Renal Corpuscle and Nephron Borders
Histologically, a nephron in the strict sense includes a double-walled epithelial cup—Bowman's capsule—and a long, unbranched tubule extending from it.
Immersed within the capsule is a vascular element: the capillary glomerulus. Together, they form the renal corpuscle (corpusculum renale). It is important to understand a histological nuance: formally, the capillary glomerulus belongs to the vascular system, but functionally they operate as a single apparatus.
Primary filtration occurs here: about 10% of blood plasma passes from the capillaries into the capsular lumen, forming the primary filtrate. Additionally, the renal corpuscle performs an endocrine function, participating in the production of renin required to control blood pressure.
Tubular Organization
The tubular portion extends from the capsule. Its task is reabsorption (the reclamation of essential substances from the primary filtrate into the secondary capillary network) and secretion (the active transport of metabolites from the blood into the lumen).
Along its length (about 5–5.5 cm), the tubule is divided into five consecutive segments:
- Proximal convoluted tubule — forms loops near its renal corpuscle within the cortex. This segment alone accounts for about 2 cm of the length.
- Proximal straight tubule — a short segment following the convoluted portion.
- Thin tubule — the descending limb of the loop of Henle, descending from the cortex into the medulla.
- Distal straight tubule — the ascending limb of the loop of Henle, returning to the cortex.
- Distal convoluted tubule — the final segment. It has a unique topographical feature: it consistently returns and contacts the renal corpuscle of its own nephron at the vascular pole (strictly between the afferent and efferent arterioles).
Epithelial Histophysiology and Excretory System
All segments of the nephron are lined with simple epithelium. The tubular cells are tightly sealed at their apical pole, isolating the lumen. At the basal pole, intercellular junctions loosen, forming a lateral intercellular space.
During reabsorption, substances first enter this lateral space, cross the basement membrane, and emerge into the peritubular (interstitial) space, from which they are taken up by capillaries. The interstitium itself (connective tissue between the tubules) in the medulla possesses an endocrine function—it synthesizes prostaglandins.
After passing through the nephron, the secondary urine enters the collecting ducts. These run perpendicular to the kidney surface: they begin in the medullary rays, descend into the medulla, and merge into papillary ducts at the apices of the pyramids. The length of the ducts is about 4.5 cm, and it is important to remember that they are independent parenchymal elements and are not part of the nephron.
Classification of Nephrons
All renal corpuscles lie exclusively in the cortex, giving it a dark red color. The presence of corpuscles in a histological section is a 100% diagnostic sign of the cortex. The type of nephron is determined by the location of the corpuscle and the length of the loop of Henle (ansa nephroni):
- Intermediate cortical (80%) — the main type. The loop of Henle is of medium size, reaching the outer zone of the medullary pyramids.
- Long / Juxtamedullary (about 20%) — corpuscles lie at the cortico-medullary junction (juxta — near). Their long loops penetrate almost the entire thickness of the pyramids.
- Short cortical (1% to 15–20%) — corpuscles located in the outer cortex. They possess a very short loop that practically does not descend deeply, remaining within the medullary rays.