The choroid plexus produces cerebrospinal fluid (CSF) by filtering blood plasma through a layer of specialized cells, then actively pumping ions across those cells into the brain’s ventricles, with water following the resulting osmotic gradient. A healthy adult produces roughly 400 to 600 ml of CSF per day, enough to completely replace the fluid surrounding the brain and spinal cord four to five times every 24 hours.
Where the Choroid Plexus Sits
The choroid plexus is a frilly, vascular tissue that lines most of the brain’s ventricles, the fluid-filled chambers deep inside the brain. It’s present in both lateral ventricles, the third ventricle, and the fourth ventricle, though it’s absent from the narrow cerebral aqueduct that connects the third and fourth ventricles and from the tips of the lateral ventricles.
Structurally, the tissue has two key layers. The inner core contains a dense network of tiny blood vessels (capillaries) that are unusually leaky, with small gaps in their walls that let water, ions, and small molecules seep through easily. Wrapped around this vascular core is a single layer of specialized epithelial cells. These cells are the workhorses of CSF production, and tight junctions between them create a seal that controls exactly what passes from blood into the ventricles. This seal is called the blood-CSF barrier, and it’s built from over 30 different types of transmembrane proteins, including members of the claudin family and junctional adhesion molecules, all anchored to the cell’s internal skeleton by adaptor proteins.
Step One: Filtering Blood Plasma
CSF production starts when blood flows into the choroid plexus capillaries. Because these capillaries have small openings (fenestrations) in their walls, water and dissolved substances pass freely out of the blood and into the connective tissue surrounding the vessels. This creates a pool of filtered fluid sitting just beneath the epithelial cell layer. At this stage, the fluid is essentially a rough filtrate of blood plasma, still containing many of the molecules the brain doesn’t need or want in the CSF.
Step Two: Active Ion Transport
The epithelial cells then do the critical work of transforming this filtrate into CSF with a carefully controlled composition. This happens through a series of ion pumps and transport proteins embedded in the cell membranes.
On the blood-facing (basolateral) side of each cell, a transporter pulls sodium and bicarbonate from the surrounding fluid into the cell. This transporter is so important that when it’s genetically removed in mice, brain ventricle size drops by about 80%, a dramatic sign that CSF production has nearly stopped. The bicarbonate entering the cell is partly supplied by an enzyme called carbonic anhydrase, which converts carbon dioxide and water into bicarbonate inside the cell. Drugs that block this enzyme significantly reduce CSF production, which is why they’re sometimes used to treat conditions involving excess fluid pressure in the brain.
On the ventricle-facing (apical) side, a sodium-potassium pump actively pushes sodium out of the cell and into the ventricle while pulling potassium back in. This pump is unusual because in most cells throughout the body, it sits on the blood-facing side. In the choroid plexus, its position on the ventricle-facing side means it’s directly secreting sodium into the CSF. Bicarbonate also exits through the apical membrane via separate transporters and ion channels, and chloride follows through its own dedicated pathways.
The net result of all this pumping is that sodium, chloride, and bicarbonate accumulate in the ventricle. CSF ends up with slightly different electrolyte concentrations than blood plasma: chloride runs about 15 to 20 percent higher, while potassium and calcium are kept lower. Protein levels in CSF are dramatically lower than in blood, since the tight junctions between epithelial cells block most large molecules from passing through.
Step Three: Water Follows the Ions
Once ions have been pumped into the ventricle, an osmotic gradient forms: the fluid in the ventricle has a slightly higher solute concentration than the fluid inside the cells. Water moves down this gradient through dedicated water channel proteins called aquaporin-1, which are densely packed into the ventricle-facing membrane of choroid plexus epithelial cells. These channels allow water to cross the membrane rapidly without requiring any energy.
Aquaporin-1 is so central to this process that when it’s deleted in mice, intracranial pressure drops measurably because CSF secretion slows. A different water channel, aquaporin-4, handles water movement in the surrounding brain tissue but is completely absent from the choroid plexus itself, highlighting how specialized these cells are for their particular job.
Production Fluctuates Throughout the Day
CSF production isn’t constant. It follows a circadian rhythm, peaking at around 2:00 a.m. with an output of roughly 42 ml per hour and dropping to its lowest point at around 6:00 p.m., when production falls to about 12 ml per hour, only 30 percent of the nighttime peak. This means the brain produces substantially more CSF while you’re asleep, which may relate to the brain’s overnight waste-clearance processes.
What Happens When Production Goes Wrong
Because the choroid plexus is a living tissue, it can develop tumors. Choroid plexus papillomas are generally benign growths that enlarge the tissue’s surface area and can cause CSF overproduction. The typical result is hydrocephalus, an abnormal buildup of fluid in the ventricles that increases pressure inside the skull. Symptoms include headache, nausea, vomiting, and lethargy. In infants, whose skull bones haven’t yet fused, the head may visibly enlarge. Hydrocephalus can also result when a tumor physically blocks the flow of CSF through the ventricles or when bleeding clogs the structures that normally reabsorb CSF back into the bloodstream.
Children who develop prolonged high intracranial pressure before treatment may experience lasting effects on vision, including optic nerve damage that doesn’t always resolve even after the tumor is removed. This underscores how tightly CSF production and drainage need to be balanced: the choroid plexus must produce enough fluid to cushion the brain, deliver nutrients, and remove waste, but even a modest excess can cause serious harm in the enclosed space of the skull.