The functional unit of the kidney is the nephron, of which there are about 1 million in each kidney. Each nephron is composed of a glomerulus, proximal tubule, loop of Henle, distal tubule and collecting duct (Fig. 9.1). The renal artery (a branch of the abdominal aorta) supplies the kidney and divides many times to form afferent arterioles, which supply the glomerular capillary tuft before draining into efferent arterioles. Efferent arterioles from (outer) cortical glomeruli drain into a peritubular capillary network within the renal cortex and then into increasingly larger branches of the renal vein. By contrast, blood from the (inner) juxtamedullary glomeruli passes via vasa recta in the medulla and returns via the cortex to renal veins that drain into the inferior vena cava. The wider diameter of the afferent compared to efferent arterioles increases the pressure of blood within the glomerulus and forces water and solutes (but not red blood cells or larger molecular weight plasma proteins) out of the glomerular capillaries into the Bowman’s capsule, forming the glomerular filtrate (about 170–180 L per day). The proximal renal tubules reabsorb most of the filtered solute required to maintain fluid and electrolyte balance, but elimination of potassium, water and non-volatile hydrogen ions is regulated in the distal tubules. As renal perfusion and glomerular filtration fall, reabsorption of water and sodium by the proximal tubules increases so that minimal fluid reaches the distal tubule. Hence, hypotensive or hypovolaemic patients cannot excrete potassium and hydrogen ions. Patients with distal tubular damage, e.g. caused by drugs, also cannot excrete potassium and hydrogen ions. Normally only about 1% of the original filtered volume, containing high concentrations of urea and creatinine, passes into the renal pelvis as urine.