Overview of Circulation
Arterial Pressure
Arterial Pressure
constant capillary flow keeps gas exchange constant and also prevents "sludging" of red cells in capillaries. Blood is like ketchup - hard to start flow when flow is stopped.
Mean arterial pressure (MAP) is calculated as 2/3 diastolic + 1/3 systolic. 120/90 MAP = 60 + 40 = 100 mmHg. 120/80 MAP = 53 + 40 = 93 mmHg. The reason MAP is not the average of systolic and diastolic is due to the fact that, at rest, 2/3 of the time is spent in diastole and 1/3 is spent in systole.
pulse pressure = systolic - diastolic changes with stroke volume & aortic compliance
Control of diastolic pressure
- heart rate - determines "run off" time during diastole
- total peripheral resistance (TPR) - determines slope of "run off"
MEMORY TOOL - PQR (ABC rule)
P = QR (use algebra to rearrange)
very useful in understanding hypertension and hypotension (shock)
R = P/Q for whole system
R = 8nl/pi r4 for single tube 2^4 = 16
where n is viscosity; l is length; and r is radius. Viscosity is greater with more red blood cells. More red cells (higher Hct) = more O2 delivery but higher resistance.
There is an optimum Hct (or hemoglobin) that is close to normal Hct. this can go up and down with blood volume
Resistance is in series; e.g. right heart and left heart or parallel; e.g., organs of body AND capillary beds.
adding a resistance in parallel ALWAYS lowers total resistance.
e.g., 1/R = 1/2 +1/2 +1/2 = 3/2 so R = 2/3 = .66
added R
1/R = 1/2 +1/2 +1/2 + 1/2 = 4/2 so R = 2/4 = .5
Flow = velocity x cross-sectional area
cm3/min = cm/min x cm2/min
student question: does density change in blood to change Reynold's No.? N = pdv/n
ans. = no
note that in air, changes in density can be important (why deep divers use helium), Helium also used clinically in patients with severe airway obstruction.
Regulation of Blood Pressure P = QR
R
slow = hormonal
fast = neural
Q
slow = hormonal
fast = neural
Cardiac Cycle
tricuspid valve then bicuspidvalve memory tool: "try it before you buy it"
Wiggers Diagram with EKG
Right heart pressures LOW due to lower resistance of lung circulation.
Importance of low right atrial pressure = avoids edema in organs and peripheral circulation. Sign of right heart failure = peripheral edema (e.g., swollen ankles)
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