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University of Florida
PCB 4723C

Topic 17 Tuesday, December 10, 2013 12:39 AM • Blood ○ Plasma is the fluid portion of blood  Mainly made of water  Proteins account for 7% of the weight and 1% is dissolved organic molecules,trace elements and vitamins, and dissolved oxygen and CO2 ○ Plasma is identical in compositionto interstitial fluid except for the presence of plasma proteins  Albumin is the most prevalent type of protein in the plasma (60%) ○ Cellular elements of blood include RBCs, WBCs, and platelets • The primary role of the circulatory system is the distribution of dissolved gases and other moleculesfor nutrition, growth, and repair ○ Secondary roles have also evolved, including fast chemical signaling, dissipation of heat by delivery of heat from the core to the surface of the body, and mediation of inflammatory and host defense responses • Blood flow is driven by a constant pressure head across a variable resistance ○ To keep things simple, first think of the left side of the heart as a constant pressure generator that maintains a steady mean arterial pressure at its exit, the aorta ○ V=IR (Ohms Law) ○ Equivalently, ΔP=FR  The pressure difference (ΔP) between an upstream point (P1) and a downstream point (P2) is equal to the product of the flow F (or Q) and the resistance R   Notice how it is the difference in pressure between the two ends of the vessel, not the absolute pressure in the vessel, that determinesthe rate of flow □ For example, if the pressure at both endns of a vessel is 100 mmHg, yet no difference exists between the two ends, there will be no flow despite the presence of 100 mmHG pressure  The equation can be rewritten to solve for flow rate, F= ΔP/R □ Flow rate is proportional to the pressure difference but inverselyproportional to the resistance • The blood can take many different pathways from the left side of the heart to the right side of the heart ○ A single capillary bed (coronarycapillaries) ○ Two capillary beds in series (glomerular and peritubular capillariasis n the kidney) ○ Two capillary beds in parallel that subsequently merge and feed into a single capillary bed in series ○ *Something blood courses from the left side of the heart directly back to the left side of the heart across shunt pathways* ○ • IN contrast, blood flow from the RIGHT side of the heart to the left side of the heart can take only a SINGLE pathway, across a single capillary bed in the pulmonary circulation • The overall resistance across a circulatory bed results from parallel and serial arrangements of branches ○ For multiple beds in series, resistance is summed  Total=R +R +R … 1 2 3 ○ For multiple beds in parallel, resistance is added as such:  • Blood pressure is the height the blood can drive a column of liquid ○ Pressure = ρgh • Pressure is always expressed as a pressure difference ○ When it is not explicitly stated, the reference pressure in human physiology is the atmosphericpressure or barometric pressure • Because a pressure difference is always between two points, and these two points are separated by a distance X and have a spatial orientation to one another, we can define a
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