Urine Formation — Tubular Reabsorption and Secretion

The roughly 180 litres of glomerular filtrate formed by the two kidneys each day is, apart from lacking large proteins and blood cells, very similar in composition to blood plasma — which means that if it were all simply voided as urine, the body would lose its entire supply of glucose, amino acids, most of its water and most of its dissolved ions within a matter of hours. This does not happen because the renal tubule reclaims the overwhelming majority of the filtrate as it flows along, through the process of tubular reabsorption, while separately adding certain other substances into the tubular fluid through tubular secretion; the fluid that finally reaches the collecting duct and beyond is therefore very different in both volume and composition from what first entered Bowman's capsule.

Reabsorption begins immediately in the proximal convoluted tubule (PCT), which reclaims roughly 70-80% of the entire filtrate — almost all of the filtered glucose and amino acids, most of the filtered Na+, K+, Cl- and bicarbonate ions, and, following these solutes osmotically, a correspondingly large volume of water. This reabsorption happens at a steady, essentially fixed rate regardless of the body's momentary hydration status, which is why it is described as obligatory reabsorption; it is made highly efficient by the dense microvilli (brush border) covering the PCT's epithelial cells, which vastly increase the surface area available for the active and passive transport of these substances back into the surrounding peritubular capillaries.

The filtrate that remains, now reduced in volume but with a similar level of dissolved salt to that in the surrounding tissue, next enters the loop of Henle, whose descending and ascending limbs have sharply contrasting permeabilities. The descending limb is permeable to water but essentially impermeable to salt, so as it carries fluid down into the increasingly salty medullary tissue described in the next section, water moves passively out of the tubule and the fluid inside becomes progressively more concentrated. The ascending limb, by contrast, is impermeable to water but actively transports Na+ and Cl- out of the tubular fluid (especially in its thick segment, where this pumping is particularly vigorous) — so that, since no water can follow the salt out here, the fluid inside the ascending limb becomes progressively more dilute as it rises back toward the cortex.

By the time the fluid reaches the distal convoluted tubule (DCT) and then the collecting duct, further reabsorption of Na+, bicarbonate and, especially, water takes place — but unlike the fixed reabsorption of the PCT, this later reabsorption is facultative, meaning it is variable and actively regulated moment to moment according to the body's actual needs: reabsorption of Na+ here is controlled by the hormone aldosterone, and reabsorption of water is controlled by ADH (antidiuretic hormone), both described in a later section. It is this facultative, hormone-controlled step, occurring last, that finally determines whether the urine produced on a given occasion will be dilute (when the body has plenty of water to spare) or highly concentrated (when water needs to be conserved). …