1. Arterial bed of the cortex
Renal blood flow is divided into two directions: the cortical system (supplying cortical and intermediate nephrons) and the juxtamedullary system (for juxtamedullary nephrons). Both systems originate from large major vessels, but differ fundamentally at the microcirculation level.
The pathway of blood in the cortical system from the hilum of the organ is as follows:
- Renal artery: enters the renal hilum.
- Interlobar arteries: run between the pyramids, heading perpendicularly toward the organ surface deep into the parenchyma.
- Arcuate arteries (aa. arcuatae): located strictly at the cortico-medullary junction, running parallel to the capsule.
- Interlobular arteries (aa. interlobulares): branch off from the arcuate arteries and extend deep into the cortex perpendicular to the surface.
- Afferent arterioles (vas afferens): terminal branches, each leading strictly to a single renal corpuscle.
2. Microcirculation of the cortex and "rete mirabile"
A "rete mirabile" is formed within the cortex. Its essence lies in the presence of two capillary beds separated not by a vein, but by an arteriole.
- Primary capillary network (glomerulus): formed by the branching of the afferent arteriole (vas afferens). Located inside the renal corpuscle.
- Efferent arteriole (vas efferens): collects filtered blood from the glomerulus and leaves the corpuscle.
- Secondary capillary network (peritubular): arises from the branching of the efferent arteriole and densely entwines the renal tubules and collecting ducts.
The hemodynamics of this segment are strictly subordinated to nephron functions. The diameter of the vas efferens is significantly smaller than that of the vas afferens. Due to this anatomical difference, a high hydrostatic pressure (about 50–60 mmHg) is generated in the glomerulus, ensuring intensive plasma filtration. Next, the blood enters the peritubular network, where pressure drops sharply to 12 mmHg. This creates ideal physical conditions for the reabsorption of fluid and useful substances from the tubular lumen back into the blood.
Venous outflow in the cortex mirrors the arterial bed in reverse. Superficial nephrons are drained via stellate venules, which empty into interlobular veins, which in turn transfer blood to arcuate and then interlobar veins.
3. Juxtamedullary circulatory system
In juxtamedullary nephrons, whose corpuscles lie at the border with the medulla, blood flow is arranged differently. The main difference is that the diameter of the efferent arteriole here is wide (comparable to the afferent one). Due to the absence of constriction, the pressure in the glomeruli remains low, and filtration is very weak.
The primary role of the juxtamedullary system is to function as a vascular shunt, dumping excess blood during renal hypervolemia.
Blood flow diagram in the shunt:
- Efferent arteriole: does not immediately break down into a dense network.
- Straight arteriole (vas recta): continuation of the efferent arteriole, descending into the medulla alongside the long loop of Henle.
- Tubular capillaries: branch off at various levels, enveloping tubular structures.
- Straight venule (venula recta): collects blood and ascends back to the layer boundary, emptying directly into the arcuate vein.
An important feature of venous outflow here is the absence of interlobular veins. The straight venules bypass them, as the interlobular veins lie significantly higher, within the thickness of the cortex.
4. Histology: identifying the cortex on a slide
When examining a histological section of the kidney (standard hematoxylin and eosin staining), the cortex is easily identified by several key features:
- Externally, the surface of the organ is covered by a dense connective tissue capsule.
- The main visual marker of the cortex is the accumulation of renal corpuscles. They appear as rounded structures with high nuclear density, making them very prominent.
- The space around the corpuscles is filled with convoluted tubules (proximal and distal).
- Vertical zones, known as medullary rays (striae medullares), are visible within the cortex. They consist of large collecting ducts and straight tubules forming loops of Henle for short and intermediate nephrons.
Briefly: answers to topical questions
Where exactly do the renal stellate venules form and collect blood from?
Stellate venules form in the superficial layers of the renal cortex. They collect blood from superficial nephrons. Subsequently, this blood enters the interlobular veins, which receive it from the stellate venules and capillaries of deeper cortical layers, ensuring venous outflow parallel to the arterial inflow.
How is the glomerular capillary wall structured at the electron-microscopic level?
The glomerular capillary wall forms a filtration barrier consisting of three layers.
- Capillary endothelium — cells contain thinned areas (fenestrae) and through-pores.
- Basement membrane — a continuous (solid) three-layer structure located between the endothelium and podocytes.
- Visceral layer of the capsule — represented by podocytes in contact with the basement membrane via processes (pedicels / foot processes).
The podocyte layer is not a continuous barrier, as free spaces (filtration slits) remain between the pedicels.
Which factors and hormones regulate the lumen of the afferent and efferent arterioles?
The lumen of the renal glomerular arterioles is regulated by changes in the tone of smooth muscle cells (SMCs) under the influence of vasoactive substances.
Factors that narrow the lumen (increase tone) of the efferent arteriole:
- Angiotensin (angiotensin II causes preferential constriction of the efferent arteriole)
- Catecholamines
- Prostaglandins
- ADH (antidiuretic hormone)
Factors that widen the lumen (decrease tone) of the afferent arteriole:
- Kinins
- Prostaglandins
To remember the branching of cortical arteries, use the mnemonic: Renal → Interlobar → Arcuate → Interlobular → Afferent.
Class and exam questions
Why is high pressure maintained in the glomerular capillaries?
The efferent arteriole is anatomically noticeably narrower than the afferent arteriole. This creates resistance to outflow and increases the pressure in the glomerulus to 50–60 mm Hg, which is necessary for plasma filtration.
What is the main function of the juxtamedullary system?
It acts as a vascular shunt. During renal hypervolemia, excess blood is bypassed through it because the pressure in its corpuscles is low and the filtration barrier functions poorly.
Where does the straight venule of the juxtamedullary nephron drain?
It empties directly into the arcuate vein at the cortico-medullary junction, completely bypassing the interlobular veins.
How to distinguish the renal cortex on a histological slide?
The main distinguishing feature of the cortex is the presence of rounded renal corpuscles densely packed with cell nuclei.
What else to study on this topic
- Structure and cellular composition of the renal corpuscle
- Histological differences between proximal and distal convoluted tubules
- Organization of the loop of Henle and its course in the medulla
- Ultrastructure of the renal filtration barrier
- Features of the blood supply to the medulla
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