The nephron consists of a glomerulus, a proximal tubule, the loop of Henle with its thick ascending limb, the distal tubule and the cortical collecting duct. In reality, the glomerulus lies a little closer to the distal tubule than shown in our drawing, enabling the juxtaglomerular apparatus, which produces renin, to sense sodium levels in the distal tubule (see later). Healthy glomeruli filter around 100 ml of blood to produce around 1 ml of urine per minute.

There are four functions relating to waste, water, electrolytes and acids:

And four hormonal functions:

We will devote the rest of this chapter to describing these functions in more detail. We do so from a clinician’s rather than a physiologist’s view in the belief that clinically relevant physiology is more easily remembered.

The waste products which are of most interest in monitoring excretory renal function are urea and creatinine. Both are freely filtered at the glomerulus. The tubules then re-absorb some urea (but not creatinine) and secrete some creatinine (but not urea). The body’s ability to excrete waste products is determined by the glomerular filtration rate (GFR). Normal GFR is approximately 100 ml/min. The human body can tolerate a substantial loss in renal function before suffering significant ill-effects and it is not until the GFR falls below 30 ml/min that things begin to go seriously wrong (Fig. 1.2).

If our glomerular filtration rate is approximately 100 ml/min and average urinary output is only 1 ml/min (1,500 ml per day), this means that 99 ml of glomerular filtrate is reabsorbed every minute. The concentration of urine in this way is driven by a series of complex mechanisms, which we can simplify as follows: