Question

A key feature of kidney function involves autoregulation to maintain a nearly constant filtration fraction over a wide range of arterial pressures. Discuss the role of the macula densa and the juxtaglomerular cells that allows this type of homeostatic control to occur

Answer 1

Renal AutoregulationRenal autoregulation is important to maintain the glomerular filtration rate (GFR). Blood flow to kidneys remains normal even when the mean arterial blood pressure vary widely between 60 mm Hg and 180 mm Hg. This helps to maintain normal GFR.

Two mechanisms are involved in renal autoregulation:

1.Myogenic response

2.Tubuloglomerular feedback.

1. Myogenic Response:-

Whenever blood flow to kidneys increases, it stretches the elastic wall of the afferent arteriole. Stretching of the vessel wall increases the flow of calcium ions from extracellular fluid into the cells. The influx of calcium ions leads to the contraction of smooth muscles in afferent arteriole, which causes constriction of afferent arteriole. So, the blood flow is decreased

2. Tubuloglomerular Feedback:--

Macula densa plays an important role in tubuloglomerular feedback, which controls the renal blood flow and GFR.

Macula densa of juxtaglomerular apparatus in the terminal portion of thick ascending limb is sensitive to the sodium chloride in the tubular fluid.

When the glomerular filtrate passes through the terminal portion of thick ascending segment, macula densa acts like a sensor.

It detects the concentration of sodium chloride in the tubular fluid and accordingly alters the glomerular blood flow and GFR. Macula densa detects the sodium chloride concentration via Na+K+ 2Cl– cotransporter.

When GFR increases, concentration of sodium chloride increases in the filtrate. Macula densa releases adenosine from ATP. Adenosine causes constriction of afferent arteriole. So the blood flow through glomerulus decreases leading to decrease in GFR. Adenosine acts on afferent arteriole via adenosine A1 receptors.

When GFR decreases, concentration of sodium chloride decreases in the filtrate. Macula densa secretes prostaglandin (PGE2), bradykinin and renin. PGE2 and bradykinin cause dilatation of afferent arteriole. Renin induces the formation of angiotensin II, which causes constriction of efferent arteriole. The dilata tion of afferent arteriole and constriction of efferent arteriole leads to increase in glomerular blood flow and GFR.