The process by which air rich in carbon dioxide is released from the lungs is called exhalation. During this time the diaphragm and intercostal muscles are stretched. The scrotum moves downward and the diaphragm moves upwards. As a result, the volume of the thoracic cavity decreases. The lungs are compressed. Air pressure inside the lungs increases (3-4 mmHg more). Air pressure in the lungs but the air pressure in the outside environment is normal. As a result, air rich in carbon dioxide is released from the lungs.

The process by which oxygen-rich air enters the lungs is called inspiration or inhalation. During this the diaphragm and intercostal muscles contract. The peritoneum rises upwards and the diaphragm descends. As a result, the volume of the thoracic cavity increases. The lungs expand. Air pressure inside the lungs is low. But the air pressure in the outside environment is normal. As a result, oxygen-rich air from the outside environment enters the lungs.

The process in which oxygen rich air enters the lungs and carbon dioxide rich air leaves the lungs is called breathing. Adults breathe 16-18 times per minute and children breathe twice per minute. However, the rate of absorption increases during exercise. Respiration is done in two stages. These are-
1. Inspiration: The process by which oxygen-rich air enters the lungs is called inspiration or inhalation. During this the diaphragm and intercostal muscles contract. The peritoneum rises upwards and the diaphragm descends. As a result, the volume of the thoracic cavity increases. The lungs expand. Air pressure inside the lungs is low. But the air pressure in the outside environment is normal. As a result, oxygen-rich air from the outside environment enters the lungs.
2. Expiration: The process by which air rich in carbon dioxide is released from the lungs is called exhalation. During this time the diaphragm and intercostal muscles are stretched. The scrotum moves downward and the diaphragm moves upwards. As a result, the volume of the thoracic cavity decreases. The lungs are compressed. Air pressure inside the lungs increases (3-4 mmHg more). Air pressure in the lungs but the air pressure in the outside environment is normal. As a result, air rich in carbon dioxide is released from the lungs.

The process in which complex food is broken down into simple food under the influence of enzymes, static energy is converted into kinetic energy and gaseous exchange takes place is called internal respiration. It occurs in the cytoplasm and mitochondria of cells. In this process glucose is oxidized to produce energy and CO2 is released. Its stages are glycolysis, Krebs cycle and gaseous exchange.

The process in which O2 and CO2 are exchanged between the respiratory organs and the atmosphere is called external respiration. It is a physical process. Enzyme has no role in it. No energy is produced in this process. This is done in two stages. Inhalation and exhalation.

Respiration is a complex process. It runs continuously in the human body. Respiration has two phases. namely-
1. External Respiration: The process in which O2 and CO2 are exchanged between the respiratory organs and the atmosphere is called external respiration. It is a physical process. Enzyme has no role in it. No energy is produced in this process. This is done in two stages. Inhalation and exhalation.
2. Internal Respiration: The process in which complex food is broken down into simple food under the influence of enzymes, static energy is converted into kinetic energy and gaseous exchange takes place is called internal respiration. It occurs in the cytoplasm and mitochondria of cells. In this process glucose is oxidized to produce energy and CO2 is released. Its stages are glycolysis, Krebs cycle and gaseous exchange.

The muscles located between the two ribs of the thorax are called intercostal muscles. During inhalation the intercostal muscles contract and the ribs rise upwards. As a result, the volume of the lungs increases and air enters the lungs. During exhalation it relaxes or expands and the parshukas descend. As a result, the volume of the lungs decreases and air escapes from the lungs.

The fleshy membrane located transversely between the thoracic cavity and the abdominal cavity is called diaphragm. It is made up of voluntary muscles. During inhalation the diaphragm contracts, descends and flattens. As a result, the volume of the lungs increases and air enters the lungs. During exhalation it expands, rises upwards and bends like a bow. As a result, the volume of the lungs decreases and air escapes from the lungs.

Muscles that help in respiration are called respiratory muscles. Respiratory muscles are- diaphragm, intercostal muscles, sternocleidomastoid, scalene, pectoralis, trapezius, latissimus dorsi, erector spinae, iliocostalis, quadratus lumborum, serratus, transverse thoracis, subclavius etc.
1. Diaphragm: The fleshy membrane located transversely between the thoracic cavity and the abdominal cavity is called diaphragm. It is made up of voluntary muscles. During inhalation the diaphragm contracts, descends and flattens. As a result, the volume of the lungs increases and air enters the lungs. During exhalation it expands, rises upwards and bends like a bow. As a result, the volume of the lungs decreases and air escapes from the lungs.
2. Intercostal muscles: The muscles located between the two ribs of the thorax are called intercostal muscles. During inhalation the intercostal muscles contract and the ribs rise upwards. As a result, the volume of the lungs increases and air enters the lungs. During exhalation it relaxes or expands and the parshukas descend. As a result, the volume of the lungs decreases and air escapes from the lungs.

Type-2 cells in the alveolar wall contain detergent-like chemicals. These chemicals are called surfactants. Detergents are chemicals like phospholipids. It reduces the surfactant tension of the alveolar wall. Prevents alveolus from collapsing. Contracts and expands the lungs with less effort. Facilitates rapid exchange of CO2 and O2. Destroys germs (bacteria). Surfactant secretion begins in the 23-week-old human fetus.