Mechanism of respiration

mechanism of respiration: inhalation & Exhalation

1. MECHANISM OF RESPIRATION Ms.Anu Sebastian: B.Pharm ; M.Pharm Assistant Professor, Dept. Of Pharmacology Nirmala College Of Pharmacy Muvattupuzha,Ernakulam,Keral

2. RESPIRATION The process of gaseous exchange in the body is called respiration. The process involves 3 steps: Pulmonary ventillation /Breathing : It is the inhalation (inflow) & exhalation(outflow) of air & involves the exchange of air between the atmosphere & the alveoli of the lungs. External/Pulmonary respiration : It is the exchange of gases between the alveoli of the lungs & blood in pulmonary capillaries across the respiratory membrane. In this process ,Pulmonary capillary blood gains O2 & loses CO2

3. 3. Internal /Tissue respiration : It is the exchange of gases between blood in systemic capillaries & tissue cells. In this step, blood loses O2 & gains CO2. Pressure Changes During Pulmonary Ventillation Air moves into the lungs when the air pressure inside the lungs is less than the air pressure in the atmosphere. Air moves out of the lungs when the air pressure inside the lungs is greater than the air pressure in the atmosphere.

4. MECHANISM/PHYSIOLOGY OF RESPIRATION INHALATION Breathing in is called inhalation. Boyle’s law states that the pressure in a closed container is inversily proportional to the volume of the container. Just before inhalation the pressure inside each lungs is equal to the atmospheric pressure 760 mmHg. For air to enter into the lungs, the pressure in the lungs should be less than the atmospheric pressure. In order to decrease the lung pressure as per B oyle’s law, the volume should be increased. The volume of lungs is increased by the contraction of diaphragm & external intercostal muscles.

5. Diaphragm is a dome shaped skeletal muscle that forms the floor of the thoracic cavity. During inhalation, Contraction of diaphragm( 75%) Under normal conditions, diaphragm descends by 1 cm creating a pressure difference of about 1-3 mmHg due to which about 500ml of air enters into the lungs. Under stressful conditions, the diaphragm descends by about 10cm which produces a greater pressure difference of about 100 mmHg & thus inhalation of about 2-3 L of air occurs.

6. Contraction of External intercostal muscles(25%) When the external intercostal muscles contract, the ribs are pulled upwards, thereby decreasing the intrathoracic pressure ,which increases the volume of thoracic cavity. Contraction of the external intercostals is responsible for about 25% of the air that enters the lungs during normal quiet breathing. Intrapleural / Intrathoracic pressure It is the pressure difference between the 2 pleural layers. Just before inhalation it is 756mmHg—Expansion of thoracic cavity—Decreases the pressure to 754mmHg—the parietal pleura is pulled outwards—along with that visceral pleura & lungs also pulled out—increases the thoracic volume.

7. So by all the above 3 ways the intrapulmonary pressure drops from 760mmHg to 758mmHg. Due to this pressure difference between atmosphere & lungs, the air diffuse into the alveoli until the pressure difference exists. EXHALATION Breathing out is called exhalation /Expiration. It is a passive process(No muscle contraction) During exhalation the elastic recoiling of chest wall & lungs takes place. Relaxation of diaphragm makes it acquire its characteristic dome shaped structure while relaxation of the external intercostals depresses the ribs.

8. As a result , the volume of thoracic cavity decreases followed by a decrease in the volume of lungs. This results in an increased pressure within the alveoli of the lungs than the atmospheric pressure. As a result, air flows from the alveoli to the atmosphere During quiet breathing exhalation lasts for about 3 seconds. During forceful breathing exhalation becomes active due to the contraction of abdominal & internal intercostal muscles. Factors Affecting Pulmonary Ventillation Surface tension of the alveolar fluid Surface of the alveoli consists of a layer of fluid known as alveolar fluid.

9. The force exerted by alveolar fluid on the luminal surface of alveoli is referred as surface tension. This surface tension should be overcome for expanding the lungs during inhalation & exhalation. Compliance of the lungs The force or effort required to expand the lungs & chestwall is known as compliance. In the lungs, compliance is related to two principal factors: Elasticity & Surface tension. The lungs normally have high compliance & expand easily because elastic fibres in lung tissue are easily stretched & surfactants in alveolar fluid reduces surface tension.

10. Airway Resistance The rate of airflow through the airways depends on both the pressure difference & the resistance. The walls of the airways, especially the bronchioles offer some resistance to the normal flow of air into & out of the lungs. As the lungs expand during inhalation, the bronchioles enlarge because their walls are pulled outward in all directions. Larger diameter airways have decreased resistance. Airway resistance then increases during exhalation as the diameter of bronchioles decreases.

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