Juxtaglomerular Apparatus Definition
The juxtaglomerular apparatus is a specialized organ situated near the glomerulus of each nephron (juxta = near).
Table of Contents
Structure Of Juxtaglomerular Apparatus
The juxtaglomerular apparatus is formed by three different structures.
- Macula densa
- Extraglomerular mesangial cells
- Juxtaglomerular cells.
Read And Learn More: Medical Physiology Notes
1. Macula Densa
Macula densa is the terminal portion of a thick ascending segment that runs in between afferent and efferent arterioles of the same nephron. Actually, it is very close to the afferent arteriole. In this part of the segment, which lies close to the afferent arteriole, the cuboidal epithelial cells Glomerular mesangial cells, or intraglomerular mesangial are tightly packed.
2. Extraglomerular Mesangial Cells
- These cells are situated in the triangular region bound by afferent arteriole, efferent arteriole, and macula densa.
- These cells are also called agranular cells, lacis cells, or Goormaghtigh cells.
- Besides extraglomerular mesangial cells, there is another type of mesangial cell situated in between glomerular capillaries called glomerular mesangial cells.
Glomerular Mesangial Cells
- Glomerular mesangial cells or intraglomerular mesangial cells surround the glomerular capillaries and form a cellular network that supports the capillary loops.
- These cells are contractile in nature and play an important role in regulating glomerular filtration.
- Contraction of mesangial cells decreases the surface area of glomerular capillaries and thereby the glomerular filtration is regulated.
- The Glomerular Mesangial Cells are also phagocytic and secrete a matrix of glomerular interstitium, prostaglandins, and cytokines.
3. Juxtaglomerular Cells
- The wall of the afferent arteriole, before entering the Bowman’s capsule is thickened like a cuff.
- It is called a polar cushion or Tolkien. It is formed by juxtaglomerular cells which are the specialized type of smooth muscle cells derived from the tunica media and tunica adventitia of the wall of the afferent arteriole.
- Because of the presence of secretory granules in their cytoplasm, the juxtaglomerular cells are also called granular cells.
Functions Of Juxtaglomerular Apparatus: The primary function of the juxtaglomerular apparatus is the secretion of hormonal substances. It also regulates the glomerular blood flow and glomerular filtration rate.
Secretion Of Renin: The juxtaglomerular cells secrete renin. Renin is a peptide with 340 amino acids. Along with angiotensins, renin forms the renin-angiotensin system which is a hormone system that plays an important role in the maintenance of blood pressure.
Stimulants for Renin Secretion
Secretion of renin is stimulated by four factors:
- Fall in arterial blood pressure
- Reduction in the ECF volume
- Increased sympathetic act4ity
- Decreased load of sodium and chloride in macula densa.
Renin-Angiotensin System
- When renin is released into the blood, it acts on a specific plasma protein called angiotensinogen or renin substrate. It is the a2 globulin.
- By the activity of renin, the angiotensin- singer is converted into a decapeptide called angiotensin
- Angiotensin 1 is converted into angiotensin 2 which is an octapeptide by the activity of angiotensin-converting enzyme (ACE) secreted from the lungs.
- Most of the conversion of angiotensin I into angiotensin 2 takes place in the lungs.
- Angiotensin 2 has a short half-life of about 1-2 minutes. Then it is rapidly degraded into a heptapeptide called angiotensin 3 by angiotensinases which are present in RBCs and vascular beds in many tissues.
- Angiotensin 3 is converted into angiotensin 4 which is a hexapeptide.
- Actions of Angiotensins Angiotensin I is physiologically inact4e and serves only as the precursor of angiotensin 2.
Angiotensin 2
Angiotensin 2 is the most active form. Its actions are:
1. On blood vessels:
- Angiotensin 2 increases arterial blood pressure by directly acting on the blood vessels and causing vasoconstriction. It is a potent constrictor of arterioles. Earlier, when its other actions were not found it was called hypertension.
- It increases blood pressure indirectly also by increasing the release of noradrenaline from postganglionic sympathetic fibers. Noradrenaline is a general vasoconstrictor.
2. On adrenal cortex:
- It stimulates zona glomerulosa of adrenal cortex to secrete aldosterone.
- Aldosterone acts on renal tubules and increases the retention of sodium which is also responsible for the elevation of blood pressure.
3. On kidney:
- Angiotensin 2 regulates the glomerular filtration rate in two ways:
- It constricts the efferent arteriole which causes a decrease in filtration after an initial increase
- It contracts the glomerular mesangial cells leading to a decrease in the surface area of glomerular capillaries and filtration
- It increases sodium reabsorption from renal tubules. This action is more predominant on proximal tubules.
4. On brain:
- Angiotensin 2 inhibits the baroreceptor reflex and thereby indirectly increases the blood pressure. The baroreceptor reflex is responsible for decreasing blood pressure.
- It increases water intake by stimulating the thirst center It increases the secretion of corticotropin-releasing hormone (CRH) from the hypothalamus. CRH in turn increases the secretion of adrenocorticotropic hormone (ACTH) from pituitary
- It increases the secretion of antidiuretic hormone (ADH) from the hypothalamus.
Other actions: Angiotensin 2 acts as a growth factor in heart and it is thought to cause muscular hypertrophy and cardiac enlargement.
Angiotensin 3: Angiotensin 3 increases blood pressure and stimulates aldosterone secretion from the adrenal cortex. It has 100% adrenal cortical stimulating activity and 40% vasopressor activity of angiotensin 2.
Angiotensin 4: It also has adrenal cortical stimulating and vasopressor acl4ities.
Secretion Of Other Substances
- The extragranular mesangial cells of the juxtaglomerular apparatus secrete prostaglandin.
- In vitro, secretion of cytokines like IL-2 and TNF by the mesangial cells is observed recently.
- Macula densa secretes thromboxane A2. The kidney also secretes 1,25-dihydroxycholecalciferol (calcitriol).
Regulation Of Glomerular Blood Flow And Glomerular Filtration Rate
Macula densa of juxtaglomerular apparatus plays an important role in the feedback mechanism called tubuloglomerular feedback mechanism, which regulates the renal blood flow and glomerular filtration rate.
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