Respiratory system is a catabolic process of release of energy mostly by the oxidation of foods. Oxygen obtained from the surrouding medium is utilised in the production of 'metabolic water', an end product of the 'burning of calories'. The direction offlow of blood in the gills and the direction offlow of water are in 'counter current flow' mechanism for efficient 'oxygenation' of blood. Birds have developed a technique of continuous exchange of gases even during expiration, with the help of a unique feature - the presence of 'air sacs' and parabronchi'. Insects and some other arthropods supply oxygen directly to each cell via the tracheae and tracheoles. Thus every cell can virtually breathe air directly, making the tracheate animals' very active.
At a height of about 6000 m the pC>2 bcomes almost half of what it is at the mean sea level, hence the mountain sickness' in people ascending mountains. The 'ribcage' and the 'diaphragm' help mammals breathe in air more effectively. Homeostasis of oxygen and carbon dioxide are under the control of the respiratory centre. Inhalation and exhalation are under the control of the medulla oblongata. There is apneumotaxic centre in the pons and it controls the rate anddepth of breathing. Man cannot hold his breath for long. He is forced to breathe in. Elephant seal, a mammal can remain under water for up to two hours. The muscles of the elephant seal like in some other aquatic mammals contains myoglobin (muscle haemoglobin) which has more affinity for oxygen.
All living cells need constant supply of energy in order to carryout various metabolic activities. Energy is produced by the oxidation of micromolecules of digested food such as the glucose and amino acids, and fatty acids. These are the transported to the body cells through the circulatory system. Oxygen is utilized by organisms to breakdown stored food materials such as glucose, fatty acids etc. Carbon dioxide which is harmful (dissolves in water to form carbonic acid, which lowers the pH of blood threatening homeostasis) is also released during the above catabolic reactions. It is therefore, evident that 02 has to be continuously provided to the body cells and COa produced by the cells has to be sent out. Respiration is a vital feature of life. The process of exchange of 02 from the medium (air or water) with COa produced by the cells at the same time is called breathing' (Ref: NCERT Vol I, Page 268). However, the process of respiration during which the air is inhaled into the lungs and exhaled out of the lungs is called 'breathing'. Many authors described the process of gaseous exchange as 'external respiration' or 'ventilation' because it emphasizes that the entry of oxygen and the exit of COa happen at places other than the energy releasing sites. More accurately the "processes leading to including the chemical breakdown of food materials to provide energy for life is called the respiration. As that the chemical breakdown of the nutrients occurs inside the living cells of every organism, it is called internal respiration or cellular respiration.
Anaerobic respiration is the incomplete break down of organic molecules with less yield of energy; it takes place in the absence of in organisms such as yeast, bacteria, muscles(under certain conditions) etc.
C6H12Oe-> 2COa+ 2C2H5OH(ethanol)+2ATP
C6H1206-» 2C3H603(lactic acid)+2ATP
Aerobic (requiring oxygen) respiration yields more energy (grdually) due to complete breakdown of organic molecules, utilizing oxygen.
C6H1206+6H20+602 -> 6C02+12H20+36ATP
