Kangaroo rats have evolved highly specialized kidneys that have allowed them to greatly concentrate their urine. This high urine concentration allows them to use much less water than most other mammals in order to excrete waste. In fact, individuals of the Dipodomy genus on a dry diet have been reported to have higher concentrations of urea in their urine than any other mammal (Howel and Gersh, 1935). Dipodomys have apparently evolved even more efficient kidneys than their close relatives Perognathus, whose urine concentrations are less than half that of Dipodomys. (Macmillen & Hinds, 1983). The way in which these rodents are able to form such highly concentrated urine can be seen by examining the anatomy of their kidneys and the relative sizes of kidney structures. On a macroscale, the kidney is divided into the cortex, the medulla, and the renal pelvis (Figure 1).
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Figure 1. Diagram of the kidney showing major parts. Taken (and modified) from U.S. National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program - Diagram Source Figure 2. Diagram of the Nephron showing major parts. Taken (and modified) from National Kidney and Urologic Diseases Information Clearinghouse - Diagram Source
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The functional unit of the kidney is the nephron.(Figure 2). Blood entering the kidney passes through a dense capillary bed called the glomerulus. The glomerulus is surrounded by a tubular structure called Bowman's capsule. Blood plasma passes from the renal capillaries in the glomerulus and into Bowman's capsule. Ions, such as sodium, glucose, and carbonate, and water then begin to be reasbsorbed into blood vessels in the lumen of proximal convoluted tubule. The majority of these nephron regions lie in the cortex of the kidney. However, the majority of the water reabsorption in the kidneys occurs in the Loop of Henle, which makes up the renal medulla. Further reabsorption of water occurs in the distal convoluted tubules and in the nephric duct systems leading to the bladder and urethra. Much of this ion and water reabsorption requires active transport, particularly by sodium-potassium pumps, and therefore presents energetic requirements on the organism (Barac-Nieto, 2004).