Filtration Fraction · What Is It, How to Calculate, and More

The filtration fraction (FF) represents the portion of blood plasma that gets filtered by the glomerulus relative to the total renal plasma flow (RPF). FF is a measure of the kidney 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, on 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. Each nephron is composed of a renal corpuscle and a set of kidney tubules, all surrounded by a network of blood vessels known as peritubular capillaries. 

Filtration takes place in the glomerulus, a ball-shaped cluster of capillaries enclosed by the Bowman’s capsule. Blood enters the glomerulus via the afferent arteriole and exits through the efferent arteriole. As blood flows through the glomerular capillaries, water and small solutes 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. Along the way, molecules, such as ions and water, are reabsorbed back into the bloodstream, whereas others are secreted into the tubules from the surrounding capillaries. 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. GFR depends mainly on the amount of blood that gets to the kidneys through the renal artery. 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 volume 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 kidneys use autoregulatory mechanisms to maintain a stable GFR over a wide range of conditions. 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.  

When GFR drops, three main mechanisms act to restore it. First, there is the myogenic response, which refers to the relaxation of the afferent arteriole to allow 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.