SB 3.26.59

nāḍyo ’sya nirabhidyanta
 tābhyo lohitam ābhṛtam
nadyas tataḥ samabhavann
 udaraṁ nirabhidyata
nāḍyaḥ — the veins; asya — of the universal form; nirabhidyanta — became manifested; tābhyaḥ — from them; lohitam — blood; ābhṛtam — was produced; nadyaḥ — the rivers; tataḥ — from that; samabhavan — appeared; udaram — the stomach; nirabhidyata — became manifested.
The veins of the universal body became manifested and thereafter the red corpuscles, or blood. In their wake came the rivers (the deities presiding over the veins), and then appeared an abdomen.

Blood veins are compared to rivers; when the veins were manifested in the universal form, the rivers in the various planets were also manifested. The controlling deity of the rivers is also the controlling deity of the nervous system. In Āyurvedic treatment, those who are suffering from the disease of nervous instability are recommended to take a bath by dipping into a flowing river.