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Procedure Renal Physiology

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Renal Physiology


  1. Demonstrate the function of the kidney when presented with an excess salt or water load.
  2. Quantify variations in urine content when presented with an excess salt or water load.
  3. Understand the role of the kidney in maintaining homeostasis
  4. Use urinalysis strips to examine and understand kidney function.

Kidney Regulation of Osmolality

The organs responsible for the main function of the urinary system are the kidneys. These paired organs, located in the abdominal cavity, are responsible for filtering blood and then modifying the resultant filtrate to form urine. The process of filtration occurs continuously, with the plasma passing through the kidneys about 60 times a day. The resulting 180 liters of filtrate formed is ultimately converted to approximately 1.5 liters of urine. This is due to two other kidney functions- reabsorption and secretion- which together bring the formation of low volume and (usually) highly concentrated urine (to 1,200 mOsm). All of these functions take place in the functional unit of the kidney, the nephron. Each kidney has about one million nephrons.

One of the kidney’s main functions is the regulation of the osmolality of the body fluids at around 300 mOsm/L. Regulation of osmolality will be demonstrated in the following activity by presenting the kidney with an excess water or salt load and recording its response as reflected in the concentration and volume of urine produced.

Kidney Regulation of Osmolality

  1. Limit your fluid intake on the day of the experiment Empty your bladder 1 hr to 2 hr before the laboratory start time and record the exact time. Do not save this urine sample.
  2. On entering the laboratory, take a urine collection bottle to the restroom and void into the bottle, emptying your bladder fully. Record the time. This sample is the control urine.
  3. Return to the laboratory and immediately drink the solution assigned to you as quickly as possible. The class will be divided into two groups as follows;
  1. Group I drinks 800ml of water
  2. Group II drinks 800ml of isotonic sports drink
  3. Group III drinks 80 ml of salty water (7g in 80ml)
  1. Every 30 minutes after drinking the solution, empty your bladder into a cleaned collection bottle. If you are unable to void retain your urine until the next 30 min collection time.
  2. Analyze the urine from each collection for the following;
  1. Specific Gravity. Pour some of the urine into a urinometer cylinder and measure the specific gravity as described in the section of activity on specific gravity.
  2. Chloride concentration. Place 10 drops of urine into a test tube. Add 1 drop of 20% potassium chromate. Add 2.9% silver nitrate solution drop by drop, shaking constantly during the addition. Tally the number of drops of silver nitrate needed to change to a brown color. Each drop added represents 1g/l (1mg/ml) of sodium chloride in the urine. Calculated the total amount of NaCl for each 30 minute period and the total for the experiment for each of the three conditions. Also express the total NaCl as a concentration in mg/ ml.
  1. Record all the data on the worksheet and graph the changes over time as follows. Three graphs (volume, specific gravity, and chloride concentration) each with three lines (one for each group).


The kidneys are the chief regulators of the internal environment of the body. They do this by regulating the concentration of ions, water, and pH in the various body fluids. In addition, they provide for the elimination of the waste products of metabolism. Each of the million nephrons in the kidneys contains two main structures the glomerulus and the renal tubule. As blood passes through the kidneys, it is first filtered through the glomerulus (120 ml/min), and the filtrate passes through the renal tubule.

The tubule filtrate is similar to blood plasma in composition expect that large molecules (molecular weights greater than 70,000) are excluded (e.g. plasma proteins). As this filtrate passes along the proximal and distal tubules, most of the water is reabsorbed, and many essential substances are actively or passively reabsorbed into the bloodstream. Toxic by-products of metabolism and substances in excess are retained in the filtrate or secreted into the filtrate and finally are excreted in the urine (1ml formed/ min). Thus, the final concentration of the urine is quite different from that of the glomerular filtrate and reflects the integrity of kidney function and changes in blood composition.



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