Structure and Function of the Nephron
The nephron is a long tube that runs from the cortex into the medulla and back again to the cortex before joining another tube called the collecting duct. The nephron starts as a small cup-like structure known as the Bowman’s capsule and leads into what is known as the first convoluted tubule (also known as the proximal convoluted tubule). It descends into the medulla as the loop of Henle then back into the cortex to become the distal convoluted tubule. These tubules drain into the collecting duct. Several convoluted tubules drain into each collecting duct, these collecting ducts empty into the renal pelvis. From the renal pelvis the excretory product, urine, drains into the ureter. The ureter from each kidney empties into the bladder, and urine leaves the body via the urethra.
There is a branch of the renal artery, the afferent arteriole, entering the small cup-like space of the Bowman’s capsule as a network of blood capillaries. This network is known as the glomerulus. Emerging from this network, the capillaries re-unite to form a small arteriole, known as the efferent arteriole. As the efferent arteriole continues it twines around the proximal and distal convoluted tubule. The efferent arteriole divides into capillaries at several points along the length of the tubules, absorbing various substances. These capillaries eventually reunite to drain into the renal vein. The efferent arteriole is smaller than the afferent arteriole. This difference in diameter helps to raise the glomerular pressure and aids in ultra filtration.
Some animals do not have a well developed kidney; they may have structures called nephridia. Animals such as earthworms that are simple tube-like structures have nephridia that have the same role as the more complex nephrons in the kidneys.