This question requires a basic understanding of general chemistry and/or general physics. Remember that gas will only move from an area of high pressure to low pressure; thus, if air is moving out of the lungs, the pressure inside of the lungs must be greater than the pressure outside of the lungs. The point at which air does not move in or out of the lungs is a signal that the pressure of the gas inside of the lungs is equal to that of atmospheric pressure.

In a biological sense, remember that the diaphragm contracts to cause inhalation, which results from negative or decreased pressure in the lungs. When the diaphragm relaxes, the pressure in the lungs must increase again. The increase in pressure forces the air out of the lungs and back into the atmospheric environment.

Increasing the volume of a container (in this case, the lungs) while keeping the contents (air molecules) the same will decrease the pressure. If no barrier is present (as when holding your breath), pressure will tend to equalize between areas of differing pressure. In order to equalize the pressure, air molecules from outside the body rush into the expanded lungs. The concentration of oxygen does not impact inhalation, and cilia are not used to inhale.

Inhalation happens by making the pressure in the lungs lower relative to the pressure outside the body. When the diaphragm contracts, it increases the volume of the thoracic cavity. By Boyle's law, there is an inverse relationship between pressure and volume of a gas. Thus the pressure of the thoracic cavity decreases and since air will flow from high to low pressure, this pulls air into the lungs during inhalation.

During inspiration, the lungs expand as the diaphragm contracts and internal intercostal muscles relax. As the volume of the thoracic cavity increases, its pressure decreases. This creates a pressure gradient, driving air from an area of high pressure (the environment) into the area of low pressure (the lungs).

When you are not breathing, your respiratory system cannot perform its function. Unwanted gases, such as carbon dioxide, cannot be removed from the system and necessary gases, such as oxygen, cannot enter the system. This causes a buildup of carbon dioxide in the body, which leads to acidosis. Carbon dioxide is converted to carbonic acid via carbonic anhydrase. This carbonic acid builds in the blood, lowering its pH.

Air enters the body of most terrestrial vertebrates through the nose or the mouth; the air then passes through the trachea to narrower tubes called the bronchi, to still narrower tubes called the bronchioles. The bronchioles "dead end" into structures called alveoli, which is where gas exchange of oxygen and carbon dioxide takes place with the blood in adjacent capillaries.

The lungs contain the bronchioles, the alveoli, and part of the bronchi. The trachea carries inhaled air into the bronchi, but it is not actually enclosed by the bronchi— the lungs only enclose structures that arise after the bronchi branch away from the trachea.

By folding the respiratory surface into the body, terrestrial animals increase the humidity of the environment of the respiratory surface, which will minimize evaporation and maintain moisture. Note that water loss via evaporation and perspiration accounts for the majority of water loss in terrestrial mammals.

During inspiration, the lungs expand as the diaphragm contracts and internal intercostal muscles relax. As the volume of the thoracic cavity increases, its pressure decreases. This creates a pressure gradient, driving air from an area of high pressure (the environment) into the area of low pressure (the lungs).

When you are not breathing, your respiratory system cannot perform its function. Unwanted gases, such as carbon dioxide, cannot be removed from the system and necessary gases, such as oxygen, cannot enter the system. This causes a buildup of carbon dioxide in the body, which leads to acidosis. Carbon dioxide is converted to carbonic acid via carbonic anhydrase. This carbonic acid builds in the blood, lowering its pH.

Air enters the body of most terrestrial vertebrates through the nose or the mouth; the air then passes through the trachea to narrower tubes called the bronchi, to still narrower tubes called the bronchioles. The bronchioles "dead end" into structures called alveoli, which is where gas exchange of oxygen and carbon dioxide takes place with the blood in adjacent capillaries.

The lungs contain the bronchioles, the alveoli, and part of the bronchi. The trachea carries inhaled air into the bronchi, but it is not actually enclosed by the bronchi— the lungs only enclose structures that arise after the bronchi branch away from the trachea.

By folding the respiratory surface into the body, terrestrial animals increase the humidity of the environment of the respiratory surface, which will minimize evaporation and maintain moisture. Note that water loss via evaporation and perspiration accounts for the majority of water loss in terrestrial mammals.

The left atrium receives blood from the pulmonary veins, which carry blood that was freshly oxygenated from the lungs to the heart. The partial pressure of oxygen is always highest soon after oxygenation, thus blood returning from the lungs would have a high partial pressure.

The superior and inferior vena cavae return deoxygenated blood from the body to the heart, and would have very low oxygen partial pressures. The right atrium receives this deoxygenated blood from the vena cavae and transfers it to the right ventricle. From the ventricle, the deoxygenated blood is transported to the lungs via the pulmonary arteries. It only becomes oxygenated again after reaching the lungs, and then returns to the heart through the pulmonary veins.

This question requires a basic understanding of general chemistry and/or general physics. Remember that gas will only move from an area of high pressure to low pressure; thus, if air is moving out of the lungs, the pressure inside of the lungs must be greater than the pressure outside of the lungs. The point at which air does not move in or out of the lungs is a signal that the pressure of the gas inside of the lungs is equal to that of atmospheric pressure.

In a biological sense, remember that the diaphragm contracts to cause inhalation, which results from negative or decreased pressure in the lungs. When the diaphragm relaxes, the pressure in the lungs must increase again. The increase in pressure forces the air out of the lungs and back into the atmospheric environment.

Increasing the volume of a container (in this case, the lungs) while keeping the contents (air molecules) the same will decrease the pressure. If no barrier is present (as when holding your breath), pressure will tend to equalize between areas of differing pressure. In order to equalize the pressure, air molecules from outside the body rush into the expanded lungs. The concentration of oxygen does not impact inhalation, and cilia are not used to inhale.

Inhalation happens by making the pressure in the lungs lower relative to the pressure outside the body. When the diaphragm contracts, it increases the volume of the thoracic cavity. By Boyle's law, there is an inverse relationship between pressure and volume of a gas. Thus the pressure of the thoracic cavity decreases and since air will flow from high to low pressure, this pulls air into the lungs during inhalation.

The left atrium receives blood from the pulmonary veins, which carry blood that was freshly oxygenated from the lungs to the heart. The partial pressure of oxygen is always highest soon after oxygenation, thus blood returning from the lungs would have a high partial pressure.

The superior and inferior vena cavae return deoxygenated blood from the body to the heart, and would have very low oxygen partial pressures. The right atrium receives this deoxygenated blood from the vena cavae and transfers it to the right ventricle. From the ventricle, the deoxygenated blood is transported to the lungs via the pulmonary arteries. It only becomes oxygenated again after reaching the lungs, and then returns to the heart through the pulmonary veins.

This question requires a basic understanding of general chemistry and/or general physics. Remember that gas will only move from an area of high pressure to low pressure; thus, if air is moving out of the lungs, the pressure inside of the lungs must be greater than the pressure outside of the lungs. The point at which air does not move in or out of the lungs is a signal that the pressure of the gas inside of the lungs is equal to that of atmospheric pressure.

In a biological sense, remember that the diaphragm contracts to cause inhalation, which results from negative or decreased pressure in the lungs. When the diaphragm relaxes, the pressure in the lungs must increase again. The increase in pressure forces the air out of the lungs and back into the atmospheric environment.