Question.

Ultrafiltration in the renal glomerulus is facilitated by all of the following EXCEPT which one?

a.

    1. fenestrations within the glomerular capillary endothelium

b.

    1. positive charge of the glomerular capillary basement membrane

c.filtration slits in the epithelium of the Bowman’s capsule

d.high hydrostatic pressure in the glomerular capillaries

e.podocytes

 

 

Answer.

The answer is b. The glomerular basement membrane is negatively charged.

 
Discussion.

Within the kidney, each nephron begins with a Bowman’s capsule (fig 1.). The Bowman’s capsule is a hollow spherical space connected to the proximal tubule and which surrounds a network of capillaries called the glomerulus (Fig 1). The glomerular capillaries are supplied with blood by the afferent arterioles and drained by the efferent arterioles.

Blood flowing through the glomerular capillaries is filtered by a process of ultrafiltration, with the resulting filtrate passing into the capsular space and on into the proximal tubule. Filtration occurs across a specialised membrane (called the glomerular filtration barrier) created by the combination of glomerular endothelium, the glomerular basement membrane and the closely apposed epithelium of the Bowman’s capsule. The filtrate must pass, in order from plasma to tubular fluid, through:

• the glomerular capillary endothelium is highly permeable compared to most capillary endothelia due to the presence of numerous fenestrations (pores) of around 80nm in diameter. These fenestrations allow passage of water, ions and small molecules across the endothelium but retain all cells and most large macromolecules within the capillary
• the basement membrane of the capillary endothelium is comparatively wide, contains ECM secreting mesangial cells, and carries a negative charge which repels positively charged proteins
• and finally, the capsular epithelium made up of specialised cells called podocytes. These cells wrap round the glomerular capillaries, their interdigitating foot processes creating narrow filtration slits of around 25nm in width. Due to their narrow width and negative charge, the podocyte filtration slits prevent nearly all proteins from crossing into the capsule.
The composition of the filtrate entering the Bowman’s capsule, and which flows into the proximal tubule of the nephron, is very similar to blood plasma in terms of water, ions and small molecules (those less than 4 – 8nm in size, such as urea, creatinine, peptides and glucose) but is devoid of any cells and contains virtually no proteins or other large molecules. It is a plasma ultrafiltrate.
Fluid movement across the glomerular filtration barrier is occurs by bulk flow and is driven by the high hydrostatic pressures of the glomerular capillaries.

Figure 1. Glomerular filtration membrane of the Bowman’s capsule

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