Breathing and Exchange of Gases

Introduction

Breathing, also known as respiration, is the process of exchanging O2 from the atmosphere with CO2 produced by the cells. The breathing mechanism varies in different groups of animals; lower invertebrates like sponges, flatworms, coelenterates breathe through diffusion over the entire body surface. Earthworms breathe through their moist skin and cuticle, insects have tracheal tubes, aquatic arthropods, and molluscs have gills (branchial respiration). In contrast, terrestrial animals have vascularised bags called lungs (pulmonary respiration) for the exchange of gases. Fishes breathe through their gills, whereas reptiles, birds, amphibians and mammals breathe through their lungs. Frogs and other amphibians may also breathe through their moist skin (cutaneous respiration).

The steps of respiration are as follows:

1. Breathing or pulmonary respiration is the process of inhalation and exhalation of O2  and  CO2, respectively.
2. Gas diffusion between the alveolar membrane and blood.
3. Gas is transported through the blood to the body cells.
4. O2 and CO2 diffusion take place between blood and tissues.
5. Cells use O2 for catabolic activities and the release of CO2.

Organs involved in respiration in human

1. External nostrils- A pair of external nostrils are present at the first end of the respiratory passage.
2. Nasal Chambers- There are two nasal chambers with epithelial lining of cavities known as the respiratory epithelium. This lining helps air humid and brings air temperature up to the body temperature. The mucus is produced by goblet cells and glands, making the surface sticky for trapping dust particles in the air.
3. Internal nares- These are the posterior opening of nasal cavities that lead to the nasopharynx
4. Nasopharynx- It is a part of the pharynx where only air passes through it and opens into the trachea through the glottis of the larynx region
5. Larynx- It is also known as a soundbox or voice box because it helps in the production of sound. It is made up of cartilage, present at the upper part of the trachea with an opening called glottis, which is covered by the epiglottis.
6. Trachea- It is also known as the windpipe. It is a straight tube extending up to the mid-thoracic cavity. This tube divides into right and left bronchi. It is an incomplete cartilaginous ring that prevents it from collapsing.
7. Bronchi- It undergoes repeated divisions to form branches called bronchioles, which give rise to a number of very thin irregular walled balloon-like structures called alveoli. They are well-supplied blood vessels.
8. Alveoli- There are about 300 million alveoli present in two lungs; richly supplied blood vessels helps in the exchange of gases. Surfactant is a surface-active agent lecithin secreted by type II alveolar epithelial cells; it reduces the surface tension between alveolar fluid and air and also prevents collapsing of alveoli.
9. Lungs- A pair of lungs is present in humans in an airtight chamber known as thoracic cavity or chest cavity. Each lung is enclosed by two membranes known as pleural membranes; the outer membrane is in close contact with the thoracic cavity, whereas the inner pleural membrane is in close contact with the lung surface. In between the two, there is a pleural fluid that helps to lubricate the pleural membranes so that they may slide over each Other without friction during the breathing process.

Mechanism of breathing

The breathing mechanism involves two processes: inspiration and expiration. When the pressure inside the lungs (intrapulmonary pressure) is less than the atmospheric pressure, inspiration can occur. Similarly, when the intrapulmonary pressure exceeds the ambient pressure, expiration occurs. The diaphragm and a specialised collection of muscles between the ribs called the external and internal intercostal muscles to help create such pressure gradients. The diaphragm contracts, increasing the volume of the thoracic chamber in the anteroposterior axis, which starts the process of inspiration. The ribs and sternum are lifted up by the external intercostal muscles, increasing the volume of the thoracic chamber in the dorso-ventral axis. The increase in thoracic volume results in a comparable increase in pulmonary volume. When pulmonary volume is increased, the intrapulmonary pressure falls below atmospheric pressure, forcing air from outside to enter the lungs (inspiration). The diaphragm and intercostal muscles relax, allowing the diaphragm and sternum to return to their natural positions and reducing thoracic and pulmonary volume. As a result, the intrapulmonary pressure rises slightly over atmospheric pressure, causing air to be expelled from the lungs (expiration).

A spirometer is used to quantify the volume of air involved in breathing movements and helps in the clinical assessment of pulmonary functioning. A healthy person breathes 12-16 times/ min.

Exchange of gaseous

The principal sites of gas exchange are alveoli. Also, gases are exchanged between blood and tissues as well. In these sites, O2 and CO2 are exchanged mostly through simple diffusion based on a pressure/concentration gradient. pO2 in the atmospheric air (159 mm Hg) is higher than that in the alveoli, and pO2 in alveoli is higher than that in the deoxygenated blood in the capillaries of the pulmonary arteries. 

Gases diffuse from higher partial pressure to their lower partial pressure; therefore, O2 from atmospheric air enters the alveoli then to blood, whereas the CO2 movement is in the opposite direction. The pCO2 is higher (45 mm Hg) in deoxygenated blood than that in the alveoli (40 mm Hg), and it is further low in atmospheric air (0.3 mm Hg). Therefore, CO2 moves from deoxygenated blood to alveoli and finally to the atmosphere.

CO2 has a 20-25 times higher solubility than O2; hence, the amount of CO2 that can diffuse through the diffusion membrane is substantially larger than O2. However, the diffusion membrane is composed of three major layers- the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement membrane supporting the squamous epithelium and it is surrounded by the single layer of endothelial cells. Its entire thickness is less than a millimetre (0.2 mm). As a result, all the elements in our body favour O2 diffusion from alveoli to tissues and CO2 diffusion from tissues to alveoli.

Transport of O2

1. In a dissolved state – 3% of oxygen is carried in the dissolved state through plasma.
2. As oxyhemoglobin- About 97% of oxygen is carried by RBC in the form of oxyhaemoglobin in the blood. The capacity of one gram of haemoglobin to combine with oxygen is 1.34 ml. ,i.e., about 20 ml of O2/ 100 ml of blood. During exercise, the oxygen level falls to about 4.4 ml/ 100 ml, approximately 15 ml of oxygen transported by haemoglobin.

Transport of CO2

1. In dissolved form through plasma, 7% of carbon dioxide gets transported in dissolved form, and it is carried in the solution to the lung. CO22 has higher solubility than O2; therefore, only about 3% is transported.
2. About 70% of carbon dioxide is converted to HCO3- and transported in plasma as bicarbonate ions. CO2 diffuses in the RBC where it binds with water forming carbonic acid (H2CO3), which quickly dissociates into hydrogen and bicarbonate ions in presence of carbonic anhydrase, which is unstable and diffuses into the plasma and is carried to the lungs. Here, the process is reversed at the alveolar site, which means that CO2 and  H2O are diffused from blood to the lungs, where pCO2 is low.
3. About 20-25 % is transported as carbaminohaemoglobin by RBCs as carbaminohaemoglobin(HbCO2).

Disorders of the respiratory system

1. Asthma is a breathing problem that causes wheezing and causes inflammation of the bronchi and bronchioles.
2. Emphysema is a chronic lung disease in which the alveolar walls are destroyed, resulting in a reduction in the respiratory surface. Cigarette smoking is one of the leading causes of this.
3. Occupational respiratory disorders, for example silicosis and asbestosis occur to thos working in factories that involve grinding or stone-breaking, produce so much dust that the body gets affected, especially lungs. Long-term exposure can cause inflammation, fibrosis and cancer. 
4. Pneumonia is the infection of the lungs caused by bacteria that leads to the accumulation of mucus and lymph in alveoli, which impairs the exchange of gases, mainly O2 level falls. 

Conclusion 

Living cells utilise O2 for metabolism, produce energy, and release CO2, which is harmful. All invertebrates and vertebrates respire either through the body surface, skin, cuticle, gills or lungs. The rate of breathing in aquatic organisms is much faster than in terrestrial organisms. Breathing and exchange of gases are one of the important proc