Mechanism of Breathing, Transport of Gases, Role of Respiratory Pigments in Humans

Mechanism of Breathing in Humans

Breathing is a procedure in which fresh air containing more oxygen is pumped into the lungs and air with more concentration ofCO2 is pumped out of the lungs. To put it simply breathing is a mechanical procedure including two stages, inspiration or inhalation and expiration. During inhalation, fresh air moves in and in expiration air with low O2 and high CO2concentration vacates the lungs.

During rest breathing rate is rhythmically at the frequency of 15 to 20 times per minute in humans. To understand the system of breathing we should keep in mind three elements related to lungs and associated structures.

  1. Lungs are spongy in nature. The lungs themselves neither pull air in nor can they press it out. During inspiration, passive expansion of flexible lungs happens and expiration is due to a passive contraction of lungs.
  2. The floor of the chest cavity is the diaphragm, which is a muscular sheet. The shape of the diaphragm is more domelike when its muscles are relaxed. On the other hand, when the muscles of the diaphragm contract its shape becomes less domelike.
  3. Walls of the chest cavity are composed of ribs and intercostal muscles. When muscles between the ribs contract, the ribs are elevated and when muscles in between ribs are relaxed the ribs settle down.

Throughout inspiration, the space inside the chest cavity is increased in two ways. Firstly, the muscles of ribs contract and raise the ribs upwards and forwards and secondly, the muscles of the diaphragm also contract and diaphragm end up being less domelike.

This downward motion of the diaphragm and external and upward movement of the ribs causes a boost in the chest cavity and decreases pressure. When the pressure from the lungs is removed, they broaden. With the expansion of the lungs, vacuum is created inside the lungs in which the air rushes from the outside due to higher air pressure. This is called inspiration.



Throughout expiration the muscles of ribs are relaxed and the ribs move downward and inward. In this way from the sides of the chest cavity, space becomes less. At the same time, the muscles of the diaphragm also relax becoming more domelike and the chest cavity is also lowered from the floor. This reduction in space of the chest cavity applies pressure on the lungs. When lungs have pressed the air inside the lungs moves out of the lungs and this is expiration.

Transport of Respiratory Gases

Consumption of oxygen and release of carbon dioxide by blood travelling through capillaries of alveoli is brought about by the following factors.

  1. Diffusion of oxygen in and CO2 out occurs because of the difference in partial pressures of these gases.
  2. Within the rich network of blood vessels surrounding the alveoli, blood is distributed in incredibly thin layers and, therefore, exposed to the big alveolar surface area.
  3. Blood in the lungs is separated from the alveolar air by incredibly thin membranes of the capillaries and alveoli.
Transportation of Oxygen

In humans the respiratory pigment is haemoglobin. It is present in red blood cells. Haemoglobin easily integrates with oxygen to form bright red oxyhaemoglobin. Oxyhaemoglobin is unstable and splits into the typical purple-red coloured haemoglobin and oxygen in the conditions of low oxygen concentration and less pressure.

Carbonic anhydrase enzyme present in R.B.C. facilitates this activity. In this way haemoglobin acts as an efficient oxygen carrier. A small percentage of oxygen likewise gets dissolved in the blood plasma.

Haemoglobin can take in optimal oxygen at the water level. The maximum quantity of oxygen which normal human blood takes in and brings at the sea-level is about 20ml/100ml of blood. This is the maximum capability of haemoglobin for oxygen when it is completely oxygenated. Under normal conditions, the blood of alveoli of the lungs is not totally oxygenated. When an oxygen tension is 115mm mercury, haemoglobin is 98 percent saturated and, therefore, includes 19.6 ml of oxygen per 100ml of blood.

This indicates that haemoglobin can be almost completely oxygenated by an oxygen pressure of 100 mm mercury, which exists in the lungs. Any higher oxygen pressure would have the very same outcome.

When oxygen pressure falls below 60 mm mercury, as in numerous cells and tissues, the oxygen saturation of haemoglobin decreases very greatly. This leads to the release of large quantities of oxygen from haemoglobin. In this way in the tissue where the oxygen tension is low oxyhaemoglobin dissociates rapidly.

There are 3 essential elements which affect the capacity of haemoglobin to integrate with oxygen.

  1. Carbon dioxide

When CO2 pressure increases, the oxygen tension reduces, the capability of haemoglobin to hold oxygen becomes less. In this way, increased CO2 tension favours the greater liberation of oxygen from the blood to the tissue.

  1. Temperature

The rise in temperature level also triggers a reduction in the oxygen-carrying capability of the blood, e.g., in the increased muscular activity.

  1. pH

The pH of blood also influences the degree to which oxygen binds to haemoglobin. As the pH of the blood declines, the amount of oxygen bound to haemoglobin likewise declines. This occurs because decreased pH results from a boost in hydrogen ions, and the hydrogen ions combine with the protein part of the haemoglobin particles, triggering a decrease in the ability of haemoglobin to bind oxygen. On the other hand, a boost in blood pH leads to an increased capability of haemoglobin to bind oxygen.

Transport of Carbon Dioxide

Carbon dioxide is more soluble than oxygen and dissolves easily in the tissue fluid surrounding the cells. From the tissue fluid, dissolved CO2 passes to the plasma within the blood capillaries. Carbon dioxide is transported in the blood in a number of different states.

  1. Some of the CO2 (about 20%) is brought as carboxyhaemoglobin. Carboxyhaemoglobin is formed when carbon dioxide integrates with the amino group of haemoglobin.
  2. Other plasma proteins likewise carry about 5% carbon dioxide from the body fluids to the capillaries of lungs.
  3. About 70% of carbon dioxide is carried as bicarbonate ion combined with sodium in the plasma. As carbon dioxide from tissue fluid enters the blood vessels it combines to form carbonic acid.


The carbonic acid divides rapidly and ionizes to produce hydrogen ions and bicarbonate ions.


When blood leaves the capillary most of the carbon dioxide remains in the kind of bicarbonate ions. All these reactions are reversible. In the lung’s bicarbonate ions combine with hydrogen ions to form carbonic acid which splits into water and CO2. It is this carbon dioxide which diffuses out from the blood vessels of the lungs into the space of alveolar sac.


  1. A small amount of CO2 is likewise carried by corpuscles combined with potassium.
Carbon Dioxide Concentration

In Arterial and Venous Blood, it has been discovered that arterial blood consists of about 50 ml of carbon dioxide per 100 ml of blood whereas venous blood has 54 ml of carbon dioxide per 100 ml of blood. In this way, every 100 ml of blood uses up simply 4 ml of CO2 as it passes through the tissues and gives of 4 ml of CO2 per 100 ml of blood as it passes through the lungs.

Role of Respiratory Pigments

Numerous types of respiratory pigments exist in different animals. The pigment combines with oxygen reversibly and increases the oxygen carrying capacity of the blood.

Haemoglobin is the most crucial protein present in numerous animals including humans. Haemoglobin in man increases the oxygen carrying capability of the blood to about 75 times.


Myoglobin is haemoglobin-like iron-containing protein pigment taking place in muscle fibres. Myoglobin is likewise called muscle haemoglobin. It works as an intermediate substance for the transfer of oxygen from haemoglobin to aerobic metabolic procedures of the muscle cells. It can also store some oxygen.

Myoglobin includes simply one polypeptide chain associated with an iron-containing ring structure which can bind with one molecule of oxygen. The affinity of myoglobin to integrate with oxygen is much greater as compared to haemoglobin.