What Is It, How to Calculate, and More
Author: Anna Hernández, MD
Editors: Ahaana Singh, Jaclyn Kiser, PA
Illustrator: Abbey Richard
Copyeditor: Joy Mapes
What is the filtration fraction?
The filtration fraction (FF) represents the portion of blood plasma that gets filtered by the glomerulus, a part of the kidney, relative to the renal plasma flow (RPF). FF is a measure of the kidney’s function, and although it is typically not used in routine clinical practice, it can be a helpful measure to understand how the kidneys respond under certain conditions.
The kidneys are two bean-shaped organs located in the abdomen, at either side of the lower spine. Within each kidney, there are millions of functional units called nephrons that clear the body of harmful substances and produce urine, among many other functions. Each nephron is composed of a renal corpuscle and a set of kidney tubules, which are surrounded by an intricate network of blood vessels known as peritubular capillaries.
Renal filtration takes place in the renal corpuscle, consisting of the glomerulus, which is a ball-shaped network of blood vessels, and the Bowman’s capsule, a double-walled sac that surrounds the glomerulus. Blood enters the glomerulus through a blood vessel called the afferent arteriole, and then it leaves the glomerulus through the efferent arteriole. As blood flows through the glomerulus, water and some solutes in the blood are filtered into the Bowman’s capsule, creating an ultrafiltrate of blood. This filtrate then travels through the renal tubules, where urine is ultimately produced.
Inside the renal tubules, some molecules, such as ions and water, are reabsorbed from the tubule back to the peritubular capillaries, whereas other substances are secreted from the peritubular capillaries into the tubule. This process of tubular reabsorption and secretion prevents the loss of a great amount of water and adjusts the composition of urine based on what the body needs.The glomerular filtration rate (GFR) measures the amount of filtrate produced by the kidneys per minute, and it is one of the main measures of kidney function. The GFR depends on several factors, including the amount of blood that gets to the kidneys through the renal artery, otherwise known as renal blood flow. Of this renal blood flow, only the plasma, which is the non-cellular portion of blood, can pass through the glomerulus. Plasma makes up about 55% of the blood and, since the glomerulus filters around 20% of that plasma at a time, only a small fraction of the renal blood flow is actually filtered across the glomerulus. The filtration fraction (FF) takes into account what portion of plasma entering the kidneys is filtered across the glomerulus, in relation to the total renal plasma flow (RPF).
How do you calculate the filtration fraction?
The filtration fraction (FF) is the ratio between the glomerular filtration rate (GFR) and renal plasma flow (RPF). A healthy individual has a GFR of around 120 ml/min (milliliters per minute, or about ⅓ ounce per minute) and an RPF of around 600 ml/min. This results in a FF of 0.2 or 20%. The remaining 80% of renal plasma flow will then be part of the pericapillary blood flow surrounding the renal tubules after leaving the glomerular capillaries.
Does an increased filtration fraction cause more reabsorption?
Yes, increased filtration causes more tubular reabsorption through a phenomenon called glomerulotubular balance. When the FF increases, the amount of protein-free plasma that is filtered across the glomerulus increases as well. As a result, the remaining blood that leaves the glomerulus has a higher relative protein concentration, which, in turn, favors the reabsorption of water from the tubules back into the peritubular capillaries, thereby increasing tubular reabsorption.
What happens when the glomerular filtration rate (GFR) is low?
The kidneys are able to maintain a stable GFR over a wide range of conditions due to a series of autoregulatory mechanisms. These mechanisms protect the kidneys against sudden changes in blood pressure, which could affect the renal blood flow and, consequently, lead to a decrease or increase in the GFR.There are three main mechanisms of kidney autoregulation that occur when the GFR is low. First, there is the myogenic mechanism, which refers to the relaxation of the afferent arteriole in response to a decrease in blood pressure. This allows more blood to reach the glomerulus. Second, there is the tubuloglomerular feedback, which adjusts the vascular resistance of the afferent arteriole in response to changes in the amount of solutes that reach the renal tubules. Finally, there is the activation of the renin-angiotensin-aldosterone system (RAAS), which leads to an increase in the body’s blood pressure, ultimately restoring renal blood flow and returning the GFR to normal levels. Overall, these mechanisms help preserve the GFR by increasing the amount of blood that gets to the glomerulus when blood pressure drops below a certain threshold.
What are the most important facts to know about the filtration fraction?
The filtration fraction (FF) is the portion of plasma that is filtered across the glomerulus relative to the renal plasma flow (RPF). In a healthy individual, the usual filtration fraction is around 0.2, or 20% of the total renal plasma flow. The FF can be calculated by the ratio between the glomerular filtration rate (GFR) and renal plasma flow (RPF).
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Resources for research and reference
Dalal, R., Bruss, Z., & Sehdev, J. (2020, August 11). Physiology, renal blood flow and filtration. In StatPearls [Internet]. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK482248/
Feher, J. (2017). Regulation of fluid and electrolyte balance. In Quantitative human physiology (2nd ed., pp. 740-751). Elsevier. DOI: 10.1016/B978-0-12-800883-6.00074-4Hall, J. (2015). Guyton and Hall textbook of medical physiology (13th ed.). Philadelphia, PA: Elsevier.