![]() Pressure receptors (barareceptors), located in the walls of the large arteries in the thorax and neck, are important for short-term blood pressure regulation. When these factors increase, blood pressure also increases.Īrterial blood pressure is maintained within normal ranges by changes in cardiac output and peripheral resistance. Four major factors interact to affect blood pressure: cardiac output, blood volume, peripheral resistance, and viscosity. Blood pressure is measured with a sphygmomanometer and is recorded as the systolic pressure over the diastolic pressure. Pulse pressure is the difference between systolic pressure and diastolic Diastolic pressure occurs during cardiac relaxation. Systolic pressure is due to ventricular contraction. In common usage, the term blood pressure refers to arterial blood pressure, the pressure in the aorta and its branches. It can be felt where an artery is close to the surface and rests on something firm. Pulse refers to the rhythmic expansion of an artery that is caused by ejection of blood from the ventricle. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues, causing a localized reduction in blood flow. Instead, venous return depends on skeletal muscle action, respiratory movements, and constriction of smooth muscle in venous walls. When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance. Blood flow through the veins is not the direct result of ventricular contraction. Very little pressure remains by the time blood leaves the capillaries and enters the venules. As vessel diameter decreases, the resistance increases and blood flow decreases. In blood vessels, most of the resistance is due to vessel diameter. Resistance is a force that opposes the flow of a fluid. Blood flow is slowest in the capillaries, which allows time for exchange of gases and nutrients. As the total cross-sectional area of the vessels increases, the velocity of flow decreases. The rate, or velocity, of blood flow varies inversely with the total cross-sectional area of the blood vessels. ![]() Blood flows in the same direction as the decreasing pressure gradient: arteries to capillaries to veins. Like all fluids, blood flows from a high pressure area to a region with lower pressure. Pressure is a measure of the force that the blood exerts against the vessel walls as it moves the blood through the vessels. Blood Flowīlood flow refers to the movement of blood through the vessels from arteries to the capillaries and then into the veins. The net result of the capillary microcirculation created by hydrostatic and osmotic pressure is that substances leave the blood at one end of the capillary and return at the other end. Fluid movement across a capillary wall is determined by a combination of hydrostatic and osmotic pressure. Oxygen and carbon dioxide move across the capillary wall by diffusion. Substances pass through the capillary wall by diffusion, filtration, and osmosis. In addition to forming the connection between the arteries and veins, capillaries have a vital role in the exchange of gases, nutrients, and metabolic waste products between the blood and the tissue cells. Acknowledgements Physiology of Circulation Roles of Capillaries
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