ABSTRACT
The kidneys are symmetrical organs in humans and are roughly the size and weight of the pig kidneys one might see in a butcher’s shop. They lie outside the abdominal peritoneal cavity under the diaphragm (the structure separating the thorax from the abdomen). The right kidney lies lower than the left. Each has an outer portion termed the ‘cortex’ and an inner one termed the ‘medulla’. Within the cortex each kidney contains approximately one million filtering units (glomeruli) comprising a leash of small blood vessels (capillaries) through which blood passes. A large volume of blood – a quarter of cardiac output or approximately 1-1.5 litres – passes through the two million glomeruli each minute. The pressure gradient between the blood within the glomerular capillaries and the space outside them provides the driving force for ultrafiltration of fluid across the glomerular capillary walls. This fluid is virtually free of red blood cells and other formed elements of the blood and protein, but does contain waste products of body metabolism, water and electrolytes. Many drugs (in particular those that are soluble in water) are removed from the body by this process of glomerular filtration. Fine tubules lead from the extracapillary space and dip down into the medulla of the kidney. During the process of passage of fluid through the renal tubules, modification of the contents of the tubular fluid are made by reabsorption into the blood stream or secretion from the blood stream into the tubular fluid. Immediately after the extraglomerular capillary space, the tubule is termed a ‘proximal tubule’ and further down the system there is a ‘distal tubule’. Distal tubules lead into collecting ducts which ultimately open into the main collecting system of the kidneys at the renal papillae. Tubular fluid enters the calyces of the kidney and then passes on into the renal pelvis, down the ureter and into the bladder from where urine is voided. The rate of filtration across glomerular capillaries in the average human being is approximately 180 litres per day. The vast majority of this ultrafiltrate is reabsorbed into the circulation. Eventual daily urine volume is usually of the order 1-1.5 litres and varies according to fluid intake and losses in sweat, from the gut and by other routes. From this it can be seen that even modest reductions in renal reabsorptive capacity may have a major impact upon the volume of urine produced. It is not surprising that an early feature of kidney damage is the production of an excess volume of urine (polyuria) and the need to get up at night to pass urine (nocturia), although other causes for these symptoms exist as well. Gross anatomy of the urinary tract is shown in Figure 1, microscopical anatomy in Figure 2 and a higher power microscopical view of a glomerulus in Figure 3, all in schematic form.
