<Li> Stores waste product (mainly urea and uric acid) before it and other products are removed from the body . </Li> <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>

Where does the first step in the production of urine occur
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