Mechanics, Ventilation, and Blood Flow Flashcards
general functions of the lungs
- ventilation - air comes in and out of the lungs
- gas exchange - O2 and CO2 move in and out of the blood
- oxygen delivery - O2 moves into the tissues
- acid elimination - exhalation of CO2
- regulation - how the amount of ventilation is adjusted
anatomy of the airways - upper airway
“air conditioning”: warms, humidifies, and filters the air
inhalation & the muscles involved
*inhalation is an ACTIVE process requiring muscle contraction
*the DIAPHRAGM is the major muscle of inhalation: the diaphragm contracts downward, allowing the lungs to inflate because the pressure is decreased
*external intercostals also contribute to inhalation by pulling up and out
tidal volume
air that moves into lung with each quiet respiration
inspiratory reserve volume (IRV)
air that can still be breathed IN after normal inspiration
expiratory reserve volume (ERV)
air that can still be breathed OUT after normal expiration
residual volume (RV)
air in lung AFTER MAXIMAL EXPIRATION
inspiratory capacity
*inspiratory reserve volume + tidal volume (IRV + Vt)
*total air that can be breathed in (normal inspiration + max inspiration)
functional residual capacity
*residual volume + expiratory reserve volume (RV + ERV)
*volume of gas in lungs after normal expiration; outward pulling force of chest wall is balanced with inward collapsing force of lungs
vital capacity
*inspiratory reserve volume + tidal volume + expiratory reserve volume (IRV + Vt + ERV)
*maximum volume of gas that can be expired after a maximal inspiration
total lung capacity
*IRV + Vt + ERV + RV = VC + RV
*volume of gas present in lungs after maximal inspiration
work of breathing
*refers to the energy expended or O2 consumed by respiratory muscles to produce the ventilation needed to meet the body’s metabolic demand
*comprises the work needed to overcome both elastic recoil and airway resistance
*minimized by optimizing respiratory rate and tidal volume
*work of breathing is increased in both restrictive and obstructive diseases
respiratory system compliance
*describes the ease of expanding the lungs
*compliance = change in volume / change in pressure
*normal compliance = 80-100 mL/cmH20
*HIGH compliance: means it requires smaller pressure changes to expand the lungs (“easy breathing”)
*LOW compliance: means it requires larger pressure changes to expand the lungs (“hard breathing”)
why is force needed to stretch the lungs
- architecture: elastic fibers in the lungs
- surface tension: air/tissue or air/water interface is unstable
end result - the lungs want to collapse; we need force to keep them open
hysteresis
*lung inflation follows a different pressure-volume curve than lung deflation, due to the need to overcome surface tension forces in inflation
pulmonary surfactant
*a hydrophobic mix of phospholipids and proteins, produced by type II alveolar cells (type II pneumocytes), that:
1. causes hysteresis of the volume-pressure curve
2. stabilizes alveolar size
3. reduces work of breathing
4. keeps alveoli dry
5. DECREASES ALVEOLAR SURFACE TENSION
overall - surfactant prevents collapse of alveoli
neonatal respiratory distress syndrome
*surfactant deficiency → increased surface tension → alveolar collapse
*most commonly occurs in premature babies
lecithin:spingomyelin ratio
*a screening test used to assess fetal lung maturity via amniocentesis (L:S ratio of 2.2 or more indicates adequate lung maturity)
*if test is low, then administration of antenatal corticosteroids can prompt increased surfactant production to enhance lung maturity (if pre-term delivery is expected)
airflow resistance and its affect on the work of breathing
*resistance changes with flow characteristics (laminar vs. turbulent flow):
-laminar flow: pressure and flow are linearly related
-turbulent flow: pressure and flow are exponentially related
*it takes MORE PRESSURE to achieve the same flow rate with turbulent flow (the resistance is greater with turbulent flow)
minute ventilation (VE)
*total volume of gas entering the lungs per minute
*VE = Vt x RR [minute ventilation = tidal volume x respiratory rate)
physiologic dead space
*anatomic dead space (where no alveoli are present; i.e. the conducting zone) PLUS alveoli that don’t receive blood flow
*apex of healthy lung is largest contributor of alveolar dead space
*normal: 0.2-0.3
formula for calculation of physiologic dead space (Vd)
Vd = Vt x [(PaCO2 - PeCO2) / PaCO2]
PaCO2: arterial carbon dioxide pressure
PeCO2: exhaled carbon dioxide pressure
Vt = tidal volume
alveolar gas equation
*allows the calculation of PAO2 (alveolar oxygen pressure):
PAO2 = fIO2(Pb - 47) - (PaCO2/R)
fIO2: fraction of inspired oxygen (how much oxygen is in the air we breathe; usually 21%)
Pb: barometric pressure (760 mmHg at sea level)
PaCO2: arterial carbon dioxide pressure
*R: respiratory quotient (CO2 produced/O2 consumed)
A-a gradient
*refers to the difference between the alveolar (A) and arterial (a) oxygen pressures
*A-a gradient = PAO2 - PaO2 [i.e. alveolar oxygen pressure minus arterial oxygen pressure]
*normal A-a gradient is 0-5 mmHg
*the small normal gradient occurs partly because of oxygen-poor blood that enters the left ventricle directly through THEBESIAN VEINS
*note - A-a gradient generally INCREASES with normal aging
pulmonary circulation
*pulmonary arteries (carry blood AWAY from the heart, toward the lungs) contain oxygen-POOR blood
*pulmonary veins (carry blood TOWARD the heart after it receives oxygen in the lungs) contain oxygen-RICH blood
note - pressure is much lower in pulmonary circulation than it is in systemic circulation
bronchial circulation
*separate from pulmonary artery circulation
*under systemic arterial pressure
*supplies blood to the actual tissues of the respiratory system (bronchi, conducting airways, etc)
*1-3% of total pulmonary blood flow
*not normally significant in gas exchange, BUT: often the culprit in massive hemoptysis
systemic vascular resistance = ?
= (mean aortic pressure - mean vena cava pressure) / cardiac output
pulmonary vascular resistance = ?
= (mean pulmonary artery pressure - pulmonary capillary wedge pressure) / cardiac output
= (mPAP - PCWP) / CO
how does pressure in the pulmonary circulation remain low, despite the lungs receiving 100% of cardiac output?
*recruitment (as pressure increases, more capillaries participate in receiving blood flow)
*distention (as pressure increases, the capillaries increase in size to accommodate more blood flow)
*overall - recruitment and distention maintain the low pressure of pulmonary circulation
normal ventilation and perfusion gradients
*blood flow is affected by gravity (the bottom of the lung has the most blood flow)
*zone 1 = top of lungs (PA > Pa >Pv)
*zone 2 = middle of lungs (Pa > PA > Pv); ideal physiology
*zone 3 = bottom of lungs (Pa > Pv >Pa)
note - Pa (arterial pressure); PA (alveolar pressure); Pv (venous pressure)
normal V/Q ratio distribution
*bottom of lungs receives most blood flow
*bottom of lungs receives most ventilation
*TOP OF LUNGS has the highest V/Q ratio (because there is very little blood flow there)
what is the V/Q ratio
*the ratio between ventilation and blood flow
