Gas Exchange Principles

What is the physical process that facilitates gas exchange?

Diffusion of oxygen and carbon dioxide from an area of high concentration to an area of lower concentration to facilitate metabolism

Occurs between alveoli and pulmonary capillaries until partial pressures reach equilibrium

Why are partial pressures of oxygen and carbon dioxide important?

As oxygen leaves blood, carbon dioxide is taken up by tissues

Amount of oxygen and carbon dioxide in blood varies due to metabolism level

During exercise, more oxygen needs to be added and carbon dioxide removed to maintain homeostasis. This is due to higher metabolic demand

What is the equation for alveolar ventilation (V Little A)?

(Tidal volume - Dead space volume) x Respiratory Rate

What determines the partial pressure of oxygen and carbon dioxide in the alveoli?

• Alveolar Partial carbon dioxide is directly proportional to carbon dioxide/alveolar ventilation

• Partial alveolar oxygen is directly proportional to alveolar ventilation divided by oxygen consumption

• Oxygen consumption and carbon dioxide production= metabolism

• Gas exchange driven by alveolar ventilation

• Higher metabolism= higher carbon dioxide = more oxygen consumption

What is the normal valeres of the partial pressures of oxygen and carbon dioxide in arterial blood?

Carbon dioxide: 35 to 45 mmHg

Oxygen: 80 to 100 mmHg

What is the normal values for the partial pressure of oxygen and carbon dioxide in the mixed venous blood?

Oxygen: 35 to 40 mmHg

Crbon dioxide: 41 to 51 mmHg

Why is the diffusion of carbon monoxide measured in the pulmonary function testing instead of oxygen?

• Haemoglobin combines rapidly with carbon monoxide

• Loss of carbon monoxide from alveoli into pulmonary capillary blood is limited by diffusion

• When patient exhales, alveolar sample is measured for carbon monoxide concentration, Amount of carbon monoxide lost by diffusiom ca be calxulates.

Why is helium added to inhaled air in pulmonary function tests?

• Presence of helium allows calculation of alveolar volume.

• Allows calculation of functional residual capacity, residual volume and total lung capacity

What is hypoxic vasoconstriction?

Small pulmonary arteries constrict in response to low alveolar oxygen levels

How does hypoxic vasoconstriction improve the matching of ventilation and perfusion with areas of poor ventilation?

• When ventilation decreased in an alveoli= carbon dioxide increase, oxygen decrease

• Redirects blood by constricting arterioles to better ventilated alveoli

What is type 2 respiratory failure with reference to arterial blood gas levels?

• Hypoventilation

• Elevated partial pressure of carbon dioxide and low partial pressure of oxygen in arterial blood

• Carbon dioxide >45-50mmHg

• Oxygen: <60 mmHg

What is type 1 respiratory failure with reference to arterial blood gas levels?

• More common

• Partial pressure of oxygen is low and carbon dioxide is normal in arterial blood

• Oxygen: < 60mmHg

• Carbon dioxide: < 45mmHg

What are the 2 main causes of type 1 respiratory failure?

• Ventilation/ Perfusion Mismatch

• Shunt: deoxygenated blood passes from right side of the heart to the left without being oxygenated in the lungs, thus the heart is pumping deoxygenated blood to the body. Bypasses ventilated alveoli