Urine Formation

Urine is not made in a single step. It is produced through three linked processes that occur in different regions of the nephron: glomerular filtration, reabsorption and secretion.

Glomerular filtration

The first step is the filtering of blood, carried out by the glomerulus. The kidneys work through an enormous volume of blood: on average about 1100 to 1200 ml is filtered every minute, which is roughly one-fifth of the blood pumped out by each ventricle of the heart in that minute.

Filtration is driven by the blood pressure inside the glomerular capillaries. The blood is pushed across three layers:

• the endothelium of the glomerular blood vessels,
• the epithelium of Bowman's capsule, and
• a basement membrane sandwiched between these two.

The epithelial cells of Bowman's capsule, called podocytes, are arranged in an intricate way that leaves narrow gaps between them known as filtration slits or slit pores. The membranes filter so finely that almost every component of the plasma passes into the lumen of Bowman's capsule — the notable exception being the plasma proteins, which are held back. Because the filtering is this fine, the process is described as ultrafiltration.

Glomerular filtration rate and its regulation

The volume of filtrate produced by the kidneys each minute is the glomerular filtration rate (GFR). In a healthy person the GFR is about 125 ml per minute, which adds up to a striking 180 litres per day.

The kidneys can regulate this rate themselves. One efficient control is the juxta glomerular apparatus (JGA), a specialised, sensitive region formed where the distal convoluted tubule and the afferent arteriole come into contact and their cells become modified. If the GFR falls, the JG cells release renin, which acts to boost glomerular blood flow and so restore the GFR to normal.

Reabsorption

Comparing the huge daily filtrate volume of 180 litres with the roughly 1.5 litres of urine actually released shows that nearly 99 per cent of the filtrate must be taken back into the body. This recovery is reabsorption, performed by the tubular epithelial cells along the different segments of the nephron, using either active or passive mechanisms:

• Substances such as glucose, amino acids and sodium ions are reabsorbed actively. …