Every Breath You Take
GAS EXCHANGE IN HUMANS
The active mechanism that pushes air into and out of the lungs is often termed an aspiration pump. In the case of humans, the lungs are not the pump; the thoracic cavity is.
Thoracic and Pleural Cavity
To illustrate this arrangement and describe the thoracic cavity, it is easiest to start from the outside of the body and work inward. From the outside, the first layer is the skin and muscles that form the body of the torso. Next is the rib cage, which can expand and contract to aid inhalation and exhalation.
Lining the rib cage and forming the outer boundary for the pleural cavity (potential space between two pleura of the lungs) is a thin layer of mesothelium called the parietal pleura (the parietal peritoneum of the pleural cavity). Not only does this layer of mesothelium seal the pleural cavity, it creates a low friction and nonadhesive surface that presses against the lungs.
Another layer of mesothelium, found on the surface of the lung tissue itself, is called the visceral pleura. With these two mesothelial layers adjacent to each other, intrapleural space is created. The air pressure in the intrapleural space is always slightly lower than the pressure inside the lungs (intrapulmonary pressure). Therefore, the visceral pleura is always pressed against the parietal pleura and obliterates any space between the two. This makes it a “potential space” that only exists if either pleura becomes damaged and the pressure equalizes.
Anatomy of a Word
pneumothorax
Pneumothorax is a condition in which damage to the body wall (and parietal pleura) allows the intrapleural pressure to equalize with the atmospheric pressure, causing the lungs to pull away from the pleural wall (collapse).
The pleural cavity is basically an isolated cavity that surrounds the lungs, which, under normal conditions, does not receive or lose any air. Within this cavity is the interior of the lungs, which can freely exchange air with the external environment through the bronchiolar tree. To cause air to move into the lungs, the intrapulmonary pressure must be lower than the atmospheric pressure. To cause air to move out of the lungs, the intrapulmonary pressure must be higher than the atmospheric pressure.
Inhalation
Lung tissue is not muscular and therefore cannot dilate or constrict on its own. The pump that drives ventilation is a collection of muscles in the thoracic cavity. By expanding or constricting the thoracic space, the intrapleural pressure is decreased or increased. When the thoracic cavity expands, the pressure within the intrapleural space decreases and the lungs expand. When the thoracic cavity contracts, the pressure within the intrapleural space increases and the lungs contract.
One of the main muscles involved in ventilation is the diaphragm. This dome-shaped muscle is under autonomous control (but can be voluntarily controlled) and forms the inferior boundary of the thoracic and pleural cavities. When relaxed, the superior dome portion of the diaphragm projects upward into the thoracic cavity. When contracted, the diaphragm flattens and moves downward, which increases the volume of the thoracic cavity.
Additionally, one of the two sets of muscles between the ribs (the external intercostals) contracts. This force causes the rib cage to hinge upward, and in doing so expands the thoracic cavity laterally. Both of these actions create the lower pressure required to expand the lungs and thus aspirate air into the lungs during an inhalation.
During periods of activity when respiratory rates increase, inhalation is deeper and more rapid and requires additional muscular support. This comes in the form of various accessory muscles that are attached near the top of the rib cage and include the sternocleidomastoid, parasternal muscles, and the scalenes.
Exhalation
Compared to inhalation, exhalation is simple and passive. To reduce the size of the thoracic cavity after an inhalation, all the muscles involved in inhalation simply relax and let gravity pull the rib cage back downward. After the inhalation, the diaphragm returns to its normal dome shape and further constricts the thoracic cavity, generating high pressure inside the lungs. Therefore air is pushed outward.
As in active inhalation, active exhalation requires additional muscular assistance. The internal intercostals are arranged in a different orientation to the externals such that when they contract they assist in the more rapid lowering of the rib cage and the more forceful exhalation.
At this point in the respiration process, gases move across the respiratory membrane (by diffusion) and interact with blood elements for transportation into or out of the body.
Blood-Air Barrier
Air and blood never naturally mix, but they must come within close proximity to each other for diffusion of gases to effectively and efficiently occur. Therefore, each alveolus contains capillaries, the only blood vessels that allow the exchange of gases. The lining cells of the alveoli and the endothelial cells of the capillaries compose the thin blood-air barrier.
In the alveoli, these cells are flattened, much like the capillary endothelial cells, and are called type I pneumocytes. Another cell type present at the alveolar level is the type II pneumocytes (also called great alveolar cells). These are huge rounded cells that bulge into the lumen of the alveoli. They do not aid in the exchange of gases, but rather produce a substance called surfactant that assist in keeping the alveoli open. At 0.5 micrometers in diameter, the surface tension forces of water would be sufficient to collapse the alveoli inward upon themselves. But the phospholipid-rich surfactant interacts with the water molecules, while at the same time their hydrophobic chains keep other water molecules at a distance. In this way, the pressure required to keep the alveoli open is all but eliminated in the presence of surfactant.
External Respiration
External refers to the gases and their location. In this case, the external air has simply been inhaled into the lungs to the depth of the alveoli. However, the air is still just that, atmospheric or “external” air that must exchange gases with the blood.
The movement of gases is purely by diffusion. Therefore, the pressures of the gases in the blood versus the pressure in the air determines the direction of diffusion. Within the alveoli, O2 has a partial pressure of approximately 105 mmHg, while that of the blood (just returning in the veins) is at its lowest at 40 mmHg. This drives the diffusion of O2 from the alveoli and into the blood. Conversely, CO2 pressure in the alveoli is at 40 mmHg, which is lower than that found in the blood (46 mmHg). This therefore drives CO2 from the blood and into the alveoli, where it can be expelled from the body with the next exhalation.
Internal Respiration
During internal respiration, the gases are transferred to the tissues and cells. At the tissue level, pressure inequalities drive oxygen out of the blood and into the tissues and CO2 returns to the blood from the cells. Although the direction is opposite from what occurred in the lungs during external respiration, the direction of movement is still from high pressure to low pressure.
Gas Transport
While some of the gases that dissolve into the blood remain in the liquid plasma, many will be bound and transported (oxygen to hemoglobin) or may be processed and transported in an alternate form (CO2conversion to bicarbonate ion).
Oxygen
Red blood cells are the main transportation mode for O2 throughout the human body. Oxygen diffuses into the red blood cell and binds to an iron atom, which is held in place by the heme group of the larger protein hemoglobin. In the lungs, deoxyhemoglobin (hemoglobin lacking oxygen) binds to oxygen and becomes oxyhemoglobin so it can be transported throughout the body. Under atmospheric conditions, hemoglobin will exist in a state where 97 percent of the hemoglobin molecules are oxyhemoglobin. In fact, the love hemoglobin has for oxygen is so great that even if the partial pressure of oxygen in the air is decreased from 100 mmHg to 60 mmHg, hemoglobin remains approximately 90 percent saturated.
At internal respiration, the oxygen gradient overcomes this affinity for hemoglobin and is unloaded to the tissues from the blood by diffusion. For normal activity, approximately 20–22 percent of oxygen is unloaded to the tissues. This may at first seem like a waste, but it is in fact a reserve. Under heavy exercise conditions, up to 80 percent of oxygen may be unloaded to the tissues.
Carbon Dioxide
Although some deoxyhemoglobin can bind to CO2 and become carbaminohemoglobin, this only accounts for a small fraction of the transported CO2 in the blood. The majority (70 percent) of CO2 is transported in the blood stream as bicarbonate ions that are dissolved into the plasma.
Carbon dioxide enters the blood stream and then the red blood cell via diffusion. Under conditions of high CO2 levels, such as what exists at the tissue level during internal respiration, carbonic anhydrase facilitates the conversation of CO2 into carbonic acid, which rapidly and spontaneously dissociates into hydrogen and bicarbonate ions. Some of the H+ ions bind to hemoglobin, while others are transported into the plasma where they cause a decrease in blood pH. The bicarbonate ions are also transported outside of the cell in a process called the chloride shift. During this process, bicarbonate moves outward while the chloride ion is transported inward to offset the charge difference created by transporting of bicarbonate.
Once the blood returns to the lungs and during external respiration, this entire process is reversed.