Respiratory Physiology: Mechanisms II Flashcards
What three opposing force must be overcome for inspiration to occur?
- Elastic recoil
- Inertance or impedance of acceleration of the respiratory system
- Frictional resistance
_______________ is determined by airflow and not by a change in lung volume.
Frictional resistance
The average velocity of air is equal to
Overall flow rate/ Cross-sectional area
Which type of airflow is being described below?
Airflow occurs at low flow rates and in a pattern that is parallel to the walls of the cylinder.
Laminar
Which type of airflow is being described below?
Velocity of airflow increases at points of bronchial division- the flow stream becomes unstable and eddy currents cause distibances in flow.
Transitional flow
Which type of airflow is being described below?
Increased flow rates; currents parallel and perpendicular to the walls of the tube
Turbulent flow
A reynolds number greater than _______ results in turbulent flow.
2000
Which regions are more likey to experience turbulent flow?
Conducting zone larger airways
Example: Nose, trachea, bronchi
NOTE: Small airways of gas exhange have laminar flow
How is turbelent flow produced?
By the friction between moving air and the wall of the tube. As flow increases the eddy currents occur at transitional points
What is the relationship of gas viscocity and driving pressure in cases of laminar flow, turbelent flow, and transitional flow, respectively?
Laminar Flow
- Driving pressure is directly proportional to gas viscocity
Turbulent flow
- Driving pressure is proportional to square of flow and is dependent of gas density
Transitional flow
- Driving pressure is proportional to both gas density and gas viscocity
How is reynold number calculated?
r= radius
v= average velocity
d=density
η= viscosity
Flow is mostly ___________throughout the bronchial tree
transitional
What is one reason why identifying small airway disease is challenging?
Laminar flow, which is present in small airways, is silent.
NOTE: Breathing sounds heard with a stethoscope are due to turbulent flow
Stridor
Noisy breathing with high-pitched crowning sound
What is the cause for stridor?
Turbelent air currents attempt to force their way through narrow breathing passages
Laryngomalacia
Soft, immature catilage of the upper larynz collapses inward during inhalation, causing airway obstruction
NOTE: Laryngomalacia is one of the most common causes of chronic stridor
How is resistance calculated?
Airway resistance
Frictional resistance of the entire airway from the tip of the nose (or mouth) to the alveoli
Pulmonary Resistance
Frictional resistance afforded by the lungs and airways combined
Chest Wall Resistance
Frictional resistance of the chest wall and abdominal structures
Are bronchioles in parallel or series?
Parallel
How can resistance of a lower airway be calculated?
R= Rlarge + Rmedium + Rsmall
NOTE: The 1st eight genetations of the conductin system (trachea, bronchi) provide the majority of resitance in the airways. Small airway DO NOT make a significant contribution
Do small airways make a significant contribution to the overall total airway resistance of the bronchial tree?
No because the airflow velocity is very low due to the increased cross-sectional area. Also, because bronchioles are in parallel not in series.
NOTE: An individual bronchiole has a high resistance but in total the resistance is small.
Which component of the lower airway provides the greatest resistance?
Bronchi
What factors increase airway resistance?
Airway mucus
Edema
Contraction of the bronchiole smooth muscle
Density and viscosity of the moving gas
NOTE: There is a slightly higher airway resistance during exhalation, than during inhalation, because the flow rate is slightly slower.
Airways lengthen during _________ (inspiration/expiration) and shorten during ______ (inspiration/ expiration) with the phases of respiration.
Lengthen; Shorten
What factors contribute to airway resistance?
Number, length, and cross-sectional area
NOTE: Of these factors, cross-sectional area is the most important because resistance is inversely proportional to the 4th power of the radius.
What are the two opposing forces that control airway size?
- Smooth muscle and elastic forces
- Tend to constrict airways and decrease the cross-sectional area
- Outward traction of the airway
- Due to the interdependence of alveoli and terminal bronchioles and because of positive transpulmonary pressure
Conductance
1/ Airway resistance
_____________ (increased/decreased) gas density increases airway resistance.
Increased
Status Asthmaticus
A clinical condition characterized by bronchospasm, edema, and mucus in the airway resulting in increased airway resistance
Treatment for Status Asthmaticus
O2-He low density gas mixture
How does elastic recoil affect the caliber of the airway?
- Direct traction on small intapulmonary airways
- Effects on intrapleural pressure
At FRC, what is the normal airway resistance?
1-3 c, H2O/ L.sec
NOTE: Airway resistance is higher in children due to small overall airway size
Which neurohumoral molecules cause vasoconstriction of the airway smooth muscles?
Histamine
Ach
Leukotrienes
NOTE: These molecules are released from mast/epithelial cells
_____________ (parasympathetic/sympathetic) nervous system causes constriction of airways, while __________ nervous system causes dilation of airways.
Parasympathetic; sympathetic
What drug can be used to access airway hyperresponsiveness?
Metacholine (a histamine-like compound)
Parasympathetic leads to airway __________ (constriction/relaxation), blood vessel _________ (dilation/ constriction) and __________ (increased/decreased) glandular secretion.
constriction; dilation; increased
Sympathetic leads to airway __________ (constriction/relaxation), blood vessel _________ (dilation/ constriction) and __________ (increased/decreased) glandular secretion.
Relaxation; constriction, decreased
Which factors can lead to constriction of airways?
Parasympathetic stimulation
Acetylcholine
Histamine
Leukotrienes
Thromboxane A2
Serotonin
a-adrenergic agonists
Decreased PCO2 in small airways
Which factors can lead to dilation of airways?
Sympathetic stimulation B2 receptors
Circulating B2 agonists
NO
Increased PCO2 in small airways
Decreased PCO2 in small airways
Dynamic compliance
The change in volume of the lungs divided by the distending pressure during the course of a breath.
How is surface tension related to compliance?
An increase in surface tension results in a decrease in compliance. This is because an increase in surface tension leads to an increase in pressure. Rememeber, pressure and compliance are inversely proportional, so an increase in pressure would result in a decrease in compliance.
Is surfactant more available at larger volumes or lower volumes?
Larger. This facilitates expansion of the lungs. Less surfactant is available during tidal volume.
REMEMBER: Tidal volume is the lung volume representing the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied.
What is the relationship between compliance, resistance, and volume?
- If two tubes have the same compliance but a different resistance, the volume does not change, if both are given sufficient time to fill
- If two tubes have difference compliances, the volume will change
NOTE: Increased respiratory frequency in patients with small airway obstruction do not allow insufficient time to fill and will not contribute to dynamic compliance
How do sighs and yawns help to increase dynamic compliance?
By restoring the surfactant layer during the increase tidal breath
Explain expiratory flow limitations.
During expiration, as the lung volume decreases so does the flow rate.
Why is expiratory flow limited?
This occurs when the floppy distensible tubes become compressed during normal expiration. Pressure outside the airway exceeds to pressure inside the airway
How and when expiratory flow limitation occurs is important in various disease states, such as Chronic Obstructive Pulmonary Disease. Why?
- During normal conditions, collapse near tubes with cartilage; dynamic airway compression
- In disease state, collapse occurs near small airways devoid of cartilage, leading to premature airway closure
The cross-sectional area of the airways decreases toward the trachea and the velocity _________ (increases/decreases)
Increase
What happens to tracheal dimensions and air velocity during cough?
- At start of a cough, subject takes a deep breath ~1.5 time VT
- Causing an increase in elastic recoil; the glottis closes and chest wall muscle contract causing an increase in pleural pressure.
- The glottis opens and air is forcefully expelled.
- During cough, the non-cartilaginous posterior membranous region of the trachea is compressed causing tracheal diameter to narrow.
- The turbulent airflow at the site of compression is the sound that we refer to as “cough”.
How does the relationship between airway closure and volume levels differ between healthy subjects and subects with COPD?
In healthy subjects, airway closure occurs at low volumes. In subjects with COPD, airway closure occurs at much higher volumes.
What are the two components that contribute to the work of breathing?
-
Elastic component
- Overcoming the recoil of the chest wall and lung parenchyma
- Work to overcome the surface tension
-
Resistive component
- Tissue (lung) and airway resistance
How is work of breathing calculated?
W= Pressure x Change in volume
- Pressure is the change in transpulmonary pressure required to overcome the elastic and resistance work of breathing
- Volume the volume of gas moving into and out of the lung
Mechanical work of a normal lung
Mechanical work in a lung with reduced compliance
Mechanical work associated with increased airway resistance
Breathing pattern associated with pulmonary fibrosis?
Shallow/rapid pattern
Breathing pattern associated with obstructive pulmonary disease
Breath more deeply and slowly
Normal respiratory rate
breaths/min-
