Pulmonary Physiology I Flashcards
What are the two vital purposes of the lungs?
- ) Gas exchange
2. ) Role of ventilation in controlling blood pH
The exchange of O2 for CO2 occurs within structures called
Alveoli
In order for air to reach the alveoli, which path does it have to travel?
Trachea, bronchi, bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts, and finally alevoli
Resistance to flow is inversely proportional to
Cross-sectional area of the vessel
Airway resistance decreases as air approaches the
Alveoli
The ducts which compromise the bronchial tree are ringed with
Smooth muscle (under autonomic control)
We are negative pressure breathers. What this means is that under physiologic conditions, a negative pressure differential between the atmosphere and the alveoli allows for
Movement of air into the lungs
There are opposing elastic recoil forces inherent to the
Chest wall and alveoli
The chest wall wants to recoil
Outward (expand)
The alveoli want to recoil
Inward (Collapse)
Alveoli and the chest wall are separated by membranes which create an
Intrapleural space
At resting lung volumes, what is the
- ) Intra-alveolar pressure
- ) Intrapleural pressure
- ) 0 cm H2O
2. ) some negative value usually around -5 cm H2O
The difference between the intra-alveolar pressure and the intrapleural pressure creates the
Transpulmonary (transmural) pressure of 0 - (-5) = 5cm H2O
The transpulmonary pressure is sufficient to provide a slight outward tug on the alveoli and keep them from
Collapsing
Occurs as a result of chest wall expansion which causes the intrapleural pressure to become more negative, thereby increasing the transpulmonary pressure
Inspiration
This pulls on the alveoli essentially from all directions, increasing alveolar diameter and dropping
Intra-alveolar pressure below zero
Boyles law tells us that pressure is inversely proportional to
Volume
By convention, we assume that unless told otherwise that the atmospheric pressure equals
Zero
Thus a pressure gradient exists favoring the movement of air into the
Alveoli
Inspiration is an active process that requires contraction of the
Diaphragm
A normal non-stressed expiration is passive, owing to the relaxation of the
Diaphragm
Chest wall collapse causes the intrapleural pressure to become
Less negative (or even very positive with a forced expiration)
This alleviates the outward pull on the
Alveoli
Alveolar diameter then decreases, thus elevating
Intra-alveolar pressure
Once alveolar pressure supercedes that of atmospheric pressure, air moves
Outward (i.e. down the pressure gradient)
Keeps the lungs somewhat expanded during ventilation
Transpulmonary pressure
What happens if air is allowed to enter the intrapleural space?
Intrapleural pressure reaches or exceeds that of atmospheric pressure. Lung can collapse
Often caused by a chest wall trauma which creates open communication between the atmosphere and the intrapleural space
Simple pneumothorax
During a simple pneumothorax, as the chest wall expands during inhalation, the intrapleural pressure does not decrease in the affected
Hemithorax
Thus, the normal driving force for alveolar expansion (transpulmonary pressure) is not
Generated
Thus, as a result of the loss of the normal tug of war that exists between lung elastic recoil and chest wall expansion, we will eventually see
Lung collapse in the affected hemithorax
Results from some internal insult to the visceral or parietal pleura, or some region of the tracheobronchial tree
Tension pneumothorax
The nature of this damage is such that a one-way valve is created which enables
Air to enter but not exit the pleural space
Over time, intrapleural pressures rise which causes resistance to
Lung inflation
This will likely cause
collapse of lung ipsilateral to the region of injury
The pressure increase in the injured hemithorax can then cause deviation of the trachea toward the
Contralateral hemithorax
In addition to this deviation, we can also see compression of the
Heart, vena cava, and contralateral lung
If severe enough, venous return and cardiac output can be compromised so as to result in
Hypotension and hemodynamic instability
Defined as the change in lung volume (L) divided by the change in transpulmonary pressure (cm H2O)
Lung Compliance
An indicator of how efficiently the lung inflates and deflates (i.e. the work of breathing)
Lung compliance
When plotted using an X-Y graph, where the X-axis is transpulmonary pressure and the Y-axis represents lung volume, a very compliant lung is shown by a
Steep compliance curve (relative to normal)
In other words, in a very compliant lung, a large change in volume occurs with a
Small change in pressure
A steep compliance curve is clearly seen when dealing with
Emphysema
-a manifestation of COPD
Why is too compliant of a lung a bad thing?
Harder to expel air
Shows that a much greater change in pressure is required to move a given volume
Flat compliance curve
A flat compliance curve is clearly observed in
Lung fibrosis (a form of restrictive lung disease)
Abnormally high or abnormally low lung compliance both increase the work of
Breathing
The sound that is heard due to vibrations of the ventricular and large vessel walls due to recoil of arterial and ventricular blood against the semilunar valve leaflets
S2
Normal conditions where A2-P2 are fused on expiration but split during inspiration
Physiologic splitting
Occurs due to the decreased intrathoracic pressure that is generated during inspiration
Physiologic splitting
Decreased intrathoracic pressure allows for increased venous return to the right atrium, which in fact prolongs
Diastolic filling of RV
This causes a lengthened systolic ejection from the RV and thus we see a delay in closure of th
Pulmonic valve
Also low intrathoracic pressure increases capitance of the
Pulmonary arteries and veins
Increased capitance of the pulmonary arteries and veins allows more blood to flow into the pulmonary system this also results in delayed closure of the
Pulmonic valve
As mentioned, the decrease in intrathoracic pressure lowers pulmonary vein pressure and reduces left heart diastolic filling. With less diastolic volume, we see faster closing of the
Aortic semilunar valve
Reduced left ventricular output during inspiration slightly lowers
Systolic BP
During expiration, we see an increase in
Intrathoracic pressure
The increase in intrathoracic pressure from expiration causes venous return to the right heart to be
Reduced
When venous return to the right atrium is reduced, we get quicker RV output and thus an earlier
P2
The so-called respiratory centers are housed within the
Medulla
These respiratory centers consist of several regions that include the
Dorsal respiratory group (DRG) and Ventral respiratory group (VRG)
Largely responsible for coordinating inspiratory function
DRG
Assumes control over both inspiration and expiration
VRG
The rythm of respiration is due to the highly coordinated activation/inactivation of respiratory neurons within the
Pre-Bötzinger complex of the VRG
Axons from the DRG and VRG descend the spinal cord and activate neural tracts which ultimately innervate the
Muscles of respiraiton
Which motor neurons mediate inspiration?
Phrenic and external intercoastal
Contraction flattens and lowers the diaphragm, thus increasing the volume of the
Thoracic cavity
This is complemented by contraction of so-called accessory muscles which include the
External intercostals and sternocleidomastoid
Raise and enlarge the rib cage
External intercostals
Elevates the sternum
Sternocliedomastoid
Expiration is normally
Passive
However, contraction of the internal intercostals, rectus abdominis, internal and external obliques, and transversus abdominis muscles support
Forceful expiration
Bronchial smooth muscle is innervated by the
Vagus Nerve (X)
Vagal innervation of bronchial smooth muscle enables PARASYMPATHETIC nervous system dependent
Constriction of bronchial smooth muscle
Binds to the type 3 cholinergic-muscarinic receptors expressed in bronchial smooth muscle
Acetylcholine
In addition to type 3 cholinergic-muscarinic receptors, bronchial smooth muscle also expresses
Type B2 adrenergic receptors
Within bronchial smooth muscle, the activation of B2 adrenoreceptors induces
Dilation (opens airway)
Albuterol and salmeterol are B2 agonists that are used in the treatment of
Asthma and COPD
Since bronchial smooth muscle is not innervated by the SNS, SNS-dependent bronchiolar dilation relies upon
Circulating epinephrine
Responsible for obtaining atmospheric O2 and eliminating CO2
Lungs
This process is known as
Ventilation or respiration
Gas exchange within the lungs occurs at the level of the
Alveolus
A sac-like structure located within the respiratory zones of the lungs
-the terminal point reached by inspired air
Alveolus
The alveoli are surrounded by
Capillaries
Capillary blood flow ot alveoli is known as
Perfusion (Q)
Air flwo to and from alveoli is known as
Ventilation (V)
The amount of air traversing the nose and/or mouth with each breath
Minute ventilation
Does the entire volume of air that is inspired with each breath reach the alveoli?
No
Thus, alveolar ventilation is less than the
Minute volume
For this reason, the conducting pathways (where gas exchange does not occur) are referred to as the
Anatomic dead space
The other type of dead space is known as the
Alveolar dead space
Any significant amount of alveolar dead space is pathologic. It develops when there are regions of
Ventilated but not perfused alveoli
A person with impaired cardiac output may be likely to develop significant
Alveolar dead space
Simply the sum of anatomic plus alveolar dead spaces
Physiologic dead space
A healthy adult has approximately how much dead space per pound of body weight?
1mL per pound
Transfer of gas across blood-gas barrier and blood-tissue barrier occur via diffusion as regulated by
Fick’s Law
Fick’s law shows that the amount of gas transferred is proportional to what three things?
- ) Area of tissue layer
- ) Diffusion constant
- ) Difference in partial pressure (P1-P2)
Fick’s law shows that the amount of gas transferred is inversely proportional to the
Tickness
Proportional to the gas solubility but inversely proportional to the square root of it’s molecular weight
Diffusion constant
About 20 times more soluble in liquids than H2O
CO2
