RESPIRATORY

SYSTEM

GAS EXCHANGE

PULMONARYGAS EXCHANGE

• The movement of O2 and CO2 in and out of the capillaries both in the lungs and in the peripheral tissues depends on gas diffusion.

• Diffusion is driven by partial pressure gradients.

• The lungs are well adapted for gas diffusion, with a very large alveolar surface area and a very thin layer of fluid and tissue separating alveolar gas from pulmonary blood.

GAS EXCHANGE IN THE LUNGS

• Gas exchange in the lungs depends on the partial pressure gradients between alveolar air and pulmonary arterial blood for O2 and CO2.

• Effective gas exchange also requires easy diffusion between alveoli and blood and an appropriate balance between ventilation and perfusion within the lungs.

GAS EXCHANGE IN THE LUNGS• Although the average total pressure in the alveoli is

equal to that in the atmosphere, alveolar air differs from the air we breathe (the inspirate) in a number of ways.

• Firstly, it has a higher water vapour pressure, since the inspired gases become fully saturated as they pass through the airways.

• The other major differences reflect constant removal of O2 by diffusion into the pulmonary blood and constant addition of CO2 from the same source. This reduces the PO2 and elevates the PCO2 as compared with humidified air.

• The limits the maximum possible alveolar PO2 to about 150 mmHg.

• Normally ventilation is homeostatically regulated so that alveolar PO2 and PCO2 remain relatively constant.

PULMONARY BLOOD GASES

• Gas diffusion gradients in the lung are determined by the differences between the partial pressures within the alveoli and those within the pulmonary capillaries.

• Pulmonary arterial blood entering the capillaries contains partial pressures of O2 and CO2, which are determined by the levels in systemic veins from peripheral tissues.

PULMONARY BLOOD GASES

• Venous blood is mixed in the right ventricle, giving a PO2 in the pulmonary arteries of 40 mmHg and a PCO2 of 45 mmHg.

• Since the PO2 in the alveoli is 103 mmHg, O2 diffuses into the pulmonary blood.

PULMONARY BLOOD GASES

Carbon dioxide, also driven by a partial pressure gradient, diffuses in the opposite direction, from the capillaries (PCO2 = 45 mmHg) into the alveoli (PCO2 = 40 mmHg).

PULMONARY BLOOD GASES

• Gas diffusion across the combined alveolar and capillary wall system is rapid, so that pulmonary blood normally equilibrates with alveolar gases before leaving the pulmonary capillary.

• Thus, the gas pressures in pulmonary venous blood equal those in the alveoli.

PULMONARY DIFFUSION

• Pulmonary gases must diffuse through several structures between the alveoli and the capillary blood. The total thickness of the diffusion barrier can be as little as 0.2 nm.

• Coupled with the large alveolar surface area (about 70 m2), this short diffusion distance makes the lungs very efficient gas exchange units, i.e. they have a high diffusion capacity.

GAS EXCHANGE IN PERIPHERAL TISSUES

• The pressure gradients driving peripheral gas exchange are kept constant, with a PO2 of about 98 mmHg and a PCO2 of 40 mmHg.

• Tissue gas pressures may vary widely from organ to organ and time to time, depending on the balance between blood flow and local metabolic activity, which consumes O2 and produces CO2.

GAS TRANSPORT IN BLOOD

• Both O2 and CO2 are transported between the lungs and the tissues in the blood.

• The majority of the O2 in blood is transported within red cells and is bound to haemoglobin, with only a negligible additional amount dissolved in the plasma.

• The normal haemoglobin concentration is about 15 g/dl in males and 13 g/dl in females.

• Since each gram of haemoglobin can carry 1.34 ml of O2.

OXYGEN DISSOCIATION CURVE

This sigmoid, or S-shaped curve describes the relationship between the partial pressure of O2 and the concentration of O2 in blood.

CARBON DIOXIDE TRANSPORT

Carbon dioxide is carried in three main forms in the blood:

• Transport as bicarbonate ions (HCO3) accounts for about 60-70% of the total CO2 carried in the blood. The gas diffuses into the red blood cells where it reacts with water to form carbonic acid.

• Transport as carbamino groups are formed by a reaction between CO2 and amino acid residues in peptides and proteins. About 20-30% of circulating CO2 is in this form.

• Transport as dissolved CO2. This only accounts for about 10% of the total CO2 transport.

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