Different types of breathing mechanisms are seen among different groups of animals. Animals like sponges, flatforms, etc., use their entire body surface for breathing, earthworms use their skin while insects use the tracheal system for breathing. Pisces and aquatic arthropods like fish have gills for breathing, while amphibians and mammals use lungs for breathing.

Human Respiratory System

Pharynx: Commonly known as the passage for food and air

Trachea: It is a straight tube extending up to the mid-thoracic cavity.

Larynx: It is a cartilaginous box that helps in sound production and is also called Soundbox.

Bronchi: These are the two large tubes that carry air from the windpipe to the lungs. The human system has a left and right main bronchus in each lung.

Alveoli: where the lungs and the blood exchange carbon dioxide and oxygen during the breathing process.

 

Some following steps involve the respiration process.

• Breathing by which oxygen is drawn in, and carbon dioxide is released out.
• Diffusion of Oxygen and Carbon dioxide across the alveolar membrane
• Blood transport Oxygen and Carbon dioxide
• Diffusion of Oxygen and Carbon dioxide between tissues and blood
• Utilization of Oxygen by the cells for catabolic reactions and it results in a release of Carbon Dioxide

There are two stages in Breathing:

1. Inspiration: In this stage, the atmospheric air is taken into the body.
2. Expiration: Alveolar air is released in this stage of the process. 

• A pressure gradient between the lungs and atmosphere controls air movement into and out of the lungs. Inspiration occurs when the pressure inside the lungs is less, while expiration occurs when the pressure is more inside the lungs than outside.
• The diaphragm and internal and external intercostal muscles help in the generation of a pressure gradient.
• The contraction of the diaphragm increases the volume of the thoracic cavity resulting in a decrease in pressure than the atmospheric pressure, which forces the air from outside to move into the lungs, i.e. inspiration.
• Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions and reduces the thoracic volume leading to an increase in intrapulmonary pressure to slightly above the atmospheric pressure, i.e. expiration.
• A spirometer helps in the clinical assessment that measures the volume of air involved in breathing movement.

Respiratory Volumes and Capacities

Tidal Volume(TV)	Approx. 500ml
Inspiratory Reserve Volume (IRV)	2500ml-3000ml
Expiratory Reserve Volume (ERV):	1000ml-1100ml
Residual Volume (RV)	1100ml-1200ml
Inspiratory Capacity (IC)	TV+IRV
Expiratory Capacity (EC)	TV+ERV
Functional Residual Capacity (FRC)	ERV+RV
Vital Capacity (VC)	ERV+TV+IRV
Total Lung Capacity	VC+RV

The exchange takes place primarily at alveoli, but it also occurs between tissues and blood. Carbon dioxide and Oxygen are exchanged at these two places by diffusion, mainly through the pressure gradient. The solubility of gases and the diameter of membranes are essential factors that can have an impact on the rate of diffusion.

Partial pressure is known as pressure contributed by an individual gas in a mixture of gases. It is represented as pO₂ and pCO₂.

• The solubility of carbon dioxide is 20-25 times faster than oxygen.
• The diffusion membrane comprises three primary layers: the endothelium of alveolar capillaries, the thin squamous epithelium of alveoli, and the basement substance in between these two layers.

Blood is the medium that transports Oxygen and Carbon dioxide. About 97 percent of oxygen is transported by red blood cells, while the rest of the 3 percent of oxygen is carried in a dissolved state via the plasma. Nearly twenty to twenty-five percent of carbon dioxide is taken by RBCs- seventy percent as bicarbonate while the remaining is carried through plasma.

Transport of Oxygen

Oxyhaemoglobin is a combined form of haemoglobin in RBC with oxygen. Haemoglobin molecules can carry maximum wall molecules of oxygen. The binding of oxygen with haemoglobin is related to the partial pressure of oxygen. The sigmoid curve is formed with the percentage of saturation of haemoglobin and partial pressure of oxygen. This curve is called the oxygen dissociation curve.

Transport of Carbon dioxide 

Carbamino-haemoglobin is haemoglobin that carries carbon dioxide, which is associated with the partial pressure of carbon dioxide. When the partial pressure of carbon dioxide is more and partial pressure of oxygen is less, more binding of CO₂ occurs whereas, when the pCO₂ is low and pO₂ is high, dissociation of CO₂ from the carbamino-haemoglobin takes place.

The human body has the ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissue, which is done by the neural system. The respiratory rhythms centre is present in the middle region of the brain that is responsible for this regulation. Pneumatic is another centre present in the pons region of the brain which can moderate the function of the respiratory rhythm centre. A chemosensitive area is situated adjacent to the rhythm centre, which is highly sensitive to carbon dioxide.

1. Asthma: it is the difficulty in breathing due to inflammation of the bronchi

2. Emphysema: it is a chronic disorder due to which the respiratory surface is decreased.

3. Occupational Respiratory Disorder: it is caused by certain industries like stone breaking in which the body's defence mechanism can not fully cope with the situation, which causes severe lung damage.

Here are the class 11 Biology NCERT solutions breathing and exchange of gases.

1. Explain the effects on the Respiratory System of a man while climbing a hill?

 When the altitudes increase, the oxygen level decreases and results in a lack or shortage of oxygen in hilly areas. The amount of oxygen also reduces in the blood, which causes increased respiratory rate in the human body. At the same time, in order to increase the oxygen supply to the blood, the heart starts beating faster.

2. Explain Transport of Gases?

Blood is the medium of transport for oxygen and carbon dioxide. About 97% of oxygen is transported by red blood cells, while the remaining 3% of oxygen is carried in a dissolved state through the plasma. Nearly 20-25% of carbon dioxide is transported by RBCs, whereas 70% is carried as bicarbonate while the remaining is carried in a dissolved state through plasma.

3. Explain the site of the Gaseous Exchange in an insect?

The tracheal system is a network of tubes in insects where the exchange of gases takes place. There are tiny holes present on the insect’s body edge called stigmas. From spiracles connected through a network of tubes, oxygen enters the insect body, and carbon dioxide exits through other paths.