17. Flow is proportional to pressure difference b/w entrance and exit points of a tube
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Editor's Notes
*need oxygen supply…O2 diffuses to cells….hence distance from O2 source becomes imp….average distance from O2 source is about 100 micrometer…this optimal distance is cuz of CVS
rhoedes
*Fluid cannot move through a system unless some energy is applied to it. In fluid dynamics, this energy is in the form of a difference in pressure, or pressure gradient, between two points in the system. Pressure is expressed as units of force, or weight, per unit area. A familiar example of this is in the pounds per square inch (psi) recommendation stamped on the side of tires. The psi indicates the pressure to which a tire should be inflated with air above atmospheric pressure. Inflating a tire to 32 psi signifies that 32 more pounds press against every square inch of the inner tire surface than against the outside of the tire.The pressure exerted at any level within a column of fluid reflects the collective weight of all the fluid above that level as it is pulled down by the acceleration of gravity. It is defined aswhere P = pressure, Ï = the density of the fluid, g = the acceleration of gravity, and h = the height of the column of fluid above the layer where pressure is being measured. The force represented by pressure in a fluid system is often described as the force that is able to push a column of fluid in a tube straight up against gravity. In this way the magnitude of the force resulting from fluid pressure can be measured by how high the column of fluid rises in the tube In physiological systems, this manner of expressing pressure is designated as centimeters H2O, or the more convenient mm Hg, because mercury is much denser than water and therefore will not be pushed upward as far by typical pressures seen within the cardiovascular system.Without going into mechanistic details at this time, arterial pressure peaks shortly after the heart contracts and pumps blood into the aorta and falls to a lower value when the heart relaxes between beats and is therefore not pumping blood into the aorta. The peak pressure during contraction of the heart is called the systolic pressure and is typically about 120 mm Hg in humans, whereas the minimum arterial pressure value during relaxation of the heart is called the diastolic pressure and is about 80 mm Hg. Thus, if one end of a tube were to be inserted into the aorta with the other end connected to a column of mercury sitting perpendicular to the ground at the level of the heart, that column of mercury would rise 120 mm during systole and fall to 80 mm during diastole. In clinical practice, human arterial pressure is reported as systolic over diastolic pressure or, in this example, 120/80. (Our mean arterial pressure is not the arithmetic mean of systolic and diastolic pressure but is instead about 93 mm Hg, because the time the heart spends relaxing is longer than the time it spends contracting and ejecting blood into the aorta.)
Compliance, therefore, is related to the ease by which a given change in pressure causes a change in volume.In biological tissues, the relationship between DV and DP is not linear. As shown in Figure 1, compliance (which is the slope of the line relating volume and pressure) de-creases at higher volumes and pressures.Another way to view this is that the “stiffness” of a cardiac chamber or vessel wall increases at higher volumes and pressures.
*Systolic & Diastolic P are peak/lowest arterial pressuresIn reality arterial pressures vary around average values from heartbeat to heart beat & minute to minuteFor complex reasons, compliance (TPR) does not significantly influence MAP. So e.g. In arteriosclerosis, pulse P raises a lot, not MAP!!
*means increases or decreases in CO have proportionate effects on MAP