Glomerular Filtration Rate – but Easier to Understand

By now, we all know that the major function of filtration by the urinary system occurs within the glomerulus of the nephrons, the functional unit of the kidneys. Hence, the term “Glomerular Filtration.”

Zooming in and examining the structure of the nephron, we learned that the glomerulus is a specialized bundle of capillaries situated between the afferent and efferent arterioles and contained within the Bowman’s capsule. Collectively, this section of the kidney is referred to as the renal corpuscle.

Review: Blood Supply to the Kidneys

From a previous video on kidney blood supply, we have seen how blood flows from the heart down through a system of arteries to the kidney’s tiny functional units or nephrons via the afferent arterioles.

The blood goes through glomerular filtration and then, continues to flow into the efferent arteriole, to the peritubular capillaries, then leading to a system of veins back to the heart.

What happens during Glomerular Filtration?

Glomerular filtration is the first step in urine formation. As the term suggests, this process occurs in the glomerulus of the nephrons. Blood enters the afferent arteriole into the glomerulus, and at this stage, undergoes filtration to eliminate excess water and waste products from the blood.

The nephron with emphasis on the renal corpuscle.
The nephron with emphasis on the renal corpuscle.

The glomerulus is a special bundle of vessels with a thin and permeable membrane that allows for smaller molecules (sizes of < 1.8nm) from the blood to get filtered out. If the filtrate is found to include large proteins, it could signal a problem in the urinary system. The membrane is also made of negatively charged glycoproteins such that negative molecules cannot be filtered out, as well.

This filtrate then moves down into the rest of the renal tubules to undergo the remaining processes of reabsorption and secretion.

During the filtration process, larger molecules such as blood and other proteins stay in the blood vessels to continue and move into the efferent arteriole.

Glomerular Filtration Rate is then defined as the rate at which all functional nephrons from both kidneys are able to filter in one minute. Or, in other words, it measures the filtrate volume formed by both kidneys per minute.

Effects of Afferent and Efferent Arteriolar Vasconstriction on Glomerular Filtration Rate

At certain times, our body may need to increase the filtration rate to get rid of certain substances. Sometimes, there may be a need to decrease filtration to keep certain substances in the blood.

Using a simplified illustration of the renal corpuscle below, we will show how arteriolar vasoconstriction affects the rate of glomerular filtration.

Blood flows normally from the afferent to the glomerulus and to the efferent arteriole. Normal filtration rate is achieved.
Normal blood flow from the afferent to the efferent arteriole. A normal amount of filtrate is produced from the glomerulus.

A constricted afferent arteriole results in less blood entering the glomerulus. As a result, there is less pressure inside the glomerulus causing less filtration to occur, and so, less filtrate is produced. Less blood also leaves the efferent arteriole.

Effect of afferent arteriolar vasoconstriction on glomerular filtration rate. There is increased vascular resistance, decreased renal blood flow and decreased GFR. As a result less glomerular filtrate is produced.
Effect of afferent arteriolar vasoconstriction on glomerular filtration rate.

On the other hand, if the constriction occurs on the efferent arteriole, blood can’t leave the glomerulus easily, pressure builds up causing the filtration rate to increase. The fluid inside the glomerular walls is being squeezed and pushed through the membrane producing higher than normal amounts of filtrate.

Smooth muscles make up the walls of the afferent and efferent arterioles. By constricting and relaxing these muscles, the diameter of these arterioles can be influenced and in return affects the glomerular filtration rate.

Summary

  • Glomerular Filtration occurs in the glomerulus within the renal corpuscles of the kidneys.
  • Glomerular Filtration Rate may be influenced by constricting and relaxing the smooth muscles around the afferent or efferent arterioles. Depending on which arteriole is constricted will increase or decrease the filtration rate.

Infographic

Under resting conditions, our heart pumps about 5 liters per minute of blood; 20% of which (or 1 liter) gets filtered through the kidneys. The average values of filtrates produced are as follows: 1) Men: 125 mL/min filtrate produced (range of 90 to 140 mL/min) equivalent to 180 L/day. 2) Women: 105 mL/min filtrate produced (range of 80 to 125 mL/min) equivalent to 150 L/day. Only one percent of the formed filtrates are excreted as urine on a daily basis. This equates to only about one to two liters of urine produced daily. The rest are returned to the circulation by reabsorption.
Urine in Numbers: How much urine do we make in a day?

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