Mechanism of Respiration in Human

Respiration is simply defined as the exchange of oxygen and carbon dioxide into the body. 

Physiology of respiration can be described under following phases:

1. External respiration
2. Transport of oxygen
3. Internal respiration
4. Transport of carbon dioxide
   

1. External respiration

   It is the first step of respiration.
   It occurs in lungs.
   It is the exchange of gases by diffusion between alveoli and blood in blood vessel, across respiratory membrane.
   During inspiration, oxygen is taken into the lungs. The partial pressure of oxygen in lungs is higher than that in blood capillaries.
   Therefore, oxygen diffuses from the lungs to the blood.
   Similarly, the venous blood contain high level of CO₂ in comparison to lungs. Due to high partial pressure of CO₂ in blood and low CO₂ in lungs, the CO₂ diffuses from the blood into the lungs in order to maintain the equilibrium.

2. Transport of oxygen

   Oxygen is transported from lungs to the tissue in blood.
   Plasma and RBC are helpful in transporting the oxygen.
   Water present in plasma has poor solubility for oxygen, so it carries only 3% of total oxygen.
   Most of the oxygen (about 97% of total oxygen) is transported by RBC.
   RBC has a respiratory pigment hemoglobin which combines with oxygen to form oxyhemoglobin due to high affinity of hemoglobin with oxygen.
   Almost all oxygen are transported in this form, from lungs to tissue.
   Hb₄ + 4O₂                                  Hb₄O₈
   Oxyhemoglobin compound is highly unstable compound and thus can dissociate quickly as it reaches to tissue.
   NOTE: One hemoglobin molecules contain 4 heme group; each heme can combine with oxygen. Therefore, one Hb can combine with 4 molecules of oxygen. A normal adult person has 15 gram hemoglobin per 100ml of blood. 1 gm of Hb can carry upto 1.34 ml of oxygen. 100 ml of blood conatains about 20ml of oxygen.

3. Internal respiration

   This process takes place inside the cells in various tissue, hence called tissue respiration.
   It includes two steps:
   i. Dissociation of oxyhemoglobin
   As the oxyhemoglobin reaches to the tissue, it quickly dissociates into free oxygen and hemoglobin.
   Oxygen enters into tissue, whereas the hemoglobin returns back to the RBC to pick up more oxygen.
   Hb₄O₈                                                  Hb₄ + 4O₂
   ii. Oxidation of food/ Tissue respiration
   The oxygen oxidizes the glucose in presence of respiratory enzyme and releases energy, water and CO₂. Energy is stored in the form of ATP in mitochondria to perform the metabolic activities whereas the CO₂ is expelled from the tissue.
   C₆H₁₂O₆                                  6CO₂ +6H₂O + Energy

4. Transportation of carbon dioxide

   Carbon dioxide is the waste product which is produced in the body as a result of tissue respiration. Since CO₂ is very toxic, it is essential to remove out from the body. CO₂ is transported from tissue to lungs in following ways:
   i. In the form of carbonic acid
   CO₂ combines with water of RBC to form carbonic acid. The process is catalyzed by an enzyme carbonic anhydrase found in RBC.
   About 7% of the total CO₂ are transported in this form.
   CO₂ + H₂O      carbonic anhydrase        H₂CO₃ (carbonic acid)
   ii. In the form of bicarbonates
   Carbonic acid quickly ionizes to form ions i.e. bicarbonate and hydrogen ions.
   Bicarbonates ions are pumped through RBC membrane into plasma.
   Bicarbonates combines either sodium or potassium of plasma to form sodium bicarbonate or potassium bicarbonate respectively.
   CO₂ is carried from tissue to the lungs through plasma.
   About 70% of CO₂ are transported in this form.
   These bicarbonates are carried through plasma to lungs where they combine with hydrogen ion and form water and carbon dioxide. The CO₂ is expelled out during expiration.
   iii. In the form of carbamino hemoglobin compound
   CO₂ combines with amino group of haemoglobin to form a complex caramino hemoglobin compound.
   CO₂ + HHb-NH₂                                  Hb-NH-COOH
   About 20-23% of CO₂ are transported in this form.

   In this way all the CO₂ produced in tissue are transported to lungs form where it is expelled out through expiration.

Mechanism of Respiration in Human

Tags: Mechanism of Respiration in human, Physiology of Respiration in human, Steps of respiration