Typically understated in function, our kidneys regulate the composition and volume of the blood. Our two kidneys, along with their corresponding ureters, the bladder, and the urethra, make up our urinary or renal system. Although our kidneys are only about 1 percent of our total body weight, they receive about 20 to 25 percent of our left ventricle's cardiac output. Amazingly, our kidneys will filter and process approximately 47 gallons (180 liters) of blood-derived fluid daily.
Each one of our two kidneys is home to about one million tiny blood processing units called nephrons. Each nephron will engage in two basic operations. First, they filter plasma into a series of tubes; second, they will process the filtered fluid. As you might expect, the filtered plasma-derived fluid not only contains water but also small substances dissolved within, such as electrolytes, amino acids, and glucose. Cells (e.g., RBCs, WBCs) and most proteins in our blood are too large and are not filtered out of the blood.
There are two possible fates for the components of the filtered fluid. They can either be returned to the blood or not and ultimately become a component of urine. Normally, the reuptake of substances such as glucose and amino acids back to the blood is extremely efficient. Contrarily, the reuptake of water and electrolytes is more regulated. For example, if the concentration of sodium is too high in the blood, then less sodium will be returned to the blood and more will go into urine so that an optimal blood level is achieved. On the other hand, if the level of sodium in the blood is low, then more of the filtered sodium is returned to the blood and less is lost in the urine. As you might expect, the processes engaged in reabsorbing glucose, amino acids, electrolytes, and other desired substances require a lot of energy (ATP). Because of this normal kidney operations make a significant contribution to our total daily energy use.
Our kidneys filter our blood, collecting excessive substances and cell waste materials in urine for removal.
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