Circulatory System (HB)

The circulatory system is a closed hydraulic system. It consists of a pump (the heart), a container (the blood vessels), and a fluid (the blood). This system moves oxygenated blood and nutrients to the cells throughout the body and collects waste, which it moves to the appropriate organ for disposal. A defect to any one of the components can cause significant problems for your patient.

The system is under pressure, and the body does its best to maintain this pressure at a constant level, which is known as blood pressure. Too high or low of a pressure and our patient can experience a number of problems ranging from changes to mental status to death.

We generically call this problem “shock.” We further differentiate shock by the part of the circulatory system that it affects:

• Volume shock is fluid related.
• Cardiogenic shock is related to the heart.
• Vascular shock is a problem with the blood vessels.

Think of it like a garden hose full of water with the ends attached to each other. It is under constant pressure, just like your vascular system. If you rhythmically stomp near the middle of that hose with your foot, the pressure beyond your foot will increase briefly and move the fluid through the hose–just like the beating of your heart.

Two numbers normally represent blood pressure, and you have likely seen this at a doctor’s office. The top number is the “systolic pressure” and is analogous to the beat of the heart (or stomping on the hose). It is the high pressure that moves the blood forward through the circulatory system. The bottom number is the “diastolic pressure” and the constant pressure that the system is under. The systolic pressure must be higher than the diastolic pressure to move the blood around–the difference between the two is called the pulse pressure.

Suppose the systolic is too close to the diastolic. In that case, there will not be enough pressure to force the exchange of oxygen and carbon dioxide and nutrients and wastes in the cells–this would be “inadequate perfusion pressure.” If the systolic pressure is a lot higher than the diastolic pressure, it can cause the vessels to leak fluid, and in some cases, cause an aneurysm. If the diastolic is constantly high (a person with high blood pressure), it can force the blood vessels to become hardened, leading to further problems.

Back to the example of the hose. Imagine that there is a hole in that hose, and fluid is leaking out. You are still stomping at the same rate. What happens to the pressure inside that hose? It drops, right? To increase that pressure, you need to stomp faster and harder. But as you do that, the fluid drains out more quickly. This condition is volume shock. You need to stop the leak and restore fluid to correct the problem.

For the next example, reset the system. Now you will start stomping on that hose, but only half as hard as you did before. Maybe instead of stomping, you are just rolling your foot along a section of hose. Again, what happens to the pressure? Now the pulse pressure is too low. When this occurs in a patient, we will see inadequate perfusion pressure–exchange at the cells will be impacted. This condition is called cardiogenic shock, and we need to fix the pump. Unfortunately, fixing the pump in the field is difficult to impossible.

In a final example, reset the system, and imagine that we have the same volume as before and the same size and strength pump, but the hose has doubled in diameter. While the pump can move the fluid that is there, we will still lack pressure because it relies on the container being a fixed size.

Each of these forms of shock can stem from a number of reasons, and all of them have a cause that is more common than others. In a normal, healthy person, the body will manage an insult to any part of the circulatory system by compensating for the problem. The body may shift fluid around, dilate or constrict the blood vessels, or speed up/slow down the heart. This action is done to maintain that constant blood pressure and is called “compensated shock.” If the source of the problem is not corrected, eventually, the body will be unable to keep up, and systems will start failing. This failure is called “decompensated shock” and indicates you have very little time before your patient dies.