Academic Reading — Test 138

Reading passage

Although they are each no larger than a clenched fist, the two kidneys perform some of the most demanding work in the human body. Positioned towards the back of the abdomen, just beneath the lower ribs, they receive a remarkably large share of the blood the heart pumps. At any given moment, roughly a fifth of the body's blood is passing through them. Over the course of a single day, the kidneys process the entire blood supply many times over, removing waste products and surplus water while carefully retaining the substances the body still needs. In doing so, they keep the chemical composition of the blood within the narrow limits on which life depends. The basic working unit of the kidney is the nephron, and each kidney contains around a million of them. A nephron has two principal parts: a tiny knot of blood vessels called the glomerulus, and a long, winding tube known as the tubule. Blood arrives at the glomerulus under pressure, and this pressure forces water and small dissolved molecules out through the thin vessel walls and into the tubule. This first step is called filtration. Importantly, the filter is selective by size rather than by usefulness. Large items such as blood cells and most proteins are too big to pass and remain in the bloodstream, whereas water, salts, glucose and waste molecules cross over freely. The fluid that enters the tubule at this stage is therefore not yet urine; it is a watery mixture that still contains many valuable substances. If filtration were the whole story, the body would lose enormous quantities of useful material every day. The second stage, reabsorption, prevents this loss. As the filtered fluid travels along the tubule, the cells lining it actively reclaim glucose, much of the salt, and the great majority of the water, returning these to the surrounding blood vessels. Glucose, in a healthy person, is recovered completely, which is why none normally appears in urine. Roughly ninety-nine per cent of the water that is initially filtered is eventually reabsorbed, so that only a small, concentrated remainder leaves the body. A third process, secretion, works in the opposite direction: certain substances, including some drugs and excess acids, are moved from the blood into the tubule so that they too can be excreted. Together, filtration, reabsorption and secretion fine-tune the final composition of the urine. Beyond cleaning the blood, the kidneys are the body's chief regulators of fluid balance. The volume of water in the body must stay relatively constant despite wide variations in how much a person drinks and how much is lost through sweat. The kidneys achieve this largely by adjusting how much water they return to the blood during reabsorption. This adjustment is controlled by a chemical messenger called antidiuretic hormone, often shortened to ADH, which is released by the brain when the body is short of water. ADH instructs the tubules to reabsorb more water, producing a smaller amount of darker, concentrated urine. When a person has drunk plenty of fluid, ADH levels fall, less water is reclaimed, and the urine becomes paler and more plentiful. In this way the colour and volume of urine offer a rough but genuine indication of the body's state of hydration. The kidneys also help to govern blood pressure and the balance of important salts. When blood pressure drops, the kidneys release an enzyme called renin, which sets off a sequence of chemical reactions that tightens blood vessels and prompts the body to retain salt and water. Because salt attracts water, controlling the amount of salt in the blood is closely linked to controlling its volume, and therefore to blood pressure itself. The kidneys additionally manage levels of substances such as potassium, which must be kept within tight bounds because both too much and too little can disturb the rhythm of the heart. This regulation continues quietly and constantly, without any conscious effort on our part. Given the breadth of these responsibilities, it is unsurprising that kidney disease can be so serious. When the nephrons are damaged, the kidneys may filter less effectively, allowing waste to build up in the blood while useful proteins begin to leak into the urine. Such damage often develops slowly and without obvious symptoms, which is why doctors sometimes test blood and urine to detect problems before a patient feels unwell. In cases of severe failure, the work of the kidneys must be taken over artificially by a machine, in a treatment known as dialysis, or replaced through a transplant. These measures, however valuable, are imperfect substitutes for the quiet efficiency of two healthy organs that, day and night, keep the internal environment of the body in balance.