Just like every other organ and tissue in the body, the brain needs oxy- gen- and nutrient-rich blood to function properly. Because the brain is so crucial to the body’s survival, it receives a disproportionate amount of blood. Though it takes up only about 2 percent of the body’s weight, the brain receives 15 to 20 percent of the body’s entire blood supply, and 25 percent of its oxygen supply. The body will deprive other parts of the body of blood to ensure that the brain has what it needs.
The heart “feeds” the brain by sending blood through vessels both on the surface of the brain and deep inside it. Two pairs of arteries branching out from the aorta—the internal carotid arteries and vertebral arteries—supply the brain with blood. Carotid arteries send blood to the front of the brain, and vertebral arteries send blood to the back of the brain.
Blood flow into the brain’s tissues is a bit different than it is in other parts of the body. Elsewhere in the body, nutrients, oxygen and waste products can move freely in and out of the capillaries. This is not true in the brain. The brain has its own checkpoint, the blood-brain barrier, a semi-permeable system that lets only certain substances pass into the brain. This barrier protects the brain against viruses, toxins, hormones, and other substances in the blood that might harm the brain’s delicate tissues.
Considering how essential nutrient-rich blood is to the brain’s function, any disruption in blood flow can pose a serious risk. A blockage in the brain’s blood supply from a clot either in the brain or from elsewhere in the body is called a stroke. A stroke deprives the affected part of the brain of oxygen. Without oxygen, the brain’s cells will die. If too many brain cells die, thought and virtually every other function will come to a halt. Two primary risk factors for stroke are high blood pressure and heart disease, which illustrates the close relationship between heart and brain health.