Macula densa

As macula densa ( " tight spot" ) refers to cells of the straight part of the distal tubule ( ' pars recta ') of Tubulussystems the kidney, which bear the afferent blood vessel ( afferent ) of the renal corpuscle. In this area, the tubular cells are highly prismatic and narrow. This means that the nuclei of the individual cells are closer together, which is why this area according to the histological staining in light microscopy preparation appears darker (hence the name " tight spot ").

The macula densa is part of the juxtaglomerular apparatus of the kidney and is a chemoreceptor. He used to determine the sodium - and chloride -ion concentration gradient between the blood in the afferent and the urine in the renal tubule.

Mechanism

Glomerular feedback

The cells of the macula densa have an ion transporters (Na / K / 2Cl - symporter ). This ion transport system, which are located in the lumen of the distal tubule, transported into the cells of the macula densa, and so the concentration of ions, especially sodium and chloride concentration, as measured in urine. This measurement is performed indirectly by the rate of transport. At high ion concentrations, that is hyperosmolar urine, the macula densa adenosine is secreted from the cells, which results in contraction of smooth muscles in the afferent. The afferent ( afferent ) vessel narrows and the glomerular filtration rate ( GFR) decreases. As a result of urine flow decreases through the loop of Henle, so that more ions can be reabsorbed and decreases the concentration of ions in the distal tubule. In hypoosmolar urine, the opposite effect is established.

Renin secretion

If a hypoosmolarer urine measured by the chemoreceptors of the macula densa, this leads to renin release from the granular cells of the glomerular arterioles in the circulating blood and thus into the systemic circulation. Renin is an enzyme which cleaves the decapeptide angiotensin I from the angiotensinogen prohormone. Angiotensin I is enzyme (ACE), cleaved by another peptidase, angiotensin converting to the octapeptide angiotensin II. Angiotensin II has a strong vasoconstrictor ( " vasoconstrictive " ) and leads to a systemic increase in blood pressure. In the kidney is constricted vessel especially the efferent ( located behind the glomerulus ). The pressure in the glomerulus increases and thus also the GFR and the amount of formed primary urine.