Cerebral Blood Flow

Goals

• To provide an overview of the vascular system of the brain

Reading

• Watch: online mini-lecture on the brain's blood supply found on Canvas.

The Vascular System

The vascular system of the brain is a major topic usually left out introductory courses. However, it is an important topic with which to have some familiarity. The vascular system brings glucose and oxygen to the brain, which are its principal sources of energy. Techniques like Positron Emission Tomography (PET) and functional Magnetic Resonance Imaging (fMRI) depend upon the fact that blood flow and energy delivery changes with regional brain activity. Thus, by 'following the energy’, one can image where in the brain functional activity is occurring in near real time.

A second reason for studying the vascular system is that strokes are an unfortunate common occurrence in humans that have provided important information about the functional role of particular brain regions. Ischemic strokes occur when an embolus (usually some plaque related to atherosclerosis) is carried along in a blood vessel until it reaches a point where the vessel is too narrow for the embolus to proceed. There, the embolus stops, and prevents glucose and oxygen to reach the downstream brain structures that are nourished by the, now blocked, blood vessel. The deprived cells in that part become infarcted (they die) and there is a corresponding loss of function (e.g., paralysis, speech impairment, partial blindness, etc). By knowing where the infarcted brain tissue is located, one can associate its loss to the loss of function.

A primer of the vascular system

In high-school biology, you learned that the vascular system is also known as the circulatory system because blood circulates in a loop through the body with the heart as its pump. The goal is for the hemoglobin cells in blood to bind oxygen in the lungs, and for the blood to absorb glucose from the small intestine, and then to carry these fuels to tissues throughout the body. As oxygen is exchanged with bodily tissues, the blood picks up the waste product of metabolism (e.g., after giving up oxygen, hemoglobin binds the waste carbon dioxide). The carbon dioxide is the released on the blood's return trip to the lungs, where a fresh supply of oxygen is bound to the hemoglobin.

The basic circuit was illustrated in a slide from my lecture:

Aorta —> Arteries —> Arterioles —> Capillaries —> Venules —> Veins —> Vena cava —> Right atrium —> Right ventricle —> Pulmonary artery —> Lung capillary bed —> Pulmonary vein —> Left atrium —> Left ventricle —> Aorta

The exchange of glucose and oxygen with body tissues occurs in the very thin walled capillaries. The arteries bring oxygenated blood from the heart to the capillaries, and the veins collect the deoxygenated blood from the

Key concepts of the vascular system

There are vast changes in scale in the vascular system. The flow in the large arteries is pulsatile (this is why blood spurts from an arterial wound) and reaches 90 cm/s in the aorta. However, the flow is smoothed by successive changes in the caliber of the arteries as they branch to smaller and smaller vessels. By the time blood reaches the capillaries, its velocity has been reduced to 1 mm/s.

Very small changes in a vessels diameter can cause a big change in blood flow. The diameter of the aorta in the chest is ~ 3 cm. The diameter of a small artery (arteriole) just upstream from a capillary is 100-300 um (micron). The normal diameter of a capillary in the brain is < 10 um, just about the diameter of a red blood cell.

The vascular system of the human brain

Blood flow into the brain

There are two sources of blood flow into the brain.

1. The internal carotid arteries (one per hemisphere) enter the brain through the neck and provide the majority of blood supply to the anterior parts of the cerebral cortex. This is sometimes referred to as the ['anterior circulation'.
2. The vertebral arteries combine into the basilar artery which enter the brain along the spinal cord. The basilar artery gives off smaller arteries as it ascends in the brain. It also gives rise to the posterior cerebral arteries which provides the bulk of the posterior circulation to the occipital lobes, cerebellum, and brain stem.

Circle of Willis

The internal carotid arteries and basilar artery meet in a ring like arrangement called the Circle of Willis. A number of arteries branch from the Circle of Willis and provide blood supply to specific parts of the brain.

Blood supply to cortex

In most areas of the brain, the arteries run along the surface of cortex. Small arteries called arterioles enter the brain tissue through at a 90 degree angle to the surface (remember that cortex folds, so the cortical surface may be infolded). The arterioles give rise to dense capillary beds in the cortical layers.

Blood flow out of the brain

The capillaries converge onto small veins called venules which return to the cortical surface and collect into larger veins. These veins eventually lead to a large channel that runs down the midline of the cerebral hemispheres called the sagittal sinus (another sinus, called the 'straight sinus' runs between the base of the telencephalon and the cerebellum). The blood in the sinuses collect into the jugular vein (one per hemisphere). The jugular veins provide the only outflow of blood from the brain back into the general circulation.

Blood flow is related to function

One of the reasons that strokes are so devastating is that, while the brain commands 20% of our total metabolism, the brain stores very little energy. Energy is stored in the form of glycogen in muscles, this is not true of the brain. Therefore a continuous source of energy must be provided to the brain. Interruption of blood flow to the brain causes a loss of consciousness within seconds.

The management of blood flow to particular parts of the brain is a fascinating topic beyond the scope of this course. However, it has become clear that the local activity in neurons alters the supply of blood to those neurons. Thus, one can localize functional brain activity by localizing changes in blood flow and metabolism.

I provided one example in lecture that depicts the selective dilation of a single pial artery in response to sciatic nerve stimulation, while other arteries supplying other nearby brain regions were unaffected. We will discuss this in more detail when we consider imaging methods that depend upon blood flow.