Academic Reading — Test 95

Reading passage

Each human kidney contains roughly one million microscopic filtering units known as nephrons, and it is within these structures that the blood is cleansed of the waste products generated by everyday metabolism. A nephron begins with a tight cluster of capillaries called the glomerulus, which is cradled inside a cup-shaped chamber. As blood is driven through this cluster under pressure, water and small dissolved substances are forced out of the vessels and into the chamber, while larger components such as proteins and blood cells are held back. The watery liquid that emerges from this initial stage is called the filtrate. It is important to recognise that the filtrate is not yet urine; it still contains a great many substances that the body cannot afford to lose, and these must be recovered before the liquid leaves the kidney. The recovery takes place along a slender, winding tube that extends from the glomerular chamber. As the filtrate travels down this tube, useful materials including glucose, certain salts and the bulk of the water are reabsorbed back into the surrounding capillaries, a process driven partly by active transport that requires energy. At the same time, the tube performs the opposite task of secretion, by which additional unwanted compounds are added to the filtrate from the blood. Through this balance of reabsorption and secretion, the nephron finely adjusts the final composition of the urine and, in doing so, helps to regulate the volume of fluid in the body and the concentration of salts within it. A healthy adult produces only a litre or two of urine each day, even though the kidneys filter around 180 litres of fluid in that time, which shows how much is reclaimed. When the kidneys fail, these waste products are no longer removed and begin to accumulate to dangerous levels in the bloodstream. For most of medical history such failure was simply fatal. The situation began to change in the early twentieth century, when researchers established that certain molecules would pass through a thin membrane from a region of high concentration to one of low concentration, a movement known as diffusion. If blood could be brought close to a cleansing fluid, separated only by such a membrane, the accumulated wastes might cross out of the blood while the valuable cells and proteins remained behind. This principle, identical in spirit to the filtering action of the nephron, lies at the heart of every dialysis machine. The first device capable of treating a human patient was built by the Dutch physician Willem Kolff during the Second World War. Working under the severe shortages of occupied Holland, Kolff wound long lengths of sausage casing, which served as the membrane, around a wooden drum that rotated through a bath of cleansing fluid. Blood drawn from the patient flowed through the casing, gave up its wastes to the surrounding bath, and was returned to the body. His earliest attempts ended in disappointment, and it was not until 1945 that a patient treated with his apparatus survived an episode of kidney failure. The achievement was remarkable given that it was accomplished with improvised materials at a time of war. A persistent obstacle in the early years was the difficulty of gaining repeated access to a patient's blood vessels. Each treatment destroyed the artery and vein that were used, so the number of sessions any individual could receive was strictly limited, and long-term treatment remained impossible. The breakthrough came in 1960, when a team in Seattle developed a small tube, made of a newly available plastic, that could be left permanently in place to join an artery to a vein. This device, known as a shunt, allowed a patient to be connected to the machine again and again over months and years. For the first time, people with permanent kidney failure could be kept alive almost indefinitely, and dialysis was transformed from a short-term rescue into a long-term therapy. Modern haemodialysis retains the essential logic of these early machines, even though the equipment is now compact, automated and far safer. Patients with failed kidneys typically attend a centre several times a week, where their blood is passed through a cartridge containing thousands of hollow fibres bathed in dialysis fluid. The treatment cannot reproduce every subtle function of a living nephron, and it remains a demanding routine rather than a cure, but it sustains the lives of millions of people worldwide who would otherwise have no prospect of survival.