Simple Pharmanotes: Gas Exchanges Between the Blood, Lungs, and Tissues.

Gas Exchanges Between the Blood, Lungs, and Tissues:

Pulmonary artery carries deoxygenated blood to the lungs where it forms respiratory membranes (basement membranes of blood capillary and alveoli are fused with each other) with alveoli.
Because of differences in partial pressure of oxygen and CO2 in blood and alveoli, the gases move from higher concentration to lower concentration.
Deoxygenated blood contains less partial pressure of Oxygen than alveoli and hence oxygen diffuses in the blood.
Deoxygenated blood contains higher CO2 partial pressure as compared to alveoli and hence diffuses from blood into alveoli.
The resulting oxygenated blood is sent to the heart by pulmonary veins.

The heart pumps oxygenated blood into the aorta which supplies oxygenated blood to the entire body through its branches.
The capillary carrying oxygenated blood reaches tissues where partial pressure of oxygen is less as compared to blood, this results in diffusion of oxygen from blood into tissues.
The partial pressure of CO2 is higher in tissues as compared to in blood capillaries, this results in diffusion of CO2 from tissue to blood capillaries.
Now this deoxygenated blood is collected via superior vena cava and inferior vena cava and transported to the heart.
The heart transfers this collected deoxygenated blood to the lungs via the pulmonary artery for oxygenation.

The oxygen has little water solubility and hence dissolves in little amounts in plasma which diffuses directly into the cells.
Around 2% of oxygen is dissolved in plasma.
About 98% of oxygen in blood combines with Hemoglobin to form a complex called “Oxyhemoglobin”.
Oxyhemoglobin dissociates radially on reaching the cells having lower Oxygen partial pressures and oxygen diffuses in the cells.

Carbon dioxide is formed in the cell as an end product of many reactions and is one of the major excretory products of cells.
The CO2 dissolves in little amounts in the interstitial fluid surrounding the cells, which diffuses easily in the blood capillaries (around 2%).
Majority of CO2 diffuses directly in Red Blood Cells from tissues due to concentration differences.
The large concentration of CO2 (70%) reacts with water in RBC to form Carbonic Acid (H2Co3).
The formed carbonic acid is broken down to water and bicarbonate ions (HCO3-) ions by the action of the enzyme “Carbonic Anhydrase”.
The bicarbonate ions enter the plasma from RBC and maintain the pH of blood.
Some part of Co2 entered in RBC (20%) reacts with hemoglobin and forms a complex called “Carbaminohemoglobin”.
On reaching the lungs, the dissolved CO2 in plasma directly diffuses in alveoli.
The carbaminohemoglobin complex dissociates as partial air pressure of CO2 is higher in alveoli than in blood; the free CO2 diffuses into the alveoli for exchange with oxygen.
The bicarbonate ions dissolved in plasma renters RBC and combines with water to form Carbonic acid, the formed carbonic acid dissociates into water and carbon dioxide.
The formed carbon dioxide diffuses into alveoli.