BOWMAN'S CAPSULE: STRUCTURE, HISTOLOGY, FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2024

The Bowman 's capsule represents the initial segment of the tubular component of the nephron, anatomo-functional unit of the kidney in which are carried out the processes for the production of urine with which the kidney contributes to the preservation of homeostasis of the organism.

It was named in honor of the English ophthalmologist and anatomist Sir William Bowman, who discovered its existence and published its histological description for the first time in 1842.

Illustration of a nephron (Source: Artwork by Holly Fischer via Wikimedia Commons)

There is some confusion in the literature regarding the nomenclature of the initial segments of the nephron, including Bowman's capsule. Sometimes it is described as a different part of the glomerulus and constituting with it the renal corpuscle, while for others it functions as a member of the glomerulus.

Regardless of whether in anatomical descriptions the capsule forms part or is part of the glomerulus, the fact is that both elements are so closely associated in their structure and function, that the term glomerulus awakens in those who think about it the idea of ​​a small sphere with its vessels..

If not, the capsule would simply be a receptacle into which the filtered fluid is poured into the glomerulus, but it would have no part in the glomerular filtration process itself. Which is not the case, since it, as will be seen, is part of that process to which it contributes in a special way.

Structure and histology

Bowman's capsule is like a tiny sphere whose wall invaginates into the vascular sector. In this invagination, the capsule is penetrated by the ball of capillaries, which originates in the afferent arteriole and which supplies blood to the glomerulus, from where the efferent arteriole also exits, which draws blood from the glomerulus.

The opposite end of the capsule, called the urinary pole, appears as if the wall of the sphere had a hole to which the end of the first segment that initiates the tubular function itself is connected, that is, the proximal convoluted tubule.

This outer wall of the capsule is a flat epithelium and is called the parietal epithelium of Bowman's capsule. It changes in structure by transitioning to the proximal tubule epithelium at the urinary pole and to the visceral epithelium at the vascular pole.

The invaginate epithelium is called visceral because it surrounds the glomerular capillaries as if they were a viscera. It is made up of cells called podocytes that embrace, covering them, the capillaries and that have very particular characteristics.

The podocytes are organized in a single layer, emitting extensions that interdigitate with the extensions of neighboring podocytes, leaving spaces between them called slit pores or filtration slits, which are solutions of continuity for the passage of the filtrate.

Structure of the kidney and a nephron: 1. Renal cortex; 2. Marrow; 3. Renal artery; 4. Renal veins; 5. Ureter; 6. Nephrons; 7. Afferent arteriole; 8. Glomerulus; 9. Bowman's capsule; 10. Tubules and bundle of Henle; 11. Peritubular capillaries (Source: File: Physiology_of_Nephron.svg: Madhero88File: KidneyStructures_PioM.svg: Piotr Michał Jaworski; PioM EN DE PLderivative work: Daniel Sachse (Antares42) via Wikimedia Commons)

The podocytes and the endothelial cells they cover synthesize a basement membrane on which they rest and which also has solutions of continuity for the passage of water and substances. Endothelial cells are fenestrated and also allow filtration.

So these three elements: capillary endothelium, basement membrane and visceral epithelium of Bowman's capsule, together constitute the membrane or filtration barrier.

Features

The capsule is associated with the glomerular filtration process. On the one hand, because it is part of the epithelial covering of podocytes that surrounds the glomerular capillaries. It also contributes to the synthesis of the basement membrane on which this epithelium and the glomerular capillary endothelium rest.

These three structures: capillary endothelium, basement membrane and visceral epithelium of Bowman's capsule, constitute the so-called filtration membrane or barrier, and each of them has its own permeability characteristics that contribute to the overall selectivity of this barrier.

In addition, the volume of fluid that penetrates Bowman's space, together with the degree of stiffness that opposes the outer capsular wall, determines the genesis of an intracapsular pressure that contributes to modulate the effective filtration pressure and to push the fluid along the associated tubule.

Determinants of the magnitude of glomerular filtration

A variable that collects the magnitude of the glomerular filtration process is the so-called glomerular filtration volume (GFR), which is the volume of fluid that is filtered through all the glomeruli in a unit of time. Its average normal value is about 125 ml / min or 180 L / day.

The magnitude of this variable is determined from the physical point of view by two factors, namely the so-called filtration or ultrafiltration coefficient (Kf) and the effective filtration pressure (Peff). That is: VFG = Kf x Peff (equation 1)

Filtration coefficient (Kf)

The filtration coefficient (Kf) is the product of the hydraulic conductivity (LP), which measures the water permeability of a membrane in ml / min per unit area and unit of driving pressure, times the surface area (A) of the filtering membrane, that is, Kf = LP x A (equation 2).

The magnitude of the filtration coefficient indicates the volume of liquid that is filtered per unit of time and per unit of effective driving pressure. Although it is very difficult to measure directly, it can be obtained from equation 1, dividing VFG / Peff.

The Kf in glomerular capillaries is 12.5 ml / min / mmHg per c / 100g of tissue, a value about 400 times higher than the Kf of other capillary systems in the body, where about 0.01 ml / ml can be filtered. min / mm Hg per 100 g of tissue. Comparison showing glomerular filtering efficiency.

Effective filtration pressure (Peff)

The effective filtration pressure represents the result of the algebraic sum of the different pressure forces that favor or oppose filtration. There is a hydrostatic pressure gradient (ΔP) and an osmotic pressure gradient (oncotic, ΔП) determined by the presence of proteins in plasma.

The hydrostatic pressure gradient is the pressure difference between the interior of the glomerular capillary (PCG = 50 mm Hg) and the space of Bowman's capsule (PCB = 12 mm Hg). As can be seen, this gradient is directed from the capillary to the capsule and promotes the movement of liquid in that direction.

The osmotic pressure gradient moves fluid from lower osmotic pressure to higher. Only particles that do not filter have this effect. Proteins don't filter. Its ПCB is 0 and in the glomerular capillary ПCG is 20 mm Hg. This gradient moves liquid from the capsule to the capillary.

The effective pressure can be calculated by applying Peff = ΔP – ΔП; = (PCG-PCB) - (ПCG-ПCB); = (50-12) - (20-0); = 38-20 = 18 mm Hg. Thus, there is an effective or net filtration pressure of about 18 mm Hg, which determines a GFR of about 125 ml / min.

Filtration index (IF) of substances present in plasma

It is an indicator of the ease (or difficulty) with which a substance in plasma can cross the filtration barrier. The index is obtained by dividing the concentration of the substance in the filtrate (FX) by its concentration in the plasma (PX), that is: IFX = FX / PX.

The range of IF values ​​is between a maximum of 1 for those substances that filter freely, and 0 for those that do not filter at all. Intermediate values ​​are for particles with intermediate difficulties. The closer to 1 the value, the better the filtration. The closer to 0, the more difficult it filters.

One of the factors that determines the IF is the size of the particle. Those with diameters less than 4 nm filter freely (IF = 1). As the size grows closer to that of albumin, the IF reduces. Albumin-sized or larger particles have IFs of 0.

Another factor that contributes to determine IF is negative electrical charges on the molecular surface. Proteins have a lot of negative charge, which adds to their size to make it difficult to filter them. The reason is that the pores have negative charges that repel those of the proteins.

References

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2. Guyton AC, Hall JE: The Urinary System, in Textbook of Medical Physiology, 13th ed, AC Guyton, JE Hall (eds). Philadelphia, Elsevier Inc., 2016.
3. Lang F, Kurtz A: Niere, in Physiologie des Menschen mit Pathophysiologie, 31 th ed, RF Schmidt et al (eds). Heidelberg, Springer Medizin Verlag, 2010.
4. Silbernagl S: Die funktion der nieren, in Physiologie, 6th ed; R Klinke et al (eds). Stuttgart, Georg Thieme Verlag, 2010.
5. Stahl RAK et al: Niere und ableitende Harnwege, in Klinische Pathophysiologie, 8th ed, W Siegenthaler (ed). Stuttgart, Georg Thieme Verlag, 2001.