What is Glomerular Filtration Rate?
Glomerular Filtration Rate (GFR) is the amount of fluid that filters into the Bowman’s capsule per unit of time. Clearance is the standard metric to measure the GFR. Clearance refers to the amount of a solute in the filtration fraction that is NOT reabsorbed back into the blood; therefore, the amount present in urine. Glucose has a 0% clearance because all glucose reabsorbs, and NO glucose is in healthy urine. Inulin clearance is the gold standard for measuring GFR because it has 100% clearance. However, inulin is not made in the body and is difficult to test for, so doctors rarely use inulin clearance to measure GFR. Creatinine is a waste product from the normal breakdown of creatine phosphate (an energy-releasing molecule) in muscle tissue. Creatinine has almost a 100% clearance rate, and its routine use is to measure GFR because most of it excretes from the body, and it involves simple blood and urine tests.
Any solute that is reabsorbed back into the blood is not useful in measuring GFR. One hundred percent of the filtered glucose and amino acids are reabsorbed back into the blood; therefore, these solutes are not present in the urine, which makes measuring GFR not possible. Urea is both absorbed and secreted, so the amount of urea in the urine varies. NaCl is reabsorbed back into the blood, and the amount reabsorbed depends on the osmolarity of the blood and the presence of the hormone aldosterone. K+ is reabsorbed into the blood, and the amount secreted into the nephron varies due to blood osmolarity and the presence of aldosterone. And, the amount of H+ secretion varies because it is dependent on blood pH.
Calculating GFR
A few variable need to be defined before we calculate GFR:
- Ucr – Creatinine concentration in the urine
- Pcr = Creatinine concentration in the blood plasma
- V = The volume of urine produced per minute
Creatinine\:Clearance\:(ml/min)=\frac{Ucr\times V}{Pcr}Let’s do a sample problem:
Patient 1546, a 196-lb 41-year-old male, is found to have low mean arterial pressure of 49 mmHg and is complaining of lower back pain (a possible symptom of kidney disease). His doctor orders a creatinine clearance test as part of a comprehensive physical exam. The results show him to have a creatinine blood plasma concentration of 0.018 mg/ml, and in 24 hours, he produces 1100ml of urine with a creatinine concentration of 1.50 mg/ml. Calculate creatinine clearance for this patient.
- First, you need to convert hours into minutes. The test is 24 hours, so this is 1440 minutes
- Now you need to find the volume of urine per minute: 1100 ml /1440 minutes = 0.76 ml/min
- Solve for GFR:
\frac{(1.5\:\:\bcancel{mg/ml}\times 0.76 \:\:ml/min}{0.018 \:\:\bcancel{mg/ml}} = \colorbox{yellow}{63\: ml/min}Since a GFR of 63 ml/min well below normal values (120-130 ml/min). To raise the GRF, which picture of autoregulation below demonstrates how to increase this patient’s GFR.
If you picked figures B or D, you are correct. Figure B shows the constriction of the efferent arteriole, which will reduce the blood flow leaving the glomerulus, increasing GFR. Figure D shows the dilation of the afferent arteriole, which will increase the blood flow to the glomerulus, increasing GFR.
Kidney Disease
A wide variety of factors can lead to kidney disease. The two primary causes of chronic kidney disease are diabetes and high blood pressure. The main cause of acute kidney disease is an infection, such as a urinary tract infection, gonorrhea, or chlamydia.
Our GFR decreases with age, so high blood sugar and blood pressure can significantly decrease kidney function in our twilight years. GFR and the amount of protein in the urine are the two main metrics for determining kidney disease. There should be no protein present in the urine. If protein is present, it usually means that a person has kidney disease. For example, if a patient has a normal GFR of 120 ml/min, but there are trace amounts of protein in the urine, this may signal the beginning of kidney disease. However, a person is usually on the verge of kidney failure if their GFR is less than 15 ml/min and they have small amounts of protein in the urine.
Since no protein should be in the urine, a person with a normal GFR but a high protein concentration will usually have moderate kidney disease. And a patient with a low GFR and high protein levels in the urine will most likely be on dialysis.
Dialysis
People with severe kidney disease and kidney failure usually have to go on dialysis for the rest of their lives or until a kidney becomes available for transplant. Hemodialysis is the most common type of dialysis, and it requires a patient to attend a dialysis clinic or hospital multiple times a week for treatment. Hemodialysis takes hours, and it involves a patient’s blood being filtered through a machine that acts like a kidney. The machine regulates the amount of water and solutes in the blood plasma and removes metabolic waste.
Peritoneal dialysis does not require trips to a clinic or hospital because the patient’s peritoneal cavity replaces the kidneys. The peritoneal cavity surrounds the intestines, so a catheter must be implanted into a person’s abdomen. A patient hooks a bag of dialysis solutions with a similar composition to blood plasma to their catheter. The dialysis solution leaks into the peritoneal cavity, and the patient leaves the solution in the cavity for a few hours. Water and nutrients from the dialysis solutions diffuse into the blood and waste products, and extra electrolytes diffuse into the cavity. After a few hours, the patient attaches an empty bag to their catheter, and the fluid (which is urine-like in composition) is drained into the bag.
