RENAL GLOMERULUS: STRUCTURE, FUNCTIONS, PATHOLOGIES - BIOLOGY - 2025

The renal glomerulus is the initial segment of the nephron, which in turn represents the anatomical and functional unit of the kidney. To form a nephron, the glomerulus continues with a long tube in which different segments can be recognized, the last of which ends in a collecting duct.

A collecting duct can receive tubes from many nephrons and join with others to form the papillary ducts. In these, the renal function itself concludes, because the liquid that they pour into the calyces is already final urine that continues its course through the urinary tract without further modifications.

Structure of a renal glomerulus (Source: OpenStax College via Wikimedia Commons)

A cross section of the kidney shows a superficial band called the cortex and a deep band known as the medulla. Although all of the glomeruli are in the cortex, it is said that 15% are juxtamedullary (next to the medulla) and 85% are cortical properly.

The main function of the kidney is to process the blood plasma along the nephrons to extract from it a liquid volume that will be excreted in the form of urine, and in which the excesses of some normal components of plasma and other plasma products will be contained. waste.

Kidney anatomy (Source: Grinny Manyform via Wikimedia Commons)

The glomerulus represents the structure where the onset of kidney function takes place. There the first contact between the vascular and blood systems and the nephron system itself occurs, which will deal with the processing of the plasma supplied by the first two.

Structure

In a histological section already under magnification, the glomeruli are seen as spherical structures of about 200 µm in diameter. Closer examination shows that each glomerulus actually represents the junction of a vascular component and an epithelial tubular component.

Vascular component

The vascular component is seen as penetrating through a segment of the sphere known as the vascular pole, while, in the opposite segment, the urinary pole, the small sphere seems to arise from a narrower tube, the proximal tubule, the beginning of the tubular system. Properly said.

The vascular component is a bunch of ball-shaped capillaries that originate in a small arteriole called afferent (which reaches the glomerulus) and end in another called efferent (which leaves the glomerulus). Capillaries are called glomerular capillaries.

At the vascular pole, the afferent and efferent arterioles are close together, forming a kind of "stem" from which the capillaries depart and return to form loops. In this stem and between the internal faces of the loops there are cells that, due to their location between the vessels, are called mesangial.

The vascular organization of the kidney is very particular and different from that of other organs, in which the capillaries have a nutritional function and originate in arterioles, but end in venules that leave the tissues joining in progressively larger veins to return to the heart.

The kidney, due to its function, has a double capillarization. The first is precisely that of the glomerular capillaries, which begins and ends in vessels of the same type; organization known as the arteriolar portal system, and from which the fluid whose processing will end up in urine is filtered.

The second capillarization is of the efferent arterioles and forms a peritubular network that leads to venules and allows everything reabsorbed by the tubules to return to the blood; or it provides them with a material that, being found in plasma, must be secreted for its final excretion with urine.

Epithelial tubular component

This is the so-called Bowman's capsule, which is the initial, blind and dilated end of the tubule that continues the nephron. At the vascular pole, the wall of the capsule seems to invaginate to cover the glomerular capillaries.

This fact makes the vascular and tubulo-epithelial components of the glomerulus closely associated anatomically so that the endothelial wall of the capillary is covered by a basement membrane on which the epithelium of the capsule rests.

Features

Renal function begins in the glomerulus with the filtration of a certain volume of plasma, which leaves the vascular bed and enters the tubular system through the barrier constituted by the superposition of the capillary endothelium, the basement membrane and the epithelium of the Bowman's capsule.

These three structures have certain solutions of continuity that allow the movement of water in the sense that the responsible pressure gradients determine, in this case from the capillary to the tubular space. This fluid is called the glomerular filtration or primary urine.

The glomerular filtrate does not contain blood cells or plasma proteins or other large molecules. It is, therefore, plasma with all those small components such as ions, glucose, amino acids, urea, creatinine, etc. and other endogenous and exogenous waste molecules.

After entering Bowman's capsule, this filtrate will circulate through the tubules and will be modified by the processes of reabsorption and secretion. Everything that remains in it at the end of its tubular transit will be eliminated with urine. Filtration is thus the first step in renal excretion.

Variables related to glomerular function

One of them is the glomerular filtration volume (GFR), which is the volume of plasma that is filtered in all the glomeruli in the unit of time. This amount goes around 125 ml / min or 180 L / day. This volume is reabsorbed almost everything, leaving between 1 and 2 liters daily eliminated as urine.

The filtered charge of a substance “X” is the mass of that substance that is filtered in the unit of time and is calculated by multiplying the plasma concentration of that substance (PX) by the VFG. There are as many filtered loads as there are substances that are filtered.

The filterability index of plasma substances is a variable that gives an idea of ​​the ease with which they cross the filtration barrier. It is obtained by dividing the concentration of the substance in the filtrate (FX) by its concentration in the plasma (PX). That is: FX / PX.

The value of this last variable ranges between 1 and 0. One for those substances that filter freely and whose concentrations in both compartments are equal. Zero for those substances that do not filter and whose concentration in the filtrate is 0. Intermediate values ​​for those that filter in part.

Pathologies

The term glomerulopathy refers to any process that affects one or more of the glomerular components and adversely modifies filtration, including the decrease in its volume and the loss of selectivity, allowing particles that normally do not pass through.

The nomenclature and classification of the pathological processes that affect the glomerulus is somewhat confusing and complex. Many, for example, make glomerulopathy and glomerulonephritis synonyms, and others prefer to reserve the latter term for cases with obvious signs of inflammation.

We speak of primary glomerulopathies or glomerulonephritis when the damage is confined to the kidneys and any systemic manifestation that appears, such as pulmonary edema, arterial hypertension or uremic syndrome, is a direct consequence of glomerular dysfunction.

Primary are glomerulonephritis: by Immunoglobulin A (IgA), membranous, minimal changes, focal-segmental sclerosing, membranous-proliferative (types I, II and III) and postinfectious or post-streptococcal.

In the case of the so-called secondary glomerulopathies, the glomeruli represent only one of the altered components in a disease that affects multiple organ systems and in which there are signs of primary damage in other organs. Many diseases are included here.

To name a few: Systemic lupus erythematosus, diabetes mellitus, glomerulonephritis associated with systemic vasculitis, anti-basement membrane antibodies, hereditary glomerulopathies, amyloidosis, glomerulonephritis associated with viral or non-viral infections and many more.

References

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