The urinary system, composed of kidneys, ureters, bladder, and urethra, is responsible for the excretion of waste products in urine and regulation of important ions and molecules in the body. The biochemical components of the basal lamina and varying cell types found in the renal corpuscle create the filtering mechanism that initiates this process. Ionic channels and pumps found in the nephron allow the final composition of the urine to be modified according to the needs of the body. This is regulated by various hormones that help regulate sodium, potassium, chloride, calcium, magnesium, ammonia/acid–base balance, total body water, blood pressure, and other molecules and ions. The renin–angiotensin–aldosterone hormonal system as well as vasopressin and atrial naturetic peptide are keys in this process. Finally, the kidneys are essential for red blood cells production via erythropoietin synthesis and also add an essential biochemical step in the activation of vitamin D.
BASIC ANATOMY AND PHYSIOLOGY
The urinary system, embryologically derived along with the reproductive system from the intermediate mesoderm, consists of the kidneys and associated blood vessels, ureters, bladder, and urethra. The kidney (Figure 18-1) contains hundreds of thousands to millions of nephrons and associated tubules and collecting ducts where blood is filtered by the renal corpuscles for waste products; fluid balance (and, secondarily, blood pressure) is adjusted; electrolytes, glucose, amino acids, and other molecules are balanced to appropriate levels; acid–base levels are maintained; and the process of urine production is completed. Additionally, the enzyme renin, the hormone erythropoietin, and the final active form of vitamin D are produced by the kidney.
Basic Anatomy of the Kidney. The kidney contains the renal artery and vein, which transport blood to and from the pelvis and into a calyx and medulla where filtration occurs. Urine produced by this process exits via the ureters. [Reproduced with permission from Kibble JD and Halsey CR: The Big Picture: Medical Physiology, 1st edition, McGraw-Hill, 2009.]
The structure of the renal corpuscle (Figure 18-2A) allows the kidneys to perform its initial filtering function. The overall structure is composed of a glomerulus, which receives blood via a small collection of capillaries bringing blood to (afferent) and away from (efferent) the renal corpuscle, and the surrounding Bowman’s capsule along with the nephron. The glomerulus forms the first part of the filtration apparatus with fenestrations (“openings” from the Latin for window) formed between the cell membranes of its endothelial cells. The fenestrations are relatively large, though, and only filter material the same size or smaller than blood cells (Figure 18-2A–B). Next is a three-layered structure called the basal lamina (also called the basement membrane), which is approximately six times thicker than other basement membranes ...