The blood supply to the brain is incredibly designed. The two internal carotid arteries join what is know as the circle of Willis. This structure allows blood to be evenly circulated to all areas of the brain independent of a discrepancy of blood from the two internal carotid arteries. For example, feeling the carotid pulse in your neck, say on the right, this restricts the blood flow within the right carotid artery, this does not lead to a compromise in circulation to the right side of the brain. This is due to the circle of Willis which leads to equal distribution of blood around the whole of the cortex irrespective of the amount of blood being supplied from each internal artery.
A common condition that affects this blood supply is stroke. First of all there are two types of stroke; ischaemic and haemorrhagic.
Ischaemic strokes result from a blockage in one of the arteries. This can be due to atherosclerosis which is a thickening of the wall of a blood vessel due to excess cholesterol building up in the artery wall. This atherosclerosis can build and build until it eventually ruptures. Platelets within the blood stream sense this as outside material and form a clot. Another risk factor is hypertension, this high pressure of the circulation pushes the clot off the atherosclerotic plaque and carries it down the artery until the clot becomes stuck in the small diameter blood vessel. This blocks the blood supply to the rest of the artery and causes the tissue that is supplied by the distal part of the artery to become ischaemic and die.
Haemorrhagic strokes on the other hand are due to one of the vessels bleeding due to a rupture in the arterial way through all three layers. Again, a risk factor for this in hypertension as the high pressure causes stress and sheering forces on the internal wall of the vessel. Smoking is also a major risk factor in both type of stroke. In terms of haemorrhagic stroke this is due to a weakening of the blood vessel wall due to the toxins within smoke.
The effects of a stroke present in different ways and this is due to the location of the infarct or the haemorrhage. Look out for next week’s blog where we will look at different symptoms and how clinicians can use these to determine location of the infarct or haemorrhage.