Pulmonary Physiology 3 Flashcards
Diffuses much quicker than O2 but has a lower arteriole to vein gradient
CO2
Intense exercise reduces the time available for
Oxygenation
Exercise increases blood flow and thus decreases
Capillary transit time
In a healthy person with a normal environment, exercise will not change
PaO2
Exercise increases blood flow and reduces capillary transit time, normally this is not a problem unless there is a
Diffusion barrier or low PAO2 (hypoxia)
Under normal circumstances, PaO2 within the alveolar capillary reaches its maximum within approximately
0.25 seconds
This is explained by the fact that deoxy-Hb,sucha as what exists in venous RBCs has a high affinity for
O2
Once the O2 binding sites on Hb are saturated, we see an increase in
PaO2 (because PaO2 represents dissolved O2)
The alveolar-to-blood gas pressure gradient is the driving force for O2 diffusion from the
Alveolus
Once PaO2 is near 100 mmHg, a significant pressure gradient no longer exists and we see no further diffusion of O2 from the alveolus to the
Capillaries
During periods of increased cardiac output, blood moves through the capillaries more rapidly. This results in reduced
Perfusion limitation of O2 transfer
Because the rate of O2 diffusion is really fast and the formation of oxyhemoglobin occurs within 100ths of a second, the only limitation to oxygenation is the
Rate of capillary blood flow
The number of RBCs passing through a capillary per unit time
Rate of capillary blood flow
Keep in mind that in a healthy lung, total O2 transfer is elevated with increased cardiac output due to recruitment of previously non-perfused capillaries which augments the surface area for
O2 diffusion
Oxygen is carried in the blood in which two forms?
- ) Dissolved in plasma
2. ) Bound to Hb (SaO2)
Incorporates these variables plus the amount of Hb
Total blood O2 (CaO2)
The amount of gas dissolved in liquid is proportional to its partial pressure and temperature in equilibrium with
Gas
The higher the partial pressure of alveolar O2, the higher the
O2 in solution in blood
At normal body temperature (37 C) for each mmHg of PO2, there is
0.003 ml O2 / dl Blood
Thus at normal conditions, what is the mL O2 / dL blood?
0.3mL O2 / dL blood (dissolved O2)
Dissolved O2 can only meet a small portion of O2 demand, this is why it is critical that we have
Hb
A complex of Heme (iron porphyrin) with a protein globin
Hemoglobin
The oxygen binding capacity of Hb is
1.39 mL O2 / g Hb
Therefore under normal conditions, how much O2 can be carried in the arterial blood in the form of HbO2?
20.7 mL O2 / dL blood
Under normal conditions, the Hb is completely
Saturated
Oxygen saturation (SaO2) is commonly determined in the clinic with a pulse oximeter, which detects the color of
Hemoglobin
Non oxygenated Hb conformation appears
Purple (thus cyanosis is purple-blue)
Oxygenated Hb has which color?
Red
Saturation can also be used to determine concentration if you also know the
Concentration of Hb (usully 15g/dL)
If you know the O2 binding capacity and the concentration of Hb, PO2, and SaO2, you can calculate
O2 concentration
O2 concentration is proportional to PO2 in blood until
Hb binding sites are saturated
Upon saturation, the curve basically
Plateaus
In an O2 dissociation curve total CaO2 is primarily due to that complexed to
Hb
-dissolved O2 makes up only a small fraction
In an O2 dissociation curve at exceedingly low PO2, the O2 dissociation curve is
Less steep
In an O2 dissociation curve with increased PO2, the plot becomes steeper as
More O2 is bound to Hb (thus increasing the binding capacity)
In an O2 dissociation curve the steep portion of the curve represents
The ability of Hb to off-load O2 at the tissue level as well as up-load O2 at alveolar capillary level
In an O2 dissociation curve at normal PaO2, Hb is
100% saturated with O2. Increasing PO2 can not augment saturation
Note that normal arterial blood is represented on the
Upper, flat portion of the O2 dissociation curve
On the O2 dissociation curve, we can see that lowering PO2 to approximately 60-70 mmHg will
Not effect CaO2 (SaO2)
This phenomenon enables us to go to reasonably high altitudes and be exposed to other moderate hypoxic conditions without a significant loss in the ability to
Deliver O2 to tissues
The sigmoidal shape of the oxygen dissociation curve shows a rapid increase in blood O2 (oxyhemoglobin) with increased
PO2 (up to approximately 70 mmHg)
After 70 mmHg, further increase in PO2 will not appreciably increase
O2 content
The steep central portion of the sigmoidal curve shows that Hb will readily unload O2 at the level of peripheral tissues in response to a small drop in
Capillary PO2
Moderate hypoxia or small decreases in ventilation and/or diffusion will not dramatically reduce
Oxygenation
At a constant PO2, a shift in the O2 dissociation curve to the RIGHT shows that
Increased unloaidng of O2 from Hb will occur
What 4 things will cause a rightward shift in the O2 dissociation curve, indicating a change of affinity of O2 for Hb?
- ) Reduced pH
- ) Increased PCO2
- ) Increased DPG (increased with hypoxia)
- ) Increased temperature
At a constant PO2, a leftward shift in the dissociation curve signifies that
O2 has higher affinity for Hb (like in fetal circulation)
CO2 is transported in the blood in which 3 forms?
- ) Dissolved
- ) Bicarbonate (HCO3-)
- ) Carbamino compounds
Accounts for 10% of total CO2 expired by the lungs
-20x more soluble than O2
Dissolved CO2
The reaction of H2O with CO2 to form H2CO3 and eventually HCO3- and H+ occurs in the
RBCs
What percentage of CO2 blown off at the lungs is transported as HCO3-?
-Converted to CO2 upon reaching the lungs
60%
Carbon dioxide that is combined with terminal amino groups of proteins, particularly Hb, are known as
Carbamino compounds
Readily form in RBCs in the presence of CO2 and protonated deoxy-Hb
Carbamino compounds
Carbamino compounds account for what percentage of expired CO2?
30%
The dissociation of CO2 form Hb is affected by
-due to competitive binding sites on Hb
O2 sat
The lower the PO2 and Hb binding, the greater the binding of
CO2 to Hb
At the level of the alveolar-capillary unit, high PO2 induces
Dissociation of CO2 from Hb for expiration
Enzymes, ligands, receptors, ion channels, transporters, and structural proteins are examples of molecules, the functions of which, will be affected by
Alterations in molecular charge
The principal chemoreceptors
Carotid chemoreceptors
Together, the peripheral chemoreceptors sense changes in
PCO2, pH, and PO2, with increased PCO2 as the main driving force
As PCO2 rises even a couple of mm Hg above the normal range, receptor activity is increased sending afferent signals from the
- ) Aortic receptors via?
- ) Carotid receptors via?
- ) CN X
2. ) Hering’s Nerve
Hering’s nerve essentially runs as a branch along
CN IX
These afferents are wired into ventilatory regulatory centers within the
Medulla
The peripheral chemoreceptors are more or less insensitive to changes in PO2 above approximately
50-60 mmHg
Thus, the predominant driving force to increase ventilation is
Hypercapnia as opposed to hypoxemia
Moderate decrease in PO2 will in fact induce an elevation in
Ventilation
However, this effect is transient since the rapid expulsion of CO2 via increased ventilation raises
-counteracts increase in ventilation
pH
Central chemoreceptors do not respond to
Hypoxemia
Instead, these receptors are exclusively designed to sense elevated
PCO2 within CSF (most likely as increase [H+])
The largest organ, the function of which is affected by fairly robust hydrostatic and pleural pressure gradients
The lung
In order for gas exchange to occur, the alveoli must be sufficiently
Ventilated
Importantly, alveoli ventilation (4-6 L/min) must also be matched with
Perfusion of alveoli
In general, pulmonry blood flow (perfusion) is set by which 3 things?
- ) Local hydrostatic pressures
- ) Alveolar pressures
- ) Vascular resistance
In general, ventilation is dependent on which 3 things?
- ) Pleural hydrostatic gradients
- ) Airway geometry
- ) Regional tissue compliances
Equals the entire output of the right ventricle, which matches cardiac output
Pulmonry blood flow
Consist of relatively thin walls when compared to those comprising the systemic vasculature
Pulmonary blood vessels
This histologic feature is largely due to a low compliment of
Vascular smooth muscle
Because of this, pulmonary vessels offer less resistance to
Flow
Highly distensible and under relatively low pressure
Pulmonary Vessels
The primary determinants of mean pulmonary artery pressure are
LA pressure, pulmonary blood flow, and pulmonary vascular resistance (PVR)
Itself is affected by lung volume, alveolar and interpleural pressures, right ventricular output, and gravity
PVR
The difference between the inside and outside pressures of a vessel
Transmural pressure gradient
If there is a large pressure gradient, pulmonary vessel diameter will
Increase
Will cause compression or collapse of a vessel
A negative transmural pressure
With inspiration, the alveoli expand, this is combined with an elongation and reduction in diameter of
Alveolar capillaries
This reduction in diameter causes an increased resistance to
Blood flow
With expiration, alveoli collapse, this is combined with a shortening and increased in diameter of
Alveolar capillaries
This increased diameter causes a decrease in resistance to
Blood flow
Actually distend with inspiration and compress with exhalation
Large pulmonary arteries and veins
An interesting phenomenon occurs during physical exertion whereby cardiac output can increase as much as 5-7x yet we do not see a correspondingly large increase in
Mean pulmonary arterial pressure
This trait enables pulmonary vascular resistance to decrease with elevations in
-thus providing a greater matching of ventilation and perfusion
Pulmonary blood flow
Pulmonary capillaries posses a high
Critical opening pressure
That is, at resting cardiac output, when less ventilation is required, many of these vessels which surround less ventilated alveoli are
Constricted and relatively non-perfused
However, with increased metabolic demand, increased flow due to increased cardiac output forces these vessels to
Open
There is a regional distribution of
Pulmonary blood flow
Pulmonary blood pressure increases towards the
Base of the lung
This increased pressure recruits more vessels in this region to distend, allowing for
Disproportionately greater flow to base of the lung
Lower blood pressures are present towards the
Apex of the lung
Lower blood pressures are present outwards the apex of the lung, such that at rest, the critical opening pressure for these vessels is
Not achieved
There are several factors which actively regulate
PVR
