Mechanics, Ventilation, and Blood Flow

Question 1

general functions of the lungs

Answer 1

1. ventilation - air comes in and out of the lungs
2. gas exchange - O2 and CO2 move in and out of the blood
3. oxygen delivery - O2 moves into the tissues
4. acid elimination - exhalation of CO2
5. regulation - how the amount of ventilation is adjusted

Question 2

anatomy of the airways - upper airway

Answer 2

“air conditioning”: warms, humidifies, and filters the air

Question 3

inhalation & the muscles involved

Answer 3

*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

Question 4

tidal volume

Answer 4

air that moves into lung with each quiet respiration

Question 5

inspiratory reserve volume (IRV)

Answer 5

air that can still be breathed IN after normal inspiration

Question 6

expiratory reserve volume (ERV)

Answer 6

air that can still be breathed OUT after normal expiration

Question 7

residual volume (RV)

Answer 7

air in lung AFTER MAXIMAL EXPIRATION

Question 8

inspiratory capacity

Answer 8

*inspiratory reserve volume + tidal volume (IRV + Vt)
*total air that can be breathed in (normal inspiration + max inspiration)

Question 9

functional residual capacity

Answer 9

*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

Question 10

vital capacity

Answer 10

*inspiratory reserve volume + tidal volume + expiratory reserve volume (IRV + Vt + ERV)
*maximum volume of gas that can be expired after a maximal inspiration

Question 11

total lung capacity

Answer 11

*IRV + Vt + ERV + RV = VC + RV
*volume of gas present in lungs after maximal inspiration

Question 12

work of breathing

Answer 12

*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

Question 13

respiratory system compliance

Answer 13

*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”)

Question 14

why is force needed to stretch the lungs

Answer 14

1. architecture: elastic fibers in the lungs
2. surface tension: air/tissue or air/water interface is unstable

end result - the lungs want to collapse; we need force to keep them open

Question 15

hysteresis

Answer 15

*lung inflation follows a different pressure-volume curve than lung deflation, due to the need to overcome surface tension forces in inflation

Question 16

pulmonary surfactant

Answer 16

*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

Question 17

neonatal respiratory distress syndrome

Answer 17

*surfactant deficiency → increased surface tension → alveolar collapse
*most commonly occurs in premature babies

Question 18

lecithin:spingomyelin ratio

Answer 18

*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)

Question 19

airflow resistance and its affect on the work of breathing

Answer 19

*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)

Question 20

minute ventilation (VE)

Answer 20

*total volume of gas entering the lungs per minute
*VE = Vt x RR [minute ventilation = tidal volume x respiratory rate)

Question 21

physiologic dead space

Answer 21

*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

Question 22

formula for calculation of physiologic dead space (Vd)

Answer 22

Vd = Vt x [(PaCO2 - PeCO2) / PaCO2]

PaCO2: arterial carbon dioxide pressure
PeCO2: exhaled carbon dioxide pressure
Vt = tidal volume

Question 23

alveolar gas equation

Answer 23

*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)

Question 24

A-a gradient

Answer 24

*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

Question 25

pulmonary circulation

Answer 25

*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

Question 26

bronchial circulation

Answer 26

*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

Question 27

systemic vascular resistance = ?

Answer 27

= (mean aortic pressure - mean vena cava pressure) / cardiac output

Question 28

pulmonary vascular resistance = ?

Answer 28

= (mean pulmonary artery pressure - pulmonary capillary wedge pressure) / cardiac output

= (mPAP - PCWP) / CO

Question 29

how does pressure in the pulmonary circulation remain low, despite the lungs receiving 100% of cardiac output?

Answer 29

*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

Question 30

normal ventilation and perfusion gradients

Answer 30

*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)

Question 31

normal V/Q ratio distribution

Answer 31

*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)

Question 32

what is the V/Q ratio

Answer 32

*the ratio between ventilation and blood flow