Cardiovascular Physiology —
Flow, Pressure, & Resistance

The Circulation

Circulatory system as a circuit

systemic circuit

pulmonic circuit

Circulatory system elements

arteries

rapid-transit passageways

pressure reservoirs

arterioles

major resistance vessels

control determines distribution of cardiac output

capillaries

sites of exchange

walls thin:  1 µm

small diameter:  7 µm

venules

veins

capacitance vessels

venous return

Distribution of blood in each element

heart

7%

pulmonary vessels

9%

arteries

13%

arterioles & capillaries

7%

veins

64%

Cross-sectional area of each element

aorta

2.5 cm²

small arteries

20 cm²

arterioles

40 cm²

capillaries

2500 cm²

venules

250 cm²

small veins

80 cm²

caval veins

8 cm²

Pressures in each element

aorta

120 mm Hg

capillaries

35–10 mm Hg

caval veins

0 mm Hg

Theory of circulatory system function

rate of blood flow to each tissue is precisely controlled by tissue need

cardiac output controlled by sum of local flows

arterial pressure controlled independently

Blood flow

Ohm’s law ¹

F = ΔP/R

measurement of flow

ml · min⁻¹
l · min⁻¹
ml · sec⁻¹

laminar flow

parabolic velocity profile

turbulent flow

Reynolds’ number:
Re = (v · d · ρ)/η

Blood pressure

standard units

mm Hg

1 mm Hg = 1.36 cm H₂O

Resistance to blood flow

R = ΔP/F

1 PRU = (1 mm Hg)/(1 ml · sec⁻¹)

R (in dyne · sec⁻⁵) = 1333 × (mm Hg · ml⁻¹ · sec⁻¹)

total peripheral resistance

total pulmonary resistance

Conductance

measure of blood flow for a given ΔP

conductance = 1/R

effect of vascular diameter

conductance ∝ diameter⁴

Poiseuille's law

expresses the relationship of all factors in total blood flow

mean velocity v = (ΔP · r²)/(8 · η · l)

F = v · π · r²

substitute v, and F = (ΔP · π · r⁴)/(8 · η · l)

solve for R

resistance of vessels in series:  R_total = R₁ + R₂ + R₃ + …

resistance in parallel:  start with conductances

C_total = C₁ + C₂ + C₃ + …

hematocrit & blood viscosity

3× at normal hematocrit

10× at 70 {polycythemia}

pressure & flow

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