Breathing and Mechanism of Gas Exchange Respiration in Rabbit

The respiration occurs in two phases namely 1. Inspiration and 2. Expiration. The oxygen is taken inside during inspiration while carbon dioxide is expelled out during expiration.

Inspiration:

During inspiration, the external intercostal muscles contract, so the ribs are bent forward and outward and the sternum moves forward. At the same time the diaphragm contracts and becomes flat. As a result the thoracic cavity increases in all dimensions. The volume of the lungs also increases as the air rushes into them through the respiratory passage. This occurs until the pressure of air in the lungs becomes equal to that of the atmosphere. The air passes through external nares, nasal chamber, internal nares, pharynx, glottis, larynx, trachea, bronchi, bronchioles, air sacs, and ultimately into alveoli. While air passes through nasal chambers, it becomes warm, moist, sterilized and free from dust particles.

The exchange of gases takes place in the alveoli. The blood flowing in the walls of the alveoli is very nearer to the fresh air. Oxygen from the air diffuses into the blood and the carbon dioxide of the blood diffuses into the air through the thin walls of alveoli. Thus the air of alveoli becomes impure and the carbon dioxide is to be sent outside immediately.

Expiration:

During expiration the intercostal muscles relax. The ribs are bent inwards and backwards and the sternum comes to its normal position. The diaphragm also relaxes and becomes dome-shaped. As a result, the volume of the thoracic cavity decreases, producing a huge pressure on the lungs which are then compressed. Thus the air with carbon dioxide is expelled from the lungs into the bronchi, trachea nasal passage, and ultimately to the outside through external nostrils.

During expiration, the lungs never become empty and some amount of residual air always remains in them. This air is called Residual air.

Transport of Respiratory Gases

The intake of oxygen and the expulsion of carbon dioxide by the blood of alveolar capillaries can be explained by diffusion. The gases pass from regions of high pressure to those of low pressure. The partial pressure of O₂ in the alveolar air is about 100 mm Hg. In contrast, that of the atmosphere is 159 mm Hg. The partial pressure of O₂ in the blood of alveolar capillaries is 40 mm Hg. Accordingly, the oxygen diffuses from the alveoli into the blood of alveolar capillaries.

Similarly, the partial pressure of carbon dioxide in the alveolar capillaries is 46 mm Hg while that of alveoli is 40 mm Hg. This results in the diffusion of CO₂ out of the blood into the alveoli. Thus the diffusion gradient in the diffusion of O₂ and CO₂ determines the direction of their flow.

Transport of oxygen:

Oxygen from the capillaries of alveoli is transported to the level of tissues through the blood. Hemoglobin present in R.B.C. helps in the transportation of O₂ to different organs. Haemoglobin is made up of four subunits, each of which comprises a haeme and a polypeptide chain (globin). The haeme consists of a porphyrin ring with one atom of iron in the center. The iron of each haeme unit can combine with one molecule of oxygen. Thus each hemoglobin molecule can carry four molecules of oxygen. The oxygen molecules are added one at a time.

Hb₄ + O₂ → Hb₄O₂

Hb₄O₂ + O₂ → Hb₄O₄

Hb₄O₄ + O₂ → Hb₄O₆

Hb₄O₆ + O₂ → Hb₄O₈

The combination of the first subunit of Haemoglobin with O₂ increases the affinity of the second, and oxygenation of the second increases the relationship of the third, and so on. The oxyhemoglobin is transported, to the tissues along with blood. The oxygen pressure in the arterial blood is 100 mm Hg while in the cells it is 1 to 40 mm Hg. Accordingly, O₂ is dissociated from the oxyhemoglobin and diffuses out from the blood through the capillary walls into the cells. This is made possible because the combination of O₂ with hemoglobin is reversible. The reduced hemoglobin is further transported through the blood to the lungs to repeat the cycle once again. The O₂ diffused into the tissues is involved in the oxidation of carbohydrates to release CO₂ along with energy.

At high O₂ pressure, the hemoglobin combines with O₂ to form oxyhemoglobin. At low O₂ pressure, oxyhemoglobin dissociates to liberate O₂ that diffuses into tissues. At any given O₂ concentration, there is a definite proportion between the amount of hemoglobin and oxyhemoglobin.

Transport of carbon dioxide:

Carbon dioxide is released from the tissues as a result of metabolic activities and diffuses into the blood. Carbon dioxide is transported in the blood in the following three ways.

1. Transport of CO₂ in the form of carbonic acid:
   • As carbon dioxide enters the blood from the tissues, it combines with water in the plasma to form carbonic acid. About 5% of CO₂ is carried in the plasma as carbonic acid.
   • CO₂ + H₂O → H₂CO₃
2. Transport of CO₂ as carbaminocompounds:
   • About 10% of the CO₂ is transported in the form of carbaminocompounds. It combines directly with the amino group of (-NH₂) hemoglobin to form carbaminohaemoglobin.
   • CO₂ + Hb.NH₂ → Hb.NH.COOH
3. Transport of CO₂ as bicarbonates:
   • About 85% of the total CO₂ is carried in the form of bicarbonates in the plasma and red blood cells. As CO₂ enters the blood cells from the tissues, it combines with water to form carbonic acid (H₂CO₃). It dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-). The latter diffuses into the plasma and forms sodium and potassium bicarbonates with sodium and potassium ions.
   • H₂CO₃ → H⁺ + HCO₃^-
   • Na⁺ + HCO₃ → NaHCO₃
   • K⁺ + HCO₃ e → KHCO₃
   • The red blood cells contain an enzyme called carbonic anhydrase. This increases the speed of reaction between CO₂ and H₂O resulting in the formation of H₂CO₃ that is further converted into bicarbonates. Carbonic anhydrase also catalyzes the reversible reaction which is the splitting of carbonic acid into water and carbon dioxide.
   • H₂CO₃ → CO₂ + H₂O