The functional unit of the kidney is the nephron, which consists of a glomerulus, proximal tubule, loop of Henle, distal tubule, and collecting duct system. The glomerulus is composed of glomerular capillaries and Bowman's capsule. The juxtaglomerular apparatus is one component of an important feedback mechanism that regulates renal blood flow and glomerular filtration rate. The structures that compose the juxtaglomerular apparatus include the macula densa, the extraglomerular mesangial cells, and the renin-producing granular cells. The lower urinary tract is composed of the ureters, bladder, and urethra.
Micturition is the process of emptying the urinary bladder.
The rate of glomerular filtration is calculated by measuring the clearance of inulin or creatinine. Renal clearance equations can be used to determine whether a substance undergoes either net reabsorption or secretion by the nephron. The first step in the production of urine is the formation of an ultrafiltrate of plasma by the glomerulus. Starling forces across the glomerular capillaries drive an ultrafiltrate of plasma from the glomerular capillaries into Bowman's space. The ultrafiltrate is devoid of cellular elements, contains very little protein, but otherwise is identical to plasma.