OVERVIEW OF THE URINARY SYSTEM / 698 GENERAL STRUCTURE OF THE KIDNEY / 699 Capsule / 699 Cortex and Medulla / 700 Kidney Lobes and Lobules / 701 The Nephron / 702 General Organization of the Nephron / 702 Tubes of the Nephron / 703 Types of Nephrons / 703 Collecting Tubules and Ducts / 703 Filtration Apparatus of the Kidney / 705 Mesangium / 710 Juxtaglomerular Apparatus / 711 KIDNEY TUBULE FUNCTION / 714 Proximal Convoluted Tubule / 715 Proximal Straight Tubule / 716 Thin Segment of Loop of Henle / 717 Distal Straight Tubule / 718 Distal Convoluted Tubule / 718 Collecting Tubules and Collecting Ducts / 719 INTERSTITIAL CELLS / 720 HISTOPHYSIOLOGY OF THE KIDNEY / 720 BLOOD SUPPLY / 721 LYMPHATIC VESSELS / 723 NERVE SUPPLY / 723 URETER, URINARY BLADDER, AND URETHRA / 723 Ureters / 725 Urinary Bladder / 726 Urethra / 726 Folder 20.1 Functional Considerations: Kidney and Vitamin D / 699 Folder 20.2 Clinical Correlation: Antiglomerular Basement Membrane Antibody-Induced Glomerulonephritis; Goodpasture Syndrome / 712 Folder 20.3 Clinical Correlation: Examination of the Urine—Urinalysis / 714 Folder 20.4 Clinical Correlation: Renin–Angiotensin–Aldosterone System and Hypertension / 714 Folder 20.5 Functional Considerations: Structure and Function of Aquaporin Water Channels / 717 Folder 20.6 Functional Considerations: Hormonal Regulation of Collecting Duct Function / 721
The urinary system consists of the paired kidneys; paired ureters, which lead from the kidneys to the urinary bladder; and the urethra, which leads from the bladder to the exterior of the body.
The kidneys conserve body fluid and electrolytes and remove metabolic waste.
Like the lungs and liver, the kidneys retrieve essential materials and dispose of wastes. They conserve water, essential electrolytes, and metabolites, and they remove certain waste products of metabolism from the body. The kidneys play an important role in regulating and maintaining the composition and volume of extracellular fluid. They also are essential in maintaining acid–base balance by excreting hydrogen ions when bodily fluids become too acidic or excreting bicarbonates when bodily fluids become too basic.
The kidneys are highly vascular organs; they receive approximately 25% of the cardiac output. The kidneys produce urine, initially a glomerular ultrafiltrate of the blood or primary urine, which is then modified by selective resorption  FOLDER 20.1 Functional Considerations: Kidney and Vitamin D