Question

1) How are oxygen and carbon dioxide transported in the blood.

2) Give an illustrated account of the structure and function of the gas exchange region of the lung.

Answer 1

In the lungs, exchange of respiratory gases takes place between the alveoli of lungs and the blood. Oxygen enters the blood from alveoli and carbon dioxide is expelled out of blood into alveolus. Exchange of gases between blood and alveoli takes place through respiratory membrane.

Respiratory membrance is a membranous structure through which exchange of respiratory gases takes place. It is formed by epithelium of respiratory unit and endothelium of pulmonary capillary. Epithelium of respiratory unit is a very thin layer. Since, the capillaries are in close contact with this membrane, alveolar air is in close proximity to capillary blood. This facilitates gaseous exchange between air and blood. Respiratory membrane is formed by different layers of structures belonging to the alveoli and capillaries.

Layers of respiratory membrane

1. Alveolar portion - a. Layer of surfactant

b. Fluid layer that lines the alveoli

c. Alveolar eoithelium which rest on basement membrane

2. Interstitial space between alveolar and capillary portion.

3. Capillaryn portion - a. Basement membrane of capillary

b. Capillary endothelial cells

Diffusion of Oxygen from Atmospheric Air into Alveoli -

Partial pressure of oxygen in the atmospheric air is 159 mm Hg and in the alveoli, it is 104 mm Hg. Because of
pressure gradient of 55 mm Hg, oxygen easily enters from atmospheric air into the alveoli.

Diffusion of Oxygen from Alveoli into Blood

When blood passes through pulmonary capillary, RBC is exposed to oxygen only for 0.75 second at rest and only for 0.25 second during severe exercise. So, diffusion of oxygen must be quicker and effective. Fortunately, this is possible because of pressure gradient. Partial pressure of oxygen in the pulmonary capillary is 40 mm Hg and in the alveoli, it is 104 mm Hg. Pressure gradient is 64 mm Hg. It facilitates the diffusion of oxygen from alveoli into the blood.

DIFFUSION OF CARBON DIOXIDE

Diffusion of Carbon Dioxide from Blood into AlveoliPartial pressure of carbon dioxide in alveoli is 40 mm Hg whereas in the blood it is 46 mm Hg. Pressure gradient of 6 mm Hg is responsible for the diffusion of carbon dioxide from blood into the alveoli.

Diffusion of Carbon Dioxide from Alveoli into Atmospheric Air

In atmospheric air, partial pressure of carbon dioxide is very insignificant and is only about 0.3 mm Hg whereas,in the alveoli, it is 40 mm Hg. So, carbon dioxide enters passes to atmosphere from alveoli easily