Filtration:  The Kidneys


Can you imagine the implications of an industrial filter that could be used in a process for more than 70 years without ever being replaced or cleaned?


Believe it or not, the human kidney is capable of such a feat.  The kidney is an amazing filtration device in that they can be used continuously for the duration of a human life without becoming clogged. If you’re interested, learn more about the workings of the kidney below….



What is the primary function of the kidney in the body?

Although the Kidneys are Tiny Organs They Receive 25% of the Cardiac Output in the body.  The two kidneys are only 0.4% of the body weight but receive about 25% of the blood.  Their most important function in the body is to regulate the composition and volume of body fluids.  Blood flows in and out of kidney leaving behind the 1% which becomes urine.  The urine produced flows through ureters to the bladder and through urethra to outside world. 

The typical kidneys filter approximately 180 liters of plasma/day and each of the 3 liters of plasma gets filtered about 60 times.  To replace this much water you would have to drink a 12 ounce soft drink every 3 minutes of the day;  fortunately 99% of the filtrate gets reabsorbed, leaving 1.5-2 liters of urine per day.  This is an incredible amount of filtration that is performed every day.

What is the kidney comprised of?

We have a total of 2 million nephrons in the 2 kidneys when we are young

The distinctive feature is that  the tubule makes a sharp bend at the loop of Henle which allows tubule fluid to move downward into regions of increasing osmotic pressure (see diagram below).  After the bend the tubule fluid moves upward through regions of decreasing osmotic pressure.  The Glomerulus has large pores, allowing filtration of large volumes of fluid.  The number of nephrons declines with age, to about 50% at age 60; this causes the GFR to drop to 50% of the value in a young person

The Kidneys use active transport (especially of Na) to set up osmotic gradients which is represented in the figure below.  The kidney takes advantage of the osmotic pressure difference between tubule fluid and interstitial fluid to move water out of the tubule.  By changing the permeability of the collecting duct the kidney is able to make concentrated or dilute urine by osmosis

Source [10]


How does the kidney filter, absorb, and secrete fluid?

In kidney filtration, which takes place in the glomerlus, about 20% of the plasma that passes through the kidney gets filtered into the nephron.  Filtration is driven by the hydrostatic pressure of the blood, and causeswater and small molecules to be filtered, while blood cells and large molecules like  proteins do not pass through the filter.

Source [10]

In reabsorption & secretion, as the filtrate passes down the nephron most of it is reabsorbed into the blood.  Reabsorption and secretion require a large amount of every which results in the kidney being one of the most metabolically active organs in the body

The overall net Process is displayed below:

Source [10]

At what rate is the kidney capable of filtering all of these fluids?

The rate at which the kidney filters blood plasma is called the glomerular filtration rate (GFR), and is relatively easy to measure.  It is a good way of assessing the kidney function.  See description below for more details:

Two substances are used to measure GFR:

If a fluid is Filtered + reabsorbed: C will be less than the GFR, while if a fluid is filtered only, C = GFR (about 120 mL/min)   Filtration followed by secretion causes C to be be higher than the GFR

This is shown below:

Source [10]