2. Urea  Urea: By-product of amino acids catabolism  Plasma concentration reflects the amount of protein  Urea enters the renal tubule by filtration  50% of urea is reabsorbed  Remainder is excreted in urine

3. Uric Acid  Uric Acid: Product of the metabolism of certain organic bases in nucleic acids  Active transport reabsorbs the uric acid in glomerular filtrate  Small amount is secreted into the renal tubule and is excreted in urine

4. Urine Composition  Reflects the amounts of water and solutes that the kidneys eliminate from the body  Retains in the internal environment to maintain homeostasis  Due to variations in dietary intake and physical activity, the composition may differ form time to time  Water (95%), urea and uric acid  Electrolytes, amino acids

5. Urine Composition  Volume of urine produced is usually between 0.6 and 2.5 liters per day  Depends on fluid intake, environmental temperature, relative humidity, emotional condition, respiratory rate, body temperature  Normal output is 50~60 milliliters per hour  Less than 30 ml per hour may indicate kidney failure

6. Review  The urea enters the ______________ by filtration  ___________ is a by-product of amino acid catabolism  ____________ is a product of the metabolism of certain organic bases in nucleic acids  Urine is about 95% __________, and usually contains _______ and ___________ acid  Urine composition reflects the amounts of _____ and solutes that the _________ must eliminate from the body or retain in the _________ ___________ to maintain ________________.