Most of my predecessors like to describe their time as a PhD student as an "adventure", I prefer to refer it as a rollercoaster: In the beginning you are overwhelmed with a strong urge to scream in terror and you just wish it to be over as soon as possible…until the adrenaline starts to kick in, and you begin to actually enjoy the ride. Suddenly the twists and turns become more predictable, giving you a sensation of being in greater control of the situation, hence boosting self-confidence, and even making you look forward for the next loop. You feel alive. And like most rollercoasters, you rarely ride it alone. This thesis, that so much hard work has taken, is dedicated to all the people who were, in one way or another, next to me, and who have helped me make this 3-year ride more bearable.First and foremost are my parents, who were able to guide me both personally and professionally to become the best doctor I could aspire to be.Professionally I will always be grateful to both of my directors: Prof. Miguel Angel Miranda Alonso and Dr. Ignacio Váya Pérez. They were one of the few who, since the very beginning, had treated me like an adult, a professional in the making, as opposed to a teenager that needed to be disciplined. Thank you for your guidance, respect and encouragement, and your constructive criticism. I would never have gotten this far without you.