood.
· Additional secretion occurs in the tubules. Some waste products, acids, and other substances move into the tubules.
· The remaining substances are excreted. Final urine travels through the ureters to the bladder.
A normal filtration barrier restricts the passage of blood cells and large proteins. If this barrier is damaged, protein may leak into the urine. This is why urine tests provide different information from blood tests. NIDDK explanation of kidney function
Why must mineral excretion be regulated?
Minerals are substances the body needs, so they should not simply be eliminated. The kidneys regulate how much is excreted according to intake and the body’s condition. Blood concentrations are not fixed at a single value but are maintained within a certain range.
Substance | What it does in the body | What to watch for when the balance is disrupted
Sodium | Helps regulate fluid volume | Excessive intake contributes to fluid retention and increased blood pressure
Potassium | Supports nerve, muscle, and heart activity | Problems may occur if the blood concentration is too high or too low
Phosphorus | Is a component of bone | Accumulation in kidney disease contributes to bone metabolism disorders
Calcium | Is needed for bone formation, muscle activity, and other functions | It is also affected by vitamin D and other hormones
Blood concentrations are not determined by food intake alone. The kidneys’ ability to excrete substances and the medications being taken also have an effect. Dietary restrictions should be determined according to blood test results. NIDDK dietary guidance for chronic kidney disease
How do the kidneys affect anemia and bone health?
Through hormones, the kidneys are involved in red blood cell production and bone metabolism. These roles cannot be explained solely by their waste-excretion function. Kidney dysfunction can therefore cause problems beyond changes in urine.
Red blood cell production and blood-forming hormones
Erythropoietin secreted by the kidneys stimulates red blood cell production in the bone marrow. When the kidneys are damaged, production of this hormone may decrease. Red blood cells are produced directly by the bone marrow, not the kidneys.
Iron deficiency may also contribute to anemia in chronic kidney disease. Inflammation or bleeding may also be a cause. Therefore, anemia should not be assumed to result solely from a lack of erythropoietin. NIDDK on anemia in chronic kidney disease
Vitamin D activation and bone metabolism
The kidneys help convert vitamin D into its active form, calcitriol. Active vitamin D supports calcium absorption and bone metabolism. When kidney function declines, problems with phosphorus excretion may also develop.
This process is regulated by calcium, phosphorus, and several hormones working together. It is not a problem that can be resolved simply by taking more vitamin D. Supplement use should be determined with a healthcare professional based on test results. NIDDK on mineral and bone disorder in chronic kidney disease
An easily overlooked function: regulating blood acidity
The kidneys help regulate blood acidity by excreting acids produced by the body. Protein metabolism and other processes produce acidic substances. The kidneys process these substances while also helping conserve bicarbonate.
If this function is insufficient, metabolic acidosis may develop. This condition cannot be identified by the sourness of food or the color of urine. Blood tests and the clinical condition must be evaluated together. IQWiG explanation of kidney function, KDIGO 2024 clinical practice guideline
Which tests assess kidney function?
Kidney health is assessed using blood and urine tests together. Early chronic kidney disease may not cause noticeable symptoms. If you have diabetes or high blood pressure, work with your healthcare team to determine when to be tested.
Test | What it checks | Limitations in interpretation
eGFR, the estimated glomerular filtration rate | Estimates the blood-filtering function | Does not represent every kidney function with a single value
ACR, the urine albumin-to-creatinine ratio | Measures how much albumin is passing into the urine | An abnormal result may require repeat testing and evaluation of the cause
Blood tests for electrolytes, hemoglobin, and other values | Check for mineral imbalances, anemia, and other problems | Do not independently establish the cause of kidney disease
Albumin is a type of protein found in the blood. Albuminuria may occur even when eGFR remains relatively preserved. The two tests assess different aspects of kidney health. NIDDK guidance on kidney disease testing
Diagnostic criteria for chronic kidney disease
Chronic kidney disease is a kidney abnormality that affects health and persists for at least 3 months. The KDIGO 2024 guideline uses abnormalities of kidney structure or function as its basis. In adults, the following findings are used in diagnostic evaluation.
· A glomerular filtration rate less than 60 mL/min/1.73m².
· Albuminuria with a urine ACR of at least 30 mg/g.
· Other evidence of kidney damage, such as structural abnormalities identified on imaging.
Do not assume chronicity based upon a single abnormal level for eGFR and ACR — KDIGO 2024 Clinical Practice Guideline for Chronic Kidney Disease, Practice Point 1.1.3.2
This means that a chronic condition should not be assumed based on a single abnormal test result. Previous test results, medical history, and other information are used to assess whether the abnormality has persisted. If acute injury is suspected, evaluation should begin without waiting 3 months. KDIGO 2024 clinical practice guideline
Guidance by condition: diet and fluid management
Diet and fluid management vary according to kidney function and treatment status. Having chronic kidney disease does not mean that all foods must be restricted. Make adjustments based on current test results.
Current condition | Management considerations
Early chronic kidney disease | Identify which nutrients actually need to be restricted
High blood potassium | Check the potassium content of foods and salt substitutes
Edema or fluid overload | Determine daily fluid intake with your healthcare team
Chronic kidney disease before dialysis | Adjust protein intake according to nutritional status
Undergoing dialysis | Account for protein loss caused by dialysis
Reducing protein too much can lead to malnutrition. People undergoing dialysis may sometimes need to consume more protein. Check the ingredients of potassium-containing salt substitutes before use. NIDDK dietary guidance for chronic kidney disease
Common mistake: equating urine volume with filtration function
It is inaccurate to assess kidney function solely by the amount or appearance of urine. The ability to excrete water and the ability to process waste products are not the same measure. Avoid the following interpretations.
Common interpretation | Correction
If urine is being produced, the kidneys are normal | Early kidney disease may have no symptoms
All filtered water becomes urine | Most of it is reabsorbed in the tubules
All minerals should be removed | They must be regulated within the range the body needs
The kidneys directly produce red blood cells | Kidney hormones signal the bone marrow to produce them
One test confirms chronic kidney disease | The duration and other evidence must be evaluated together
These distinctions connect the way the kidneys work with testing guidelines. In particular, the need for testing should not be ruled out based solely on urine volume. If you have risk factors, discuss whether you need blood and urine tests. NIDDK testing guidance
How is declining kidney function treated?
Treatment focuses on managing the cause while reducing functional decline and complications. Not every case of chronic kidney disease progresses rapidly to kidney failure. Managing blood pressure and blood glucose and using appropriate medication can help slow progression.
Medication review is also part of management. Some anti-inflammatory pain relievers can damage the kidneys. Show your healthcare team a list of everything you take, including over-the-counter medications and supplements. NIDDK guidance on managing chronic kidney disease
Comparison of kidney failure treatments
For kidney failure, treatment options such as dialysis or transplantation are determined individually. Overall health and living circumstances are considered together. Each treatment replaces kidney function in a different way.
Treatment | How it works | What to know
Hemodialysis | Cleans the blood using a filter outside the body | Replaces some kidney functions
Peritoneal dialysis | Removes waste products using the peritoneum and dialysis fluid | Does not replace every kidney function
Kidney transplant | A donated kidney functions inside the body | Ongoing treatment is still needed after transplantation
Conservative management | Manages symptoms and complications without dialysis or transplantation | Discuss treatment goals thoroughly with your healthcare team
Even with dialysis, anemia or bone metabolism problems may require separate management. The timing of dialysis is not determined by a single test value. Symptoms and overall condition are evaluated together. NIDDK guidance on choosing treatment for kidney failure
Injoys
