<P> Average urine production in adult humans is about 1--2 litres (L) per day, depending on state of hydration, activity level, environmental factors, weight, and the individual's health . Producing too much or too little urine requires medical attention . Polyuria is a condition of excessive urine production (> 2.5 L / day). Oliguria when <400 mL (millilitres) are produced, and anuria one of <100 mL per day . </P> <P> The first step in urine formation is the filtration of blood in the kidneys . In a healthy human the kidney receives between 12 and 30% of cardiac output, but it averages about 20% or about 1.25 L / min . </P> <P> The basic structural and functional unit of the kidney is the nephron . Its chief function is to regulate the concentration of water and soluble substances like sodium by filtering the blood, reabsorbing what is needed and excreting the rest as urine . </P> <P> In the first part of the nephron, Bowman's capsule filters blood from the circulatory system into the tubules . Hydrostatic and osmotic pressure gradients facilitate filtration across a semipermeable membrane . The filtrate includes water, small molecules, and ions that easily pass through the filtration membrane . However larger molecules such as proteins and blood cells are prevented from passing through the filtration membrane . The amount of filtrate produced every minute is called the glomerular filtration rate or GFR and amounts to 180 litres per day . About 99% of this filtrate is reabsorbed as it passes through the nephron and the remaining 1% becomes urine . </P>

Where does reabsorption take place in the urinary system