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A venule is an extremely small vein, generally 8–100 microme- ters in diameter. Postcapillary ve- nules join multiple capillaries exit- ing from a capillary bed. Multiple venules join to form veins. The walls of venules consist of endo- thelium, a thin middle layer with
a few muscle cells and elastic fi- bers, plus an outer layer of connec- tive tissue fibers that constitute a very thin tunica externa.
A vein is a blood vessel that con- ducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens.
Many veins have valves to prevent back flow of blood, whereas ve- nules do not. In terms of scale, the diameter of a venule is measured in micrometers compared to milli- meters for veins.
20.2 Blood Flow, Blood Pressure, and Resistance
20.2 OBJECTIVES
1. Distinguish between systolic pressure, diastolic pressure, pulse pressure, and mean arterial pressure
2. Describetheclinicalmeasurementofpulseandbloodpressure
In addition to their pri- mary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approxi- mately 64 percent of the blood volume at any given time. Their ability to hold this much blood is due to their high capacitance, that is, their capacity to distend (expand) readily to store a high volume of blood, even at a low pressure. The large lumens and relatively thin walls of veins make them far more distensible than arteries; thus, they are said to be capacitance vessels.
Blood flow refers to the movement of blood through a vessel, tissue, or organ, and is usu- ally expressed in terms of volume of blood per unit of time. It is initiated by the contrac- tion of the ventricles of the heart. Ventricular contraction ejects blood into the major arter- ies, resulting in flow from regions of higher pressure to regions of lower pressure, as blood encounters smaller arteries and arterioles, then capillaries, then the venules and veins of the venous system. This section discusses a number of critical variables that contribute to blood flow throughout the body. It also discusses the factors that impede or slow blood flow, a phenomenon known as resistance.
As noted earlier, hydrostatic pressure is the force exerted by a fluid due to gravitational pull, usually against the wall of the container in which it is located. One form of hydro- static pressure is blood pressure, the force exerted by blood upon the walls of the blood vessels or the chambers of the heart. Blood pressure may be measured in capillaries and veins, as well as the vessels of the pulmonary circulation; however, the term blood pres- sure without any specific descriptors typically refers to systemic arterial blood pres- sure—that is, the pressure of blood flowing in the arteries of the systemic circulation. In clinical practice, this pressure is measured in mm Hg and is usually obtained using the bra- chial artery of the arm.
When systemic arterial blood pressure is measured, it is recorded as a ratio of two num- bers (e.g., 120/80 is a normal adult blood pressure), expressed as systolic pressure over diastolic pressure. The systolic pressure is the higher value (typically around 120 mm Hg) and reflects the arterial pressure resulting from the ejection of blood during ventricular contraction, or systole. The diastolic pressure is the lower value (usually about 80 mm Hg) and represents the arterial pressure of blood during ventricular relaxation, or diastole.
The difference between the systolic pressure and the diastolic pressure is the pulse pres- sure. For example, an individual with a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg would have a pulse pressure of 40 mmHg. Generally, a pulse pres- sure should be at least 25 percent of the systolic pressure. A pulse pressure below this level is described as low or narrow. This may occur, for example, in patients with a low stroke volume, which may be seen incongestive heart failure, stenosis of the aortic valve, or sig- nificant blood loss following trauma. In contrast, a high or widepulse pressure is common in healthy people following strenuous exercise, when their resting pulse pressure of 30–40 mm Hg may increase temporarily to 100 mm Hg as stroke volume increases. A persistently
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State of Alaska EMS Education Primer - 2016
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