How the Renal System Works
Your kidneys initially filter your blood. The liquid portion of your blood is mostly water, but it also includes proteins, glucose, cholesterol (related to fat), and ions (charged particles), known as electrolytes. Some important electrolytes are sodium, potassium, calcium, and phosphate. Electrolytes are necessary components of almost all bodily functions, so before they are discarded in your urine, your kidneys ensure that enough electrolytes remain in your blood. To do this, the liquid removed from your blood is further modified in the tubular portion of the nephron by processes called reabsorption and secretion. The final product is excreted from your kidneys, and eventually leaves your body through the process of urination.
About 20% of the blood pumped every minute by your heart goes directly to both of your kidneys. The blood enters your kidney through the renal artery and makes its way to the glomeruli of your nephrons, where filtering takes place. In general, only small molecules are filtered through the glomerulus, leaving larger molecules (the plasma proteins) and the cells in your blood. The liquid filtered out of your blood is known as filtrate and this will eventually become urine.
The glomeruli in your kidneys’ nephrons continuously filter your blood so that your entire blood volume is filtered multiple times each day. Up to 48 gallons of blood are filtered each day, yet only about 4 to 8 cups of this filtrate is excreted as urine. This means that the majority of filtrate is recovered by your body in a process known as reabsorption.
Reabsorption occurs because the filtrate contains elements your body still needs, like water, electrolytes, glucose, amino acids, and other nutrients. The blood vessels that surround your nephrons reabsorb these substances as needed, so they are not excreted in your urine.
Secretion takes place in the tubules of your kidney. It is the process of transporting unwanted molecules out of your bloodstream and into the filtrate. Substances that are secreted are proteins and foreign molecules, including any drugs you may have taken (most drugs are cleared from the bloodstream either by the kidneys or the liver). Secretion and reabsorption occur simultaneously. After filtration, reabsorption and secretion, the remaining contents gather in the collecting duct of the nephron where the filtrate is prepared for excretion.
The filtrate, now known as urine, is composed of the end-products of unneeded amino acids, electrolytes, and other molecules that are toxic to your body. It exits your kidneys through your ureters, on its way to your urinary bladder.
Urine is stored in your urinary bladder until it is filled. The urinary bladder can hold up to 2 to 3 cups of urine. In order to urinate, muscles in your abdomen must relax, allowing the urine to flow from your urinary bladder, through your urethra, and out of your body.