 This video will cover the following objective for the respiratory system, list the major functions of the respiratory system, describe the forces that allow for air movement into and out of the lungs, describe the mechanisms that drive breathing, describe the process of gas exchange, define and contrast the process of external and internal respiration. The major organs of the respiratory system are the lungs as well as the airways that connect in and out of the lungs, and the muscles of respiration that are responsible for ventilation, for pulmonary ventilation, drawing air in and out of the lungs. So the airways are connected to the nasal cavity and oral cavity and the pharynx is the throat, which the pharynx connects to both the nasal and oral cavities, as air is being drawn in through the nasal cavities, it will be humidified, warmed and filtered, and then it passes down through the pharynx into the larynx. Larynx is also commonly referred to as the voice box, and it contains the vocal cords responsible for sound production. And then air continues from the larynx down into the lower respiratory tract, starting with the trachea, which is the large airway which then branches into the right main bronchus and left main bronchus, each bronchus branches further into secondary tertiary bronchi in the lungs, which branch further into bronchioles, which are small airways that eventually terminate in air sacs known as the alveoli, where the external respiration, gas exchange occurs, where gas, like oxygen, moves from the atmospheric air inside of the alveoli into the blood to be transported inside of the red blood cells. In the role of the muscles in the respiratory system, like the diaphragm, the diaphragm contracts in order to increase the volume of the thoracic cavity where the lungs are located, and as that volume increases, there's a decrease in pressure inside of the lungs that causes air to flow in, increasing the volume of the lungs. Ulfaction is one of the functions that the respiratory system contributes to in order to draw air in. The respiratory system can draw air in through the nasal cavity and then molecules that are in the air can dissolve in the mucus of the olfactory epithelium and bind to the olfactory receptors, which relay information in through the cranial nerve one into the olfactory bulbs and olfactory tracts that carry the information into the olfactory cortex in the temporal lobe where the sense of smell is being processed. Sound production for speech is another major function of the respiratory system. In the larynx, there are vocal cords that are elastic ligaments that vibrate as air is forced in between these bands of elastic connective tissue. That vibration creates the sound that we use for speech. This should be a familiar diagram of the path of blood flow through the pulmonary and systemic circulation and so the pulmonary circuit is the blood supply to and from the lungs. The pulmonary arteries are carrying de-oxygenated blood to the lungs and pulmonary veins drain oxygenated blood back from the lungs to the left atrium of the heart. External respiration is the aspect of gas exchange which occurs in the lungs and so is a major function of the respiratory system is gas exchange and in particular external respiration where we have gas exchange between the blood of the pulmonary capillaries and air inside of the lungs, the air inside of the alveoli, which are the small sac-shaped terminal ends of the airways inside of the lungs. The air inside of the lungs is exchanging with gas that's dissolved in the blood across the wall of the alveoli and across the wall of the capillary. In external respiration oxygen moves from the alveoli or alveolus, as we can see on this diagram, is the singular of alveoli. Oxygen is moving from an alveolus into the blood plasma and then that oxygen in the blood plasma can move into the red blood cell and oxygen will then bind to hemoglobin inside of the red blood cell where hemoglobin will function to help transport oxygen in the blood. Carbon dioxide, which is found inside of the red blood cell and also dissolves in the plasma, moves the other direction across the capillary wall from the blood toward the alveolus. And so carbon dioxide is moving out of the blood into the lungs during external respiration and oxygen is moving from the lungs into the blood during external respiration. Internal respiration refers to gas exchange between the blood within the systemic capillaries and the interstitial fluid of tissues throughout the body. As cells throughout the body use oxygen for aerobic cellular respiration, the partial pressure of oxygen in the interstitial fluid decreases, and as these cells produce ATP from aerobic cellular respiration, they also produce the metabolic waste product carbon dioxide and so carbon dioxide concentration of the interstitial fluid increases, and the partial pressure of carbon dioxide in the interstitial fluid increases. So normally the partial pressure of carbon dioxide increases to around 46 millimeters of mercury in the interstitial fluid and the partial pressure of carbon dioxide in the pulmonary or in the systemic capillaries is initially around 40 millimeters of mercury coming in from the arteries. But as carbon dioxide diffuses into the blood, the blood carbon dioxide partial pressure rises to around 46 millimeters of mercury and the blood oxygen partial pressure is normally around 100 millimeters of mercury in the blood coming from the arteries into the systemic capillaries. Then the partial pressure of oxygen in the interstitial fluid is usually below 40 millimeters of mercury and oxygen will diffuse out of the blood into the interstitial fluid until the partial pressure of oxygen in the blood arrives at around 40 millimeters of mercury in the blood that's deoxygenated blood draining into the venules to return back to the to the right atrium of the heart through veins. The fact that the respiratory system allows us to remove carbon dioxide from the blood enables another major function of the respiratory system, which is acid base homeostasis. The respiratory system contributes to the balance of the pH of the body by removing carbon dioxide from the blood because carbon dioxide reacts with water in the blood forming carbonic acid and carbonic acid can release a hydrogen ion into the surrounding solution lowering the pH. The enzyme carbonic anhydrase is responsible for interconversion of the carbonic acid with water and carbon dioxide and so the chemical reaction that converts carbonic acid to carbon dioxide and water is very quick and is catalyzed by this enzyme carbonic anhydrase, which is one of the fastest enzymes that humans have ever studied. This is a reversible reaction. So if we have excess carbon dioxide, that carbon dioxide can be converted into carbonic acid by this enzyme, carbonic anhydrase, and if we have excess carbonic acid, carbonic acid can be converted into carbon dioxide by this enzyme. So it can move either direction and when the pH of the body is becoming very acidic as the pH is becoming too low, it will stimulate an increase in the respiratory drive leading to increased pulmonary ventilation, that is increased rate of airflow in and out of the lungs, which speeds up gas exchange, helping to remove carbon dioxide from the body and this will have the effect of helping to raise the pH of the blood restoring homeostasis. So here's a diagram just summarizing the mechanism through which the respiratory system can contribute to the rapid regulation of acid-base balance in the body. If the pH of the body becomes too low, which is also known as acidosis, there are chemoreceptors that are detecting that change in pH. The central chemoreceptor in the brain is very sensitive to a change in the pH of the cerebrospinal fluid and if that falls too low, it will lead to a stimulation of the respiratory drive leading to increased respiratory rate, removing carbon dioxide from the body, helping to lower the carbonic acid concentration of the blood, and this brings the pH of the body back to the homeostatic set point around a pH of 7.4. And the opposite will happen if the pH of the body becomes too high. If the pH is above 7.4, this is alkalosis. So there's a little bit of room, so above 7.45 would be alkalosis, a normal range would be around 7.35 to 7.45, but if it gets above that 7.45, the pH of the body is too high known as alkalosis, which is also going to be detected by receptors, the central chemoreceptors in the brain are going to respond to that alkalosis, leading to a decrease in the respiratory rate, which causes the carbon dioxide to accumulate in the blood. It slows the rate at which carbon dioxide is being removed from the blood. So the blood carbon dioxide concentration rises and the blood carbonic acid level rises and as that carbonic acid level rises, that causes a decrease in the pH of the blood, helping to restore the homeostatic set point around a pH of 7.4. Inspiration is the process that moves air into the lungs. So breathing in is the act of inspiration. And inspiration is an act of process that requires contraction of the inspiratory muscles, or the muscles of inspiration, which include the diaphragm and the external intercostal muscles. When the diaphragm and external intercostal muscles contract, the volume of the thoracic cavity increases, which causes the pressure inside of the lungs, which is known as intrapulmonary pressure, or intra-alveolar pressure. This intrapulmonary or intra-alveolar pressure decreases as the thoracic volume increases. And when the intrapulmonary pressure is lower than the atmospheric pressure, air will flow into the lungs. Expiration is essentially the opposite. When the diaphragm and external intercostal muscles relax, the lungs can recoil, and the volume of the thoracic cavity is decreased. This leads to an increase in the pressure inside of the lungs. And when the intrapulmonary pressure is higher than the atmospheric pressure, air will flow out of the lungs in the process of expiration. And so expiration is a passive process that can occur just by relaxing the diaphragm and external intercostal muscles, although it is possible to produce more forceful expirations by contracting accessory muscles, such as the abdominal muscles, if you contract your rectus abdominis and the abdominal obliques and transverse abdominis. Those muscles will compress the abdominal cavity, which will help to decrease the volume of the thoracic cavity and produce more forceful expiration. Pulmonary ventilation is normally regulated subconsciously, with an involuntary mechanism, coordinated by respiratory control centers of the brainstem. These control centers are found specifically within the medulla oblongata and the pons. Within the medulla oblongata, there are two regions, the dorsal respiratory group and the ventral respiratory group that are the respiratory control centers. The dorsal respiratory group contains the cell bodies of the motor neurons that extend their axons out through the spinal nerves to stimulate contraction of the diaphragm and the external intercostal muscles, leading to inspiration. The ventral respiratory group contains a cluster of neurons known as the pre-Botsinger complex that generates the breathing rhythm to regulate the activity of the dorsal respiratory group. The ventral respiratory group also contains motor neurons that excite contraction of accessory respiratory muscles. For example, the internal intercostal muscles are accessory muscles of expiration that contract to produce more forceful expirations. The respiratory control centers in the medulla oblongata are in turn regulated by respiratory control centers in the pons known as the abnustic and pneumotastic centers. The abnustic center excites the dorsal respiratory group, stimulating an increased respiratory drive and the pneumotastic center has an inhibitory effect decreasing the respiratory drive. The respiratory control centers of the brainstem are also regulated by the cerebral cortex enabling voluntary control over the respiratory muscles. The hypothalamus also contains control centers that can influence pulmonary ventilation, enabling emotions to have an influence over our pulmonary ventilation such as when anger causes a cessation of ventilation or when anxiety causes hyperventilation. As the major functions of the respiratory system include maintenance of the acid-base balance of the body or maintenance of the pH of the body, there are central chemoreceptors in the medulla oblongata that are very sensitive to changes in the pH of the cerebrospinal fluid. And this is the primary mechanism controlling the respiratory drive which regulates the pulmonary ventilation rate. If the blood carbon dioxide concentration becomes elevated which is known as hypercapnia, carbon dioxide diffuses into the cerebrospinal fluid where it reacts with water in a reaction catalyzed by the enzyme carbonic anhydrase to form carbonic acid then carbonic acid releases hydrogen ions into solution lowering the pH of the cerebrospinal fluid. This decrease in pH is detected by the central chemoreceptors leading to an increased respiratory drive stimulating an increase in respiratory rate and or tidal volume increasing the pulmonary ventilation rate increasing the rate at which carbon dioxide is removed from the blood in order to restore the homeostatic set point of the cerebrospinal fluid pH. In response to low blood carbon dioxide concentration which is known as hypocapnia the pH of the cerebrospinal fluid will increase. The central chemoreceptors will respond to this increased pH by decreasing the respiratory drive leading to a decreased pulmonary ventilation rate in order to restore the homeostatic set point of the cerebrospinal fluid pH. There are also peripheral chemoreceptors located in the aortic arch and the carotid sinus near the location of the bowel receptors that monitor mean arterial pressure by detecting stretching of the large arteries the peripheral chemoreceptors monitor the pH of the arterial blood carbon dioxide concentration and oxygen concentration of the arterial blood while the central chemoreceptors provide the primary mechanism regulating pulmonary ventilation under normal circumstances in an abnormal circumstance such as at high altitude where the partial pressure of oxygen in the atmosphere is lower leading to a decreased rate of oxygen diffusion into the blood a low oxygen concentration of the arterial blood known as hypoxia is detected by the peripheral chemoreceptors receptors in the carotid sinus relay this information in through cranial nerve number nine the glasopharyngeal nerve and chemoreceptors located in the arch of the aorta relay this information in through cranial nerve number ten the vegas nerve this information is relayed into the control centers in the medulla oblongata and leads to an increase in the pulmonary ventilation rate in order to restore the homeostatic set point for the blood oxygen concentration