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We determined the spin exchanges between the Cu²⁺ ions in the kagomé layers of volborthite, Cu₃V₂O₇(OH)₂·2H₂O, by performing the energy-mapping analysis based on DFT+U calculations, to find that the kagomé layers of Cu²⁺ ions are hardly spin-frustrated, and the magnetic properties of volborthite below ~75 K should be described by very weakly interacting antiferromagnetic uniform chains made up of effective S = 1/2 pseudospin units. This conclusion was verified by synthesizing single crystals of not only Cu₃V₂O₇(OH)₂·2H₂O but also its deuterated analogue Cu₃V₂O₇(OD)₂·2D₂O and then by investigating their magnetic susceptibilities and specific heats. Each kagomé layer consists of intertwined two-leg spin ladders with rungs of linear spin trimers. With the latter acting as S = 1/2 pseudospin units, each two-leg spin ladder behaves as a chain of S = 1/2 pseudospins. Adjacent two-leg spin ladders in each kagomé layer interact very weakly, so it is required that all nearest-neighbor spin exchange paths of every two-leg spin ladder remain antiferromagnetically coupled in all spin ladder arrangements of a kagomé layer. This constraint imposes three sets of entropy spectra with which each kagomé layer can exchange energy with the surrounding on lowering the temperature below ~1.5 K and on raising the external magnetic field <i>B</i>. We discovered that the specific heat anomalies of volborthite observed below ~1.5 K at <i>B</i> = 0 are suppressed by raising the magnetic field <i>B</i> to ~4.2 T, that a new specific heat anomaly occurs when <i>B</i> is increased above ~5.5 T, and that the imposed three sets of entropy spectra are responsible for the field-dependence of the specific heat anomalies.