Exchange of Gases in Respiration: Definition and Mechanism

What is Respiration and Exchange of Gases?

o The complete purpose of the respiration process is to exchange gases and transport oxygen to every part of the body.

o Exchange of gases occurs in the alveoli of the lungs. The air that reaches the alveoli is by the pulmonary ventilation.

o Oxygen and carbon dioxide are exchanged in the membrane where the alveolar and capillary walls meet.

o The blood carries the oxygenated blood to the body and carbon dioxide away from the body.

Exchange of Gases Process

The exchange of gases in lungs have many gaseous law and principles to be followed. The following are the laws

• Gas Laws and Compression

o Gases have pressure i.e., the force exerted by the gases to what they are in contact with. The air we breath in is the mixture of gases and the pressure exerted by the air is the atmospheric pressure.

The partial pressure is the pressure of the one type of gas of the mixture. And the total pressure is the sum of all partial pressure of the gases in the mixture.

Partial pressure is responsible for the gaseous movement. Gas move from its higher partial pressure to its lower partial pressure.

Higher gas partial pressure is directly proportional to the rapid movement of gases.

• Perfusion and Ventilation

o Gas exchange in lungs has two aspects that play a major role – ventilation and perfusion.

o Perfusion is the flow of blood in the capillaries whereas, ventilation is known to be the movement of air into and out of the lungs.

o Ventilation and perfusion have to be compatible for efficient gas exchange.

o Partial pressure of oxygenated pulmonary venous blood is 100mm Hg whereas partial pressure of oxygen 104mm Hg in alveolar air.

o Oxygen enters the alveoli at high rate when the ventilation is sufficient and partial pressure of oxygen remains high in the alveoli.

o When the ventilation is not sufficient then it is observed that the partial pressure of alveoli drops.

• Partial pressure

o Oxygen does not diffuse from alveoli to the blood if the partial pressure isn’t large enough.

o Diameter of airways regulate the ventilation whereas diameter of blood vessels regulates the perfusion.

o The diameter of bronchioles is known to be sensitive to the partial pressure of carbon dioxide this will help in exhaling the carbon dioxide out of the body.

o Partial pressure of carbon dioxide when is higher in alveoli causes to increase in the diameter of bronchioles and decrease the supply of oxygen to the blood and allow carbon dioxide to exhale from the body at higher rate.

• Exchange Site

o Exchange of gases occur at two sites in the body – lungs and tissue.

o Lungs, where the carbon dioxide is released at the respiratory membrane and oxygen is taken in.

Tissue, where the oxygen is released and carbon dioxide is taken it.

Gaseous Respiration

o External respiration and internal respiration both have part in the exchange of gases.

o External respiration conducts the exchange of gases with external environment and site is alveoli.

o Internal respiration occurs in the tissue and is the exchange of gases with internal environment.

o By the process of simple diffusion gases are exchanged and do not require energy.

o As the gas exchange through the membrane follow the pressure gradients.

o Respiratory membranes are permeable to gases and are very thin like the capillary membranes.

o The lungs provide a large surface are throughout it.

External Respiration

o Deoxygenated blood is carried by the pulmonary artery from the heart to the lungs.

o Pulmonary artery branches to form capillary network and form the pulmonary capillaries and creates respiratory membrane with alveoli.

o Gas exchange occurs when the blood is pumped through capillary network.

o Oxygen dissolves with hemoglobin and some directly dissolves with the plasma and oxygenated hemoglobin returns to the heart via pulmonary veins. And carbon dioxide moves from blood to alveoli.

o The difference of partial pressure of oxygen in alveoli (104 mm Hg) to the blood (40 mm Hg) is 64mm Hg. This large difference results in the rapid flow of oxygen across the membrane.

o Partial pressure difference of carbon dioxide is 5 mm Hg. The partial pressure of carbon dioxide in blood is 45 mm Hg whereas alveoli is 5 mm Hg, but the solubility of carbon dioxide is more than that of oxygen.

o Hence the movement is rapid for both oxygen and carbon dioxide.

Internal Respiration

o Diffusion is the same process followed by the internal respiration than the external respiration due to partial pressure gradient but opposite to the respiratory membrane.

o Partial pressure of oxygen in tissue is 40 mm Hg and blood is 100 mm Hg, this results in the pressure gradient and blood to dissociate from hemoglobin and diffuse out of blood.

Cellular Respiration

o Results is continuous production of carbon dioxide and partial pressure of carbon dioxide is lower in blood than tissue which cause carbon dioxide to diffuse out of the tissue and carried by hemoglobin or plasma to the lungs. This then flows to lungs and oxygenated by the external respiration.

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