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The kidneys are reddish brown, bean-shaped organs situated in the abdominal cavity, one on either side of the vertebral column in the lumbar region of the body.
They lie asymmetrically, the right kidney being lower than the left as the right side of the abdominal cavity is occupied by the liver.

Each kidney is 10 cm long, 6 cm wide and 4 cm thick and weighs 200 – 250 g in adults. A thin, tough, fibrous whitish capsule envelops each kidney. The kidney adapted by the following feature to perfom its functions:-

1. Its tubules are long and coiled; providing a large surface area for efficient reabsorption  and enough time is allowed for useful products is allowed for them to be reabsorbed.

2. The tubules are supplies with blood capillaries that ensure constant removal of reabsorbed substances hence creating room for more reabsorption.

3. The efferent vessels leaving the glomerulus have a narrower lumen than the afferent tubule. This helps to build up a higher pressure necessary for ultra-filtration.

4. There exists micro villi in he inner surface of the epithelial lining of the proximal convoluted tubules that provides a larger surface area for efficient reabsorption.

5. The cells in the proximal convoluted tubule have many mitochondria; this provide energy for active reabsorption of amino acids, mineral salts and glucose.

6. The loop of Henle has a U- shape to ensure maximum absorption of water hence reducing its wastage.

7. The afferent vessel that enters the kidney is highly branched to form glomerulus. This provide large surface area for ample blood to flow into the kidney for filtrati

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