Renal glomerulus

The renal glomerulus (from glomus, ball) is a dense spheroidal network of arterial capillaries, responsible for filtering the blood.

The nephron

Each of the body's two kidneys contains approximately one and a half million nephrons. The nephron is considered the functional unit of the kidney, since it alone is able to perform all the functions to which the kidney is responsible. Each single nephron can be didactically subdivided into sections:

Renal corpuscle: is formed by the renal glomerulus and the Bowman's capsule; the latter is a hollow spherical structure with a blind end, which envelops the glomerulus to collect the filtrate. Together the renal glomerulus and the Bowman's capsule form the renal corpuscle, also known as the Malpinghi or Malpighian corpuscle
Tubular elements: the filtrate collected by the Bowman's capsule is channeled into a series of canaliculi, where it is deprived of substances useful for the organism (reabsorption) and enriched with those present in excess or considered dangerous (secretion). The continuous system of canaliculi it is divided into three sections - proximal tubule, loop of Henle, distal tubule - each of which is specialized in the reabsorption and / or secretion of particular components of the blood

As explained above, the quantity of any substance present in the urine (excreted load) is the result of the following expression:

Load Excreted (E) = Filtered Load (F) - Reabsorbed Load (R) + Secreted Load

For didactic purposes, in the image above the nephron appears unfolded, when in reality it twists and folds in on itself several times (image below).

The renal corpuscle

At the two ends of the renal glomerulus we find the two arterioles that put it in communication with the circulatory system. Upstream we find an "arteriole, called afferent, which carries the blood to be filtered; downstream we find an" arteriole, called efferent, which carries the partially filtered blood in a network of capillaries distributed around the tubular elements.

In this way the peritubular capillaries originating from the efferent arteriole can collect the blood components reabsorbed by the tubules and secrete the substances that must be removed from the blood, then excreted from the organism with the urine.

As shown in the figure above:

the afferent arteriole has a greater caliber than the efferent.
in juxtamedullary nephrons, the long peritubular capillaries that penetrate deep into the medullary area of the kidney are called vasa recta.

The blood flowing from the peritubular capillaries is collected in venules and small veins that flow into the renal vein to carry the blood outside the kidney.

The renal glomerulus: what are its functions?

The renal glomerulus acts as a filter for the blood that passes through it.

Filtration is a passive, relatively non-specific process, which marks the first stage of urine formation. As we will see better in the next chapter, glomerular capillaries are called fenestrated, since they have relatively large pores through which many of the components can pass. some blood.

In particular, the renal glomerulus can be compared to a large mesh sieve, capable of retaining only proteins and blood cells. For this reason the filtrate collected in the Bowman's capsule, called ultafiltrate or pre-urine, has a composition very similar to that of plasma (liquid part of the blood), but without the plasma proteins.

Overall, the volume of renal ultrafiltrate is about 120-125 ml per minute, that is, about 170/180 liters per day. Since the amount of urine excreted is more than 100 times lower, it is evident how the tubular system reabsorbs the overwhelming majority of glomerular ultrafiltrate.

Along the tubular path, the ultrafiltrate undergoes a series of changes that lead to a production of concentrated (definitive) urine of approximately 1 / 1.5 liters per day.

The filtration barriers

The blood is pushed by the hydrostatic pressure against the capillary walls of the glomeruli, favoring the passage of many of its components into the Bowman's capsule, where they collect forming the ultrafiltrate (or pre-urine). To make this passage, the blood components must pass through three different filtration barriers:

the capillary endothelium: as anticipated, the glomerular capillaries are fenestrated capillaries, with large pores that allow most of the blood components to filter through the endothelium. The diameter of these pores allows the passage of many substances, making it too small only for some plasma proteins and for blood cells (collectively called corpuscular elements), which remain in the blood. In particular, under normal conditions the fenestrated capillaries allow the filtration of molecules with a diameter of less than 42 Å. Although the albumin molecule is smaller (36 Å), under normal conditions it cannot cross the capillary endothelium because it is blocked by fixed negatively charged proteins which reject it (since albumin is also negatively charged).

As shown in the figure, so-called mesangial cells are present in the spaces surrounding the renal glomeruli. These are specialized cells, capable of modifying the blood flow through the capillaries by contracting (therefore increasing it) or relaxing (decreasing it). Mesangial cells are also responsible for phagocytosis and secrete cytokines associated with immune and inflammatory processes.
the basal lamina: the fenestrated endothelium of the blood capillaries rests on a thin basal lamina, called dense lamina, which separates the capillary endothelium of Bowman's capsule. The basal lamina is made up of glycoproteins and collagen-like material (proteoglycans); both components are negatively charged, thus helping to reject most of the plasma proteins, preventing their filtration
The epithelium of Bowman's capsule: contains specialized cells called podocytes (from podos, foot); each podocyte is characterized by cytoplasmic extensions, called pedicels, which protrude like tentacles from the cell body, enveloping the glomerular capillaries and resting directly on the dense lamina of the wall In this way, filtration slits (slit pores) are formed, delimited by a membrane.
Similarly to mesangial cells, podocytes also have contractile fibers connected to the basement membrane by proteins called integrins. The contractility of these cell types is influenced by the endocrine action of some hormones that regulate blood pressure and fluid balance in the body.

Thanks to these three barriers, the filtration of the blood components results:

free for molecules with a radius <20 Å
variable for molecules with a radius of 20-42 Å (70 - 150 Kd): filterability between 20 Å and 42 Å depends on the charge. Since most of the plasma proteins are negatively charged, the filtration barrier prevents or severely limits the filtration of proteins with a radius of 20-42 Å.
absent for molecules radius> 42Å