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| Figure 22.8 The glomerular filtration barrier. This consists of the endothelial cells, the basement membrane, and the podocytes. Macula densa cells are a part of the juxtaglomerular apparatus and sense the chloride concentration in the distal tubule and adjust the diameters of different arterioles accordingly, thus regulating the glomerular blood flow. |
| The excretory function of the kidneys includes filtration of the plasma in the glomeruli, transport of water and solutes from the tubular lumen back to the blood (tubular reabsorption),
and secretion of different substances from the tubular cells into the lumen.
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| Plasma from the glomerular capillaries is filtered into the Bowman's space. Glomerular filtration depends on the filtration surface and on the permeability of the filtration barrier, which includes the layer of endothelial cells with characteristic pores (fenestrations) that line the glomerular blood vessels, the basement membrane, and epithelial cells (podocytes) with characteristic foot processes (Fig. 22.8). The fenestrations in the endothelial layer and the spaces between foot processes of the podocytes form a sieve that filters water and small molecules. Filtration of larger molecules through this barrier is limited by their size, shape, and electric charge. For instance, at pH 7.4, most plasma proteins are negatively charged, and so is the filtration barrier; this hinders filtration of even the smallest proteins such as myoglobin (molecular mass 17 kDa), and prevents almost completely filtration of the larger (69 kDa) albumin.
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| Glomerular filtration is driven by the hydrostatic pressure in the glomerular capillaries, which is approximately 50 mmHg. The hydrostatic pressure is counteracted by the oncotic pressure of the plasma and the back-pressure (approximately 10 mmHg) of the filtrate in the glomerular capsule. Changes in glomerular filtration rate alter the total amount of filtered water and solute, but not the composition of the filtrate.
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