Figure 17

Hemodynamic mechanisms regulating sodium excretion. Many different neurohumoral mechanisms, paracrine factors, and drugs exist that can influence sodium excretion and the pressure natriuresis relationship. These modulators may influence sodium excretion by altering changes in filtered load or changes in tubular reabsorption. Filtered load depends primarily on hemodynamic mechanisms that regulate the forces operating at the glomerulus. As shown, the glomerular filtration rate (GFR) is determined by the filtration coefficient (Kf) and the effective filtration pressure (EFP). The EFP is a distributed force determined by the glomerular pressure (Pg), the pressure in Bowman's space (PB), and the plasma colloid osmotic pressure within the glomerular capillaries (Kg). The Ag increases progressively along the length of the glomerular capillaries as protein-free fluid is filtered such that filtration is greatest in the early segments of the glomerular capillaries, as designated by the large arrow. The glomerular forces, EFP, and blood flow are regulated by mechanisms that control the vascular smooth muscle tone of the afferent and efferent arterioles and of the intraglomerular mesangial cells. The filtration coefficient also is subject to regulation by neural, humoral, and paracrine influences [17]. Changes in tubular reabsorption can result from alterations of various processes governing both active and passive transport along the nephron segments. Peritubular capillary uptake (PCU) of the tubular reabsorbate is mediated by the net colloid osmotic pressure gradient (nc - rc(). As a result of the filtration of protein-free filtrate, the plasma colloid osmotic pressure entering the peritubular capillaries is markedly increased. Thus, the colloid osmotic gradient exceeds the outwardly directed hydrostatic pressure gradient (Pc - Pi). Appropriate responses of one or more of these modulating mechanisms allow the kidneys to respond rapidly and efficiently to changes in sodium chloride intake [3,17]. nB—colloid osmotic pressure in Bowman's space; ftga—colloid osmotic pressure in initial parts of glomerular cappillaries; ftge—colloid osmotic pressure in terminal segments of glomerular capillaries; RA—resistance of preglomerular arterioles; RE—efferent resistance; RV—venous resistance. (Adapted from Navar [3].)

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