pulmonary mechanics, diffusion, perfusion

lung and chest wall tissue

elastic

elasticity

ability to stretch and return to starting shape/position

lung tissue at rest

more elastic; recoil forces pull inward

chest wall tissue at rest

less elastic; recoil forces pull outward

compliance

change in vol per change in psi

determines how easily tissue (lung/chest) expands

surfactant incr alveolar compliance→decr work of breathing

air movement is dictated by

psi changes

elastic properties of lungs & chest wall

airflow resistance

substances move across pressure gradients from…

areas of high psi to low psi

for a fixed amt of gas in a container, increased container volume=

decr pressure exerted by gas

for a fixed amt of gas in a container, decreased container volume=

increased pressure exerted by the gas

changes in psi and vol are influenced by

structural stability, including resting tone of abdominals, pelvic floor, integrity of spine & rib cage

at rest

alveoli remain slightly inflated due to - intrapleural psi and surfactant

equal recoil forces=pull chest wall out, pull lungs in

equal psis=airways vs atmosphere, intrathoracic vs intrabdominal

during inspiration

contraction of inspiratory mm

outward pull of chest wall exceeds inward pull of lungs

3D incr in thoracic cavity vol

psi changes: airway psi lower than atmospheric (air→lungs); intrapleural psi more - (drive lymph flow); intrathoracic psi lower than intrabdominal (assist w/ circ from lower body to heart)

key factors in inspiration

diaphragm contract & descend→decr intrathoracic psi

resting tone of abdominal mm resists excessive diaphragm descent

diaphragm’s costal fibers elevate & expand lower rib cage laterally

external intercostals contract to further elevate ribs & counteract inward F from low thoracic psi

accessory mm contribute to pump handle movement of sternum, elevate upper rib cage

during expiration

inspiratory mm relax

inward pull of lungs > outward pull of chest wall →decr thoracic cavity vol

psi changes: airway psi > atmospheric (air out of lungs); intrapleural psi returns→resting level; intrathoracic psi > intrabdominal psi

key factors of expiration

relax inspiratory mm & elastic recoil of lung tissue reverses psi gradient

contraction of internal intercostals, abdominals aid forceful exp

intrathoracic psi can be incr by “closing can” w/ glottis, pelvic floor mm

coughing involves opening glottis→forceful contracting of exp mm, maintain pelvic floor support

recoil F at residual volume

elastic recoil of chest wall directed outward is large

recoil directed inward is small

recoil F at functional residual capacity

elastic recoils of lung & chest wall are = but opposite

recoil F at large lung volume

elastic recoil of chest wall→smaller, recoil of lung incr

recoil F at ~70% total lung capacity

equilibrium position of chest wall (recoil=0)

recoil F at total lung capacity

elastic recoil of both lung & chest wall direct inward, favor decr in lung vol

psi-vol relationships

ventilation follows net F btwn outward pull of chest wall and inward pull of lung elasticity

functional residual capacity (FRC)=point where these forces are in equilibrium

FRC represents resting vol of respiratory system

what happens if mm are too weak to gen force for inspiration?

less O2/no O2 transport to body, would require mechanical ventilation

what if compliance is decr by disease?

takes more effort to inspire

what if intrapleural psi becomes positive?

lungs collapse

what if elastic recoil of lung tissue is damaged by disease?

incr residual vol→harder to expire

what if integrity of chest wall is damaged by trauma?

lose mm integrity and support

airflow resistance

affected by length & radius of airway and air flow rate

work of breathing

mm F must be able to handle elastic, flow resistance, and inertial work

elastic fibers in lung are damaged by disease. which part of work gets harder?

expiration

chest wall becomes stiff and fibrotic. what happened to compliance? which part of work gets harder?

less compliance; inspiration

intrapleural psi is more _____ in upper lung areas and _______ as you move toward dependent areas

negative; increases

ventilation favors…

lowermost lung fields in all positions=easiest to ventilate lower lungs b/c more compliance (smaller expansion = easier “inflation”, like a balloon)

supine=posterior lung

standing/sitting upright=inferior lung

upper areas of lung have more

alveolar expansion at rest, but decreased compliance

dependent areas of lung have

less alveolar expansion at rest, but better compliance

collateral ventilation

allows air to bypass blocked alveoli (due to mucus, tumor, etc.)

diffusion

exchange of O2 from alveoli and CO2 from blood

occurs due to a pressure gradient btwn gas in air and gas in blood

gases move from high psi to low psi

pp of oxygen: inspired air

159 mmHg

pp of oxygen: alveoli (don’t memorize these numbers, just know where change occurs and that one area is higher/lower than another)

100 mmHg

pp of oxygen: venous blood

40 mmHg

pp of carbon dioxide: venous blood

46 mmHg

pp of carbon dioxide: alveoli

40 mmHg

which diffuses more rapidly, CO2 or O2?

CO2, but both are quick

factors that impair diffusion

reduced surface area (less interaction btwn air and blood)

thicker membrane tissue (farther for gases to travel)

reduced pressure gradient (pp are not significantly different btwn air and blood)

perfusion

blood circulates from R ventricle→pulmonary artery, arterioles, then pulmonary capillaries where it surrounds alveoli

blood is oxygenated via diffusion, returns to L heart via pulmonary veins

low psi system w/ thin vessel walls compared to systemic circ

gravity pulls more blood flow to dependent area

perfusion in upright position

lung apex has low perfusion, base has high perfusion

V-Q matching

oxygenation can only take place in areas that have both good ventilation and good perfusion

best matching occurs in mid-lower lung zones

ventilation-perfusion ratio (V/Q)

index of match btwn alveolar ventilation and pulmonary blood flow

averaged across entire lung is ~0.8

upright V-Q matching: independent zone

high resting V in alveoli compresses capillaries→less blood flow

ventilation much greater than perfusion

V/Q>1

upright V-Q matching: middle zone

ventilation and perfusion are similar

V/Q ~1

upright V-Q matching: dependent zone

high V of blood in capillaries compresses alveoli→more blood flow, less ventilation

perfusion much greater than ventilation

V/Q <1

physiological dead space

high ventilation, low perfusion (V/Q>1)

air is moving, but not enough blood present for gas exchange

shunt

low ventilation, high perfusion (V/Q<1)

blood is moving through, but no air is available for adequate pickup of O2 and drop off of CO2

pathology causing V/Q mismatch

physiologic dead space (ex. due to blood clot)

shunt (ex. aspiration/blockage of lung)

best V-Q matching position

upright

maximizes lung volumes and capacities

supine position causes

decreased lung and chest wall compliance

increased airway resistance and airway closure

increased work of breathing

decreased cough effectiveness

increased venous return, increased workload on R side of heart