Renal corpuscle

The renal corpuscle ( corpuscle renal, Malpighi Marcello Malpighi corpuscles after, 1628-1694 ) is a part of the nephron. Is a spherical structure in the cortex of the kidney, which is the primary urine, an ultrafiltrate of blood. It consists of a capillary glomerulus ( glomerulus, most glomeruli, or glomerulus, most glomeruli, Ugs. Miracle ball ), which is surrounded by the Bowman's capsule (after William Bowman, 1816-1892 ). Both structures form the blood - urine barrier.

Vessels

One can distinguish a urinary pole in the renal corpuscles with the opening to the vascular pole of a tubule with exactly one incoming and one outgoing blood vessel and on the other side. The feeding artery to the glomerulus is the arteriole glomerularis afferens (or short the afferent ). It originates from the renal artery ( renal artery ) via a sequence of branches into arteries intralobulares, arcuate arteries and arteries cortical radiatae that eventually leave the afferent. After passage of the capillary bed in the glomerulus, the blood flows through another arterial vessel, the arteriole glomerularis efferens (or short the efferent arteriole ) from.

The afferent juts into the Bowman's capsule into it branches out and forms a ball, the glomerulus, which is branched at a closer look from about 30, anastomosing, but parallel capillary loops there. The pressure drop during the passage of the blood through the capillaries of the parallel course low.

On the afferent called the juxtaglomerular apparatus is attached. It is a point of contact for tubule of the same nephron, which initially leads away to the center of the kidney and then looped returns to the vicinity of its starting point. Here, significant regulatory processes play from.

The efferent arteriole leads from the glomerulus away towards the center of the kidney and forms around the tubules of the nephron from which it comes, again a capillary bed.

Bowman's capsule

The Bowman's capsule Bowman's capsule also has two layers. The outer ( parietal ) Journal of the Bowman's capsule surrounds the entire nephron. It consists of a thin, single-layer squamous epithelium. The inner ( visceral) Journal of the Bowman's capsule is the capillaries directly. The specialized cells of the inner sheet are called podocytes. Between inner and outer sheet of the Bowman's capsule is formed a narrow lumen. The unfiltered portion of the blood plasma passes through the blood - urine barrier in this lumen and flows directly through the opening of the capsule in the subsequent off, composed of proximal tubule cells, proximal tubule.

Which is crucial for the function of the structure is the renal corpuscle blood - urinary barrier. It is made of capillary endothelium, podocytes and an intermediate, common basement membrane. The barrier determines which molecules are filtered and contains highly specialized structures.

• The endothelium is the fenestrated ( windowed ) type. The windows are not ( as with other fenestrated endothelia ) closed by a diaphragm. It also has a highly negatively charged glycocalyx of Sialoglykoproteinen.
• The glomerular basement membrane is particularly thick with 300 nanometers and contains numerous negatively charged proteoglycans. It is the basal laminae of podocytes and capillary endothelium, which are fused together, so that a lamina rara externa, lamina densa and a lamina rara interna is formed. The lamina densa is attributed to a mechanical barrier function.
• The podocytes have primary and secondary branches. Between these projections a slit diaphragm is formed. The very fine, extremely numerous, interrelated secondary projections cover of the Harnseite complete the basement membrane. In the slots between the toothed feet a slit diaphragm is (similar to the Adhärenskontakten, nephrin protein ). The podocytes possess a negatively charged glycocalyx.

The numerous negative charges in all layers of the blood - urine barrier to prevent, for example, at pH 7.4 negatively charged plasma proteins are filtered ( charge selectivity). In addition, basement membrane, podocytes and slit diaphragm are only permeable to molecules up to a radius of eight nanometers ( approximately 70 kDa) (size selectivity). Overall, a permselectivity of the filter is obtained by charge and size, so that, for example, albumin can pass through the filter as the main plasma 69 kDa protein, total negative charge and a molecular radius of 3.5 nanometers only to a very limited extent.

Glomerulus in the SEM; the image width is 23 microns.

Glomerulus with broken capillaries in the SEM; the image width is 11.5 microns in size.

Interior view of the windowed endothelium ( fenestrae ) in the glomerulus of the kidney in mice SEM; the image width is equal to 1.15 microns.

Mesangium

The mesangium is a special connective tissue within and outside the renal corpuscle. The so-called mesangial cells ( Mesangiozyten ) support the capillary walls, phagocytose and are also at the forwarding of information in regulatory processes ( tubuloglomeruläres feedback) involved. The extraglomerular mesangial cells are a component of the juxtaglomerular apparatus.

Function

Per minute happen in humans at about 1 liter of blood or 600 ml of blood plasma, the glomeruli of the kidneys ( Renal plasma flow ), of which about 20 %, or about 120 ml of filtered per minute ( Glomerular filtration rate ). So about 180 liters of primary urine formed per day. Of these, 80 to 90 % is reabsorbed in the proximal tubules. The hormone- regulated fine-tune ADH - dependent in the principal cells of the collecting ducts, so that a total of 99 % of water reabsorbed and about 1.5 liters of urine per day are formed.

Decisive for the filtration is the pressure difference, ie the difference of the various pressures in the capillaries and the Bowman's capsules, which are each composed of the hydrostatic and colloid osmotic pressure the. During passage through the glomerulus, the hydrostatic pressure increases practically not decrease because of the large total cross-section of parallel capillaries, the resistance is low. As an ultrafiltrate is pressed and the plasma protein to remain, while the capillary passage continuously increase the protein concentration, and thus the colloid osmotic pressure, so that the effective filtration pressure drops, reaching zero at the end, when the filtration equilibrium is reached.