Pulmonary gas exchange: the basics

Published on 09/04/2015 by admin

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Last modified 09/04/2015

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1.2 Pulmonary gas exchange

The basics

Our cells use oxygen (O₂) to generate energy and produce carbon dioxide (CO₂) as waste. Blood supplies cells with the O₂ they need and clears the unwanted CO₂. This process depends on the ability of our lungs to enrich blood with O₂ and rid it of CO₂.

Pulmonary gas exchange refers to the transfer of O₂ from the atmosphere to the bloodstream (oxygenation) and CO₂ from the bloodstream to the atmosphere (CO₂ elimination).

The exchange takes place between tiny air sacs called alveoli and blood vessels called capillaries. Because they each have extremely thin walls and come into very close contact (the alveolar–capillary membrane), CO₂ and O₂ are able to move (diffuse) between them (Figure 1).

Figure 1 Respiratory anatomy.

Pulmonary gas exchange: partial pressures

ABGs help us to assess the effectiveness of gas exchange by providing measurements of the partial pressures of O₂ and CO₂ in arterial blood – the PaO₂ and PaCO₂.

Partial pressure describes the contribution of one individual gas within a gas mixture (such as air) to the total pressure. When a gas dissolves in liquid (e.g. blood), the amount dissolved depends on the partial pressure.

Note

PO₂ = partial pressure of O₂

PaO₂ = partial pressure of O₂in arterial blood

Gases move from areas of higher partial pressure to lower partial pressure. At the alveolar–capillary membrane, air in alveoli has a higher PO₂ and lower PCO₂ than capillary blood. Thus, O₂ molecules move from alveoli to blood and CO₂ molecules move from blood to alveoli until the partial pressures are equal.

A note on … gas pressures

At sea level, atmospheric pressure (total pressure of gases in the atmosphere) = 101 kPa or 760 mmHg

O₂ comprises 21% of air, so the partial pressure of O₂ in air

= 21% of atmospheric pressure

= 21 kPa or 160 mmHg

CO₂ makes up just a tiny fraction of air, so the partial pressure of CO₂ in inspired air is negligible

Carbon dioxide elimination

Diffusion of CO₂ from the bloodstream to alveoli is so efficient that CO₂ elimination is actually limited by how quickly we can “blow-off” the CO₂ in our alveoli. Thus, the PaCO₂ (which reflects the overall amount of CO₂ in arterial blood) is determined by alveolar ventilation – the total volume of air transported between alveoli and the outside world every minute.

Ventilation is regulated by an area in the brainstem called the respiratory centre. This area contains specialised receptors that sense the PaCO₂ and connect with the muscles involved in breathing. If it is abnormal, the respiratory centre adjusts the rate and depth of breathing accordingly (Figure 2).

Figure 2 Control of ventilation.

Normally, lungs can maintain a normal PaCO₂, even in conditions where CO₂ production is unusually high (e.g. sepsis). Consequently an increased PaCO₂ (hypercapnia) always implies reduced alveolar ventilation.

Key point

PaCO₂is controlled by ventilation and the level of ventilation is adjusted to maintain PaCO₂within tight limits.

A note on … hypoxic drive

In patients with chronically high PaCO₂ levels (chronic hypercapnia), the specialised receptors that detect CO₂ levels can become desensitised. The body then relies on receptors that detect the Pao₂ to gauge the adequacy of ventilation and low Pao₂ becomes the principal ventilatory stimulus. This is referred to as hypoxic drive.

In patients who rely on hypoxic drive, overzealous correction of hypoxaemia, with supplemental O₂, may depress ventilation, leading to a catastrophic rise in PaCO₂. Patients with chronic hypercapnia must therefore be given supplemental O₂ in a controlled fashion with careful ABG monitoring. The same does not apply to patients with acute hypercapnia.

Haemoglobin oxygen saturation (SO₂)

Oxygenation is more complicated than CO₂ elimination. The first thing to realise is that the PO₂ does not actually tell us how much O₂ is in blood. It only measures free, unbound O₂ molecules – a tiny proportion of the total.

In fact, almost all O₂ molecules in blood are bound to a protein called haemoglobin Hb (Figure 3). Because of this, the amount of O₂ in blood depends on two factors:

1. Hb concentration: this determines how much O₂ blood has the capacity to carry.

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