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Our urine is produced not only to eliminate many cellular waste products, but also to control both the amount and the composition of the extracellular fluid in the body. Controlling the amount of water and chemicals in the body is essential to life, and the body does so by producing various amounts of urine so that we can either excrete the "extra" water and chemicals (mainly sodium) or conserve the water and chemicals when they are in short supply.
Therefore, the volume of urine that is excreted everyday is a reflection of how much extracellular fluid and sodium bodies have to spare. The kidney tubule regulation of the salt and water in our bodies is the most important factor in determining urine volume. Too much water and salt in our bodies is dangerous and too little water and salt is dangerous. Therefore, the level of water and salts excreted in urine - the urine volume - is adjusted to the needs of the body.
Because of the high pressure of the blood in the glomerulus, and the large size of the pores in the glomerulus/capsule-filtering device, an enormous volume of fluid passes into the kidney tubules. If this fluid were left as it is, the body would be drained dry in 30 minutes. In fact, as the fluid flows down the tubule, over 90% of the water in it is reabsorbed. The main part of this reabsorption takes place in the collecting tubes. The amount of water removed from the collecting ducts is controlled by a hormone called antidiuretic hormone (ADH) produced by the pituitary gland, situated at the base of the brain.
When the blood becomes more concentrated, as happens when an animal is deprived of water, ADH is secreted and causes more water to be absorbed from the collecting ducts so that concentrated urine is produced. When plenty of water has been consumed, and the blood is dilute, no ADH is secreted and no or little water is absorbed from the collecting ducts, so dilute urine is produced. In this way, the concentration of the blood is controlled precisely.
Summary of Process of Urine Formation
As the fluid flows along the proximal convoluted tubule useful substances like glucose, water, salts, potassium ions, calcium ions, and amino acids are reabsorbed into the blood capillaries that form a network around the tubules. Many of these substances are transported by active transport and energy is required.