Kidney. Artwork of a section through a human kidney. The two kidneys excrete urine and regulate blood and electrolyte balance. One of the million nephrons contained in each kidney is magnified at centre right. Fluid is filtered from the blood of the arteriole (red) by the mesh-like glomerulus of the Bowman's capsule (yellow). The fluid collects in the tubule (yellow) below. Nearly all water and salts are reabsorbed from the tubule and its straight section, the loop of Henle (at centre), to form urine. Urine then runs through one of the calyces (at far left) and into the ureter (not seen). The kidney consists of outer cortex (light pink) and inner medulla (dark pink).