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The Kidney

 Glomerular (Kidney) Filtration Rate

A key measurement of kidney health is its ability to perform its major function, filtering impurities out of the blood. The Glomerular filtration rate (GFR) is the technical name for this process.

The GFR is equal to the clearance rate when nothing either reabsorbed or secreted by the kidneys (a process that kidneys are constantly performing). The rate is measured by the quantity of the impurities in the urine that originated from a calculable volume of blood. The GFR is typically recorded in units of volume per time, e.g., milliliters per minute (mL/min).

In clinical practice, the substance creatinine clearance is used as a proxy to measure the filtration rate. Creatinine (a form of a substance that activates the body’s energy reserves) is at a steady-state concentration in the blood, and so measuring creatinine clearance is a more straight forward process.

Creatinine clearance rate (CCr) is calculated from the creatinine concentration in the collected urine sample (UCr), times the urine flow rate (volume over change in time (v̇)), and the plasma (the non-red blood cell component of blood) concentration of the creatinine (PCr).


In other words; how much liquid you have times, how long it took to produce it, divided by how much of the stuff (creatinine), that you are measuring is in the liquid that you have. This gives you the clearance rate


This is represented mathematically as;


One method of determining the creatinine clearance rate is to collect urine (usually for 24 hours) to determine the amount of creatinine that was removed from the blood over a given time interval.

  1. Each day is 24 hours long, and each hour is 60 minutes. How many minutes are in each day? (1,440)


If one passes 1440 milligrams (mg) in 24 hours, this is equivalent to removing 1mg/minute.

  2.  If the blood concentration of creatinine is 0.02 mg/mL, and 1 mg of creatinine is produced. How many liters of blood must be cleaned of creatinine to produce that one gram?

                                       0.02       1

                                       -----  =   ---                      0.02X = 1                          X = 50 liters of blood                        

                                         1          X


  3.   A person has a plasma creatinine concentration of 0.01 mg/ml and in 1 hour (60 minutes) produces 60ml of urine with a creatinine concentration of 1.25 mg/mL.  What is their creatinine clearance rate per minute?


Utilizing the above formulae for Ccr as a proxy for GFR, determine the stages of kidney disease for the persons based on the data below.


The Stages of Kidney Disease


GFR level (mL/min/1.73 m2) where mL = milliliters, min = minute, and 1.73m2 = body surface area


1) CKD1 – GFR above 90 mL/min/1.73m2

2) CKD2 (mild) – GFR of 60 to 89 mL/min/1.73m2

3) CKD3 (moderate) – GFR of 30 to 59 mL/min/1.73 m2

4) CKD4 (severe) – GFR of 15 to 29 mL/min/1.73 m2

5) CKD5 kidney failure - GFR less than 15 mL/min/1.73 m2


There are a number of factors that impact the result including race, gender and body size. The 1.73m2 in the above calculation stands for the standard body size. In order to account for size variation in human beings, the follow correction can be applied as an aid in determining a more accurate figure.


Where BSA (body surface area) is calculated by measuring height in centimeters (1 inch = 2.54 centimeters) and kilograms (2.2 lbs. = 1 kilogram) in the formula below:


Multiplying the height in centimeters times the weight in kilograms and then taking the square root. The square root is then divided by 60 and then placed in the corrected Ccr formula.

If a person is six feet tall;

6 x 12 inches x 2.54 = 183 centimeters in height (rounded)

If a person weighs 200 lbs;

200/2.2 = 91 kilograms (rounded)

 The square root of 183 X 91 (16,653) rounded to the nearest whole number = 129

Their BSA would 129/60 = 2.15. Plugging into the Ccr-corrected equation yields a correction factor of 0.8 (1.73/2.15).


  4.  The rate of kidney decline is different between ethnic-racial groups. Kidney function declines with age. If a white person’s kidney function rate declines at 1 mL/min per annum GRF, and African-American’s at 1.3mL/min per year. If at age 30 each group has a GRF of 100, how many years will it take for each group to reach the second stage of kidney dysfunction?

100-1(W) = 89                                   -W = 89-100                      W = 11 years

100-1.3(A) = 89                             -1.3A = 89-100                     A = 11/1.3            A = 8.5 years


 5.  How many more years will it take each person to reach Stage 3, moderate kidney dysfunction when African-American’s  kidney decline subsequently increases to 1.75mL/min per annum and the white person’s rate of decline increases to 1.25mL.min per annum rate (Whites = 24 years, African Americans = 17 years).

1.75A = (89-59)                               A = 17 years

1.25W = (89-59)                             W= 24 years


 a.  How old will the white person be when they reach the stage of moderate kidney failure?

30+11+24 = 64 years old

 b.  How old will the African American person when they reach the stage of kidney failure?

30+8.5+17 = 45.5 years old


 c.  Based on your height and weight what would be your BSA correction factor?



The kidneys are two bean-shaped organs found on the left and right sides of the body in vertebrates. Kidneys filter the blood in order to make urine, to release and retain water, and to remove waste. Each kidney feeds urine into the bladder by means of a tube known as the ureter. 

Kidneys also regulate fluid balance and blood pressure. They are also responsible for the reabsorption of water. In humans, the kidneys are located high in rear of the abdominal cavity, one on each side of the spine. Despite their relatively small size, the kidneys receive approximately 20% of the cardiac output.


Many of the kidney's functions are accomplished by relatively simple mechanisms of filtration, reabsorption, and secretion. Filtration is the process by which cells and large proteins are filtered from the blood to make an initial step in the process that creates urine. The kidney generates 180 liters (1.06 quarts = 1 liter) of filtrate a day, while reabsorbing a large percentage, allowing for the generation of only approximately 2 liters of urine.


African Americans are more at risk for kidney failure than any other race. More than 1 in 3 kidney failure patients living in the United States is African-American.


Diabetes is the #1 cause of kidney failure.  Diabetes is a group of diseases in which there are high blood sugar levels over a prolonged period. Diabetes is due to either the body not producing enough insulin (in the pancreas) or the body not responding properly to the insulin produced. Insulin regulates the body’s process of turning food, especially, glucose (sugar), from the blood into fat, liver and skeletal muscle cells.


About 1 in 9 African-American adults has diabetes. About 1 in 3 African-Americans with diabetes does not know he or she has it. African-Americans with diabetes develop kidney failure more often than whites. Diabetes causes nearly 40 percent of all cases of kidney failure in the United States.



Former NBA player forced to retire

Sean Elliott

kidney transplant recipient

Former NBA player forced to retire                 

Alonzo Mourning

kidney transplant recipient


Tracy Morgan

kidney transplant recipient



waiting for a kidney transplant


Nick Cannon

suffers from kidney disease



had kidney removed in the 1970’s


George Lopez

transplant recipient

High blood pressure is the #2 cause of kidney failure. It causes about 1 out of 4 cases (25%) in the United States. High blood pressure is a long term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. About 90–95% of cases are primary, defined as high blood pressure due to nonspecific lifestyle and genetic factors. Lifestyle factors that increase the risk include excess salt, excess body weight, smoking, and alcohol.


Almost half (over 42%) of African-American adults have high blood pressure. African-Americans are six times more likely to get kidney failure from their high blood pressure than whites.


Almost 1 in 5 African-Americans is uninsured. As a result, their health care choices may be limited.


Calculations of kidney performance are an important part of physiology and can be estimated.

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