The cardiovascular system: what it is and how it works

The cardiovascular system: what it is and how it works

Dale Pinnock
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Published by
Quadrille Publishing
978 184949 542 4

Having a basic understanding of the cardiovascular system will enable you to start to build a clear picture of what is going on in your body, how small changes in your diet and lifestyle will have a great impact upon it, and especially how your current diet and changes you make to it may affect your specific issues. ‘The cardiovascular system’ refers to the heart, the blood vessels and their contents.

The blood

The most obvious place to start. This tissue is the whole reason the circulatory system exists and finding out what it is, the components in there and which plays what roles will be useful later.

One of the primary functions of the blood is as a transport system. It brings oxygen and nutrients to the cells and tissues of the body. The nutrients we take in – vitamins, minerals, amino acids, fats, glucose, or their by-products – play vital roles in the daily operations of every cell in every tissue in every system. These nutrients and their by-products get where they need to go via the blood. The blood also carries away waste. Our cells are very good at housekeeping; they process waste and throw it out as rubbish to be carried away in the circulatory system.

The blood is made up of several components:

–Plasma: This is the liquid portion of the blood, and makes up around 55 per cent of blood volume. It has very little colour – just a subtle pale yellow tinge – and is mostly water with a bundle of proteins, clotting factors and nutrients suspended in it. It also carries antibodies and other important elements for our immune function.

–Erythrocytes: Otherwise known as red blood cells. These are the familiar disc-like cells that we often see in images and animations of the blood. Their main job is to transport oxygen to our tissues. Red blood cells contain a protein-based structure called haemoglobin. This is known as a metalloprotein (a protein that binds to metal), as iron makes up an important part of its structure. The iron in haemoglobin actually binds to oxygen to carry it around the bloodstream, where it can be deposited to cells and tissues. This is why people who have serious anaemia or iron deficiency become very tired and fatigued, as their capacity to deliver vital life-giving oxygen to cells is diminished. If cells don’t get enough oxygen, their ability to create energy and perform many important functions is greatly impaired and severe fatigue and malaise soon set in.

–Leukocytes: Otherwise known as white blood cells, these are the second most prominent type of cell in our blood. They are essentially the army of our immune systems, patrolling the body on the look out for anything that is upsetting the peace.

They can rapidly identify invaders that are trying to cause infection or damage. They can also identify our own cells that are suffering for whatever reason. They can tell if one of our cells has become infected and is in trouble. Or they can identify cells that are going through pathological changes, such as the changes that occur during the initiation of cancer. When they make this identification, they can set about a series of events that can deal with it. Some incidents can be dealt with by leukocytes there and then; others may require the leukocytes to recruit help and back-up.

There are several different types of white blood cells that do slightly different jobs. I won’t go into all the details now but, as we go on, I will touch on the subtle differences as they become relevant to the whole picture of cardiovascular health.

–Thrombocytes: Also called platelets, these are the third cell type that make up the non-liquid portion of our blood. Their role is to carry out what is called haemostasis. This is basically stopping bleeding at sites of injury. When you cut yourself, the blood doesn’t keep oozing out of your body without stopping; we’d soon be in trouble at a very young age if that were the case. This is all thanks to our thrombocytes.

They stop the bleeding by rushing to the area of damage and forming a platelet plug. This is as it sounds, a clumping together of these cells to plug the wound. When this occurs, platelets send out a series of chemical messengers. Clotting factors (substances that assist with the clotting process) that are circulating in the plasma are sensitive to these signals and, when they get to the area of the platelet plug, they begin to lay down a fibrous structure called fibrin, which forms a mesh around the plug and strengthens it.

This series of events is an important thing to remember, as it is a vital part of understanding some of the things that take place in the body in cardiovascular disease.

–The heart: This astounding pump system is so complex that it is beyond even the best human engineers. There have been numerous attempts to replicate it, all of which have failed miserably. There are artificial systems that can do its job during surgical procedures, but nothing that comes close to mirroring its functionality. About the size of a closed fist, the heart takes the deoxygenated blood (blood that has delivered all of its vital oxygen to the tissues) that is in your veins to the lungs to become oxygenated, before it is taken back off to the tissues of our body once again. It is divided into four chambers: two atria and two ventricles. Between each atrium and ventricle there is a one-way valve that prevents the backward flow of blood, ensuring the pump works as an effective continuous one-way system, with blood flowing in, then out in a perfectly orchestrated fashion.

The heart is divided in half, with two chambers per half. The right and left sides of the heart have two distinct jobs to do. The right side brings in blood that has low levels of oxygen and sends it to the lungs to get its oxygen levels topped up and also to remove its carbon dioxide. The left side of the heart takes the blood that has been freshly oxygenated and pumps it back out to the rest of the body, sending vital oxygen to our cells.

–The blood vessels: Our blood vessels (arteries and so on) resemble a network of incredibly complex plumbing. Thousands of vessels run through our body, some as thick as a hose pipe, others thinner than a single hair, delivering blood, oxygen and nutrients to our tissues. The thicker ones are called arteries, the next size down are arterioles, with the smallest and finest being capillaries.

Understanding the structure of the blood vessels and how they work is a vital part of understanding the events that take place in cardiovascular disease, and to start seeing how diet and lifestyle may offer both prevention and intervention.

Blood vessels are made up of several layers that all have different functions to carry out. Of these layers, the two I want you to become most familiar with – and those that I am going to discuss most frequently – are the endothelium and the smooth muscle layer.

–Smooth muscle: The bulk of our blood vessel walls is formed from smooth muscle. Smooth muscle is a type of involuntarily muscle (that means it reacts to environmental and chemical changes, rather than our conscious choice to move it, as we would a muscle in our legs).

Blood vessels need to be incredibly responsive to the constantly changing environment of our bodies and the continual fluctuation in our tissues’ needs for oxygen and nutrients. To be this responsive they must change size and shape very quickly.

The smooth muscle in the blood vessel walls can rapidly contract and relax to allow this change to occur. This has great relevance to heart disease as will be described in the next section.

–Endothelium: The endothelium is an incredibly thin yet unbelievably supple and complex inner skin that lines our blood vessels.

At face value level, the endothelium acts as a physical barrier between the blood vessel’s contents and the rest of the vessel structure. This in itself is vitally important, as there are many potentially damaging components that can be in our circulation that could affect the health of the vessel.

The endothelium also regulates many aspects of blood vessel function, anything from responding to hormonal signalling to even controlling the activity of the smooth muscle described above. The health of the endothelium is of vast importance to cardiovascular health in general, and will be a recurrent theme in this book.

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