Key physiology

Key physiology

By
Dale Pinnock
Contains
0 recipes
Published by
Quadrille Publishing
ISBN
978 184949 542 4

OK, so I’m not going to expect you to suddenly become a heart surgeon or anything, but I do firmly believe that you should be as informed as possible about the inner workings of your body. There are a few aspects of how the cardiovascular system works that will really help you both to start to put the information in this book into some kind of context and to bring the bigger picture together. The more you can grasp what is going on, the better you can understand how nutrition is a key part of the solution. There are a few main areas, things that you would have heard about over and over – either from your doctor or in the news – that are vital to understand. The first of these is:

–Blood pressure: We all know by now that, if blood pressure is too high, we have a problem. The British Heart Foundation estimates that there could be in excess of five million people in the UK with undiagnosed high blood pressure. That is a serious number. But what is blood pressure? What does the term mean and why is it such a big deal?

Blood pressure is basically the pressure that circulating blood places on the blood vessel walls. There has to be a certain amount of pressure in our vessels so that each contraction of the heart can push the blood to where it needs to go. As the blood is moved along, it exerts pressure against the vessel wall. That’s it. So, why does it matter how high it is?

The higher your blood pressure is, the more force is placed upon the blood vessel walls, which figures. The vessel walls are designed to withstand a vast amount of force, but not a limitless amount. We get to a point where too much pressure is a problem. If your blood pressure is high enough for long enough, then the vessel becomes more susceptible to damage and any areas that have already been damaged from previous events run the risk of getting worse.

What the numbers mean

When we have our blood pressure checked, we are told (sometimes) a couple of numbers, then there is usually a suggestion as to whether they are good or bad. But what do the numbers mean? Following a blood pressure measurement, you will be told that your blood pressure is ‘something over something’; 120 over 80 for example. The first number (systolic), represents the pressure exerted on the vessel wall when the heart squeezes and a large volume of blood is forced through the blood vessel. The second (diastolic) number points to the pressure put upon the vessel wall when the heart is at rest, or between beats. So which number is the most important? Well, in most cases it is the first one, the number that shows how much of a beating the vessel wall is taking under the most amount of force. The higher this is, the more risk there is of vascular injury, heart attack or stroke. More pressure = more chance of damage. (However, according to Blood Pressure UK, recently it has been thought that in those less than 40 years old, diastolic pressure is a greater predictor of risk. This may be because it can show that there is less flexibility in the vessel wall than expected, or that a kind of hardening has started to take place.)

What controls blood pressure

The main driver of blood pressure is a pair of responses called vasoconstriction and vasodilation. Vasoconstriction makes the vessel get smaller and narrower. Vasodilation makes it get larger and wider. As a vessel constricts and gets narrower, the pressure within it gets much higher, as a specific volume of blood is having to be forced through a smaller space. When a vessel dilates and gets wider on the other hand, the pressure within it drops as there is more space for the blood to fill, hence less force exerted on the blood vessel wall. A healthy vessel is moving between these two states constantly to keep our blood flowing along nicely.

Everything from physical activity to the health of our blood vessels will determine the rate and extent to which these changes occur. In a healthy individual, the move between vasodilation and vasoconstriction should be smooth and highly responsive. If you recall from the previous section, the bulk of our blood vessel walls are made up of layers of smooth muscle. This smooth muscle is the key component of the blood vessels that allows them to widen and constrict so readily. The muscle contracts; the vessel gets narrower. The muscle relaxes; the vessel gets wider.

Think back to the previous section and you will recall that I placed great importance on the endothelium, that thin but vitally important inner skin that lines our blood vessels. Well, this seemingly simple structure is one of the major controllers of vasodilation. This is due to a highly active chemical that the endothelial cells produce, called nitric oxide. When the endothelial cells produce nitric oxide, it leaves the endothelial cells and then migrates out deep into the vessel walls, where it encourages the smooth muscle of the blood vessel to relax, which then allows the vessel to widen. Vasoconstriction is caused by a number of chemical factors, from calcium flowing into the muscle, through to neurotransmitters. Calcium, for example, causes muscle fibres to move together and muscles to contract, which in turn will narrow the blood vessel wall.

In terms of how nutrition can influence this whole picture, nitric oxide production and vasodilation is the most relevant part. The section of this book that describes cardiovascular disease processes will give you a good idea of how things can start to go wrong with the endothelium and the knock-on effects of that through the system as a whole, then when we get to explore the role of nutrition in cardiovascular health, we will begin to see how certain nutrients and dietary patterns can influence this. Hopefully, by building the picture piece by piece, you will finish reading the book with a better understanding of what’s going on and what you can do about it.

Blood clotting

This is a normal and absolutely vital response to injury and, without it, we would be in big trouble. It basically describes a series of events that stem bleeding when a vessel is injured. This can be the obvious type of injury, such as when you cut yourself, or silent internal injuries, such as damage to a blood vessel wall or a ruptured plaque (more on that later).

Sticking with the cut finger, you will realise that, when you cut yourself, you don’t carry on bleeding to the point that you turn white and keel over. After a minute or so, the bleeding stops; give it a good few hours and a scab will start to form. This is the whole process of coagulation in action.

When an area of a blood vessel becomes damaged, whether a kitchen knife ploughs through it or an atherosclerotic plaque ruptures (again, more on that later), a response is set in motion. Thrombocytes (platelets) begin to clump together around the site of injury to form a platelet plug. When this occurs, platelets send out a series of chemical messengers. Clotting factors that are circulating in the plasma are sensitive to these signals and, when they get to the area of the plug, they begin to lay down a fibrous structure called fibrin, which forms a mesh around the plug and strengthens it. This is essentially like a scab; the masking tape over the leak in the hose pipe. A makeshift repair job that stems the bleeding, while your body begins to repair damaged tissues. While this sequence of events is designed to save your life, as we’ll see, when the process happens within a blood vessel, there is the potential for it to be life threatening.

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