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Renal Glomerular Filtration

The first stage of the filtration process in the kidney takes place in the structure known as the glomerulus, in the part of the nephron located in the cortex.

The blood arrives from the afferent arteriole into the small glomerular capillaries inside the Bowman's capsule. Because the capillaries are sandwiched between two constrictive arterioles, the strong hydrostatic pressure "pushes" water, ions, and other small molecules through a filtering membrane into the nephron canal and farther into the proximal tubule. This is a passive process, with the membrane acting like a special mesh to facilitate filtration of the metabolites in the bloodstream.

The amount of filtrate going through the glomerular membrane in one minute for both kidneys is called the Glomerular Filtration Rate, or GFR. From the glomerular filtration rate directly depends the volume of urine being excreted; a higher filtration rate results in a high volume of water being transferred to the tubules and ultimately to the urine collecting duct.

To demonstrate the efficiency of glomerular filtration, up to 180 liters are filtered every day by the kidney for an average individual, although 99% of the water is reabsorbed, restraining to a typical daily volume of urine of 1-2 liters.

The GFR mainly depends on the difference between the outward pressure (the pressure exerted by the heartbeat and the constricting arteriole) and the inward pressure (the hydrostatic pressure exerted back from the tubule in the Bowman's capsule). This difference is called the net filtration pressure.

To measure the filtration rate, we can also use the loss of creatinine in the bloodstream. Creatinine is a byproduct of metabolic processes in the muscles and is almost entirely filtered out in urine by the kidneys. By measuring the concentration of creatinine in the plasma and the urine as well as the volume of urine, we can estimate the glomerular filtration rate:

Image containing different equations. - The amount of creatinine entering the kidney equals the glomerular filtration rate times the concentration of plasma creatinine. - The amount of creatinine leaving the kidney in the urine equals the urine production rate times the concentration of urinary creatinine. If we assume that the amount of creatinine leaving the kidney equals the amount entering via glomerular filtration, then we have: - glomerular filtration rate times the plasma creatinine concentration equals the urine production rate times the urinary creatinine concentration - and the glomerular filtration rate equals the urine production rate times the concentration of urinary creatinine divided by the concentration of plasma creatinine. The rates are measured in milliliters per minute and the concentrations in millimoles per liter.

Figure 1. Glomerular Equations

However, this is an approximation since some of the creatinine is actively secreted out in the nephron not the result of the passive glomerular filtration, contributing to the final concentration of creatinine in the urine output.