The physics of the circulatory system, as of any fluid system, are very complex. That said, there are many physical factors that influence blood pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs, etc.
Some physical factors are:
Rate of pumping.
In the circulatory system, this rate is called heart rate, the rate at which blood (the fluid) is pumped by the heart. The higher the heart rate, the higher (potentially, assuming no change in stroke volume) the blood pressure.
Volume of fluid.
In the case of the circulatory system, this is blood volume, the amount of blood present in the body. The more blood present in the body, the higher the rate of blood return to the heart and the resulting cardiac output. There is some relationship between dietary salt intake and increased blood volume, potentially resulting in higher blood pressure, though this varies with the individual and is highly dependent on autonomic nervous system response.
In cardiac physiology, the rate and volume of flow are accounted for in a combined fashion by cardiac output, which is the heart rate, the rate of contraction, multiplied by the stroke volume, the amount of blood pumped out from the heart with each contraction. It represents the efficiency with which the heart circulates blood throughout the body.
Resistance. In the circulatory system, this is the esistance of the blood vessels. The higher the resistance, the higher the blood pressure. Resistance is related to size (The larger the blood vessel, the lower the resistance), as well as the smoothness of the blood vessel walls. Smoothness is reduced by the buildup of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure. Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing blood pressure.
Viscosity, or thickness of the fluid. If the blood gets thicker, the result is an increase in blood pressure. Certain medical conditions can change the viscosity of the blood. For instance, low red blood cell concentration, anemia, reduces viscosity, whereas increased red blood cell concentration increases viscosity. Viscosity also increases with blood sugar concentration'visualise pumping syrup. (It was thought that aspirin and other drugs decreased the viscosity of blood, but this has been found not to be so; "blood thinners" reduce the tendency of the blood to clot, not viscosity.)
In practice, each individual's autonomic nervous system responds to and regulates all these interacting factors so that, although the above issues are important, the actual blood pressure response of a given individual varies widely because of both split-second and slow-moving responses of the nervous system and end organs. These responses are very effective in changing the variables and resulting blood pressure from moment to moment.
Source http://www.wikipedia.org
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