The Kidney
The kidney is a retroperitoneal organ responsible for fluid balance, waste excretion, and endocrine function (renin, erythropoietin, vitamin D activation). The functional unit is the nephron.
Microscopic Structures
| Structure | Key Feature |
|---|---|
| Renal Corpuscle | Glomerulus + Bowman’s capsule; site of blood filtration. |
| Proximal Tubule | Simple cuboidal with prominent brush border (reabsorption). |
| Loop of Henle | Concentrates urine via the countercurrent multiplier system. |
| Distal Tubule | Lacks brush border; sensitive to aldosterone. |
The Juxtaglomerular Apparatus (JGA)
The JGA is critical for blood pressure regulation and consists of three cell types:
- Macula Densa: Specialized cells in the distal tubule that sense NaCl concentration.
- Juxtaglomerular (JG) Cells: Modified smooth muscle cells in the afferent arteriole that secrete renin.
- Extraglomerular Mesangial Cells: Provide structural/functional communication between the macula densa and JG cells.
High-Yield Exam Pearl:
The macula densa acts as a sensor: low NaCl delivery to the distal tubule (indicating low GFR/hypotension) triggers the release of renin from the JG cells, initiating the RAAS cascade to increase blood pressure.