Juxtaglomerular apparatus




The juxtaglomerular apparatus is a structure in the kidney that regulates the function of each nephron, the functional units of the kidney. 


The juxtaglomerular apparatus is named because it is next to the glomerulus.


The juxtaglomerular apparatus consists of three types of cells:


the macula densa, a part of the distal convoluted tubule of the same nephron,


juxtaglomerular cells, (also known as granular cells) which secrete renin and 


extraglomerular mesangial cells


The juxtaglomerular apparatus is formed by the distal convoluted tubule and the glomerular afferent arteriole. 


It is located near the vascular pole of the glomerulus and its main function is to regulate blood pressure and the filtration rate of the glomerulus. 


The macula densa has a collection of specialized epithelial cells in the distal convoluted tubule that detect sodium concentration of the fluid in the tubule. 


As the sodium level elevates the macula densa cells trigger contraction of the afferent arteriole, reducing flow of blood to the glomerulus and the glomerular filtration rate. 


The juxtaglomerular cells of the afferent arteriole secrete renin when blood pressure in the arteriole falls. 


The juxtaglomerular cells of the afferent arteriole are derived from smooth muscle cells, 


Renin increases blood pressure via the renin-angiotensin-aldosterone system. 


Lacis cells, also called extraglomerular mesangial cells, are flat and elongated cells located near the macula densa. Their function remains unclear.


The juxtaglomerular apparatus is part of the nephron, and is next to the glomerulus. 


The juxtaglomerular apparatus is found between afferent arteriole and the distal convoluted tubule of the same nephron. 


Its location is critical to its function in regulating renal blood flow and glomerular filtration rate.


The renin-angiotensin system is activated when juxtaglomerular cells are poorly perfused.


Renin is produced by juxtaglomerular cells. 


These cells are located in the tunica media of the afferent arterioles as they enter the glomeruli.


The juxtaglomerular cells secrete renin in response to:


Stimulation of the beta-1 adrenergic receptor


Decrease in renal perfusion pressure 


Decrease in NaCl concentration at the macula densa, often due to a decrease in glomerular filtration rate.


Extraglomerular mesangial cells are located in the junction between the afferent and efferent arterioles. 


Extraglomerular mesangial  cells have a contractile property similar to vascular smooth muscles and play a role in regulating GFR by altering the vessel diameter. 


Where the afferent arterioles enter the glomerulus and the efferent arteriole leaves it, the tubule of the nephron touches the arterioles of the glomerulus from which it arose. 


In the wall of the distal convoluted tubule, at the point above, there is a modified region of tubular epithelium called the macula densa.


Cells in the macula densa respond to changes in the sodium chloride levels in the distal tubule of the nephron, referred to as the tubuloglomerular feedback loop.


The macula densa’s detection of elevated sodium chloride, leads to an increase in GFR.


The increased salt concentration causes several cell signals to cause the adjacent afferent arteriole to constrict. 


The afferent arteriole constriction decreases the amount of blood coming from the afferent arterioles to the glomerular capillaries, and decreases the amount of fluid that goes from the glomerular capillaries into the Bowman’s space and therefore the glomerular filtration rate (GFR)).


When the sodium concentration is decreased less sodium is reabsorbed in the macular densa cells. 


The macular densa cells increase the production of nitric oxide and prostaglandins to vasodilate the afferent arterioles and increase renin release.


Excess secretion of renin by the juxtaglomerular cells can lead to excess activity of the renin–angiotensin system, hypertension and an increase in blood volume. 


Increased renin production may occur due to narrowing of the renal artery, or a tumor of juxtaglomerular cells that produces renin, leading  to secondary hyperaldosteronism, which will cause hypertension, high blood sodium, low blood potassium, and metabolic alkalosis.


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