The Cardiac Cycle, Pump Function & Blood Pressure

Hemodynamics

P

R= Q

P1 P2

Q = blood flow rate (mL/sec) P= pressure difference between ends of vesselsR = resistance

Vessel diameter/radius is the most critical factor

determining blood flowD = 1, Q = 1 mL/sec

D = 2, Q = 16 mL/sec

D = 4, Q = 256 mL/sec

Figure 23.9 The physics of flow through tubes

Poiseuille’s Equation

R =radius, v = blood viscosity, l = length

increase radius, increase blood flow

Q πΔPr4

8vl

Blood Pressure Level (mmHg)

Category Systolic Diastolic

Normal < 120 and < 80

Prehypertension 120-139 or 80-89

High Blood Pressure

Stage 1 Hypertension

140-159 or 90-99

Stage 2 Hypertension

>159 or >99

Categories for Blood Pressure Levels in Adults

Determinants of Systolic Pressure

• Stroke Volume

• Heart Rate

• Distensibility of walls of aorta

• Ejection velocity

• peripheral “run off”

• Blood volume remaining in aorta before next ejection

Determinants of Diastolic Pressure

• Systolic Pressure

• Aortic Distensibility

• Rate of peripheral “run off”

• Heart Rate (duration of diastole)

Effects of Posture on BP

• Supine mean arterial BP=100 mmHg

• Upright pressure varies with level above or below the heart– For every 1 cm above heart, pressure

decreases by .77 mmHg.– For every 1 cm below heart, pressure

increases by .77 mmHg.