Respiration
Respiration is a process that releases energy from food substances in all of the living cells. 

Chemical reactions in living cells that break down nutrient molecules to release energy in the presence of oxygen are known as aerobic respiration reactions. 

Chemical reactions in living cells that break down food molecules to release energy in the absence of oxygen are known as anaerobic respiration reactions. These release much less energy per glucose molecule compared to aerobic respiration. 

There are two types of anaerobic respiration:

1. In humans, glucose is broken down into lactic acid with the help of an enzyme, releasing a small amount of energy. C6H12O6-> 2C3H5OH + energy 

2. In microorganisms, glucose breaks down into alcohol and carbon dioxide, releasing a small amount of energy. C6H12O6 -> 2C2H5OH + 2CO2 + energy. Microorganisms can respire both aerobically and anaerobically. 

Lactic Acid and Oxygen Debt 

Muscle cells in the body respire the most. During vigorous exercise, lactic acid builds up in the muscles and blood. Lactic acid needs to be broken down as it is a poisonous compound. To break the lactic acid down, we need oxygen. This extra amount of oxygen which is required to break down the lactic acid is known as oxygen debt

Human Gaseous Exchange 

Gaseous exchange means taking in oxygen and releasing carbon dioxide, while cellular respiration refers to the burning of oxygen with glucose and the release of energy, giving out oxygen. 

The rate and depth of breathing increase after exercise. When a person exercises, the cells respire more efficiently, increasing the production of carbon dioxide in the blood. Thus, more oxygen will diffuse in, and the stimulus for breathing increases. 

The inner lining of the trachea is called ciliated epithelium. There are two cells present in the inner lining of the trachea. One of them is ciliated cell, while the other is goblet cell. Goblet cells produce a sticky substance known as mucus. Mucus traps the pathogens which results in the narrowing of the airways. Therefore, cilia beat in a continuous fashion to sweep mucus with trapped pathogen towards the throat. From the pharynx, it enters the oesophagus. It then enters the stomach where it is digested by hydrochloric acid. When a person smokes, the sticky tar goes in their body sticking to the cilia. Therefore, there will be no sweeping action making the airways narrow and blocked. 

Features of Gaseous Exchange Surfaces in Humans: 


There are cells present in the lungs with a large surface area. If all these cells are put together, they would be about the size of a tennis court. 

The cell surface membrane is thin. This takes lesser time for the diffusion of oxygen and carbon dioxide. 

The association with the blood Capillaries provides a good blood supply. 

Lungs can hold large amounts of air providing good ventilation. 

Associated Organs 


Ribcage:

The rib cage protects the lungs. During the process of breathing in, the rib cage lifts upwards and outwards, resulting in the enlargement of the chest cavity to hold more air. While breathing out, the ribcage will go back to its original position which is inwards and downwards. 

Intercostal Muscles: 

These are an antagonistic pair of muscles. They are of two types: internal muscles and external muscles. When you breathe in, external Intercostal muscles contract while the internal Intercostal muscles relax. This process is vice versa for breathing out. 

Diaphragm:

Diaphragm is a dome-shaped tissue that separates the chest cavity from the abdomen. When you breathe in, it will contract and flatten. When you breathe out, it will relax and return to its dome shape. 

When you breathe in, the chest cavity volume increases, thus the air pressure will drop. This will cause the air from outside to diffuse in. This phenomenon is known as the ventilation of the lungs. 

Inspired air comprises of 21% oxygen, 0.04% carbon dioxide, and 78% nitrogen. Expired air has 16% oxygen, 4% carbon dioxide, and 78% nitrogen.

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