Pulmonary Ventilation (Physiology)

꩜ To supply respiring tissues with oxygen

꩜ To remove excess carbon dioxide

꩜ By maintaining partial pressure gradients between alveolar air and blood

꩜ By ventilating gas exchange surfaces, which involves moving air from the atmosphere to the alveoli and then out again

꩜ Gas exchange relies on partial pressure gradients, ensuring that oxygen is supplied to alveoli and carbon dioxide does not accumulate.

꩜ Changing metabolic demands require varying levels of gas exchange to supply oxygen and remove carbon dioxide.

꩜ This is achieved by changing the rate of alveolar ventilation to modulate partial pressure gradients between the alveoli and blood.

꩜ Hypoventilation and hyperventilation are defined as insufficient or excessive levels of breathing relative to metabolic demands.

꩜ They can be identified by the level of CO2 present within the blood.

Hypoventilation results in excessive levels of CO2 within arterial blood (PaCO2 > 6.0 kPa).

Hyperventilation results in reduced levels of CO2 within arterial blood (PaCO2 < 4.9 kPa).

꩜ Increasing the rate of ventilation increases alveolar oxygen partial pressure (PAO2) and decreases alveolar carbon dioxide partial pressure (PACO2).

꩜ Decreasing the rate of ventilation has the opposite effects.

꩜ Due to CO2's role in acid-base balance

꩜ Because of the complexities of oxygen exchange and transport

꩜ The total volume of air moved into and out of the lungs during inspiration and expiration is crucial for the quantity of oxygen reaching alveolar surfaces and participating in gas exchange.

꩜ Total lung volumes vary between individuals based on factors such as height, sex, age, and the presence of specific respiratory diseases.

꩜ Tidal Volume (VT): Volume of air moved in and out of the lungs during quiet breathing.

꩜ Inspiratory Reserve Volume (IRV): Additional volume of air that can be inhaled after a normal inspiration.

꩜ Expiratory Reserve Volume (ERV): Additional volume of air that can be exhaled after a normal expiration.

꩜ Residual Volume (RV): Volume of air remaining in the lungs after maximal expiration.

꩜ Total Lung Capacity (TLC): Total volume of air the lungs can hold after maximal inspiration.

꩜ Vital Capacity (VC): Maximum volume of air that can be exhaled after a maximal inspiration.

꩜ Functional Residual Capacity (FRC): Volume of air remaining in the lungs after a normal expiration.

Ventilation refers to the volume of fresh air that reaches respiratory surfaces over a given time.

꩜ It depends on both the volume of air inspired during each breath.

꩜ It also depends on the frequency of breathing, also known as respiratory rate, which indicates how many breaths occur per minute.

꩜ Total ventilation represents the total volume of air inspired over a given time, but it doesn't perfectly reflect the volume of air reaching respiratory surfaces and participating in gas exchange.

꩜ Gas exchange only occurs in alveoli, not in the airways, so not all inspired air contributes to gas exchange.

꩜ Air is required to occupy the airways, which do not contribute to gas exchange, known as the "dead space."

꩜ Alveoli do not completely collapse at the end of expiration, leaving a residual volume of air in the lungs.

Alveoli contain a mixture of "stale" air, which has already participated in gas exchange, and "fresh" air, which has not yet participated in gas exchange.

꩜ The fact that air enters and exits via the same path means that approximately 150mL of air from each inspiration, equivalent to the volume of the dead space, never reaches the alveoli or participates in gas exchange.

꩜ This air is the first to leave the respiratory system at the beginning of the next expiration.

꩜ To accurately calculate the volume of air reaching respiratory surfaces (alveolar ventilation), an adjustment needs to be made to account for the volume of air that does not reach the alveoli, known as the dead-space volume.

The dead-space volume is typically around 150mL in adults.

꩜ Boyle's Law states that at a constant temperature, the pressure of a gas is inversely proportional to its volume.

꩜ In other words, as the volume of a gas decreases, its pressure increases, and vice versa.

꩜ This law helps us understand how changes in volume affect pressure within a closed system, such as the respiratory system.

꩜ Pressure is the force exerted by gas molecules on the surfaces of the space in which they are contained.

꩜ Gases naturally move from areas of high pressure to areas of low pressure until an equilibrium is reached.

꩜ Gases can be moved between connected spaces by creating a pressure gradient, where there is a difference in pressure between the two spaces.

꩜ Boyle's Law describes the relationship between pressure (P), volume (V), and molar quantity (n) of gas molecules present.

꩜ Boyle's Law is derived from the ideal gas law, which describes the behavior of gases under various conditions.

꩜ Pressure (P)

꩜ Volume (V)

꩜ Molar quantity (n), representing the number of gas molecules present.

꩜ Boyle's Law states that at a constant temperature, the pressure of a gas is inversely proportional to its volume.

꩜ Humans cannot naturally modulate the pressure of the atmosphere, so the movement of air between the atmosphere and lungs relies solely on changing alveolar pressure.

꩜ To move air into the lungs during inspiration, alveolar pressure must fall below atmospheric pressure (creating a pressure gradient), and during expiration, alveolar pressure must rise above atmospheric pressure.

꩜ These pressure changes are achieved by contraction and relaxation of respiratory muscles, altering the volume of the thoracic cavity, which in turn affects alveolar pressure.

꩜ During inspiration, increases in lung volume decrease alveolar pressure, generating a pressure gradient between the alveoli and the atmosphere.

꩜ Air moves from the area of high pressure (atmosphere) to low pressure (alveoli).

꩜ The process reverses during expiration.

The lungs and chest wall are separated by a pair of serous membranes known as pleurae (singular = pleura).

꩜ The visceral pleura lines each lung, while the parietal pleura lines the thoracic cavity, surrounding the chest, diaphragm, and mediastinum.

꩜ Between the two pleurae is the sealed, fluid-filled pleural cavity.

꩜ The tissues attached to each pleura recoil in opposite directions due to their elastic properties.

꩜ This stretches the sealed pleural cavity between them, decreasing the pressure within the pleural space.

꩜ Negative intrapleural pressure is naturally sub-atmospheric due to the opposing recoil of the chest wall and lungs.

꩜ Within an enclosed space, negative pressure generates a collapsing force, pulling the surfaces of the contained space together.

꩜ Negative intrapleural pressure acts to pull the two pleurae (and consequently the lungs and chest wall) together.

꩜ The level of negative intrapleural pressure determines the force acting to pull the pleura together, which affects lung and chest wall expansion and compression.

꩜ Negative intrapleural pressure plays a key role in breathing by influencing the balance of forces acting on the lungs and chest wall.

꩜ Expansion and compression of the lungs and chest wall can be conceptualized as a tug-of-war between various forces acting upon them.

꩜ Elastic (outward) recoil of the chest wall and inspiratory respiratory muscle contraction (e.g., contraction of the diaphragm) pull the chest wall outwards and expand the thoracic cavity.

꩜ Elastic recoil of the lungs pulls the visceral pleura inwards and compresses lung volume.

꩜ Negative intrapleural pressure acts to pull the lungs and chest wall together.

꩜ The balance between these three forces determines whether the lungs expand or compress at a given time.

꩜ Contraction of respiratory muscles, such as the diaphragm, generates sufficient force to pull the parietal pleura outwards.

꩜ This action stretches the pleural cavity, decreasing intrapleural pressure.

꩜ As intrapleural pressure becomes more negative, the force pulling the two pleurae together increases.

꩜ When the force pulling the pleurae together becomes greater than the force generated by the elastic recoil (due to elastic tissue) of the lung, the visceral pleura is pulled outward, expanding the lung.

꩜ Relaxation of inspiratory respiratory muscles results in decreased outward force acting on the parietal pleura.

꩜ This reduces the force acting to stretch the pleural cavity, increasing intrapleural pressure.

꩜ The increased (less negative) intrapleural pressure no longer generates sufficient force to overcome the elastic recoil of the lung.

꩜ As a result, the visceral pleura is pulled inward along with the pleural cavity and parietal pleura, decreasing lung volume.

꩜ During a forced expiration, respiratory muscles such as the abdominals actively contract to compress the volume of the thoracic cavity.

꩜ Muscle contraction generates inward force on the parietal pleura, compressing the pleural cavity further and increasing intrapleural pressure (PIP).

꩜ This results in a more pronounced decline in lung volume, in terms of both speed and magnitude.

꩜ The overall function of the respiratory system brings together the relationships described by Boyle’s law and the role of intrapleural pressure in breathing.

꩜ Pneumothorax is a condition where air enters the pleural space due to a rupture in one of the pleural membranes.

꩜ Pneumothorax can be caused by trauma, bleb formation, or disease, which can rupture one of the pleural membranes.

꩜ In pneumothorax, the entry of air into the pleural space disrupts the naturally sub-atmospheric intrapleural pressure.

꩜ Entry of air into the pleural cavity results in its volume increasing, at the expense of the lung.

꩜ Recoil of the lungs and expansion of the chest wall during breathing can potentially draw additional air into the cavity.

꩜ Simultaneously, the expansion of the chest wall during inhalation enlarges the thoracic cavity, further lowering the intrapleural pressure and potentially drawing air into the pleural cavity, especially in cases like pneumothorax.

꩜ With the loss of negative intrapleural pressure, elastic recoil of the chest wall and lungs is no longer resisted.

꩜ This causes affected regions of the lungs to collapse.

꩜ The overall effect depends on the site and extent of the injury.

꩜ Total ventilation is the volume of air entering or leaving the respiratory system in a given time period.

꩜ Alveolar ventilation is the volume of inspired air that reaches alveoli and participates in gas exchange in a given time period.

꩜ Tidal volume is the volume of air inspired or expired per breath.

꩜ Breathing frequency is the number of breaths that take place over a given time period, usually measured per minute.

꩜ Deadspace volume is the volume of the anatomic dead space within the respiratory system, which essentially refers to the volume contained within the airways that doesn’t participate in gas exchange.