V. Anatomy and Physiology
The Urinary Tract
The Urinary System rids the body of nitrogenous wastes while regulating water, electrolyte, and acid-base balance of the blood.
Kidneys are small, dark red organs with a kidney-bean shape lie against the dorsal body wall in a retroperineal position (beneath the parietal peritoneum) in the superior lumbar region. The kidneys extend from T12 to the L3 vertebra; thus they receive some protection from the lower part of the rib cage. Because the liver crowds it, the right kidney is positioned slightly lower than the left. It is convex laterally and has a medial indentation called the renal bilum. Several structures, including the ureters, the renal blood vessels, and nerves, enter or exit the kidney at the hilum.
A transparent fibrous capsule encloses each kidney and gives a fresh kidney a glistening appearance. A fatty mass, the perineal fat capsule surrounds each kidney and acts to cushion it against blows. The renal fascia, the outermost capsule, anchors the kidney and helps hold it in place against the muscles of the trunk wall.
When a kidney is cut lengthwise, three distinct regions become apparent. The outer region, which is light in color, is the renal cortex. Deep the cortex is a darker reddish-brown area, the renal medulla. The medulla has many basically triangular regions with a striped appearance, the renal or medullary pyramids. The broader base of each pyramid faces toward the cortex, its tip, and the apex points toward the inner region of the kidney. The pyramids are separated by extensions of cortex-like tissue, the renal columns.
Medial to the hilum is a flat, basin like cavity, and the renal pelvis. The pelvis is continuous with the ureter living the hilum. Extensions of the pelvis, calyces, form cup-shaped areas that enclose the tips of the pyramids. The calyces collect urine, which continuously drains from the tips of the pyramids into the renal pelvis. Urine then flows from the pelvis into the ureter, which transports it to the bladder for temporary storage.
The kidneys continuously cleanse the blood and adjust its composition, so it is not surprising that they have a very rich blood supply. Approximately one-quarter of the total blood supply of the body passes through the kidneys each minute. The arterial supply of each kidney is the renal artery. As the renal artery approaches the hilum, it divides into segmental arteries, each of which gives off several branches called interlobar arteries, which travel through the renal columns to reach the cortex. At the cortex medulla junction, interlobar arteries give off the arcuate arteries, which curve over the medullary pyramids. Small cortical radiate arteries then branch off the arcuate arteries and run outward to supply the cortical tissue. Venous blood draining from the kidney flows through veins that trace the pathway of the arterial supply but in a reverse direction – cortical radiate veins to arcuate veins to interlobar veins to the renal vein, which emerges from the kidney hilum. (There are no segmental veins)
Nephrons and Glomeruli
Each kidney contains over a million tiny structures called nephrons. Nephrons are the structural units of the kidneys and, as such, responsible for forming urine. Each nephron consists of two main structures: a glomerulus, which is a knot of capillaries, and a renal tube. The closed end of the renal tube is enlarged and cup-shaped and completely surrounds the glomerulus. This portion of the renal tube is called the glomerular, or Bowman’s capsule. The inner (visceral) layer of the capsule is made up of highly modified octopus-like cells called podocytes. Podocytes have long branching processes called foot processes that intertwine with one another and cling to the glomerulus. Because opening, the so-called filtration slits, exist between their extensions, the podocytes form a porous, or “holey,” membrane around the glomerulus....
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