The cardiovascular system transports many compounds, including gases, nutrients, and waste products, to and from the body’s tissues through the red blood cells and plasma. Furthermore, it serves an important role in immunity through specialized cells, such as leukocytes, which help the body fight localized or systemic pathogens.3 Capillaries within the body can dilate and constrict to maintain proper body temperature. In addition, the circulatory system mediates the formation of blood clots to repair damaged vessels. These functions reflect the important jobs of the cardiovascular system, which include maintenance of blood pressure, gas and solute exchange, coagulation, and thermoregulation. It is important to recognize that, for the circulatory system to serve its predominant functions, blood pressure must be kept sufficiently high to propel blood forward. Blood pressure, therefore, provides healthcare professionals with information regarding the effectiveness of the circulatory system. In addition, high blood pressure, or hypertension, is a pathological state that may result in damage to the blood vessels and organs. Blood pressure is a measure of the force per unit area exerted on the wall of the blood vessels and is measured with a sphygmomanometer. Sphygmomanometers measure the gauge pressure in the systemic circulation, which is the pressure above and beyond atmospheric pressure (760 mmHg at sea level). Blood pressure is expressed as a ratio of the systolic (ventricular contraction) to diastolic (ventricular relaxation) pressures. Pressure gradually drops from the arterial to venous circulation, with the largest drop occurring across the arterioles. Normal blood pressure is considered to be between 90/60 and 120/80. The largest drop in blood pressure occurs across the arterioles.3 This is necessary because the capillaries are thin-walled and unable to withstand the pressure of the arterial side of the vasculature. An analogy can be drawn between circulation and an electric circuit. Much like an electromotive force (voltage) drives a current through a given electrical resistance, the pressure gradient across the circulatory system drives cardiac output through a given vascular resistance. This analogy is an important one to remember because the equations of electric circuits can be applied to the cardiovascular system. For example, Ohm’s law (V = IR) can be translated into the following equation in circulation:
ΔP = CO × TPR
where ΔP is the pressure differential across the circulation, CO is the cardiac output, and TPR is the total peripheral (vascular) resistance. It is also important to note that arterioles and capillaries act much like resistors in a circuit. When electricity travels through a wire, the wire itself provides an intrinsic level of resistance that limits the flow of electricity through it. Resistance is based on three factors: resistivity, length, and cross-sectional area. Resistivity has no obvious correlate in physiology, but the other two factors certainly do. The longer a blood vessel is, the more resistance it offers. The larger the cross-sectional area of a blood vessel, the less resistance it offers. In addition, arteries are highly muscular and are able to expand and contract as needed to change vascular resistance and maintain blood pressure. Arterioles can also contract to limit the amount of blood entering a given capillary bed (much like increasing resistance will decrease current flow to a given branch in a circuit). Finally, with the exception of the three portal systems, all systemic capillary beds are in parallel with each other. Therefore, opening capillary beds will decrease vascular resistance (like adding another resistor in parallel) and, assuming the body can compensate, increase cardiac output. Hydrostatic pressure is the force per unit area that the blood exerts against the vessel walls. This is generated by the contraction of the heart and the elasticity of the arteries, and can be measured upstream in the large arteries as blood pressure. Hydrostatic pressure pushes fluid out of the bloodstream and into the interstitium through the capillary walls, which are somewhat leaky by design.3 Osmotic pressure, on the other hand, is the sucking pressure generated by solutes as they attempt to draw water into the bloodstream. Because most of this osmotic pressure is attributable to plasma proteins, it is usually called oncotic pressure. At the arteriole end of a capillary bed, hydrostatic pressure (pushing fluid out) is much larger than oncotic pressure (drawing fluid in), and there is a net efflux of water from the circulation. As fluid moves out of the vessels, the hydrostatic pressure drops significantly, but the osmotic pressure stays about the same. Therefore, at the venule end of the capillary bed, hydrostatic pressure (pushing fluid out) has dropped below oncotic pressure (drawing fluid in), and there is a net influx of water back into the circulation.
1) Bettini, Alessandro (2016). A Course in Classical Physics 2—Fluids and Thermodynamics. Springer. p. 8
2) The Venturi effect”. Wolfram Demonstrations Project. Retrieved 2017-08-06
3) Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.; Walters, P. (2002). Molecular Biology of the Cell (4th ed.). New York and London: Garland Science.
4) Moran and Shapiro, Fundamentals of Engineering Thermodynamics, Wiley, 4th Ed, 2000.
5) Kauzmann, W. (1966). Kinetic Theory of Gasses, W.A. Benjamin, New York