10. Ventilation Distribution
⢠Ventilation distribution within the lung depends on the compliance of alveoli and the relative
distending pressure.
⢠Theoretically, the pressure within all the alveoli in the lung is constant, but the pressure outside the
alveoli is heterogenous throughout the lung, resulting in different- sized alveoli.
⢠Ventilation distribution is also affected by anatomy and flow rates.
⢠Central regions of the lung are preferentially ventilated, but as flow rates are increased, this
ventilatory difference is minimized.
⢠Simply stated, during spontaneous ventilation, more gas is distributed to gravity- dependent areas.
17. The V/Q ratio is the balance between the ventilation (bringing oxygen in to /removing đđ đ from the
alveoli) and the perfusion (removing đ đ from the alveoli and adding đđ đ). The V/Q ratio is important
because the ratio between the ventilation and the perfusion is one of the major factors affecting the
alveolar (and therefore arterial) levels of oxygen and carbon dioxide.
Variable Normal Value
PAO2 ~ 100 mm Hg
PACO2 40 mm Hg
PaO2 95 - 100 mm Hg
PaCO2 40 mm Hg
18. ⢠Decrease the V/Q ratio : A decrease in the V/Q ratio is produced by either decreasing ventilation or
increasing blood flow (without altering the other variable). These will both have the same effect - the
alveolar (and therefore arterial) levels of oxygen will decrease and the đđ đ will increase.
⢠When you consider a decrease in the V/Q ratio, all you need to remember is:
⢠Ventilation is not keeping pace with perfusion.
⢠The alveolar oxygen levels will decrease, which will lead to a decrease in arterial oxygen levels
(Pađ đ)
⢠The alveolar đđ đ levels will increase (we're not getting rid of it as fast), also leading to an
increase in arterial đđ đ .
⢠To increase the ventilation-perfusion ratio.
⢠Increase in the V/Q ratio means that ventilation is in excess of the metabolic needs being met by
perfusion, so we blow off đđ đ (lower PAđđ đ ) and increase our Pđđ đ (and Pađ đ).
20. Changing the V/Q Ratio Pathologically
⢠Increasing the V/Q ratio to infinity. Pulmonary embolism
⢠This blood will be very well oxygenated (lots of ventilation, little perfusion) and have a very low
đđ đ.
⢠Not much blood gets through to these alveoli, so the volume of blood in this condition is very
low. However, 5 liters of blood is still coming to the lungs every minute - the blood that can't get
to the area of lung affected by the embolism gets shunted to other parts of the lung (leading to
a low V/Q ratio in those parts of the lung).
⢠We wasted energy by bringing ventilation to this area - in fact, this is alveolar dead space.
21. Changing the V/Q Ratio Pathologically
⢠Decreasing the V/Q ratio to zero:
⢠In this case, we wasted cardiac effort to send the blood to the lungs even though nothing
happened to it as far as oxygen and carbon dioxide go. We call this a physiological shunt -
although the blood travelled to the lungs, it didn't get any oxygen.
⢠In contrast, an anatomical shunt occurs when the blood physically doesn't enter the lungs (e.g. a
right-to-left shunt - the blood jumps straight from the right ventricle to the left ventricle without
going to the lungs). The end result is- some of the arterial blood has very low oxygen and high
đđ đ.
25. A-a gradient
⢠Pđđ đ â Pađ đ
⢠A normal Aâa gradient for a young adult non-smoker breathing air, is between 5â10 mm Hg.
⢠It gives an idea about the cause of hypoxemia.
⢠However, the Aâa gradient increases with age.
31. đ đ and đđ đ Transport in Lungs
⢠2 modes: Convection and Diffusion
⢠The pulmonary diffusion capacity/ability of đđ đ to pass between the alveoli to blood is 20 times more
than Oxygen which allows it to diffuse across the alveolar membrane with greater efficiency.
⢠Diffusion allows for đ đ and đđ đ to be exchanged at the alveoli-pulmonary capillary interface along a
concentration gradient.
32. đ đTransport in Blood
⢠đ đ is carried in blood in 2 forms:
1. Dissolved in plasma (2%)
2. Reversibly bound to Haemoglobin (98 %)
⢠The Hb molecule consists of four intertwined subunits, each of which consists of a:
⢠Polypeptide globin chain (alpha or beta)
⢠Haem group (porphyrin ring containing a Fe2+ ion)
⢠đ đ binds reversibly to the Fe2+ ion in the haem group, each Hb molecule holding up to four đ đ
(one to each Fe2+)
34. Oxygen Haemoglobin Saturation Curve
⢠The P50 is the Pđ đ at which the Hb-đ đ saturation is 50%, normally around 3.5 kPa. It is a reference point that
describes the position of the curve and changes as the curve moves under different conditions.
35. Factors that Influence Oxygen Binding
1. Temperature
2. pH : A decrease in pH by addition of carbon dioxide or other acids causes a Bohr Shift.nA Bohr shift is
characterized by causing more oxygen to be given up as oxygen pressure increases.
3. 2,3-Diphosphoglycerate (DPG) : It is the main primary organic phosphate. DPG binds to haemoglobin
which rearranges the haemoglobin into the T-state, thus decreasing the affinity of oxygen for
haemoglobin
36. đđ đ Transport in Blood
⢠đđ đ is transported in the blood in 3 different forms :-
⢠Either dissolved in blood (5 %) or
⢠transported as bicarbonate (90 %) or
⢠as a carbo-amino (5%) compound.
42. Chemosensitive Area
⢠This area located in the Medulla is highly sensitive to PaCO2 or H+ which in turn excites the
other portions of the Respiratory Center.
⢠The sensory neurons are specifically excited by H+ ions. However, they do not cross the BBB.
But BBB is permeable to dissolved CO2.
⢠Increase in PaCO2 à Increase in CSF [H+] à Activates Chemoreceptors à Secondary
Stimulation to Medullary Centers Ă Increase Alveolar Ventilation Ă Reduce PaCO2 to
normal.
46. Respiratory Reflexes
⢠Hering- Breuer Inflation reflex : Overinflated lungs Ă stretch receptors + Ă feedback response Ă
switches off the inspiratory ramp and stops further inspiration.
⢠Hering-Breuer Deflation Reflex : Marked lung deflation causes a decrease in Expiration time.
⢠Pulmonary irritant receptors : Stimulated by some types of irritants that enter the tracheobronchial
tree which causes coughing and sneezing.
⢠J Receptors : Stimulated especially when the pulmonary capillaries become engorged with blood or
fluids which can in turn lead to dyspnea.