16.5 Structure of the nephron

• In the blood, an optimum concentration of water and salts is maintained to ensure a fairly constant water potential of blood plasma and tissue fluid.
• The homeostatic control of the water potential of the blood is known as osmoregulation.
• The fibrous capsule is the outer protective membrane of the kidneys.
• The cortex is a lighter coloured outer region of the kidneys, made up of Renal (Bowman's) capsules, convoluted tubules and blood vessels.
• The medulla is the darker coloured inner region of the kidneys, made up of loops of Henle, collecting ducts and blood vessels.
• The renal pelvis is a funnel-shaped cavity in the kidneys that collects urine into the ureter.
• The ureter is a tube within the kidneys that carries urine to the bladder.
• The renal artery is a vessel that supplies the kidney with blood from the heart via the aorta.
• The renal vein is a vessel that returns blood to the heart via the vena cava.
• There are around one million microscopic structures, known as nephrons, in each kidney.
• The nephron is the functional unit of the kidney. It is a narrow tube up to 14mm long, closed at one end, with two twisted regions separated by a long hairpin loop.
• Each nephron is made up of a Renal (Bowman's) capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.
• The Renal (Bowman's) capsule is the closed end at the start of the nephron. It is cup-shaped and surrounds a mass of blood capillaries known as the glomerulus. The inner layer of the renal capsule is made up of specialised cells known as podocytes.
• The proximal convoluted tubule is a series of loops surrounded by blood capillaries. Its walls are made of epithelial cells which have microvilli.
• The Loop of Henle is a long, hairpin loop that extends from the cortex into the medulla of the kidney and back again. It is surrounded by blood capillaries.
• The distal convoluted tubule is a series of loops surrounded by blood capillaries. Its walls are made of epithelial cells, but it is surrounded by fewer capillaries than the proximal tubule.
• The collecting duct is a tube into which a number of distal convoluted tubules from a number of nephrons empty. It is lined by epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney.
• Each nephron is associated with four different blood vessels : the afferent arteriole, the glomerulus, the efferent arteriole and the blood capillaries.
• The afferent arteriole is a tiny vessel that arises from the renal artery, and it supplies the nephron with blood. The afferent arteriole enters the renal capsule of the nephron, where it forms the glomerulus.
• The glomerulus is a highly branched knot of capillaries from which fluid is forced out of the blood. The glomerular capillaries recombine to form the efferent arteriole.
• The efferent arteriole is a tiny vessel that leaves the renal capsule. It has a smaller diameter than the afferent arteriole, and so causes an increase in blood pressure within the glomerulus. The efferent arteriole carries blood away from renal capsule and later branches to form the blood capillaries.
• The blood capillaries are a concentrated network of capillaries that surround the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. These capillaries merge together into venules (tiny veins), and in turn merge together to form the renal vein.