Blood Flow, Blood Pressure, & Resistance


A. Pulmonary Blood Flow (see figure ==>)

1. Large; full cardiac output; provide "layer of blood" around each alveolus

2. Variable, depending upon level of cardiac output (5 - 25 liters/min)

Note: Do not confuse the pulmonary circulation with the bronchial circulation. The latter is a relatively small division of the systemic circulation which supplies the airways, etc. with systemic arterial (oxygenated) blood

B. Pulmonary Blood Pressure

1. Low pressure in arteries and capillaries (compared with systemic circulation)

e.g. at rest

  Systemic Pulmonary
Arterial pressure 120/80 mmHg; 95 mmHg mean 25/10 mmHg;  15 mmHg mean
Capillary pressure 30 mmHg mean 10 mmHg mean
Venous pressure 2-5 mmHg 2-5 mmHg

2. Consequence: work of the right ventricle in pumping blood is much reduced compared to the work of the left ventricle

Note:  Work = Flow x Pressure and Pressure = Flow x Resistance

Note: The pulmonary arterial pressure need not be as high as the systemic arterial pressure to overcome the effects of gravity, nor is a high pressure needed to distribute blood flow among several vascular beds.

C. Pulmonary Vascular Resistance

1. Low, only about one-sixth to one-ninth of the systemic circulation
2. Low resistance due mainly to less small vessel smooth muscle (arterioles)

Note: Arterioles not required to distribute flow to various vascular beds, as in the systemic circulation

3. Control of Resistance

Flow vs. Pressure

a. Blood Pressure: P (or COR

Mechanisms: with increasing pulmonary arterial pressure (or cardiac output)

1) capillary recruitment (more capillaries open)
2) blood vessel dilation (vessels distend)

Significance: pulmonary circulation can accommodate a large increase in cardiac output with only a small increase in pulmonary arterial pressure

Note: The sensitivity of pulmonary vascular resistance to hydrostatic pressure in the circulation means that gravity can exert a major influence on lung blood flow distribution

b. Local "metabolic" control

1) Stimulus: alveolar PO2 , PCO2

Note: implies local underventilation compared to local circulation rate

2) Responses:

a) vascular smooth muscle constrict R (!)  (opposite of systemic circulation)
b) bronchial smooth muscle relax

3) Significance

a) Pulmonary blood flow is directed away from poorly ventilated regions to better ventilated regions of the lung

b) Ventilation is increased to underventilated regions

Note opposite responses in overventilated regions

c.    Autonomic Control:  Not so important