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Reabsorption in different segments of the tubule. The concentration of a substance X (TFX) and inulin (TF¡n) in tubular fluid can be measured via micropuncture (^ A). The values can be used to calculate the non-reabsorbed fraction (fractional delivery, FD) of a freely filtered substance X as follows:

FD = (TFx/Px)/(TFin/Pin), where Px and Pin are the respective concentrations in plasma (more precisely: in plasma water).

Fractional reabsorption (FR) up to the sampling site can then be derived from 1 - FD (^ D, columns 2 and 3, in %).

Reabsorption and secretion of various substances (see pp. 16-30, transport mechanisms). Apart from H2O, many inorganic ions (e.g., Na+, Cl-, K+, Ca2+, and Mg2+) and organic substances (e.g., HCO3-, D-glucose, L-amino acids, urate, lactate, vitamin C, peptides and proteins; ^ C, D, p. 158ff.) are also subject to tubular reabsorption (^ B1-3). Endogenous products of metabolism (e.g., urate, glu-curonides, hippurates, and sulfates) and foreign substances (e.g., penicillin, diuretics, and PAH; ^ p. 150) enter the tubular urine by way of transcellular secretion (^ B4, C). Many substances, such as ammonia (NH3) and H+ are first produced by tubule cells before they enter the tubule by cellular secretion. NH3 enters the tubule lumen by passive transport (^ B5), while H+ ions are secreted by active transport (^ B6 and p.174ff.).

Na+/K+ transport by Na+-K+-ATPase (^ p. 26) in the basolateral membrane of the tubule and collecting duct serves as the "motor" for most of these transport processes. By primary active transport (fueled directly by ATP consumption), Na+-K+-ATPase pumps Na+ out of the cell into the blood while pumping K+ in the opposite direction (subscript "i" = intracellular and "o" = extracellular). This creates two driving "forces" essential for the transport of numerous substances (including Na+ and K+): first, a chemical Na+ gradient ([Na+]o > [Na+]i) and (because [K+]i > [K+]o), second, a membrane potential (inside the cell is negative relative to the outside) which represents an electrical gradient and can drive ion transport (^ pp.32ff. and 44).

Transcellular transport implies that two membranes must be crossed, usually by two different mechanisms. If a given substance (D-glucose, PAH, etc.) is actively transported across an epithelial barrier (i.e., against an electrochemical gradient; ^ see p.26ff.), at least one of the two serial membrane transport steps must also be active.

Interaction of transporters. Active and passive transport processes are usually closely interrelated. The active absorption of a solute such as Na+ or D-glucose, for example, results in the development of an osmotic gradient (^ p. 24), leading to the passive absorption of water. When water is absorbed, certain solutes are carried along with it (solvent drag; ^ p. 24), while other substrates within the tubule become more concentrated. The latter solutes (e.g., Cl- and urea) then return to the blood along their concentration gradients by passive reabsorption. Electrogenic ion transport and ion-coupled transport (^ p. 28) can depolarize or hyperpolarize only the luminal or only the basolateral membrane of the tubule cells. This causes a transepithelial potential which serves as the driving "force" for paracellular ion transport in some cases.

Since non-ionized forms of weak electrolytes are more lipid-soluble than ionized forms, they are better able to penetrate the membrane (non-ionic diffusion; ^ B2). Thus, the pH of the urine has a greater influence on passive reabsorption by non-ionic diffusion. Molecular size also influences diffusion: the smaller a molecule, the larger its diffusion coefficient (^ p. 20ff.).

D. Reabsorption, secretion and fractional excretion

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