Distal Convoluted Tubule

The distal convoluted tubule (DCT) is the coiled part of the nephron after the ascending limb of the loop of Henle, back in the cortex. It leads into the collecting duct. Filtrate entering the DCT is dilute, because the ascending limb has removed ions but not water.The DCT makes fine adjustments to the composition of the filtrate, according to the body's needs. Like the PCT, its cells have many mitochondria and microvilli, because they carry out a lot of active transport.Functions of the DCT:• Sodium and potassium balance. Na⁺ ions are actively reabsorbed into the blood and K⁺ ions are secreted into the filtrate. The rate is increased by the hormone aldosterone from the adrenal cortex when blood Na⁺ concentration is low.
• Blood pH. If the blood is too acidic, DCT cells secrete more H⁺ ions into the filtrate and reabsorb more hydrogencarbonate ions, raising the blood pH. If the blood is too alkaline, fewer H⁺ ions are secreted.
• Calcium ions are reabsorbed under hormonal control.
• Water. The permeability of the later part of the DCT to water can be changed, so water can be reabsorbed by osmosis when the body needs to conserve it.So, unlike the PCT, which reabsorbs a large and fairly fixed proportion of the filtrate, the DCT reabsorbs or secretes variable amounts to keep the composition of the blood constant. This makes it an important site of homeostasis.Example: after a meal rich in potassium, more K⁺ is secreted into the filtrate in the DCT, so blood K⁺ concentration returns to normal and the excess leaves in urine.