Mechanics of Breathing Flashcards
define pulmonary circulation.
in pulmonary circulation deoxygenated blood from right ventricle passes to lungs via pulmonary arteries. Blood is oxygenated in pulmonary capillaries which join to form pulmonary veins. Pulmonary veins carry oxygenated blood from lungs to left atrium
Describe the characteristic features of pulmonary circulation.
1) compliance of pulmonary vessels
- thin walled
- has short and wide branches
- have larger diameter than the systemic circulation
- i.e pulmonary arterial system is thin walled and distensible
- because of this pulmonary arterial tree has large distensibility than that of the entire systemic tree.
- allows to accomodate 2/3rds pf the stroke volume output of the right ventricle
2) pressure in pulmonary circulation - In pulmonary artery: - systemic pressure - 25 mmHg - diastolic pressure - 8 mmHg - pulse pressure - 17 mmHg - mean pressure - 15 mmHg - Pulmonary capillary pressure - 7 mmHg. The mean pressure in major pulmonary veins and left atrium is 2 mmHg in recumbent position
3) blood volume of lungs: from 225 to 900 ml.
Failure of left side of heart causes blood to dam up in pulmonary circulation, leading to corresponding inc in pulmonary vascular pressure.
4) blood flow to lungs: equal to cardiac output.
all factors affecting cardiac output also affect the pulmonary blood flow
5) effect of hypoxia:
When conc of O2 in alveoli dec - adjacent blood vessels slowly constrict within 3 to 10 minutes.
opposite to the effect normally observed in systemic vessels which dilate in response to low O2
- response probably due to
release of some vasoconstrictor substance from alveolar epithelial cells in response to hypoxia.
6) effect of hydrostatic pressure on blood flow through lungs:
in standing position
- pulmonary arterial pressure in uppermost portion of lungs - 15 mm hg less than pulmonary arterial pressure at the level of heart
- pap in lowermost portion of lungs - 8 mm hg greater than
- resulting in lesser blood flow in the upper parts of the lungs and greater blood flow in the lower parts
- Pulmonary capillaries are distended by blood pressure and are compressed by
alveolar air pressure. If alveolar pressure greater than capillary blood pressure -
the capillaries close - no blood flow.
Lung therefore has three different zones of blood fl ow.
Zone 1. No blood flow at all during any part of cardiac cycle
Zone 2: Intermittent fl ow of blood in systole of heart
Zone 3: Continuous blood fl ow because pulmonary capillary pressure remains
greater than alveolar pressure during the entire cardiac cycle.
normal lungs - zone 2 & 3
7) pulmonary capillary dynamics:
net filtration pressure - very small quantity of fluid flow out from capillaries into pulmonary interstitium and alveoli.
- Because of negative interstitial fluid pressure, if fluid enters alveoli, it is sucked into interstitium through small openings between epithelial cells of alveoli. Thus normally alveoli are kept dry except for a small quantity of fluid on their surface to keep them moist.
po2 and co2 levels of pulmonary capillary blood
po2= 40 mm of hg pco2= 45 mm of hg
describe mechanism of respiration
inspiration
- respiratory centres initiate the stimuli for inspiration
- these impulses are carried via nerves to the inspiratory muscles
- the diaphragm (and/or other inspiratory muscles) contract
- the chest wall expands increasing the volume of thorax
- the intrapleural pressure becomes more negative
- the alveolar pressure decreases below the atmospheric level
- the alveoli inflate as air flows into them until the alveolar pressure reaches atmospheric pressure
expiration:
- respiratory centres terminate the inspiratory impulses
- the diaphragm (and/or other expiratory muscles) relax
- the chest wall retracts and decreases the volume of the thorax
- the intrapleural pressure becomes less negative
- the alveolar pressure increases above the atmospheric pressure
- the alveoli undergo elastic recoil forcing air out
- the alveoli deflate as air flows out until the alveolar pressure reaches atmospheric pressure.
intrapleural pressure variation during normal tidal Respiration
-5 cm of h2o to -7.5cm of h2o
compliance of lungs
- compliance means the ability to stretch (stretchability) or the ability to recoil
- extent of lung expansion for each unit increase in transpulmonary pressure (if enough time is allowed to reach equilibrium)
- normal compliance- 200 ml / 1 cm h2o
meaning - every time the transpulmonary pressure increases by 1 cm h2o the lung volume, after 10-20 seconds will expand 200 ml - compliance- ∆v/∆p
compliance curve
the two curves are called inspiratory compliance curve and expiratory compliance curve and the entire diagram is called compliance diagram of the lungs.
the characteristics of the diagram are determined by elastic forces of lungs.
1) elastic forces of lung tissue (due to elastin and collagen fibres interwoven among the lung parenchyma)
- deflated lungs - elastically contracted and kinked state
- expanded lungs - stretched and unkinked state
thereby elongating and exerting even more elastic force
2) elastic forces caused by surface tension of the fluid that lines the inside walls of the alveoli and other lung air spaces.
- the fluid-air surface tension forces in the alveoli represent about two-thirds of the total lung elasticity.
regional variation:
- basal region - gravity dependent hence more ventilation and less negative intrapleural pressure
- this results in upper region alveoli having larger volumes. less compliant as compared to those in the dependent regions, which are more compliant and are able to increase their volume with each breath.
lung compliance dec - fibrosis, silicosis and tuberculosis
lung compliance inc - emphysema
surface tension.
force that tends to minimize the surface area at the interface is known as surface tension.
caused due to unbalanced attraction of surface molecules by the molecules below the surface.
- In lungs, surface tension at the fluid-air interface of alveolus has a tendency to reduce the size of each alveolus, thus resulting into recoil tendency of the
lung.
lung volumes and capacities
static and dynamic
1) static
tidal volume - 500 ml
vol of air inspired or expired with each normal breath
IRV 3000 ml
air inspired with a maximal inspiratory effort in excess of the normal tidal inspiratory volume
ERV 1100 ml
air expired with a maximal expiratory effort in excess of the normal tidal expiratory volume
residual volume 1200 ml
vol of air remaining in the lungs at the end of maximal expiratory effort
2) dynamic
times vital capacity (FEV1 or forced expiratory volume in 1 second)
vol of air that can be expired rapidly in 1 second with a max. force following a max. inspiration
72-85%
95%
98-100%
fvc - forced vital capacity- total vol of air that can be expired with greatest force and speed after a maximal inspiration.
most useful test to detect generalised airway obstruction and differentiate between obstructive and restrictive diseases
in obstructive- fev1 percentage dec
in restrictive - fvc dec fev1 as a percentage of fvc remains the same
- maximum mid expiratory flow rate
max flow achieved during the middle third of the total expired volume.
FEF25-75%
indicates patency of small airways
between 200 ml - 1200 ml in liters per second indicates patency of larger airways. - maximum voluntary ventilation
max breathing capacity
max vol of gas that can be breathed per minute by maximal voluntary effort
peak expiratory flow rate max velocity of flow in liters per minute. pefr - peak flow meter index of patency of airways however mmefr more sensitive indicator
lung capacities
inspiratory capacity - 3500 ml
max vol of air that can be inspired after normal tidal expiration
expiratory capacity (1600 ml) max vol of air that can be expired after normal tidal inspiration
functional residual capacity (2300 ml)
amt of air that remains in the lungs at the end of normal expiration
importance
- continuous exchange is possible
- breath holding is made possible
- dilution of toxic inhaled gases
- load on the respiratory mechanism and left ventricle is decreased
vital capacity - 4600 ml
max amt of air that can be expired after a maximal inspiratory effort.
significance
- assessment of max inspiratory expiratory effort
- info on strength of respiratory m
- info on other aspects of pulmonary functions like fvc.
factors affecting: size of thoracic cavity (more in males)
age old age vc is dec
strength of respiratory m - swimmers divers inc
gravity - standing vc more
pregnancy - dec due to pushing of diaphragm
diseases - dec
total lung capacity - 5800 ml
vol of air present in the lungs at the end of maximal inspiration
ic - irv + tv ec - erv + tv vc - irv + erv + tv frc - erv + rv tlc - irv + erv + tv + rv
closing volume
refers to the lung vol that coincides with the beginning of airway closure in the dependent parts of the lungs during expiration. in normal. healthy individuals, the closing volume is less than frc and more than rv.
measurement - using the same apparatus used for anatomical dead space air.
diffusion capacity of lung for carbon monoxide
technique used to measure the capacity of the lungs to transfer gas that is inhaled to the red blood cells in pulmonary capillaries.
the o2 diffusing capacity can be calculated by measuring
1) alveolar po2
2) po2 in pulmonary capillary blood
3) rate of o2 uptake by the blood
however po2 in pulmonary capillary blood is very difficult and imprecise so we measure carbon monoxide diffusing capacity instead and then calculate
the principle of the co method is:
- a small amt of co is breathed into the alveoli and the pp of co in the alveoli is measured
- co pressure in blood is essentially zero cuz hb combines so rapidly with co that it never has the time to build up
- therefore, the pressure difference of co across the respiratory membrane is equal to its pp in the alveolar air sample
- then measuring vol of co absorbed in a short period and dividing this by the alveolar co partial pressure —> co diffusing capacity
to convert co diffusing capacity to o2 - multiply by 1.23 as the diffusion coefficient for o2 is 1.23 times that of co.
thus avg diffusing capacity for co - 17 ml/min/mm hg
and o2 - 21 ml/min/mm hg
test - single breath method
during strenuous exercise - o2 - 65 ml/min/mm hg
work of breathing
1) required to expand the lungs against the lung and chest elastic forces called compliance work or elastic work
2) required to overcome the viscosity of the lung and chest wall structures called tissue resistance work
3) required to overcome airway resistance to movement of air into the lungs called airway resistance work.
restrictive lung disease economize their ventilation by
taking rapid and shallow breaths
obstructive lung diseases economize their ventilation
by taking slow and deep breaths
