Urine is the fluid that leaves the collecting ducts and is stored in the bladder. It is what remains of the glomerular filtrate after selective reabsorption and secretion.Urine normally contains:• water
• urea, the main nitrogenous waste, made in the liver from excess amino acids
• mineral ions such as Na+, K+ and Cl−, in amounts that balance the diet
• creatinine, a breakdown product of creatine phosphate in muscles
• hormones and their breakdown products, and some drugsUrine normally does not contain glucose, amino acids, proteins or blood cells. Glucose and amino acids are all reabsorbed in the proximal convoluted tubule, and proteins and cells are too large to pass the basement membrane in the glomerulus.The volume and concentration of urine vary with water intake, sweating and temperature. Its composition is also a useful indicator of health:• glucose in urine suggests diabetes mellitus, because blood glucose is too high for all of it to be reabsorbed
• protein in urine suggests damage to the glomerulus, for example from high blood pressure or kidney disease
• hCG, a hormone made by the embryo, is small enough to be filtered, so its presence shows pregnancy; test strips detect it with monoclonal antibodies
• drugs such as anabolic steroids are small and are excreted in urine, so urine tests can detect their useWorked example: the urea concentration is 0.3 g dm−3 in plasma and 20 g dm−3 in urine. Urea is 20 ÷ 0.3 ≈ 67 times more concentrated in urine, mainly because most of the water has been reabsorbed.
• urea, the main nitrogenous waste, made in the liver from excess amino acids
• mineral ions such as Na+, K+ and Cl−, in amounts that balance the diet
• creatinine, a breakdown product of creatine phosphate in muscles
• hormones and their breakdown products, and some drugsUrine normally does not contain glucose, amino acids, proteins or blood cells. Glucose and amino acids are all reabsorbed in the proximal convoluted tubule, and proteins and cells are too large to pass the basement membrane in the glomerulus.The volume and concentration of urine vary with water intake, sweating and temperature. Its composition is also a useful indicator of health:• glucose in urine suggests diabetes mellitus, because blood glucose is too high for all of it to be reabsorbed
• protein in urine suggests damage to the glomerulus, for example from high blood pressure or kidney disease
• hCG, a hormone made by the embryo, is small enough to be filtered, so its presence shows pregnancy; test strips detect it with monoclonal antibodies
• drugs such as anabolic steroids are small and are excreted in urine, so urine tests can detect their useWorked example: the urea concentration is 0.3 g dm−3 in plasma and 20 g dm−3 in urine. Urea is 20 ÷ 0.3 ≈ 67 times more concentrated in urine, mainly because most of the water has been reabsorbed.