The renal system consists of the kidneys, ureters, bladder, and urethra. The kidney contains the nephron, the functional unit of the renal system. The nephron consists of the glomerular and peritubular capillaries and the associated tubular segments. The glomerular tuft (glomerulus) contains capillaries and the beginning of the tubule system, Bowman’s capsule.
Tubule fluid, an ultrafiltrate of plasma, is formed at the renal glomerulus and passes through the tubules. The composition of the filtrate is modified by secretion and reabsorption as it passes through the tubules of the renal cortex and medulla, ending with the collecting ducts. A second capillary bed, the peritubular capillaries, carries the reabsorbed water and solute back toward the vena cava. Filtrate from the tubules collects at the renal calyx and is transported by the peristaltic action of the ureter to the bladder. The bladder stores urine until elimination from the body through the urethra.
Renal Cortex And Medulla
Each kidney can be visually and functionally divided into an outer cortex and an inner medulla. The renal cortex contains all the glomeruli, a large portion of the peritubular capillaries, as well as the proximal tubule, distal tubule, and cortical portion of the collecting duct. The renal medulla contains the vasa recta, the loop of Henle, and the medullary portion of the collecting duct. The renal medulla has a pyramidal structure, with the collecting ducts emptying into the renal calyces.
Blood Vessels And Renal Tubules
The kidneys have an extensive vascular supply and receive about 20% of the cardiac output. The renal vascular pattern is unusual in that blood flows through two capillary beds, one with high pressure (glomerular) and the second with low pressure (peritubular), connected in series. Blood enters the kidney via the renal artery and, after a series of divisions, arrives at the glomerulus. Blood entering the glomerular capillaries must first pass through an afferent arteriole. Blood exiting the glomerular capillaries passes through a second arteriole, the efferent arteriole. Blood then flows through the peritubular capillaries, which include the vasa recta that extend into the renal medulla. Blood leaves the peritubular capillaries, collects in progressively larger venules and veins, and then exits the kidney via the renal vein.
Filtrate formed in Bowman’s capsule remains separated from the body fluid spaces by a layer of epithelial cells that extends through the remainder of the urinary system.
Consequently, renal filtrate and urine are functionally outside the body, similarly to the fluids of the GI tract. Renal tubules consist of a single layer of epithelial cells that selectively secrete or reabsorb compounds. Tubular transport represents a mechanism to reabsorb water and solutes filtered at the glomerulus before they are excreted from the body in the urine. The ureter, bladder, and urethra also have an epithelial lining, but the epithelial cells do not allow transport of water or solutes. Consequently, filtrate that exits the renal collecting duct and collects in the renal pelvis is identical to the final urine.
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