Breathing: Mechanism and Reflex Regulation Easy Notes


Breathing is the process of inflow and outflow of air between the atmosphere and alveoli of lungs. It consists two types of processes teemed as inspiration and expiration.

Inspiration means the intake of air into lungs and expiration means removal of air from the lungs. The inspiration and expiration results in the expansion and contraction of the lungs respectively.

The downward and upward movement of the diaphragm which increases and decreases the diameter of the thoracic cavity. Elevation and depression of the ribs which lengthens and shortens the thoracic cavity.

Inspiratory Mechanism

Inspiratory Mechanism is the process in which fresh air enters the lungs through a respiratory passage. The diaphragm in intercostal muscle and abdominal muscle play an important role in the process of breathing.


During breathing the diaphragm becomes flat and gets lowered by the contraction of its muscle fibres, thereby increasing the volume of the thoracic cavity in length. It is comprise of external intercostal muscles.

External Intercostal Muscles

External intercostal muscles occur between the ribs. These muscles contract and pull the ribs and sternum upward or outward. Thus, the volume of the thoracic cavity increased.

Describe the mechanism of Breathing, how breathing reflex is regulated in the mammal

Abdominal Muscle

Abdominal muscle relaxes and allows compression of abdominal organs by the diaphragm. It is also play a passive role in taking of air. The muscle of the diaphragm and external intercostal muscles are principal muscles of inspiration.

Movement of fresh air into the lungs

By the above mechanism the volume of thoracic cavity increases, as a result, there is a decrease of air pressure in the lungs. The greater pressure outside the body now causes air to flow rapidly into external noses, nasal chamberinternal noses, pharynx, glottis, larynx, trachea, bronchi, bronchioles, alveolar duct, alveoli.

Entering of air in lungs step by step from external noses to alveoli of lungs;

External noses > Nasal chamber > internal noses > Pharynx > Glottis Larynx > Trachea > Bronchi > bronchioles > Alveolar ducts > Alveoli of lungs.

Conditioning and filtration of air

The mucus secreted by the mucous gland keeps the internal lining of nasal chamber moist. Mucus check the entry of dust particles into the respiratory tract and kills the bacteria of the air. Blood vessels of nasal chamber radiate heat and warm the air passing through the nasal chamber.

Exchange of O2 and CO2 in lungs

Due to the higher partial pressure (100 mm Hg) of O2 in alveolar air, it diffuses into the blood capillaries of lungs and combines with haemoglobin to form oxyhemoglobin. The partial pressure of C02 of blood capillaries of lungs is higher (46 mm Hg) than the alveolar air (40 mm Hg), therefore C02 diffuses from the blood into the alveolar air.

Describe the mechanism of Breathing, how breathing reflex is regulated in the mammal

Expiratory Mechanism

During expiration containing C02 foul air comes out from the lungsExpiration is a passive process which takes place as follows;

  • Internal intercostal muscle contract so that they pull the ribs downward and inward decreasing the size of the thoracic cavity.
  • The muscles fibre of diaphragm relaxes making it convex decreasing the volume of the thoracic cavity
  • The abdominal muscles contract (external and internal muscles contract) to compress the abdomen and pushes it to contain towards the diaphragm.The internal intercostal muscle and abdominal muscle are called muscles of expiration.

Movement of foul air out of the lungs

The overall volume of thoracic cavity decreases and foul air goes outside from the lungs in the following manner given below.

Alveoli > alveolar duct > bronchioles > bronchi > trachea > larynx > glottis > Pharynx > Internal noses > Nasal chamber > external noses > outside.

Regulation of breathing reflexes

Respiration is an involuntary process and is carried out automatically at a constant rate under normal conditions. The contraction of various respiratory muscles at the proper time and with proper strength to secure adequate gaseous exchanges is coordinated by a complex series of reflexes maintained by the nervous system and also, to some extent by C02 concentration. 

The breathing reflexes are controlled in the following ways;

Nervous Regulation

Nervous regulation followed by these following processes;

Medullary respiratory centre is situated on the floor of forth ventricle present in the medulla.

The centre is bilateral and two halves are connected together by commissural neurons. The sides of this center are connected with the motor respiratory neurons.

The neuron cells of the centre are connected to the breathing apparatus with the motor as well as sensory or afferent and efferent nerves and thus maintain the reflex arc. These cells are sensitive to changes in the chemical organization of blood and concentration of CO2 in plasma. Each half of the respiratory centre is composed of two parts, Inspiratory centre and an expiratory centre.

Describe the mechanism of Breathing, how breathing reflex is regulated in the mammal

The expiratory centre lies above the inspiratory centre. Out of the two only one work at a time. The inspiratory centre worked in normal breathing and expiratory centre during force expiratory conditions like sneezing, laughing and coughing.

The expiratory neurons are excited by the aforesaid stimuli, whereas inspiratory neurons discharge spontaneously. 

In the wall of alveoli of lungs are present stretch receptors, which are stimulated by the expansion and relaxation of lungs and propagate inhibitory impulses to the inspiratory and expiratory parts of the respiratory centre respectively. The receptors are innervated by the branches of the vagus nerve.

In addition, a pneumotaxic centre is present in the pons, which acts as an inhibitory nerve centre and connected with both inspiratory and expiratory parts of the respiratory centre.

The respiratory centre is connected with the lungs, carotid sinuses, aortic arch etc. through afferent nerves, with the respiratory muscles through efferent nerves and with the pneumotaxic centre by both afferent and efferent nerves

The inspiratory centre propagates impulse spontaneously to the intercoastal muscles. As a result of their contraction, the thorax and lungs expand and in doing so stimulate the stretch receptors

The stretch receptors initiate impulses at a very high rate, which are propagated by afferent neurons in the vagus nerve to the pneumotaxis centre.

The activated pneumotaxic centre is inhibitory in nature and sends impulses to the expiratory centre. The expiratory centre inhibits inspiratory centre.

As a result, inspiration ends and expiration starts. This reflex was described by Hering and Breuer.

Chemical Regulation

Chemical regulation in respiration is controlled to some extent by the concentration of respiratory gases in the blood. The respiratory centre is very sensitive to C02 concentration. It Increases if tension is slight breathing becomes deep and fast permitting more CO2 to leave the blood. Similarly, O2 concentration in the blood affects the breathing rate but in opposite direction. 

Regulation of the respiratory system must be coordinated with the homeostatic mechanism for temperature control, metabolic control, water and salt balance and circulatory system activities.

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