The blood leaves the glomerulus (first capillary microcirculation) through the efferent arterioles. The efferent arteriolar near the medulla run into the medulla and form the vasa recta to supply the tubules in the renal medulla (second capillary microcirculation). The efferent arterioles near the cortex supply the tubules of the cortex.
The vasa recta run parallel to the tubules of the medulla from the cortex-medulla border to the papilla and back. This results in a countercurrent vascular architecture, which enables the high osmotic gradient in the medulla. The venae rectae drain into the arcuata veins, which drain via the interlobular veins to the renal vein.
fig. schematic drawing of the renal blood flow: see text for description.
Each kidney has about 1 million functional units, the nephrons [schematic figure of the nephron]. The number of nephrons is considerably variable, e.g. the number of nephrons correlates with birth weight. A low number of nephrons carry an increased risk for kidney disease and hypertension.
The kidney consists of about 1 000 000 renal corpuscles, which corresponds to the number of nephrons. The number of nephrons is considerably variable. Depending of the location of the glomerulus, nephrons are classified as cortical nephrons or juxtamedullary nephrons.
The afferent arterioles supply the glomerular capillaries with blood. There is a constant proportion of the blood plasma (primary urine), which is filtrated in the Bowmann space of the glomerulus. The primary urine passes from the Bowmann space into the proximal tubule.
The glomerular filtration barrier consists of the pores in the glomerular endothelial cells, the glomerular basement membrane and the slits between the foot projections of the podocytes. Due to the size of the pores and the electrical charges of the filtration barrier there is a high permeability to water and small water-soluble substances, but a low permeability to plasma proteins.
The Bowmann's capsule envelopes the glomerulus with a single layer of flat epithelium (parietal layer of the Bowmann's capsule). This epithelium continues to cover the glomerular capillaries with specialized cells (podocytes, visceral layer of the Bowmann's capsule).
Podocytes are highly specialized cells, which envelope the glomerular capillaries with foot projections. The podocytes are thus the visceral layer of the Bowmann's capsule and part of the filtration barrier of the glomerular filtration. The podocytes have lost their ability to divide; perished podocytes can only be replaced with the help of hypertrophy.
The glomerular basement membrane is approximately 250 nm thick collagen-containing membrane, which provides the stability for the filtration process, but constitutes also an element of the filtration barrier. The glomerular basement membrane is synthesized by the podocytes.
The glomerular endothelial cells are flat endothelial cells with regular pores of about 50–100 nm in diameter.
The mesangium consists of cells and matrix; it fills the space between the glomerular capillaries and extends with the vascular pedicle to the extraglomerular space. The mesangial cells have a contractile function, they insert with microfilaments to the glomerular basement membrane. The contraction of the mesangial cells controls and enables the high glomerular filtration pressure of 35–50 mmHg.
Deutsche Version: Histology der Nieren: Nephron und Glomerulus.