How the Kidneys Actually Filter Blood

A nephron does more than filter blood once. Its glomerulus filters selected plasma contents, then its tubule reabsorbs needed material and secretes other substances. These three processes create urine.

A nephron combines a filter with an adjustment tube Each kidney contains many hundreds of thousands of nephrons. A nephron begins with a glomerulus and continues as a long tubule. The glomerulus filters part of the blood plasma. The tubule then changes the filtered fluid through reabsorption and tubular secretion. Nephrons work in parallel, but they are not isolated units. Shared blood flow, hormones and local feedback influence how they function. Filtration begins at the glomerulus Blood enters the glomerulus, which is a small network of capillaries. Pressure moves water and selected dissolved substances into Bowman's capsule. The fluid crosses a three layer glomerular filtration barrier. This barrier selects substances by size, electrical charge and other physical properties. Blood cells and most large proteins normally remain in the bloodstream. Water, electrolytes, glucose, amino acids, urea and many other small substances enter the filtrate. The barrier does not recognise whether a substance is useful or waste. Filtration therefore produces an unfinished fluid rather than final urine. Filtrate is not the same as urine Filtrate is the fluid entering the tubule after glomerular filtration. Only part of the plasma reaching a glomerulus is filtered during one pass. The blood continues through small vessels around the tubule. These vessels receive reabsorbed substances and provide substances for secretion. The kidneys receive a large blood flow, and blood circulates through them repeatedly. They do not pass every blood component into the nephron. Reabsorption returns most filtered material Reabsorption moves water and dissolved substances from the tubule back into nearby blood vessels. Most filtered water and sodium return to the circulation. At usual blood glucose levels, almost all filtered glucose is reabsorbed. Glucose can appear in urine when transport capacity is exceeded or tubular handling is altered. Different tubule sections use different channels and transport proteins. Hormones adjust some later stages according to fluid and electrolyte needs. This process is selective, but the kidney does not consciously decide what the body wants. Cell transport and chemical signals produce the adjustment. Tubular secretion adds selected substances Tubular secretion moves selected substances from nearby blood into the tubule. These include acids, potassium, some medicines and several metabolic waste products. Secretion supports acid base and electrolyte control. It also helps remove substances that were not filtered sufficiently at the glomerulus. Urinary excretion reflects filtration, reabsorption and secretion together. Final urine contains water, urea, creatinine, electrolytes, acids and other dissolved substances. Urine is not simply a container for material that is always harmful. Normal urine contains variable amounts of substances that the body also needs. Blood flow and pressure affect filtration Glomerular filtration depends on blood flow, pressure across the filtration barrier and the barrier's condition. Systemic blood pressure does not pass unchanged into each glomerulus. Renal autoregulation adjusts small blood vessels to keep blood flow and filtration relatively stable during ordinary pressure changes. This protection has limits. Severe or prolonged loss of renal perfusion can reduce filtration and contribute to acute kidney injury. Obstruction, inflammation, toxins and direct tissue injury can also reduce kidney function. A reduced filtration result therefore does not prove that low blood pressure or permanent nephron loss is the cause. Remaining nephrons can adapt after nephron loss When nephron number falls, some remaining nephrons can enlarge and filter more. This adaptation is called single nephron hyperfiltration. It can preserve the total glomerular filtration rate for a time. It does not mean that kidney structure is normal or that lost nephrons have returned. Persistent hyperfiltration can also place extra strain on remaining glomeruli in some conditions. Compensation is therefore useful but not always harmless. Symptoms do not appear at one fixed amount of nephron loss. Kidney damage, filtration and symptoms can change at different rates. Kidney tests answer different questions Creatinine is a waste product measured in blood. Laboratories use it with age and sex to calculate an estimated glomerular filtration rate, shortened to eGFR. An eGFR estimates total filtration across both kidneys. It does not count individual nephrons or measure every tubular and hormone function. During rapidly changing acute kidney injury, creatinine may lag behind the current change. An automatically reported eGFR may therefore be less reliable than trends and the full clinical picture. A urine albumin:creatinine ratio, shortened to ACR, checks for albuminuria. Persistent albuminuria can indicate filtration barrier damage even when eGFR remains normal. Urine testing can also show blood, glucose or other patterns. These findings provide clues and do not identify the cause or nephron segment alone. Ultrasound can show kidney size, swelling and some obstruction. A normal ultrasound cannot prove that microscopic filtration and tubular function are normal. Know when urgent assessment matters Acute kidney injury can cause few or no symptoms. Passing much less urine, vomiting, diarrhoea, dizziness, swelling or breathlessness can provide clues. Ask for an urgent GP appointment or contact NHS 111 if you pass much less urine than usual and feel unwell. Seek prompt advice if vomiting or diarrhoea prevents you from keeping fluids down. Call 999 for severe breathing difficulty, confusion, collapse, or if someone is difficult to wake. Do not drive yourself.

Urine formation requires glomerular filtration, tubular reabsorption and tubular secretion. The kidneys regulate these processes rather than separating useful material from waste in one simple filtering step.