Basics of renal anatomy and physiology.

A kidney contains over 1 million functioning units called nephrons. Each nephron is composed of a glomerulus and tubule. The glomerulus acts to filter the blood free of cells and large proteins, producing an ultrafiltrate composed of the other smaller circulating elements. The ultrafiltrate enters the tubule, which is highly specialized at various segments, to produce the final urine by removing substances from the tubular fluid (reabsorption) or adding substances to the tubular fluid (secretion). Filtration, reabsorption, and secretion keep the organism in balance in terms of water, minerals, electrolytes, and hydrogen ion concentration and eliminate the toxic substances produced by the body. The major known hormonal functions of the kidney influence blood pressure, calcium metabolism, and red blood cell production.