Kidneys and Fluids

Your kidneys do not simply strain the whole blood supply. Each nephron filters part of the plasma, then its tubule returns needed substances and removes others. This controlled process forms urine and keeps the internal environment stable.

The urinary system has several connected parts The urinary system includes the kidneys, ureters, bladder and urethra. Most people have two kidneys behind the abdominal organs, one on each side of the spine. Blood enters each kidney through a renal artery and leaves through a renal vein. Urine drains from each kidney through a ureter to the bladder, then leaves through the urethra. The kidneys do much more than make urine. They regulate the composition and volume of body fluids while removing selected waste products. Nephrons form urine Each kidney contains many microscopic units called nephrons. A nephron includes a glomerulus, which is a capillary filter, and a tubule that adjusts the filtered fluid. The glomerulus filters water and many small dissolved substances from plasma. Blood cells and most large proteins normally remain within the bloodstream. Filtration is only the first step. The tubule uses reabsorption to return most filtered water and needed substances to the blood. Tubular secretion moves selected substances from the blood into the tubule. The fluid remaining after these adjustments becomes urine. The kidneys therefore do not consciously decide what to keep. Cells, transport proteins, blood flow and hormone signals continually alter reabsorption and secretion. Most filtered water returns to the blood The kidneys form far more filtered fluid than final urine. Most filtered water returns to the circulation as the fluid passes through the tubules. When the body needs to conserve water, hormone signals increase water reabsorption. When water is plentiful, the kidneys can produce a larger volume of more dilute urine. Urine volume also changes with drinking, sweating, illness, medicines and hormone levels. Passing more or less urine does not measure kidney function by itself. Electrolytes must stay within workable ranges An electrolyte is a dissolved mineral carrying an electrical charge. Sodium helps regulate fluid distribution, while potassium supports nerve and muscle activity. The kidneys adjust how much sodium, potassium and other electrolytes leave in urine. Other organs and hormones also contribute to this regulation. Large potassium changes can affect muscle and heart electrical activity. This does not mean that every kidney problem causes a dangerous heart rhythm. The kidneys help control acid base balance Body cells continually produce acids. The kidneys remove acid and conserve or produce bicarbonate, helping keep acid base balance within a narrow range. The lungs contribute by controlling carbon dioxide. Kidney and lung responses work over different timescales and support the same overall balance. The kidneys contribute to blood pressure control The kidneys influence blood pressure by adjusting sodium and water balance. They also release renin when selected signals suggest reduced kidney blood flow or reduced effective circulation. Renin begins a hormone pathway that can narrow blood vessels and increase sodium retention. This pathway is one part of blood pressure control, not the only controller. High blood pressure can damage kidney blood vessels over time. Kidney disease can also make blood pressure harder to control, so the relationship works in both directions. The kidneys also have hormone related roles Healthy kidneys make erythropoietin, a hormone that supports red blood cell production. Reduced erythropoietin can contribute to anaemia in more advanced kidney disease. The kidneys also help produce active vitamin D. This supports calcium balance and bone health alongside the gut, liver, bones and other hormones. These roles explain why substantial kidney disease can affect more than urine and waste removal. Kidney tests measure different features Creatinine is a waste product used in blood tests. Laboratories combine it with other information to calculate an estimated glomerular filtration rate, shortened to eGFR. An eGFR estimates overall filtration. It is not a direct measurement, and one result can change with acute illness, medicines and factors affecting creatinine production. A urine albumin:creatinine ratio, shortened to ACR, measures albumin relative to creatinine in one urine sample. Persistent albuminuria can indicate kidney damage even when eGFR remains normal. Temporary albuminuria can occur, so selected raised ACR results need confirmation. Clinicians interpret eGFR and ACR together, alongside blood pressure, background and previous results. Blood or protein detected in urine can come from several causes. A urine result does not identify the affected structure or diagnosis by itself. Many kidney problems cause no early symptoms Chronic kidney disease often causes no symptoms in its early stages. Routine blood and urine tests can therefore identify possible changes before a person feels unwell. Tiredness, swelling, breathlessness and changes in urination are not specific to kidney disease. Each has several possible causes and needs clinical context. Acute kidney injury, shortened to AKI, can also occur without obvious symptoms. When symptoms occur, they may include much less urine, sickness, swelling, breathlessness, confusion or sleepiness. Ask for an urgent GP appointment or contact NHS 111 if you pass much less urine than usual and feel unwell. Do the same if you see blood in your urine, even once. Call 999 for severe breathing difficulty, confusion, collapse, or blue, grey, pale or blotchy skin. Do not drive yourself.

The kidneys form urine through filtration, reabsorption and secretion. These processes regulate fluid, electrolytes, acid base balance and blood pressure while supporting red blood cell production and bone health.