The main roles of the respiratory system are to add O2 and remove CO2 from
pulmonary capillary blood.
PiO2 in the upper airway at sea level is about 150 mmHg.
PAO2 at sea level is always about 100 mmHg.
The oxygen content of blood in the pulmonary arteries is the same as that in the
The pH of pulmonary arterial blood is lower than the pH of pulmonary venous
Atmospheric pressure declines linearly with increasing altitude and at 18,000
feet is about 380 mmHg.
Pressure increases linearly with increasing depth under water.!
At a depth of 10 meters at an altitude of 18,000 feet, the total pressure would
be about 1140 mmHg.
Basal metabolic rate is about 250 ml/min O2 and 200 ml/min CO2 and hence
the respiratory exchange ratio, R, is 1.2.
The metabolic rate, and hence oxygen consumption, increases with exercise,
fever, inflammatory conditions and malignancy.
Ventilation is an energy requiring process and at rest is about 15% of the
body’s energy consumption.
The work of breathing involves resistive forces due to stretching of the lungs
and elastic forces due to the friction of airflow in the bronchi.
At FRC the elastic recoil of the chest wall is balanced by the elastic recoil of
the lungs and this explains the increase in FRC in pulmonary fibrosis.
Tidal volume is about 500 ml in adults at rest and is composed of alveolar
volume of about 350 ml and dead space of about 150 ml
During inspiration intrapleural pressure is more negative at the base of the
lung compared to the lung apex.
Airflow is laminar throughout the upper and lower airways.
Because a 80/20 helium/oxygen mixture is less dense than air it can be used in
situations of severe airflow obstruction to reduce resistive forces.
Spirometry measures all the lung volumes including TLC, FRC and RV.
The hallmark of an obstructive defect is a reduction in FEV1 and the forced
Alveolar hypoventilation is characterised by an elevated PaO2.
Because the pulmonary circulation is at a lower pressure than the systemic
circulation, the cardiac output from the right ventricle is lower than the left.
Pulmonary artery pressure increases markedly with exercise.
Pulsus paradoxus is a reversal of the normal swings in systolic pressure with
A reduction in regional oxygen concentration in the lungs causes dilation of
the pulmonary arterioles to that area.
Gas exchange is most efficient when there is roughly a 1:1 ratio of ventilation
and perfusion in all A/c units.
High V/Q units represent wasted ventilation and contribute to the physiologic
Low V/Q units are a common cause of hypoxaemia.
Supplemental oxygen fully corrects the hypoxaemia caused by shunts
Diffusion impairment is a common cause of hypoxaemia at rest.
The alveolar – arterial gradient for oxygen is widened if there is inefficient gas
exchange in the lungs.
Oxygen diffuses across the A-C membrane more rapidly than CO2
Oxygen transfer across the A-C membrane is perfusion limited meaning
that the amount of blood available limits the total amount of O2 transferred
rather than the diffusion capacity of the membrane.
O2 carriage in the blood is predominantly by hemoglobin and each Hb
molecule can carry a maximum of 3 molecules of O2.
1 gm of fully saturated Hb carries 1.3 litres of O2.
The O2 content of blood is influenced by PaO2, [Hb] and cardiac output.
Unloading of O2 in the tissues is aided by increased H+ and CO2.
CO2 and CO bind to the same site on Hb as O2.
CO2 is transferred in blood as HCO3 and this is formed by carbonic
anhydrase in the serum.
A respiratory acidosis is due to inadequate CO2 removal by the lungs.
The compensation for a metabolic acidosis is hyperventilation.
pH: 7.40, PaCO2: 40, HCO3: 25
pH: 7.30, PaCO2: 60, HCO3: 27
pH: 7.37, PaCO2: 60, HCO3: 33
pH: 7.28, PaCO2: 30, HCO3: 16
pH: 7.50, PaCO2: 30, HCO3: 24
pH: 7.43, PaCO2: 30, HCO3: 21
pH: 7.50, PaCO2: 46, HCO3: 30
pH: 7.05, PaCO2: 56, HCO3: 16