We know that all living beings require oxygen to survive and also are well aware of the fact that the sustenance of life is impossible in the absence of oxygen. A common man understands respiration simply as the exchange of gases, that is, intake of oxygen and the release of carbon dioxide. However, scientifically, the process of respiration is much more complex.
Respiration includes both:
a) Breathing, which involves gaseous exchange implying the intake of oxygen from the atmosphere and the release of carbon dioxide.
b) Cellular respiration which is the breakdown of simple food, thereby releasing the energy inside the cells. This energy is released in the form of ATP.
Types of cellular respiration
Depending upon the availability of atmospheric oxygen, cellular respiration is divided into two categories, which are:
- Aerobic Respiration
- Anaerobic Respiration
Let us now understand the two categories of respiration and the pathways involved therein.
The complete breakdown of respiratory substrates into carbon dioxide and water in the presence of atmospheric oxygen is known as aerobic respiration.
Aerobic respiration occurs in two steps:
- First, the glycolysis that occurs in the cell cytoplasm, and then second, the Kreb’s cycle which takes place in the mitochondria.
- In glycolysis, a glucose molecule is broken down into two molecules of pyruvic acid.
And in Kreb’s cycle, pyruvic acid is further broken down into carbon dioxide, water, and the energy molecule-ATP.
The breakdown of respiratory substrates in the absence of oxygen is termed anaerobic respiration. It involves the incomplete breakdown of glucose into ethanol and carbon dioxide through one pathway, and its breakdown into lactic acid through another.
In anaerobic respiration, the first step of glycolysis is the same as in aerobic respiration. In certain anaerobic microorganisms, the pyruvic acid thus obtained, is converted into ethanol, carbon dioxide, and energy. The anaerobic respiration also occurs inside the muscle cells during vigorous activities, when the oxygen is scanty. In such instances, pyruvic acid breaks down into lactic acid and energy.
We are now in a position to summarize the process of cellular respiration. This schematic figure shows the various pathways by which the glucose molecule can be broken down into simpler products in the presence or absence of oxygen.
There are some specific organs for gaseous exchange in different animals such as gills in fish, skin in earthworms, gills and lungs in frogs, and the tracheal system in insects. But as compared to those the respiratory system in human beings is very complex.
The respiratory system in human beings consists of a mechanism for letting the fresh air from outside flow into the lungs and a mechanism for the expulsion of air from the lungs.
Parts of the body involved in the respiratory process are:
- External Nostrils
- Nasal Cavities
- Internal nostrils
- Larynx and Trachea
The respiratory organs include a pair of lungs which have Bronchi, Bronchioles, and alveolar sac. The air is taken into the body through the nostrils. The air passing through the nostrils is filtered by fine hair that lines the passage into the lungs.
Rings of cartilage present in the windpipe ensure that the air passage does not collapse Within the lungs the passage divides into smaller and smaller tubes which terminate in balloon-like structures called alveoli. The alveoli provide a surface where the exchange of gases can take place. The walls of the alveoli contain an extensive network of blood vessels
The oxygen in the alveolar sac is taken up by the blood in the blood vessels, to be transported to all the cells in the body. The blood takes up oxygen from the alveolar sac and release back CO2 to the alveolar sac. The exchange of CO2 and oxygen from blood to alveolar sac and vice versa takes place by the process of diffusion
Mechanism of breathing-
Lungs cannot expand or contract on their own. As we breathe in, we lift our ribs and flatten our diaphragm and the chest cavity expands as a result. Because of this, the air is sucked into the lungs and fills the alveoli.
When we breathe out, the thorax cavity contracts and air moves out. The intake of fresh air into the lungs is called inhalation and expulsion of air from the lungs is called exhalation.