Blood vessels and the brain

The brain’s supply chain

Every neuron sits within a few cell-widths of a capillary. The brain’s vasculature is estimated at roughly 400 miles of vessels in an adult, and it does far more than deliver oxygen. Vessel walls are lined by the blood–brain barrier, a tightly sealed layer that decides what enters brain tissue. The spaces around vessels are the highways of the clearance system. And the slow rhythmic constriction and dilation of vessel walls, called vasomotion, is one of the forces that moves fluid through brain tissue.

What age and disease do to vessels

Arteries stiffen with age, small vessels narrow or leak, and the blood–brain barrier becomes more permeable. On MRI these changes appear as white-matter lesions and tiny bleeds collectively called cerebral small vessel disease, which is common after 70 and is a major contributor to vascular dementia and to mixed dementia alongside Alzheimer’s pathology.

Nedergaard and Goldman’s 2020 review in Science notes that small vessel disease is linked to glymphatic dysfunction in experimental models, and that declining vascular health and declining fluid clearance track each other with age. Vessels and clearance are not separate stories.

The best-supported intervention we have

The strongest evidence for protecting the brain through midlife is vascular. The SPRINT-MIND trial found that lowering systolic blood pressure below 120 mmHg, rather than the standard 140, significantly reduced the risk of mild cognitive impairment and slowed the accumulation of white-matter lesions. The 2024 Lancet Commission lists hypertension, high LDL cholesterol, diabetes, smoking, obesity and physical inactivity, all primarily vascular or metabolic, among the fourteen modifiable risk factors that together account for about 45 percent of dementia risk.

The National Institute on Aging’s practical summary is simple: controlling blood pressure helps the heart and the brain, and the earlier it starts, the better.

A barrier that can be opened on purpose

The blood–brain barrier that protects the brain also blocks most drugs. In 2024 a team at West Virginia University reported in the New England Journal of Medicine that focused ultrasound, guided by MRI and paired with microbubbles, could briefly open the barrier in specific regions in three people with Alzheimer’s disease, allowing an antibody to clear amyloid about a third faster in the treated regions than in untreated ones. The barrier closed again within 24 to 48 hours. It is a small, early study, but it shows that the brain’s vascular interface is becoming something engineers can work with rather than only around.