It is not self-evident that we, as beings that walk upright, can keep the blood pressure inside our heads high enough for the proper perfusion of our brains – and keep the blood pressure in our legs low enough to prevent varicose veins, for example.
In order to do this, our body uses a number of different mechanisms, explains Joke Keijsers. “For instance, the small arteries in our organs can constrict and can thus ensure that the pressure inside the capillaries does not increase too much. In addition, we have the so-called muscle pump: when you stand up straight, the muscular tone in your legs makes the veins constrict, so that the blood is actively pushed back to the heart. Also, veins are fitted with valves, which prevent the blood from flowing back to the organs. The veins in our legs, where the pressure is highest, contain more of these valves than the ones located closer to the heart.”
How exactly these control mechanisms work together, and how big the impact of each mechanism is, was still largely unknown. Indeed, according to Keijsers we do know a lot about arteries (the blood vessels that carry blood from the heart to the organs) but significantly less about veins (in which the blood flows back to the heart): given that life-threatening ailments occur in the arteries in particular, it is much easier to raise funds for research into arteries.
Within aerospace there are funds available for research into veins, which is why Keijsers’ PhD period consisted of a cooperation between the TU/e group of Cardiovascular Biomechanics and the Deutsches Zentrum für Luft- und Raumfahrt (DLR) in Cologne, where the biomedical engineer was stationed during the initial years of her project. “My supervisor there is a French woman who also obtained her PhD at TU/e. She conducted the measurements in trial subjects which I could use in turn for the validation of my model.”
In the first few weeks after a space mission astronauts often faint, says Keijsers. “One reason for this is the reduced muscular tone in their legs; after all, under weightlessness you lose muscular bulk.” In Cologne the same effect was produced by submitting trial subjects to prolonged bedrest. Fainting due to hypotension when standing upright also occurs in elderly people and in hemiplegic patients, she explains, which makes her research applicable on a much larger scale than for aerospace alone.
Keijsers has succeeded in modeling the blood pressure control in the legs. When her model is integrated with a model of the whole circulation, the result can be used in the future for the selection of a treatment for frequent complaints such as fainting, but also for varicose veins – which are formed by hypertension in the lower legs. “Nowadays varicose veins are filled up or removed. Before long you may be able to use the model to decide to place a new valve in a vein at a strategic site. You will need to tailor such a model to each person individually, for unlike arteries, veins have slightly different locations in all persons.”