Measuring renal plasma flow and renal blood flow

Renal blood flow refers to the amount of blood that the kidneys receive over a period of time.

Blood gets to the kidneys through the renal artery.

Blood from the renal artery flows into smaller and smaller arteries, eventually forming the tiniest of arterioles called the afferent arterioles.

After the afferent arteriole, blood moves into a tiny capillary bed called the glomerulus. The glomerulus is part of the functional unit of the kidney, called the nephron.

There’s about 1 million nephrons in each kidney, and each of them consists of a renal corpuscle - made up of the glomerulus and the Bowman’s capsule surrounding it - and a renal tubule.

Interestingly, once the blood leaves the glomerulus, it does not enter into venules. Instead the glomerulus funnels blood into efferent arterioles which divide into capillaries a second time. These capillaries are called peritubular capillaries - because they are arranged around the renal tubule.

Now, blood filtration starts in the glomerulus, where an urine precursor called filtrate is formed.

The amount of blood filtered into the nephrons by all of the glomeruli each minute is called the glomerular filtration rate, and it’s actually just a small fraction of the blood that gets to the kidneys, because the glomerulus doesn’t allow red blood cells and proteins to pass through and be excreted into urine.

So right from the start, what passes through the glomerulus is mostly plasma - which normally makes up about 55% of blood. What is more, the glomerulus only filters about 20% of that plasma in one go. So when all is said and done, of those around 1.25 liters that the heart pumps out every minute, glomerular filtration rate is normally around 125 milliliters. That plasma-derived filtrate then enters the renal tubule.

The renal tubule is made up of a proximal convoluted tubule, the nephron loop - also known as the loop of Henle - which has an ascending and a descending limb - and finally the distal convoluted tubule.

As filtrate makes its way through the renal tubule, waste and molecules such as ions and water are secreted from the peritubular capillaries into the tubule, and they are also absorbed from the tubule back into the capillaries.

The peritubular capillaries reunite to form larger and larger venous vessels.