Forms a water-specific channel (PubMed:19383790, PubMed:7559426, PubMed:8601457). Plays an important role in brain water homeostasis (PubMed:37143309). It is involved in glymphatic solute transport and is required for a normal rate of water exchange across the blood brain interface. Required for normal levels of cerebrospinal fluid influx into the brain cortex and parenchyma along paravascular spaces that surround penetrating arteries, and for normal drainage of interstitial fluid along paravenous drainage pathways. Thereby, it is required for normal clearance of solutes from the brain interstitial fluid, including soluble beta-amyloid peptides derived from APP. Plays a redundant role in urinary water homeostasis and urinary concentrating ability (By similarity). {ECO:0000250|UniProtKB:P55088, ECO:0000269|PubMed:19383790, ECO:0000269|PubMed:37143309, ECO:0000269|PubMed:7559426, ECO:0000269|PubMed:8601457}. This is the function of AQP4 (aquaporin 4, Ensembl gene identifier ENSG00000171885).