Gas Exchange Inside the Body
Gas exchange in the body does not only occurs in lungs, but also in tissues. Gas exchange occurs due to air pressure differences. High pressure gas will diffuse to low pressure gas.
1. External Respiration
External respiration is related to gas exchange process between the air in alveoli and blood in lung capillaries. The blood that goes into lung capillaries carries higher pressure CO2 than air pressure in atmosphere. Therefore, CO2 diffuses out of the blood and enters the lungs.
Most of CO2 carried by blood plasma is in the form of bicarbonate ion (HCO2-) While the rest is free CO2 that soon will diffuse out of the body. After obtaining ion hydrogen (H+) from HHb, bicarbonate ion forms carbonic acid. The carbonic acid (H2CO3) is broken down into water and carbon dioxide.
The blood entering lung capillaries contains a small amount of O2 while the air in alveoli contains a large amount of hight pressure O2 Consequently, the O2 diffuses into plasma and goes into red blood cells in lungs. Then, hemoglobin capturing O2 to form oxihemoglobin (HbO2).
2. Internal Respiration
Internal Respiration is related to gas exchange in blood of capillary vessel and tissues fluid. The blood entering capillaries contains oxihemoglobin. The oxihemoglobin will free O2 and the O2 will diffuse out of blood and goes into tissues.
After CO2 diffuses into the blood, a little part of it bonds with hemoglobin and form carboxyhemoglobin. Most of CO2 will bond with water and form carbonic acid. By the help of carbonate anhidrase enzyme, carbonic acid is soon broken down to form ion hydrogen (H-) and ion bicarbonate (HCO3-)
The formed bicarbonate ion will diffuse out of red blood cells and is carried by blood plasm. The part of globin of haemoglobin will bind hydrogen ion to form HHb, which is called reduced haemoglobin. The process of internal respiration and external respiration can be seen in following chart.