Question

Which of the following is most likely to cause an increase in the glomerular filtration rates? (a) Blockage of ureter (b) Dilation of the afferent arterioles (c) Release of renin from the juxtaglomerular apparatus (d) Volume depletion

Short answer

The choice most likely to cause an increase in the glomerular filtration rates is (b) Dilation of the afferent arterioles.

Step-by-step solution

Analyze Each Option

Examine each statement to understand if it encourages the increment in a glomerular filtration rate or not.

Evaluate Option (a)

Option (a) mentions 'Blockage of ureter'. The ureter is a tube that sends urine from the kidney to the bladder. A blockage could result in a buildup of pressure in the kidneys, but this would lower the GFR rather than increase it.

Evaluate Option (b)

Option (b) mentions 'Dilation of the afferent arterioles.' The afferent arterioles feed blood into the glomerulus. Dilation of these arterioles allows more blood to flow to the glomerulus, which leads to an increased GFR.

Evaluate Option (c)

Option (c) speaks of the 'Release of renin from the juxtaglomerular apparatus'. Renin is an enzyme that ultimately leads to the production of angiotensin II, which causes vasoconstriction. This process might initially preserve GFR in the face of low blood volume by maintaining pressure in the glomerular capillaries, but it would not typically lead to an increase in GFR.

Evaluate Option (d)

Option (d) says 'Volume depletion'. A decrease in volume would lead to a reduced GFR as less fluid is available for filtration in the kidney.

Key concepts

Understanding Afferent Arterioles

The afferent arterioles are small blood vessels that play a critical role in kidney function. Think of these arterioles as the entry gate for blood into the filtration units of the kidney called glomeruli. When they dilate or widen, they allow more blood to flow into the glomerulus.
This increase in blood flow is like having more water flow through a water filter, which can increase the filtration rate.

• Blood enters the kidney through the renal artery and flows into the glomeruli via the afferent arterioles.
• Dilation means the blood vessel widens, allowing more blood to pass through.
• More blood flowing into the glomerulus increases the glomerular filtration rate (GFR).

In terms of kidney function, a higher GFR means the kidneys are filtering blood more efficiently.
This is vital for maintaining balance in electrolytes and waste in the body.
If you're ever asked about factors that could increase GFR, always consider afferent arteriole dilation as a key one.

The Juxtaglomerular Apparatus and Its Role

The juxtaglomerular apparatus (JGA) is like a control center for your kidneys.
It is located near the glomeruli and is responsible for regulating blood pressure and filtration rates. The JGA accomplishes this by assessing the blood flow and pressure in the kidneys and adjusting accordingly.

• It includes several structures, like the macula densa and juxtaglomerular cells.
• The macula densa senses sodium concentration in the distal tubule of the nephron.
• Juxtaglomerular cells produce and release renin when needed.

By releasing various hormones and acting on blood vessels, the JGA plays a crucial role in maintaining the body's fluid balance.
While it has a significant role in preserving GFR, it doesn't directly increase it through renin during normal conditions.
Instead, it ensures that the kidneys function efficiently even when there are fluctuations in blood pressure or volume.

Renin Release: What It Means

Renin is an enzyme critical in the body's long-term blood pressure regulation and fluid balance.
When the juxtaglomerular apparatus detects low blood pressure or volume, it releases renin into the bloodstream. This sets off a chain of events called the Renin-Angiotensin-Aldosterone System (RAAS).

• Renin converts angiotensinogen, a protein produced by the liver, into angiotensin I.
• Angiotensin I is then converted into angiotensin II, a powerful vasoconstrictor.
• Angiotensin II narrows blood vessels, raising blood pressure and helping to maintain GFR.

While renin release helps to maintain adequate pressure in the kidneys, which preserves GFR, it usually does not increase GFR. Instead, it focuses on maintaining balance and ensuring the body adapts to changes in blood pressure or volume.
Hence, its primary role is seen as a protective mechanism rather than a method to enhance filtration.