A distal convoluted tubule is a twisted, tube-like structure about 0.2 inches (around 5 mm) in length, found inside a part of the kidney known as a nephron. Each kidney contains many nephrons, and these are the functional units in which blood is filtered to form urine. One nephron is made up of a renal corpuscle, containing tiny blood vessels where blood is filtered, and a renal tubule. A renal tubule consists of three different sections, which carry the filtered fluid, or filtrate, away from the kidney while processing it to create urine. The distal convoluted tubule is the section farthest away from the renal corpuscle, and the cells that line it are able to actively pump potentially harmful substances, such as ammonia, urea and certain drugs, out of the blood and into the urine.
Each kidney contains more than one million nephrons and, together, the kidneys can filter all of the body's blood within about five minutes. Blood passes into the renal corpuscle inside a nephron and enters a knot of tiny blood vessels, known as a glomerulus, at relatively high pressure. It filters through gaps in the blood vessel walls and then through slits in the wall of the capsule that cups the glomerulus, draining into a space inside the capsule. Large molecules cannot pass through and remain in the blood, while water and dissolved waste products end up inside the capsule space. From there they drain into the renal tubule and the process of urine formation takes place.
Inside the first section of the renal tubule, called the proximal convoluted tubule, useful nutrients and minerals are absorbed from the filtrate, together with water. These pass into surrounding blood vessels to be returned to the general circulation. Next, within the section known as the loop of Henle, the concentration and volume of the filtrate are regulated to maintain the body's fluid balance.
Finally, inside the distal convoluted tubule, useful substances are returned to the blood, while waste products and toxins are added to the filtrate. Hydrogen is also pumped in, making the urine pH more acidic. The distal convoluted tubule walls do not normally allow water to pass through, but a hormone known as antidiuretic hormone, or ADH, can open channels which allow water to move out, concentrating the urine.
From the distal convoluted tubule, filtrate drains into what are known as collecting ducts. These are tubes which receive filtrate from the distal convoluted tubules of many nephrons. Inside these collecting ducts, water can be absorbed to regulate the final concentration of urine produced by the kidneys. On leaving the collecting ducts, urine enters a space known as the renal pelvis, from where it passes into the bladder and is expelled from the body during urination.
