Alveoli and Gas Exchange

the DRG, allowing the individual to relax after inspiration, allowing for control over the rate of respirations.

These regions of the brain do not act in a vacuum. They respond to basic systemic stimuli. The greater the stimulus, the greater is the response. The CO2 concentration is the major factor that controls the respiratory drive in the healthy person. There are central chemoreceptors in the brain and brainstem and peripheral chemoreceptors in the aortic arch and carotid arteries. An increased CO2 level will stimulate the respiratory effort, while a decreased CO2 level will inhibit the respiratory effort. Lactic acid will lower the pH and will increase the respiratory rate; lactic acid will increase during intense exercise.

The hypothalamus and the cortex are also involved in influencing breathing. The hypothalamus responds to changes in temperature, emotions, and pain signals. These include things like the fight-or-flight response, which will act to increase the respiratory rate in response to stress.

ALVEOLI AND GAS EXCHANGE

Gas exchange is the ultimate goal of the respiratory system. Pulmonary ventilation is what it’s called when the body brings air to the alveoli for the process of gas exchange. External respiration is what it is called when the gas exchange occurs at the level of the alveoli. In order to understand this type of respiration, you need to also understand Dalton’s gas law or the law of partial pressures. It indicates that the sum total of a mixed gas pressure is equal to the sum of the partial pressures of each gas in the mixture.

Most of air is nitrogen gas at more than 78 percent. Oxygen is next at 21 percent. CO2 makes up just 0.04 percent of the total gas in atmospheric air. Water vapor actually has a higher concentration than CO2 at 0.4 percent of atmospheric air. This partial pressure is important because a gas will move toward from its area of higher partial pressure to its area of lower partial pressure.

There is also Henry’s law to consider and that states that the concentration of gas in a liquid medium is proportional to both its partial pressure and its solubility in the liquid.

Nitrogen has a very high partial pressure in atmospheric air but it has a low solubility in blood so it does not have a high blood concentration in this liquid. The major exception is scuba divers that breath in a mixture of gases that have a higher partial pressure of nitrogen underwater. This leads to a potentially fatal level of nitrogen dissolved in blood.

In alveolar air, the water vapor pressure is higher because air is humidified as it enters the lungs. The nitrogen percentage is 75 percent; the oxygen percentage is about 14 percent; the water vapor percentage is 6.2 percent; and the CO2 percentage is 5.2 percent. The partial pressure of oxygen is 104 millimeters of mercury and the partial pressure of CO2 in the alveolar air is 47 millimeters of mercury.

Gas exchange occurs at the lungs and at the tissues. External respiration is the exchange of gases at the alveoli, while internal respiration is that exchange that occurs in the tissues. Gases exchange via simple diffusion with no energy required. They follow the pathway set up by the partial pressures of the gases and by the concentration of the gas in the liquid plasma. The respiratory membrane is highly permeable to both oxygen and carbon dioxide and the alveoli offer up a large surface area.

Most oxygen is picked up by hemoglobin in the pulmonary capillaries. Some carbon dioxide is taken up by hemoglobin in the tissues but most is dissolved in plasma. Some carbon dioxide gets transferred via bicarbonate, which goes to carbon dioxide and water through the action of carbonic anhydrase in the red blood cell. The solubility of oxygen isn’t high in blood but there is a big difference in the partial pressure of oxygen in the alveoli (at 104 millimeters of mercury) versus the capillaries (at 40 millimeters of mercury). This is what allows oxygen to diffuse into the bloodstream.

The difference between the partial pressure of carbon dioxide at the alveoli is not as great at 5 millimeters of mercury. This is offset by the greater solubility of carbon dioxide in the blood versus oxygen so that carbon dioxide can leave the plasma and enter the alveolar side in the lungs.

Only about 1.5 percent of all blood oxygen is dissolved in the plasma. Most enter the RBC and get picked up by hemoglobin, which can bind up to four oxygen molecules per heme molecule. The end result is oxyhemoglobin, which is formed whenever oxygen binds to hemoglobin, contributing to its bright red color. The binding of one oxygen

molecule facilitates the binding of the other four molecules. In the same way, the dropping off of one oxygen molecule facilitates the dropping off of the other three. A hundred percent saturation comes from all four heme molecules attached to oxygen. Most blood is 95 to 99 percent saturated.

This relationship between oxygen and hemoglobin binding leads to an oxygen dissociation curve. At lower partial pressures of oxygen, there is less of a percent saturation of hemoglobin. There are two different factors that determine the dissociation of oxygen from hemoglobin. Highly active tissues, such as muscle, have a lower partial pressure of oxygen. The partial pressure of oxygen in fatty tissue is higher because less oxygen is used. Less oxygen dissociates in fatty tissue than in muscle tissue.

Higher temperatures will cause oxygen and hemoglobin to dissociate to a greater degree, while low temperatures inhibit this dissociation process. This is what’s seen in highly metabolic tissues. Hormones like growth hormone, thyroid hormone, epinephrine, and androgens will stimulate the 2,3-bisphosphoglycerate production in the RBCs. This molecule is a metabolic product of glycolysis and promotes the dissociation of oxygen from hemoglobin.

The Bohr effect indicates that there is a relationship between blood pH and oxygen’s affinity for the hemoglobin molecule. Acidotic pH levels will promote oxygen dissociation, which is what’s seen in highly metabolic tissues. Blood pH will decrease with CO2, carbonic acid, and lactic acid in the tissues. These will promote oxygen dissociation.

Carbon dioxide, as mentioned, is transported as bicarbonate, on hemoglobin, and in solution in plasma. About 7 to 10 percent is transported as solubilized CO2. Most (about 70 percent) is transported as bicarbonate and then turned into carbon dioxide and water via carbonic anhydrase in the RBCs. The bicarbonate leaves the RBCs by switching places with chloride in the plasma. The CO2 is rapidly diffused across the respiratory membrane into the alveoli, where it is exhaled. About 20 percent of CO2 gets bound by hemoglobin to make carbaminohemoglobin. It dissociates from the