Factors That Influence Arterial Blood Pressure
Arterial blood pressure depends on a variety of factors that are at work in the body at any given moment (figure 11). These factors include the pumping action of the heart, blood volume, resistance to flow, and the viscosity (thickness) of the blood. If any of these factors change, the blood pressure will also change so the flow of blood can continue as normally as possible. Let's consider an example in which a person suffers from a cardiovascular disease called arteriosclerosis, where plaque is accumula ting in and clogging the arteries. This example illustrates how resistance to flow can affect blood pressure. As the disease progresses and more plaque accumulates in the blood vessels, the flow of blood faces more and more resistance. In order to maintain an adequate flow of blood, the heart must pump "harder," and therefore the blood that leaves the heart is forced out wi th greater pressure. Thus, the blood pressure is increased. This is a case where the blood pressure is altered and stays altered over a long period of time.However, an important point to remember is that, when cardiovascular disease is not present, blood pressure very seldom stays altered over a long period of time. This is because the systems that regulate and control your blood pressure are some of the most efficient and fastest regulating systems in the body. Your blood pressure usually increases or decreases in response to some instantaneous need of the body, but then returns back to normal once other regulating mechanisms "kick in." Your body knows how important it is to keep your blood pressure as stable as possible so that appropriate blood flow through the tissues is maintained, particularly in the brain. So, whenever a blood pressure change is detected, the body immediately responds to bring it back to normal. Your body takes good care of your blood pressure. Now, let's look at the factors that can influence a change in blood pressure.
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| Figure 11. Some factors that influence arterial blood pressure. |
Formula: Cardiac output = stroke volume x heart rate
Units: ml of blood/minute = ml of blood/beat x beats/minute
The blood pressure in your body usually changes in response to changes in cardiac output. In other words, as the cardiac output increases, so does the blood pressure. If the cardiac output decreases, so does the blood pressure. (Usually, however, blood pressure returns back to normal very quickly. Remember, your body takes good care of your blood pressure.) But what would make the cardiac output increase or decrease? Look at the formula above. If either the stroke volume or the heart rate increases, so does the cardiac output and, as a result, the blood pressure rises. Conversely, if the stroke volume or the heart rate decreases, so do both the cardiac output and the blood pressure.
Friction between the blood and walls of the blood vessels creates a force called resistance. This force must be overcome by blood pressure if the blood is to continue flowing. Consequently, factors that alter resistance cause changes in blood pressure. Resistance in the systemic portion of the circulation is called peripheral resistance.
The viscosity of a fluid is related to the ease with which its molecules flow past one another. Measurement of viscosity could also be thought of as measuring how thick a fluid is. As viscosity increases, fluids flow less easily. (Think of the ketchup commercial on television where the watery ketchup pours quickly while you wait in anticipation for the other, more viscous ketchup to pour!) Blood cells and plasma proteins increase the viscosity of the blood. This prevents the blood from flowing as easily as water (remember the saying "blood is thicker than water"). Therefore, greater force is needed to propel it through the vascular system. So, it is not surprising that blood pressure rises as blood viscosity increases. Conversely, blood pressure will decrease as blood viscosity decreases.