Kidney Structure & Function Introduction Kidneys are paired organs which are located close to the dorsal (back) wall of the abdominal cavity, on either side of the spine. Their function is to filter metabolic wastes, such as urea and excess ions, from the blood. In doing so, the kidneys maintain the pH of the blood and regulate the water balance of the body. The basic structural unit of the kidney is the nephron. The human kidney consists of approximately one million nephrons. The glomeruli, the sites of blood filtration, along with the Bowman’s capsules and collecting tubules, the sites of urine production, are located in the cortex. The loop of Henle extends down into the medulla. The longer the loop of Henle, the thicker the medulla and the more efficient the reabsorption of water is. This results in a more concentrated urine. Part A Kidney Structure Aim To identify the main structural components of a mammalian kidney. Materials Mammalian kidney (eg sheep kidney) Scalpel Forceps Dissecting board Disposable gloves Procedure Place the kidney on the dissecting board. Examine its external features. It should be possible to locate and identify three tubes entering or leaving the kidney. Using your textbook to help you, draw a labelled diagram of the external features of the kidney. Use a scalpel to cut the kidney in half lengthways by making an incision just to one side of the tubes. Examine each of the cut surfaces. Try to identify the internal structures shown in Figure 1. Draw a labelled diagram of the internal structures of the kidney. Include comments about the differences in appearance of the cortex and the medulla. Dispose of your dissected kidney in the appropriate manner. Dispose of your gloves and wash your hands. Discussion Questions 1. How did you distinguish between the renal artery and the renal vein? 2. Which area of the kidney contains the glomeruli and Bowman’s capsules? 3. In which part of the kidney does the majority of water reabsorption occur? 4. What is the function of the ureters? Part B Kidney function and water loss Aim To investigate the relationship between kidney structure and the concentration of urine produced by mammals that live in different environments. Procedure Read the following information and then answer the questions. The control of water loss is very important for terrestrial (land) mammals. A study was done on nine species of mammals in which biologists measured the concentrations of urea and ions in the urine of each species. They also measured the relative thickness of the medulla in the kidneys of each species. The data they obtained is shown in Figure 2. The nine species tested are represented by letters of the alphabet. Discussion Questions 5. Describe the general relationship observed by biologists between the urea concentration and medullary thickness. 6. Consider specifically mammals a and g. Using the data to support your choice, which of these mammals would you consider to be most capable of surviving in a desert environment? 7. How would the kidney of the mammal you have just selected make it suitable for survival in the desert? 8. What other adaptations might such a mammal have to enable it to live in the desert? Figure 2. The relationship between the concentration of urea and the thickness of the medulla in various mammalian kidneys. Answers to discussion questions 1. The renal artery is the larger blood vessel as the kidney needs a rich blood supply to carry out its role of filtration 2. The cortex. 3. In the medulla, as this is where the loop of Henle extends into therefore the collecting ducts are located here. 4. The ureters transport urine from the kidneys to the bladder for storage. 5. The thicker the medulla the greater the concentration of urea and ions in the urine produced. 6. Mammal g would be most suited to living in a desert environment because it produces a more concentrated urine. This means that more water is being reabsorbed into the body, enabling greater conservation of water in a desert environment. 7. The kidney of mammal g would have a longer loop of Henle, and therefore a thicker medulla, which enables greater reabsorption of water resulting in the production of concentrated urine. 8. This mammal may also have a light coloured coat that reflects heat, an insulating coat of fur to keep the heat out, a low surface area:volume ratio, or fat tissue may be concentrated in a particular region so that it doesn’t interfere with heat loss (as seen in camels).