1. Mechanism of Breathing:
There are two phases of each breathing movement (ventilation of lung) – inspiration and expiration. Respirations in man are carried out with the help of intercostals muscles and diaphragm.
Intercostals muscles, between each pair of ribs, are of two types- external and internal. The Diaphragm consists of circular and radial muscle fibres arranged around the edge of a circular inelastic sheet of white fibres (collagen).
It is an active process as it is due to muscle contraction.
i. The external intercostals muscles contract and the internal intercostals muscles relax.
ii. This pulls the rib cage up and down.
iii. At the same time the diaphragm muscles contract, which flattens the diaphragm?
iv. Both actions increase the volume of thorax.
v. As a result the pressure in the thorax and hence the lungs, is reduced to less than atmospheric pressure.
vi. Air therefore, enters the lungs, inflating the alveoli, until the air pressure in the lungs is equal to that of the atmosphere.
It is a passive process and caused due to muscle relaxation. It is a process in which air rich in CO2 (mixed with slight O2) is exhaled out. Ribs during expiration are angled downward and external intercostal muscles are elongated forward and downward. The respiratory muscles act as follows-
i. The external intercostal muscles relax and the internal intercostal muscles contract. The ribcage drops mainly due to its own weight.
ii. At the same time, the diaphragm relaxes. The dropping rib cage forces the diaphragm into a dome shaped pushing it up into the thoracic cavity.
iii. These events reduce the volume of the thorax and raise its pressure above that of the atmosphere.
iv. Consequently air is forced out of the lungs.
The normal breathing discussed earlier is also called ‘abdominal breathing’ as it is mainly due to the movements of diaphragm which can be recognized from outside by movements of abdominal wall, and inconspicuous chest movements. However, we can voluntarily take deep breath by effort.
In the process of deep inspiration chest distension is brought about by external intercostal muscles and the abdominal muscles. In this process contraction of diaphragm is four to five times more than the normal so that the chest volume increases 15% to 20% and the movement of chest becomes apparent.
Hence this phenomenon is also called thoracic breathing. In deep and forced expiration requires strong contraction of abdominal muscles whereas normal expiration is mainly due to elastic recoil of chest wall and pulmonary alveoli.
Pulmonary volume is the volume of air in the lungs and is divided into four different types, according to the completely or incompletely filled lungs during breathing. These are –
(i) Tidal volume (TV):
The volume of air inspired or expired involuntarily in each normal breath. It is about 500 ml of air in average young adult man.
(ii) Inspiratory reserve volume (IRV):
The maximum volume of air which a person can inhale over and above tidal volume by deepest possible voluntary inspiration it is about 3000 ml.
(iii) Expiratory reserve volume (ERV):
The volume of air which can be expired over and above the tidal volume with maximum effort it is about 1100 ml.
(iv) Residual volume (RV):
The volume of air left in the lungs even after maximum forceful expiration. It is about 1200 ml.
Pulmonary capacities are the combination of two or more pulmonary volumes. It could be
(i) Inspiratory capacity:
Tidal volume + Inspiratory reserve volume. It is about 3500 ml.
(ii) Functional residual capacity (FRC):
Expiratory reserve volume + Residual volume. It is about 2300 ml.
(iii) Vital capacity:
Inspiratory reserve volume + Tidal volume + Expiratory reserve volume. This is the maximum amount of air a person can expel forcefully from his lungs after first filling these with a maximum deep inspiration. It is about 4600 ml
(iv) Total lung capacity:
Tidal volume + Inspiratory reserve volume + Residual volume + expiratory reserve volume. It is about 5800 ml.
All pulmonary volume and capacities are about 20% to 25% less in women than in men, more in atheletic people than asthenic (thin, non-exercising) people. With the exceptions of FRC and RV, all other pulmonary volumes and pulmonary capacities can be measured with the help of simple spirometer.
The lung volumes help to diagnose respiratory diseases like asthma, TB etc. (Spirometer-It is an instrument for measuring the air taken into and exhaled by the lungs).
A certain fraction of venous blood passing through the pulmonary capillaries does not become oxygenated; this fraction is called shunted blood. Also, additional blood flows through the bronchiolar vessels rather than alveolar capillaries, (normally 2% of cardiac output). This is unoxygenated shunted blood. The total quantitative amount of shunted blood per minute is called physiologic shunt.
When ventilation of some of the alveoli is great but alveolar blood flow is low, there is far more available O2 in alveoli than can be transported away from the alveoli by the flowing blood. In addition there is ventilation of anatomical dead space areas of respiratory passage also. The above two together, are called physiologic dead space. In brief dead air space is a part of inspired air left in the trachea and bronchial tree (about 150 ml), where no gaseous exchange takes place.
Breathing can be controlled by central nervous system. Man breathes 14 to 18’times/minute.
1. Respiratory centre:
Located in the medulla oblongata and pons varolii. These centres regulate the rate and depth of breathing by controlling contraction of diaphragm and other respiratory muscles. Medulla oblongata contains inspiratory centre in dorsal portion and expiratory centre in the ventral portion.
The expiratory centre is connected with vagus nerves that innervate the lungs. Pons varolii contains pneumotaxis and apneustic area. Pneumotaxis area which helps to control both the rate and pattern of breathing is present in the dorsal portion.
While the apneustic centre, which operates in association with the depth of inspiration, is present in the lower part of pons varolii.
2. Chemical control:
A chemosensitive area is situated near respiratory centre, medulla. It is highly sensitive to changes in CO2 concentration (or change in blood pH, as blood CO2 concentration influences its pH by forming HCO-3, within RBC using the enzyme carbonic anhydrase). Carbonic anhydrase is the ‘fastest’ enzyme in the enzyme world.
3. Chemoreceptor cells:
Located in carotid arteries. These cells send signals to the respiratory centre in the brain and monitor the concentration of CO2 and H+ ions.
4. Hering-Breuer reflex:
In the walls of bronchi and bronchioles stretch receptors are located and are stimulated by overstretching of the lungs.
This reflex serves as a protective mechanism for preventing excessive lung inflation but simultaneously it may increase breathing rate by reducing inspiratory time, like the pneumotaxis centre.
2. Gaseous Exchange:
The exchange of gases between the alveoli and blood in the lungs, and between the blood and the tissues is the result of difference in partial pressure of the respiratory gases, that is, oxygen and carbon dioxide, nitrogen etc.
Partial pressure of oxygen pO2 in alveolar air is 100 mm Hg and it is only 40 mm Hg in the arterial capillaries of the lungs.
Therefore, oxygen from the alveolar air rapidly diffuses due to its higher pO2 into the blood capillaries. The pO2 in the venous blood capillaries of lung is about 95 mm Hg.
Similarly the pCO2 in the blood reaching the alveolar capillaries is 46 mm Hg whereas pCO2 in alveolar air is 40 mm Hg. Therefore CO2 rapidly leaves the blood capillaries and reaches the alveoli.
The gaseous exchange between the blood and tissues is also due to the differential partial pressures. The pO2 and pCO2 of the arterial blood reaching the tissues is 95 mHg and 40 mm Hg respectively, while pO2 and pCO2 of tissues is 20 mm Hg and 52 mm Hg respectively.
Therefore, oxygen quickly leaves the blood and enters the cells whereas CO2 produced in the tissues leaves the cells and enters the blood.
A surface active agent, lines the alveoli. It is a mixture of several phospholipids, proteins and ions, secreted by type II alveolar epithelial cells.
It reduces surface tension between the alveolar fluid and air and prevents small alveoli from collapsing and increases respiratory compliance (compliance means the quality of yielding to pressure without disruption, or an ability to do so, as an expression of dispensability of an air or fluid-filled organ).