In the

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

, the macula densa is an area of closely packed specialized cells lining the wall of the distal tubule where it touches the glomerulus. Specifically, the macula densa is found in the terminal portion of the distal straight tubule ( thick ascending limb of the loop of Henle), after which the distal convoluted tubule begins. The cells of the macula densa are sensitive to the concentration of

sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

in the thick ascending loop of henle. A decrease in sodium chloride concentration initiates a signal from the macula densa that has two effects: (1) it decreases resistance to blood flow in the afferent arterioles, which raises glomerular

hydrostatic pressure Hydrostatics is the branch of fluid mechanics that studies fluids at hydrostatic equilibrium and "the pressure in a fluid or exerted by a fluid on an immersed body". The word "hydrostatics" is sometimes used to refer specifically to water and o ...

and helps return the

glomerular filtration rate Renal functions include maintaining an acid–base balance; regulating fluid balance; regulating sodium, potassium, and other electrolytes; clearance (medicine), clearing toxins; absorption of glucose, amino acids, and other small molecules; reg ...

(GFR) toward normal, and (2) it increases

renin Renin ( etymology and pronunciation), also known as an angiotensinogenase, is an aspartic protease protein and enzyme secreted by the kidneys that participates in the body's renin-angiotensin-aldosterone system (RAAS)—also known as the reni ...

release from the juxtaglomerular cells of the afferent and efferent arterioles, which are the major storage sites for renin. As such, an increase in sodium chloride concentration would result in vasoconstriction of afferent arterioles, and reduced paracrine stimulation of juxtaglomerular cells. This demonstrates the macula densa feedback, where compensatory mechanisms act in order to return GFR to normal. The release of renin is an essential component of the renin–angiotensin–aldosterone system (''RAAS''), which regulates

blood pressure Blood pressure (BP) is the pressure of Circulatory system, circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term ...

and

volume Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch) ...

Histology

The cells of the macula densa are taller and have more prominent nuclei than surrounding cells of the distal straight tubule (cortical thick ascending limb). The close proximity and prominence of the nuclei cause this segment of the distal tubule wall to appear darker in microscopic preparations, hence the name ''macula densa''.

Function

Macula densa cells sense changes in sodium chloride level, and will trigger an autoregulatory response to increase or decrease reabsorption of ions and water to the blood (as needed) in order to alter blood volume and return blood pressure to normal. A decrease in afferent arteriole diameter causes a decrease in the GFR (glomerular filtration rate), resulting in a decreased concentration of sodium and chloride ions in the filtrate and/or decreased filtrate flow rate. Reduced blood pressure means decreased venous pressure and, hence, a decreased peritubular capillary pressure. This results in a smaller capillary hydrostatic pressure, which causes an increased absorption of sodium ions into the vasa recta at the

proximal tubule The proximal tubule is the segment of the nephron in kidneys which begins from the renal (tubular) pole of the Bowman's capsule to the beginning of loop of Henle. At this location, the glomerular parietal epithelial cells (PECs) lining bowman’s ...

. Hence, a decrease in blood pressure results in less sodium chloride present at the

distal tubule The distal convoluted tubule (DCT) is a portion of kidney nephron between the loop of Henle and the collecting tubule. Physiology It is partly responsible for the regulation of potassium, sodium, calcium, and pH. On its Apical membrane, apica ...

, where the macula densa is located. The macula densa senses this drop in salt concentration and responds through two mechanisms, both of which are mediated by

prostaglandin Prostaglandins (PG) are a group of physiology, physiologically active lipid compounds called eicosanoids that have diverse hormone-like effects in animals. Prostaglandins have been found in almost every Tissue (biology), tissue in humans and ot ...

release. First, prostaglandins preferentially vasodilate the renal afferent arteriole, decreasing afferent arteriole resistance and, thus, offsetting the decrease in glomerular hydrostatic pressure caused by the drop in blood pressure. Second, prostaglandin activates prostaglandin-sensitive specialized smooth muscle cells of the renal afferent arterioles, juxtaglomerular cells (JG cells), to release

into the bloodstream. The JG cells can also release renin independently of the macula densa. There are stretch-sensitive baroreceptors lining the arterioles that will release renin if a fall in blood pressure (i.e. decreased stretch of arteriole due to less blood flow) in the arterioles is detected. Furthermore, JG cells contain beta-1 adrenergic receptors, and so activation of the sympathetic nervous system will further stimulate renin release. Thus, a drop in blood pressure results in preferential vasodilation of the afferent arterioles, increasing renal blood flow (RBF), renal plasma flow (RPF) and GFR due to greater blood flow to the glomerulus. Note that there is no change in

filtration fraction In renal physiology, the filtration fraction is the ratio of the glomerular filtration rate (GFR) over the renal plasma flow (RPF). Filtration Fraction, FF = GFR/RPF, or FF = \frac. The filtration fraction, therefore, represents the proportion o ...

, as both GFR and RPF are increased. It also results in the release of renin, which, through the

renin–angiotensin system The renin–angiotensin system (RAS), or renin–angiotensin–aldosterone system (RAAS), is a hormone system that regulates blood pressure, fluid, and electrolyte balance, and systemic vascular resistance. When renal blood flow is reduced, ...

, causes constriction of the efferent arterioles, which ultimately increases hydrostatic pressure in the glomerulus. The process triggered by the macula densa helps keep the GFR fairly steady in response to varying artery pressure. Damage to the macula densa would impact blood flow to the kidneys because the afferent arterioles would not dilate in response to a decrease in filtrate osmolarity and pressure at the glomerulus would not be increased. As part of the body's blood pressure regulation, the macula densa monitors filtrate osmolarity; if it falls too far, the macula densa causes the efferent arterioles of the kidney to contract, thus increasing the pressure at the glomerulus and increasing the glomerular filtration rate.

References

External links

* - "Mammal, kidney cortex (LM, Medium)" * - "The Nephron: Juxtaglomerular Apparatus" {{DEFAULTSORT:Macula Densa Kidney anatomy

Histology

Function

See also

References

External links