The heart, what can I say? It's pretty important! Heart disease effects many people, not just those that have heart disease, but those that have friends and family with the disease. Perhaps something that you may or may not have considered is the fact that heart disease is a huge problem in animals as well! Before getting started, I should note, strokes (as one one think of in humans) this is something that doesn't typically occur in animals. Attribute that to a very controlled diet, one that lacks a mainstream of fast food and junk, and this is a fairly good explanation as to why stoke is lacking in the field of veterinary medicine.
But, before I get into too many particulars, let's start at the beginning:
The Duties of the Heart (or the Cardiovascular System as a whole) can be broken down into 3 main actions (in order of importance):
- Maintain Arterial Blood Pressure
- Maintain Cardiac Output
- Maintain Venous Pressure
It's as simple as that for the CV (Cardiovascular) system. What do I mean by 'Cardiovascular System?' Well, the CV system not only involves the heart, but it also includes the vessels (arteries, veins, and capillaries) that carry the blood that is ejected by the heart. Now, it is easy to say, "So blood in included too, right?" While the blood is transported by the CV system, blood is a component of the circulatory system. Confusing, eh? Here is a link to an NIH diagram, just to firm up the difference. Notice, that by definition, the CV system is the 'Four chambers of the heart, the arteries, veins, and capillary networks."
Now, back to those duties.
The most important duty of the CV system is to maintain arterial blood pressure. Why is this so important? Well, maintenance of arterial blood pressure (hopefully) guarantees that the heart and brain will receive blood. Not only is blood important for the delivery of oxygen and nutrients, blood is equally important for the removal of metabolic waste. Should the brain and/or heart suffer from lack of oxygen, lack of nutrients, or damage by waste products, death will occur! By maintenance of arterial blood pressure, the CV system (at very least) maintains a living organism long enough to attempt to fix or adjust to any deficits in the remaining two duties. It's really that important!
The second most important duty of the CV system is to maintain cardiac output. Cardiac output, in simple terms, is 'flow of blood.' This differs from blood pressure in that, hmmmm.... Let's start with this: Cardiac Output is a component of blood pressure. Blood pressure can be thought of as the energy (or vigor) with which the blood is travelling. If you increase blood pressure, you will increase the amount of blood that is following to an organ. If you decrease blood pressure, you will decrease the amount of blood that is flowing to an organ. Now, let's think of what would happen with cardiac output. If we increase cardiac output (or the flow of blood from the heart) we will increase arterial blood pressure. If we decrease cardiac output, we will decrease arterial blood pressure. In all honesty, it is not as simple as this, by any mean. However, for now, let's think of it as such. In the future, we will further describe how the heart compensates with changes in blood pressure and cardiac output so that it is functioning at an optimal level (or reaching a 'normal' level - referred to as homeostasis). The maintenance of cardiac output is important for delivery of nutrients, removal of wastes, etc. At this stage, however, we more so refer to the delivery/removal to all of the body.
The third most important duty of the CV system is to maintain venous pressure. Arteries and veins, they seem so alike, so it would be logical to think that this duty should function similarly to Duty #1 (maintenance of arterial blood pressure). Unfortunately, it's not (but for a very great reason I'll explain momentarily). When it comes to venous pressure start thinking about all of the organs in the body. In particular, start thinking about the lungs. Should venous pressure increase in any of these organs (there are a variety of reason, but for now let's say a blockage), this can be very bad news. When organ pressure increases, that pressure is transferred to nearby vessels. Rather than explain Starling's Forces right now, just know that an increase in pressure in blood vessels results in plasma (or fluid) being forced into tissues. Your vessels, they have fenestrations (or 'windows'). While microscopic, these 'windows' allow red and white blood cells to move out of the blood (among other things). Because of this, and due the microscopic size of 'plasma molecules', plasma easily moves out of the blood into tissues when forced, So, back to the lungs. As anatomy goes, the lungs are extremely close to the veins of the heart. As a result, if pressure is increased, it will force plasma (or fluid) into the lungs! When this occurs, lung tissue is compromised by the edema (or fluid accumulation), meaning that is is unable to properly deliver oxygen to the body! Obviously, breathing is very important, and we certainly wouldn't want that to occur.
I know this is a lot to start with. However, understanding these cornerstones to the CV system are integral to the understanding of its physiology. The heart, it is 99% going to everything it can to maintain these 3 priorities. If you can just remember these, you may be able to determine how the heart is going to respond to a particular insult, as we go along.
Cheers!
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