respiration

 Process of respiration

Respiration is a biological process. Gas exchange can be observed in some animals externally. 

Respiration in a human is a complex process and it occurs in three stages.

1. Gas exchange between external environment and lungs
2. Gas exchange in alveoli
3. Cellular respiration 

 Intake of oxygen into lungs and removal of gaseous waste in cells occurs in external gas exchange.

The system involved in entering O2 into lungs and release of gaseous waste products  produced during biological processes is the respiratory system.

Function of the respiratory system

 

Main parts of respiratory system are nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and alveoli. Internal surface of nasal cavity is covered with mucus. Due to the presence of mucus in the nasal cavity, the lining of it is moist. There are numerous cilia present on the lining of the nasal cavity. The bacteria, dust and other wastes found in inhaled air stick onto the mucus. This prevents the entry of them into the lungs. By rhythmic  movement of cilia the waste materials are sent out. The materials that are collected at pharynx are removed out with saliva. 

The changes that take place when inhaled air passes through the nasal cavity are as follows.

• Moisturizing/ Humidifying inhaled air
• Warming up of inhaled air up to body temperature
• Removal of foreign matter from inhaled air 

Lungs are present in thoracic cavity. Thorasic cavity is protected by ribs. Inter - costal muscles are present within ribs. The lower limit of thoracic cavity is the diaphragm. Let's study about the initial activity, in the respirotry system. That is the gas exchange between external environment and lungs. 

Inspiration

During inspiration, air enters into lungs. For that, the volume of the lungs should increase. To increase the volume of the lungs, volume of the thoracic cavity should be increased. This occurs due to following changes. Inter-costal muscles contract, therefore ribs move up and sternum moves forward. 

At the same time the diaphragm contracts and reduce its curvature. Due to above activities the volume of the thoracic cavity increases and with that volume of lungs increase. So air enter into lungs through the nose. 

Expiration

For expiration to occur, the volume of the thorasic cavity should decrease to reduce the volume of the lungs.

This occurs due to the following changes. Inter-costal muscles relax. So the sternum and ribs move into its original position. The diaphragm relax and becomes curved. 

Due to these activities the volume of the lungs  decreases, thereby gas inside lungs move out through trachea and then nasal cavity.

The gas exchange that takes place in alveoli

The inhaled air finally reach the alveoli, through nasal cavity, trachea, bronchi, and bronchioles. The O2 concentration in alveoli is greater than that of the blood capillary network around it. Therefore O2 diffuse out of the alveoli into the blood capillaries. Similarly CO2 and water vapour concentration is greater in blood capillaries

Gas exchange takes place in alveoli. The place where gas exchange between external environment and blood takes place is known as respiratory surface. Accordingly, the respiratory surface of human is the wall of alveoli. The exchange of gases takes place by diffusion.

Characteristics of a respiratory surface

The adaptations of the respiratory surfaces for efficient gas exchange are  as follows.

• Surface should be moistened and permeable for gas exchange
• Surface should be thin for diffusion of gases
• A larger surface area to exchange large volume of gas according to the needs of animals
• Surface should be highly vascularized

 In many animals, body cover acts as the respiratory surface and gases exchange through the body cover. The respiratory surface of human is the wall of alveoli and  the adaptations of the alveoli for efficient gas exchange are  as follows.

• Thin alveolar wall
• Moist alveolar surface
• Presence of a blood capillary network around alveoli
• Presence of large number of alveolar sacs 

Cellular respiration 

Oxygen moved through alveoli reacts with simple organic compounds (glucose) in cells. In this chemical reaction energy is released, therefore  respiration   is the process of oxidation of simple foods to produce energy for biological activities within living  cells.

Aerobic respiration and Anaerobic respiration

We discussed the respiration that takes place inside cells in the presence of oxygen. The respiration that takes place in the presence of oxygen is called aerobic respiration. 

Some organisms can respire without O2 . Respiration carried out by organisms without O2 is known as anaerobic respiration. 

Anaerobic respiration that takes place in plants and yeast cells is known as alcohol fermentation. The anaerobic respiration that takes place inside these cells can be given by the following word equation.

Glucose  => Carbon dioxide  + Ethyl alcohol + Energy

When Yeast carries out anaerobic respiration during fermentation, CO2 and Ethyl alcohol is produced. This process is an example for alcohol fermentation.

Animals including human cells also perform anaerobic respiration. The anaerobic respiration that takes place within animal cells is referred to as lactic acid fermen- tation. The products of that is given in the equation given below. 

Glucose => Lactic acid +  Energy