Collin County Community College BIOL. 2402 Anatomy & Physiology WEEK 9 Respiratory System 1 Lung Physiology Factors affecting Ventillation 1. Airway resistance Flow = Δ P / R Most resistance is encountered at the medium sized bronchioli ( mostly smooth muscle, no cartilage ) Diameter is affected by : Symp. System : epi and norepi result in dilation ParaSymp. System : can result in constriction Irritants, histamine also cause constriction Any blockage of passageways also increases resistance ( such as thick mucus ) 2 1
Lung Physiology 2. Lung Compliance Compliance = Δ V / Δ P Lung compliance is a measure of the lung s stretchability. When compliance is abnormally low, the work of breathing is increased. 3 Lung Physiology Decreased Lung Compliance can be due to reduced lung elasticity reduced thoracic cage flexibility reduced production of surfactant In general, the higher the compliance the better since less pressure changes are needed to inflate the lungs. Diseases that cause reduced lung compliance are referred to as Restrictive Lung Diseases. Those patients require more energy and effort to breathe! 4 2
Lung Physiology An extremely high compliance is not good either! This because transpulmonary pressure never gets a chance to become large enough to exert its physiological effect. With such a high compliance, the lungs might fail to hold themselves open, and are prone to collapse. 5 Lung Physiology 3. Surfactant 6 3
Lung Physiology Surface Tension The attraction of the water molecules to each other resists expansion of the bubble (resists an increase in surface area). The surface area tends to shrink as small as possible. If alveoli were lined with water alone, they would collapse. It would require more transpulmonary pressure to open up the alveoli, thus resulting in a lower compliance 7 Lung Physiology Law of La Place as it applies to alveoli Collapsing Inward Pressure = 2. Surface Tension / radius Translation : The smaller a bubble, the greater the collapsing pressure If two bubbles of different diameter are connected to each other, and they are not lined with surfactant, the smaller one will collapse and the air will flow into the larger one. 8 4
P = 2.T/ r In the absence of surfactant, the attraction between water molecules (H-bonds) can cause alveolar collapse. By reducing the surface tension of water, surfactant helps prevent alveolar collapse. 9 Lung Physiology During normal quiet breathing, respiratory muscles work to expand the lungs and overcome airway resistance, while expiration is a passive process On average, however, inhalation only expends 3 % of total body energy due to high compliance of lungs and low airway resistance. More energy will be needed when pulmonary compliance decreases ( pulmonary fibrosis) airway resistance increases (obstructive lung disease) elastic recoil decreases (emphysema) there is a need for increased ventilation A person with obstructive lung disease may for example expend 30% of total body energy just to inhale. 10 5
Respiratory Volumes Respirometer or a Spirometer 11 Respiratory Volumes Maximum Lung volume ~ 5700 ml of air However, during quiet breathing the lungs are not close to being completely filed or completely emptied. Lungs operate at half capacity during normal breathing. The lungs can never be completely deflated, even when lungs are emptied forcefully. About 1,200 ml remains behind and provides continued gas exchange between alveoli and blood during heavy exercise. If lungs emptied all the way, the body would experience dramatic fluctuations in O2 and CO2. 12 6
The tidal volume is the amount of air moved in (or out) of the airways in a single breathing cycle. Inspiratory and expiratory reserve volumes, are, respectively, the additional volume that can inspired or expired. All three quantities sum to the lung s vital capacity. The residual volume is the amount of air that must remain in the lungs to 13 prevent alveolar collapse. Restrictive lung diseases reduce IRV Obstructive lung diseases reduce ERV FEV 1 = amount expelled in 1 sec when lungs are fully filled Healthy people: FEV 1 ~ 80% of V.C. 14 7
Respiratory Volumes Concept of Dead Space the air located in the conducting zones air that will not take part in gas exchange ~ 150 ml So if Tidal Volume is 500 ml, then only (500-150) ml takes part in gas exchange This 350 ml mixes with the 2400 ml already in the lungs Each tidal volume breath only replaces 12 % of lung volume. 350 of (2400 + 350) 15 Respiratory Volumes Fresh inspired air is diluted by the stale air remaining in the lungs from the previous breathing cycle. 16 8
Respiratory Volumes Ventilation Rates Minute Ventilation rate (MVR) The amount of air inhaled every minute MVR = breathing rate x Tidal volume Alveolar Ventilation rate (AVR) The amount of air that takes part in gas exchange AVR = breathing rate x (Tidal volume - Dead space) 17 Respiratory Volumes Increased O 2 demand and Ventilation For the heart : CO = HR x SV For the lungs : AVR = BR x (TV - Dead space) Increasing TV and/or BR will increase air supply More specifically, the body tries to make ventilation more efficient by increasing the fraction of useful air. A useful ratio is to analyze what fraction of the inspired air is actually used by the alveoli in gas exchange. AVR / MVR = (TV - Dead space) / TV 18 9
Respiratory Volumes AVM/MVR A = Fast shallow respiration : 0/6000 = 0 B = Normal respiration : 4200/6000 = 0.70 C = Slow deep respiration : 5100/6000 = 0.85 19 Respiratory Volumes Shallow faster breathing lowers the efficiency of ventilation since dead space becomes a larger fraction of tidal volume Increasing TV makes ventilation more efficient. So, it s better to increase TV than to increase BR. The body in general tries to regulate both TV and BR closely in order to match respiration with oxygen demands. Normally, dead space is fixed. But in certain conditions it can become altered. Example: breathing through a tube extends dead space why are snorkels short and fat? 20 10
Snorkel 1 cm in diameter and 0.5 meter long Volume = πr 2. L = (3.14)(1) 2.50 = 157 cm 3 Total Dead Volume = 150 + 157 = 307 ml TV = 150 BR = 12 MVR = 6000 AVR = 12 x ( 500-307) = 2316 Normal AVR/MVR = 0.70 Now AVR/MVR = 0.39 21 Pulmonary Pathologies 22 11
Gas Exchanges Provision of O 2 to the tissues is accomplished by 4 main processes. Pulmonary Ventilation movement of air in and out of the lungs. External Respiration gas exchange between lungs and blood Transport of the gases transport via blood between lungs and tissues Internal Respiration gas exchange between blood and tissues. Resp. System CardioVasc. System 23 Properties of Gases Dalton s Law Total pressure of a mixture of gases is equal to the sum of the partial pressure of each of the gases in the mixture P T = P 1 + P 2 + P 3 +.. The partial pressure of each gases is directly proportional to the percentage of that gas in a mixture. For example, if gas 1 is 30 % of the mixture, then the partial pressure of P 1 = 0.3 P T 24 12
Properties of Gases Gases in Air and their Partial Pressures Atmospheric Pressure = 760 mm Hg Gas N 2 O 2 CO 2 % 78.6 20.9 0.04 P p (mm Hg) 597 159 0.3 H 2 O 0.46 3.7 25 Properties of Gases During gas exchange, gases diffuse from an air mixture into a liquid medium. The amount that will diffuse and dissolve is determined by Henry s Law! Henry s Law The amount of gas that will dissolve in a liquid is proportional to it s partial pressure in the gas mixture above the liquid. Dissolved ml of a gas in a liquid = (K. P i )/ 760 Where K = solubility coefficient P i = Partial pressure of gas in question 26 13
Properties of Gases Dissolved ml of a gas in a liquid = (K. P i )/ 760 Solubility coefficient for following gases is such that K C02 = 20. K O2 K N2 = 0.5 K O2 This means that the relative diffusion rates of these gases into body fluids are N 2 to O 2 to C0 2 = 0.5 to 1 to 20 27 Properties of Gases ml of gas dissolved in 100 ml blood at pulmonary vein : N 2 : 1.25 O 2 : 0.29 C0 2 : 2.62 Even though the diffusion coefficient for Nitrogen is 1/2 of that of Oxygen, there is thus more Nitrogen dissolved in blood than Oxygen. Why? The answer is the same reason why Nitrogen becomes a lethal element during scuba diving and prolonged exposure to higher atmospheric pressures ( similar as opening a bottle of soda! ). 28 14
Exchange of gases External Respiration It is the Exchange of gases at the level of the alveoli (between lungs and capillary blood) Internal Respiration Exchange of gases at the level of the tissues Exchange of gases between capillary blood and tissue cells Driving force is simple diffusion 29 Exchange of gases 30 15
Composition of Atmospheric vs Alveolar air Gas N 2 % 78.6 Air P p (mmhg) 597 % 74.9 Lungs P p (mmhg) 569 O 2 20.9 159 13.7 104 CO 2 0.04 0.3 5.2 40 H 2 O 0.46 3.7 6.2 47 These Differences are due to : gas exchange in the alveoli humidification (increased P p of water) mixture with residual volumes in the lungs 31 External Respiration Factors that influence gas exchange 1. Pressure gradient Alveoli PO 2 = 104 PCO 2 = 40 Equilibrium occurs within 0.25 sec PO 2 = 40 O 2 CO 2 PO 2 = 104 O 2 CO 2 PCO 2 = 46 PCO 2 = 40 32 16
2. Thickness of Respiratory Membrane The thicker the membrane, the harder diffusion External Respiration 3. Surface Area The larger the area, the more gases can diffuse Fick s Diffusion Equation Flux = D. A. (C2-C1) / x Time to diffuse 1 µm = Time to diffuse 1 cm = 0.5 msec 14 hrs 33 External Respiration 4. Ventilation-Perfusion coupling Normally, ventilation in the alveoli is matched perfectly with blood flow through the alveolar capillaries. 34 17
External Respiration If alveoli are not functioning properly, and ventillation of those alveoli is lower than normal, the alveoli will end up with low oxygen levels and high CO 2 levels. Thus capillary blood PO 2 will also drop, and capillary PCO 2 increases. The drop on O 2 results in vasoconstriction of the capillaries. The result is less blood flow through that section of the lungs. Also, blood is diverted to better perfused sections of the lungs. The result is a better matched ventilation - perfusion! 35 36 18
External Respiration Local Pulmonary Controls Gas Composition Bronchioli Pulmonary arteries PCO 2 increases Dilate (Constrict) PCO 2 decreases Constrict (Dilate) PO 2 increases (Constrict) Dilate PO 2 decreases (Dilate) Constrict Responses in brackets indicates a weak response 37 External Respiration The ratio of O 2 consumption to ventilation determines alveolar PO 2 The ratio of CO 2 production to ventilation determines alveolar PCO 2 38 19
Internal Respiration Pressure gradient Tissue cells PO 2 = 104 PO 2 < 40 PCO 2 > 46 O 2 CO 2 PO 2 = 40 PCO 2 = 40 O 2 CO 2 PCO 2 = 46 39 External Respiration Changes in the concentration of dissolved gases are indicated as the blood circulates in the body. Oxygen disappears at active cells as it is converted to water; active cells release carbon dioxide as a byproduct 40 of fuel catabolism. 20
Internal/External Respiration and Metabolism AVR = 4000 ml/min po2 in air = 20.9 % How much oxygen is delivered to the alveoli for exchange? 20.9 % of 4000 = 840 ml/min How much oxygen is actually exchanged? po2 in exhaled air = 14.75 % = 590 ml/min 840-590 = 250 ml O2 enters blood stream / min This amount is what tissues in general require at rest (basal metabolism) = 250 ml O2 / min 41 21