3 Pulse Oximetry for the Measurement of Oxygen Saturation …
53
3.4 Mechanism of Oxygen Exchange
Air inhaled during breathing enters the lungs. Oxygen in the air is trapped by the
haemoglobin molecules and carbon dioxide from the blood is released to the air that
gets exhaled [7]. A heterogeneous collection of gas exchange units called alveoli in
the lungs surrounded by large pulmonary capillary beds aid this gas exchange through
diffusion. Diffusion of a gas requires differential partial pressure. The inhaled air (at
near sea level) is at a total pressure of 101 kPa and is made up of nearly 21%
of oxygen, 78% nitrogen (N 2 ) and small quantities of carbon dioxide, argon and
helium. The partial pressures exerted by the two main gases added together nearly
equal the atmospheric pressure. The partial pressure of oxygen (P O 2 ) of dry air at sea
level is therefore approximately 21 kPa but by the time air passes through the trachea
and reaches the alveoli, the P O 2 falls to about 13 kPa. Venous blood returning to the
lungs has a P O 2 of 5 kPa. A thin wall (about 0.5 μm thick) between the pulmonary
capillaries and the alveoli permits diffusion of gases. Since P O 2 of air in the alveoli
is 13 kPa and in the pulmonary capillaries is 5 kPa, oxygen diffuses from alveoli to
the blood in the pulmonary capillaries. On the other hand, the reverse partial pressure
gradient for carbon dioxide ensures the diffusion of carbon dioxide from the blood
to the air trapped in the alveoli. Normally partial pressure of nitrogen in the blood
and the alveolar air is about the same hence very little nitrogen is diffused in either
direction.
Thus, the blood returning to the left side of the heart to be pumped into the
systemic circulation is replenished with oxygen. If this process is normal, then P O 2
of pulmonary venous blood would be equal to the P O 2 in the alveoli. Any malfunction
would render the pulmonary vein P O 2 to be less than the P O 2 in alveoli, resulting in
reduced amount of oxygen in the arterial blood. In fact, the amount of oxygen bound
to the haemoglobin at any time is dictated by the P O 2 to which the haemoglobin
is exposed. When the arterial blood enters body cells through capillaries, wherein
the P O 2 is lower than the arterial P O 2 , oxygen is detached from the haemoglobin
and enters the cell. The total quantity of oxygen bound to haemoglobin in normal
arterial blood is approximately 19 mL per 100 mL of blood at a P O 2 of 13 kPa. On
passing through tissue capillaries this amount is reduced to 14 mL per 100 mL of
blood at a P O 2 of 5 kPa. Thus, under normal conditions, about 5 mL of oxygen is
consumed by tissues from each 100 mL of blood that passes through tissue capillaries
during each cycle. When blood returns to the lungs, approximately 5 mL of oxygen
diffuses from alveoli into each 100 mL of blood, bringing back the oxygen dissolved
in the blood to normal condition. Nearly 98% of the diffused oxygen gets bounded
to haemoglobin molecules and the remaining 2% gets dissolved in plasma. Each
haemoglobin molecule is made up of four “heme” (the iron-containing portion of
haemoglobin) groups and a protein group, known as “globin” (amino acid chains
that form a protein). Each heme unit can carry one oxygen molecule and hence
one molecule of haemoglobin can carry up to four molecules of oxygen. When
one haemoglobin molecule binds four oxygen molecules, it becomes fully saturated
53
3.4 Mechanism of Oxygen Exchange
Air inhaled during breathing enters the lungs. Oxygen in the air is trapped by the
haemoglobin molecules and carbon dioxide from the blood is released to the air that
gets exhaled [7]. A heterogeneous collection of gas exchange units called alveoli in
the lungs surrounded by large pulmonary capillary beds aid this gas exchange through
diffusion. Diffusion of a gas requires differential partial pressure. The inhaled air (at
near sea level) is at a total pressure of 101 kPa and is made up of nearly 21%
of oxygen, 78% nitrogen (N 2 ) and small quantities of carbon dioxide, argon and
helium. The partial pressures exerted by the two main gases added together nearly
equal the atmospheric pressure. The partial pressure of oxygen (P O 2 ) of dry air at sea
level is therefore approximately 21 kPa but by the time air passes through the trachea
and reaches the alveoli, the P O 2 falls to about 13 kPa. Venous blood returning to the
lungs has a P O 2 of 5 kPa. A thin wall (about 0.5 μm thick) between the pulmonary
capillaries and the alveoli permits diffusion of gases. Since P O 2 of air in the alveoli
is 13 kPa and in the pulmonary capillaries is 5 kPa, oxygen diffuses from alveoli to
the blood in the pulmonary capillaries. On the other hand, the reverse partial pressure
gradient for carbon dioxide ensures the diffusion of carbon dioxide from the blood
to the air trapped in the alveoli. Normally partial pressure of nitrogen in the blood
and the alveolar air is about the same hence very little nitrogen is diffused in either
direction.
Thus, the blood returning to the left side of the heart to be pumped into the
systemic circulation is replenished with oxygen. If this process is normal, then P O 2
of pulmonary venous blood would be equal to the P O 2 in the alveoli. Any malfunction
would render the pulmonary vein P O 2 to be less than the P O 2 in alveoli, resulting in
reduced amount of oxygen in the arterial blood. In fact, the amount of oxygen bound
to the haemoglobin at any time is dictated by the P O 2 to which the haemoglobin
is exposed. When the arterial blood enters body cells through capillaries, wherein
the P O 2 is lower than the arterial P O 2 , oxygen is detached from the haemoglobin
and enters the cell. The total quantity of oxygen bound to haemoglobin in normal
arterial blood is approximately 19 mL per 100 mL of blood at a P O 2 of 13 kPa. On
passing through tissue capillaries this amount is reduced to 14 mL per 100 mL of
blood at a P O 2 of 5 kPa. Thus, under normal conditions, about 5 mL of oxygen is
consumed by tissues from each 100 mL of blood that passes through tissue capillaries
during each cycle. When blood returns to the lungs, approximately 5 mL of oxygen
diffuses from alveoli into each 100 mL of blood, bringing back the oxygen dissolved
in the blood to normal condition. Nearly 98% of the diffused oxygen gets bounded
to haemoglobin molecules and the remaining 2% gets dissolved in plasma. Each
haemoglobin molecule is made up of four “heme” (the iron-containing portion of
haemoglobin) groups and a protein group, known as “globin” (amino acid chains
that form a protein). Each heme unit can carry one oxygen molecule and hence
one molecule of haemoglobin can carry up to four molecules of oxygen. When
one haemoglobin molecule binds four oxygen molecules, it becomes fully saturated