Respiratory System
Pulmonary Ventilation
Internal Respiration
External Respiration
Transport of Respiratory Gases
Define: (Pulmonary gas) exchange of O2 and CO2 between lungs and blood
Direction
Mechanisms
Define: Breathing. Movement of air into and out of lungs
Direction
Inspiration: air moves into lungs from atmosphere
Expiration: air moves out of lungs to atmosphere
Respiratory Zone: site of gas exchange
Diffusion Infuences
Thickness and Surface area of Respiratory Membrane: The greater the surface area of the respiratory membrane, the more gas can diffuse across it
Partial Pressure Gradients and Gas Solubilities: Steep partial pressure gradient for O2 exists between blood and lungs; Partial pressure gradient for CO2 is less steep, but diffuses in equal amounts with O2 because CO2 is 20x more soluble in plasma and alveolar fluid than O2
Ventilation-Perfusion Coupling: alveolar ventilation and pulmonary blood perfusion rates must be matched for optimal, efficient gas exchange
Carbon Dioxide Transport- (3 ways) CO2 travels from cells to blood to be exhaled through the lungs
Oxygen Transport (2 ways)- O2 travels from lungs to blood into the body tissues to be used by cells.
- Dissolved in plasma (7-10%)
- Attached to Hemoglobin (Hb) Molecules (98.5%)
- Dissolved in plasma (1.5%)
Each hemoglobin molecule can combine with 4 molecules of O2 (each oxygen can bin to one heme group). This is called oxyhemoglobin. Hemoglobin that has released oxygen for the cells to use is called deoxyhemoglobin.
Rate at which Hb reversibly binds or releases O2 is reulated by PO2, temprature, blood pH, PCO2, and BPG concentration. All factors modify hemoglobin's 3D structure.
When factors are decreased: When more oxygen is available (pulmonary capillaries), it will want to bind to hemoglobin. When one O2 molecule saturates a heme group, the hemoglobin molecules's affinity for other O2 molecules to bind increases.
Mechanisms
Boyle's Law:
P1 x V1=P2 x V2
Pressure varies inversely with volume
When factors are increased: All of these factors tend to be highest in the systemic capillaries, where oxygen unloading is the goal. As cells metabolize glucose and use O2, they release CO2, which increases the PCO2 and H+ levels in capillary blood. Both declining blood pH (acidosis) and increasing PCO2 weaken the Hb-O2 bond This is called the Bohr effect, it enhances oxygen unloading where it is most needed.
Intrapulmonary Pressure
Intrapleural Pressure
Inspiration: Lung volume increases -> Pressure decreases
Expiration: Lung volume decreases -> Pressure increases
Inspiration: chest wall expands -> pressure more negative
Expiration: chest wall recoils -> pressure returns to initial value (less negative)
- Bound to hemoglobin (about 20%)
- As bicarbonate in plasma (about 70%)
Dissolved CO2 is bound and carried in the RBCs as carbaminohemoglobin. (CO2+Hb⇌HbCO2) This reaction is rapid and does not require a catalyst. Carbon dioxide binds to amino acids of globin. Carbon dioxide rapidly dissociates from hemoglobin in the lungs, where the PCO2 of alveolar air is lower than that in blood. Carbon dioxide readily binds with hemoglobin in the tissues, where the PCO2 is higher than that in blood. Deoxygenated hemoglobin combines more readily with carbon dioxide than does oxygenated hemoglobin.
Most carbon dioxide molecules entering the plasma quickly enter RBCs. The reactions that convert carbon dioxide to bicarbonate ions for transport mostly occur inside RBCs. CO2+H2O⇌H2CO3 (carbonicacid)⇌H+ +HCO3−(bicarbonate ion). Although this reaction also occurs in plasma, it is thousands of times faster in RBCs because they contain carbonic anhydrase, an enzyme that reversibly catalyzes the conversion of carbon dioxide and water to carbonic acid. Hydrogen ions released during the reaction (as well as CO2 itself) bind to Hb, triggering the Bohr effect. In this way CO2 loading enhances O2 release. Once generated, HCO3− moves quickly from the RBCs into the plasma, where it is carried to the lungs. To counterbalance the rapid outrush of these anions from the RBCs, chloride ions move from the plasma into the RBCs. This ion exchange process, called the chloride shift, occurs via facilitated diffusion through an RBC membrane protein.
The Haldane effect: reflects the greater ability of reduced hemoglobin to form carbaminohemoglobin and to buffer H+ by combining with it. The lower the PO2 and the lower the Hb saturation with oxygen, the more CO2 that blood can carry.
Comparing the Haldane and Bohr effects: As CO2 enters the systemic bloodstream, it causes more oxygen to dissociate from Hb (Bohr effect). The dissociation of O2 allows more CO2 to combine with Hb (Haldane effect).
The bicarbonate buffer system resists shifts in blood pH. If the hydrogen ion concentration in blood begins to rise, excess H+ is removed by combining with HCO3− to form carbonic acid (a weak acid). If H+ concentration in blood drops below desirable levels, carbonic acid dissociates, releasing hydrogen ions and lowering the pH again.
Neural mechanisms that control respiration
Medullary centers
Ventral respiratory group: rhythm-generating and integrative center. When its inspiratory neurons fire, a burst of impulses travels along the phrenic and intercostal nerves to excite the diaphragm and external intercostal muscles. As a result, the thorax expands and air rushes into the lungs. When the VRG’s expiratory neurons fire, the output stops, and expiration occurs as the inspiratory muscles relax and the lungs recoil. This cyclic on/off activity of the inspiratory and expiratory neurons repeats continuously and produces a respiratory rate of 12–16 breaths per minute. This normal respiratory rate and rhythm is called eupnea
Pontine respiratory centers: influnce and modify the activity of medullary nerurons (mostly act on VRG). Act to smooth out transitions between inspiration and expiration.
Dorsal respiratory group: integrates input from peripheral stretch and chemoreceptors and communicates this information to the VRG.
Chemoreceptors: sensors that respond to chemical fluctuations. Among the factors that influence breathing rate and depth, the most important are changing levels of CO2, O2, and H+ in arterial blood.
Transpulmonary pressure: the difference between the intrapulmonary and intrapleural pressure. It keeps the air spaces of the lungs open. We cannot overemphasize the importance of negative pressure in the intrapleural space and the tight coupling of the lungs to the thorax wall. Any condition that equalizes intrapleural with the intrapulmonary (or atmospheric) pressure causes immediate lung collapse
Indicator of effective ventilation
Minute ventilation (L/min)
Rough estimate of effectiveness
Define: amount of gas that flows into or out of respiratory tract in 1 min
= Tidal volume (ml) x Respiratory rate (/min)
Alveolar ventilation rate (ml/min)
Better indicator
Define: amount of gas that flows into or out of alveoli in a particular time
= frequency (breaths/min) x (Tidal volume - dead space) (ml/breath)
Dead space (usually alveolar dead space): constant, AVR usually affected by TV rather than frequency
Total Dead Space: sum of anatomical and alveolar dead space
Anatomical: air remains in passageways -> not contribute to gas exchange
Alveolar: occupied by non-functional alveoli due to collapse or obstruction
Dalton's Law of Partial Pressures: Total pressure exerted by mixture of gases is equal to sum of pressures exerted by each gas; partial pressures of each gas is directly proportional to the percentage of that gas in the gas mixture; At high altitudes, partial pressures decline, while at lower altitudes (under water), partial pressures increase significantly
Henry's Law: states that when a gas is in contact with a liquid, the gas will dissolve in the liquid in proportion to its partial pressure; the greater the concentration of a particular gas in the gas phase, the more and the faster the gas will move into the liquid. At equilibrium, partial pressures in the two phases will be equal. The amount of each gas that will dissolve depends on solubility of the gas in liquid and the temperature of the liquid; CO2 is 20x more soluble in water than O2 and little N2 will dissolve; solubility decreases as temperature of liquid rises
Respiratory Volumes: Used to assess respiratory status
Inspiratory Reserve Volume (IRV): The amount of air that can be inspired forcibly beyond the tidal volume (2100-3200ml)
Expiratory Reserve Volume (ERV): The amount of air that can be forcibly expelled from the lungs (1000-1200ml)
Tidal Volume (TV): The amount of air moved into and out of the lungs with each breath (avg. ~500ml)
Residual Volume (RV): The amount of air that always remain in the lungs that are need to keep the alveoli open
Respiratory Capacities: Combinations of two of more respiratory volumes
Vital Capacity (VC): Sum of TV+IRV+ERV
Inspiratory Capacity (IC):Sum of TV+IRV
Total Lung Capacity (TLC): Sum of all lung volumes (TV+IRV+ERV+RV)
Functional Residual Capacity (FRC): Sum of RV+ERV
CO2 diffuses from the blood to the lungs
O2 diffuses from the lungs to the blood
Zones
Conducting Zone: Transports gas to and from gas exchange sites
Respiratory Zone: Site of gas exchange
Internal Respiration involves capillary gas exchange in body tissues.
Compared to external respiration, partial pressures and diffusion gradients are reversed.
Oxygen moves from blood to tissues because tissue PO2 is always lower than in arterial blood PO2 (40 vs. 100 mm Hg)
CO2 moves from tissues into blood because tissue PCO2 is always higher than arterial blood PCO2 (45 vs. 40mm Hg)
Venous blood returning to the heart has PO2 of 40 mm Hg and PCO2 of 45 mm Hg
Oxygen Transport
Molecular O2 is cariied in blood in 2 ways. 1.5% is dissolved in plasma and 98.5% is looslely bound to each Fe of hemoglobin (Hb) in RBCs. Each Hb can transport 4 O2 molecules.
Oxygen-Hemoglobin Dissociation Curve: Percent of Hb saturation can be plotted against PO2 concentrations. The graph is an S shaped curve. Increases in temperature, H+, and PCO2 shift curve to the right. Decreases in same factors shift curve left.
Rebecca Collette, Nghi Cao, Cindy Do, Philicia Henderson