What are the kidneys and where are they located?
The kidneys are two bean-shaped organs located deep in the back of the abdomen, on either side of the spine. The right kidney is often slightly lower than the left because of the liver. On top of the kidneys are the adrenal glands, which have other functions.
Blood enters the kidneys through the renal arteries. Millions of microscopic filtration units produce urine, which is collected in the renal pelvis and carried through the ureters to the bladder.
Structural changes such as cysts are common and often benign. Read more about kidney cyst and kidney changes.
What function do the kidneys have?
The kidneys filter the blood and excrete waste products in the urine. They also adjust how much water, sodium, potassium, and other substances the body retains or gets rid of.
The kidneys affect blood pressure through their hormonal system. They also produce erythropoietin, which stimulates the formation of red blood cells, and activate vitamin D, which is important for calcium balance and bones.
The kidneys have a large reserve capacity. This means that kidney function can deteriorate gradually without clear early symptoms.
The kidneys adjust filtration and urine concentration according to fluid intake, blood pressure, and the body's needs. Therefore, urine volume normally varies. Drinking extremely large amounts of water does not automatically improve kidney function and may be inappropriate in certain diseases.
Common kidney problems and symptoms
Kidney stones can cause severe pain in the side or back that comes in waves. Urinary tract infections can spread to the kidney and cause fever, chills, and flank pain. Chronic kidney disease often has few symptoms early on but can later cause fatigue, itching, swelling, and shortness of breath.
Blood in the urine, high blood pressure, protein in the urine, or abnormal creatinine levels may be signs of kidney or urinary tract problems. Cysts and tumors may be discovered incidentally.
Back pain is more often due to muscles and bones than to the kidneys, but fever, urinary problems and blood in the urine make a kidney cause more likely.
Diabetes, high blood pressure, cardiovascular disease and certain medications increase the risk of chronic kidney disease. Pain-relieving anti-inflammatory medications can in some situations put a strain on the kidneys, especially in cases of dehydration or already impaired function.
When should you seek care for kidney-related symptoms?
Contact your healthcare provider if you have blood in your urine, recurring flank pain, swollen legs, long-term high blood pressure, or a significant decrease in urine output. Abnormal kidney values need to be monitored even if you feel well.
Seek immediate medical attention if you have a fever and severe pain over your kidney, especially if you feel unwell or have difficulty urinating. Seek emergency medical attention if you have very little or no urine production, severe shortness of breath, severe swelling, confusion, or a rapid deterioration in your general condition.
Blood in the urine should be assessed even if it does not hurt.
How are the kidneys examined?
Blood tests with creatinine are used to calculate eGFR. Urine tests can show albumin, blood, infection, and other abnormalities. Blood pressure is an important part of the kidney assessment.
Ultrasound can show the size of the kidneys, cysts, and whether the urinary tract is dilated. CT is common for kidney stones and is also used for tumor and vascular issues. MRI can provide detailed soft tissue images and is used when CT contrast is inappropriate or when a change needs to be characterized.
In some cases, a kidney biopsy, hormone tests, or specialist assessment by a kidney or urinary tract doctor is needed.
In case of repeated abnormal tests, it is important to compare with previous values and check medications, fluid status and concomitant disease. The urine albumin-creatinine ratio can provide important risk information even when eGFR is still relatively preserved.
When might MRI of the kidneys be relevant?
MRI may be relevant to assess certain renal tumors, complex cysts, vessels, congenital anomalies, or changes that cannot be characterized by ultrasound or CT. The examination may be part of a targeted MRI abdomen.
MRI is not the first choice for suspected kidney stones; CT is often used. Kidney function must also be assessed with blood and urine tests because images do not show filtration directly.
If contrast media is considered, renal function and other risk factors need to be assessed.
Limitations and next steps
A normal ultrasound or MRI does not rule out early functional impairment. At the same time, a simple cyst may be clearly visible without affecting function. Structure and function must therefore be assessed separately.
The next steps may include blood pressure treatment, diabetes control, medication review, follow-up tests, treatment of infection, or referral to a specialist. Early detection of chronic kidney disease may make it possible to slow its progression.
Follow-up is often based on trends rather than a single test. A temporary deterioration may be due to dehydration or acute illness, while a long-term decline in eGFR requires a different assessment. Drug dosage may need to be adjusted according to kidney function.