Page 169 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
Reabsorption in different segments of the Transcellular transport implies that two
tubule. The concentration of a substance X membranes must be crossed, usually by two
(TF X) and inulin (TF in) in tubular fluid can be different mechanisms. If a given substance (D-
measured via micropuncture (! A). The values glucose, PAH, etc.) is actively transported
can be used to calculate the non-reabsorbed across an epithelial barrier (i.e., against an
fraction (fractional delivery, FD) of a freely fil- electrochemical gradient; ! see p. 26ff.), at
tered substance X as follows: least one of the two serial membrane transport
FD = (TF X/P X)/(TF in/P in), steps must also be active.
where P X and P in are the respective concentra- sive transport processes are usually closely in-
Interaction of transporters. Active and pas-
Kidneys, Salt, and Water Balance pling site can then be derived from 1 – FD (! D, in the development of an osmotic gradient
tions in plasma (more precisely: in plasma
water).
terrelated. The active absorption of a solute
such as Na or D-glucose, for example, results
Fractional reabsorption (FR) up to the sam-
+
columns 2 and 3, in %).
(! p. 24), leading to the passive absorption of
Reabsorption and secretion of various sub-
water. When water is absorbed, certain solutes
stances (see pp. 16–30, transport mecha-
are carried along with it (solvent drag; ! p. 24),
while other substrates within the tubule be-
nisms). Apart from H 2O, many inorganic ions
–
+
2+
(e.g., Na , Cl , K , Ca , and Mg ) and organic
+
2+
come more concentrated. The latter solutes
–
–
substances (e.g., HCO 3 , D-glucose, L-amino
(e.g., Cl and urea) then return to the blood
reabsorption. Electrogenic ion transport and
proteins; ! C, D, p. 158ff.) are also subject to
tubular reabsorption (! B1–3). Endogenous
ion-coupled transport (! p. 28) can depolarize
7 acids, urate, lactate, vitamin C, peptides and along their concentration gradients by passive
products of metabolism (e.g., urate, glu-
or hyperpolarize only the luminal or only the
curonides, hippurates, and sulfates) and for- basolateral membrane of the tubule cells. This
eign substances (e.g., penicillin, diuretics, and causes a transepithelial potential which serves
PAH; ! p. 150) enter the tubular urine by way as the driving “force” for paracellular ion trans-
of transcellular secretion (! B4, C). Many sub- port in some cases.
+
stances, such as ammonia (NH 3) and H are Since non-ionized forms of weak electro-
first produced by tubule cells before they enter lytes are more lipid-soluble than ionized
the tubule by cellular secretion. NH 3 enters the forms, they are better able to penetrate the
tubule lumen by passive transport (! B5), membrane (non-ionic diffusion; ! B2). Thus,
+
while H ions are secreted by active transport the pH of the urine has a greater influence on
(! B6 and p. 174ff.). passive reabsorption by non-ionic diffusion.
+
+
+
+
Na /K transport by Na -K -ATPase (! p. 26) Molecular size also influences diffusion: the
in the basolateral membrane of the tubule and smaller a molecule, the larger its diffusion
collecting duct serves as the “motor” for most coefficient (! p. 20ff.).
of these transport processes. By primary active
transport (fueled directly by ATP consump-
+
+
+
tion), Na -K -ATPase pumps Na out of the cell
into the blood while pumping K in the op-
+
posite direction (subscript “i” = intracellular
and “o” = extracellular). This creates two driv-
ing “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 mem-
+
+
brane potential (inside the cell is negative rela-
tive to the outside) which represents an elec-
trical gradient and can drive ion transport
(! pp. 32ff. and 44).
156
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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