I. Definition
A. Movement of mixed venous blood through the pulmonary capillary bed for the purpose of gas exchange between the blood and alveolar air
II. The pulmonary vascular system is a high-volume system with low capillary resistance.
A. Pulmonary blood flow is about 6 L/minute.
B. Mean pulmonary arterial pressure is 15 mmHg.
III. Regional differences in blood flow in the lungs
A. Lung bases receive a greater percentage of blood flow than do the apices.
B. Factors that affect distribution of pulmonary blood flow
1. Gravity and hydrostatic pressure differences within the blood vessels: Blood must flow against gravity to the apices when a person is in the upright position.
2. Effect of alveolar pressure: Alveolar pressure may be greater than pulmonary capillary pressure in the apical and middle regions of lungs because of the following:
a. Positive pressure ventilation
b. Decreased right ventricular preload (decreased hydrostatic pressure), dehydration, hemorrhage
c. Air trapping in chronic obstructive pulmonary disease
3. Effect of decreased PaO2 (partial pressure of oxygen in arterial blood) (alveolar gas): local reflex that causes vasoconstriction of pulmonary arterioles supplying hypoxic alveoli
I. Definition
A. Mechanical movement of air into and out of the alveoli for the purpose of gas exchange between the atmosphere and capillary blood
B. Gas flows from higher atmospheric to lower intrapulmonary pressure during inhalation.
II. Regulation of ventilation
A. CNS control
1. Brain stem centers (medulla and pons): Cells fire automatically to trigger inhalation; others fire to halt inhalation; exhalation occurs passively.
2. Cerebral cortex: allows voluntary control to override brain stem centers in response to chemical stimuli and lung inflation changes
B. Chemical regulation
1. Central chemoreceptors in medulla respond to increased PaCO2 (partial pressure of carbon dioxide in arterial blood) (hypercapnia) and decreased pH (acidosis) through medullary stimuli by increasing ventilatory depth and rate; hypercapnia is the major stimulus to alter ventilation.
2. Peripheral chemoreceptors in aortic and carotid bodies respond to decreased PaO2 (hypoxemia) by stimulating medullary centers to enhance ventilation.
3. Patients with chronically high PaCO2: Hypercapnic ventilatory drive is lost; these patients respond only to changes in PaO2 by stimulation of peripheral receptors to adjust ventilation (hypoxemic respiratory drive).
a. Supplemental O2: Administer low liter flows very carefully to prevent apnea (e.g., begin with 1-2 L/minute and assess).
b. Do not withhold O2 if needed; be prepared to assist with mechanical ventilation if respiratory drive is depressed.
III. Work of breathing (WOB)
A. Definition: amount of effort required to overcome the elastic and resistive properties of the lungs and chest wall
B. Elasticity (elastic recoil): tendency of the lungs to return to their original shape
1. Lungs try to collapse because of tension between the interstitial elastic fibers and the surface of the alveoli.
2. Chest wall attempts to resist inward-moving recoil.
C. Compliance: measure of distensibility, or how easily the lungs and thorax can be stretched; describes resistance as a result of elastic properties. Increased compliance means less pressure is needed to stretch the lungs and/or thorax.
1. High compliance: easier to expand lung tissue (e.g., chronic obstructive pulmonary disease)
2. Low compliance: stiff lungs; chest wall less distensible (e.g., pneumonia, acute respiratory distress syndrome)
IV. Alveolar ventilation (Va): amount of air that reaches alveoli and participates in gas exchange
A. PaCO2: best indicator of Va
B. Normal: PaCO2 is 35 to 45 mmHg
ALVEOLAR DIFFUSION
I. Definition
A. Exchange of O2 and CO2 across the alveolocapillary membrane
B. Oxygen diffuses down the concentration gradient from higher alveolar pressure (PaO2) to lower pulmonary capillary pressure (PaO2).
C. CO2 diffuses at a rate 20 times greater than that of O2 from capillary to alveolus.
II. A-a gradient: alveolar-to-arterial oxygen gradient
AaDO2: alveolar-to-arterial oxygen difference
A. A calculation to aid in diagnosing the degree of a patient’s hypoxemia
B. Formula: difference between partial pressure of O2 in the alveoli (PaO2) and partial pressure of O2 in arterial blood (PaO2)
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