The Pulmonary System and Exercise F23 PDF

The Pulmonary System and Exercise Chapter 10 Pulmonary Functions Gas exchange between external environment and body Supply oxygen (metabolism) Eliminate carbon dioxide (metabolism) Ventilation and diffusion Regulate hydrogen ion concentration [H+] to maintain acid-base balance...

The Pulmonary System and Exercise Chapter 10 Pulmonary Functions Gas exchange between external environment and body Supply oxygen (metabolism) Eliminate carbon dioxide (metabolism) Ventilation and diffusion Regulate hydrogen ion concentration [H+] to maintain acid-base balance Anatomy of Ventilation Nose and Mouth Trachea Bronchi Bronchioles Alveoli Mechanics of Ventilation: Inspiration Inspiration Diaphragm contracts, flattens out, moves downward Air in the lungs expands, reducing its pressure Pressure differential between lungs and ambient air sucks air in through the nose and mouth, and inflates the lungs Inspiration concludes when thoracic cavity expansion ceases, and intrapulmonic pressure increases to equal atmospheric pressure Mechanics of Ventilation: Expiration Predominantly passive process Air moves out of the lungs from recoil of stretched lung tissue and relaxation of inspiratory muscles The sternum and ribs swing down, the diaphragm up Decrease chest cavity volume and compress alveolar gas; this forces it out through respiratory tract to atmosphere Lung Volumes and Capacities Vary with Age Sex Body size and composition Stature Static lung volume Dimensional component for air movement within pulmonary tract with no time limitation for subjects Dynamic lung volume Power component of pulmonary performance during different phases of ventilatory excursion Spirometry  Static Lung Volumes Tidal volume 6 Liters 3 0 Average 600 mL (males), 500 mL (females)  Static Lung Volumes Inspiratory Reserve Volume (IRV) 6 Liters 3 0 Average 3000 mL (males), 1900 mL (females)  Static Lung Volumes Expiratory Reserve Volume 6 Liters 3 0 Average 1200 mL (males), 800 mL (females)  Static Lung Volumes Residual Lung Volume (RLV) 6 Aging changes lung volumes because of Liters decreases in lung 3 tissue elasticity and decline in pulmonary muscle power. Most likely due to sedentary lifestyle than true aging 0 Average 1300 mL (males), 1000 mL (females)  Static Lung Volumes Forced Vital Capacity (FVC) 6 Large volumes Liters probably reflect genetic endowment 3 because static volumes do not change appreciably with exercise training 0 Average 4800 mL (males), 3200 mL (females) Dynamic Lung Volumes Dynamic measures of pulmonary ventilation depend on: The maximum lung air volume expired (FVC) The speed of moving a volume of air Airflow speed depends on Airway’s resistance to smooth flow of air and resistance (“stiffness”) offered by the chest and lung tissue to changes in shape during breathing Lung compliance Dynamic Lung Volumes FEV1.0: Percentage of FVC expelled in 1 second (normal ≥ 80%) FEV1.0/FVC: Reflects expiratory power and overall resistance to air movement in the lungs; averages ~85% Maximum Voluntary Ventilation: Rapid, deep breathing for 15 seconds that is extrapolated to the volume breathed for 1 minute; ranges between 140 and 180 L·min-1 for men, 80 to 120 L·min-1 for women Pulmonary Ventilation Minute ventilation Resting values Respiratory rate ? Tidal volume ? Minute ventilation (VE) = Breathing rate x TV Increases in depth or rate or both increases VE Maximal exercise Healthy, young adults increase to 35-45 breaths/min (elites 60-70 breaths/min) TV increases to 2 L or greater Elite males athletes can reach 160-200 L/min TV rarely exceeds 55-65% vital capacity Pulmonary Ventilation Alveolar ventilation Portion of VE that mixes with air in alveolar sacs Anatomic dead space ~ 30% of resting TV ~ equal composition- dead space air and ambient air except dead space air is fully saturated with water vapor Because of dead space volume, ~70% of inspired ambient air in each resting TV mixes with existing alveolar air All 500 mL of TV enters alveoli, but only 350 mL represents fresh air Prevents drastic changes in alveolar air composition, ensuring consistency in arterial blood gases throughout breathing cycle  Condition TV (mL) RR (br/min) VE (mL/min) Dead space Alveolar ventilation ventilation (mL/min) (mL/min) Shallow 150 40 6000 150 x 40 0 breathing Normal 500 12 6000 150 x 12 4200 breathing Deep 1000 6 6000 150 x 6 5100 breathing Alveolar ventilation, not dead-space ventilation, determines gaseous concentrations at the alveolar-capillary membrane Depth versus Rate Adjustments in depth and rate maintain alveolar ventilation as exercise intensity increases Moderate exercise, trained endurance athletes ↑ TV and only minimally ↑ rate With deeper breathing, alveolar ventilation increases from 70% of minute ventilation at rest to >85% of total ventilation during exercise ↑ occurs because > percentage of inspired TV enters alveoli with deeper breathing With exercise, increasing TV results largely from Tidal encroachment volume on IRV, with plateaus ~ smaller decrease 60% of vital in ERV capacity Depth and Rate Ventilatory adjustments occur unconsciously Blend rate and TV so alveolar ventilation = alveolar perfusion Conscious attempts to modify breathing during running and other general PAs do not benefit performance Entrainment- reduces energy cost of the activity Gas Exchange  Gases Total pressure: force exerted by all gas molecules against surfaces they encounter Partial pressure: force exerted by a certain gas, if it was the only gas present % concentration * total pressure Gas concentration: amount of gas in a given volume, determined by product of gas’ partial pressure and solubility Gas Exchange Oxygen supply depends on oxygen concentration in ambient air and its pressure Ambient air remains constant: Oxygen 20.93% Nitrogen 79.04% Carbon dioxide 0.03% Small quantities of water vapor At sea level, pressure of air’s gas molecules have barometric reading of ~760 mmHg Partial Pressures Ambient Air: PO2 = 159 mmHg PCO2 = 0.2 mmHg PN2 = 600 mmHg Tracheal Air: PO2 = 149 mmHg PCO2 stays the same Alveolar Air: PO2 = 103 mmHg PCO2 = 39 mmHg Gases want to go where they are not Movement of Gas in Air and Fluids Henry’s Law The amount of a gas dissolved in a fluid depends on Partial pressure differential between the gas above the fluid and dissolved in it (major factor) Solubility of the gas in the fluid (constant) Temperature of blood (minimal changes) Pressure Differentials Net diffusion of a gas occurs only when a difference exists in gas pressure Specific gas’ partial pressure gradient represents the driving force for its diffusion Solubility Dissolving power, reflects the quantity of a gas dissolved in a fluid at a particular pressure Gas with higher solubility has a higher concentration at a specific pressure For two different gases at identical pressure differentials, solubility of each gas determines number of molecules moving into/out of fluid For each unit of pressure favoring diffusion, approximately 25x more CO2 than O2 moves into or from a fluid Gas Exchange in the Body The exchange of gases between lungs and blood and their movement at the tissue level takes place passively by diffusion In the Lungs: transfer of oxygen from the alveoli into the blood Dilution of oxygen in inspired air: water vapor saturation, oxygen continually leaves alveolar air, carbon dioxide continually enters alveolar air In the Tissues: Oxygen leaves capillary blood and flows toward metabolizing cells, while carbon dioxide flows from the cell into the blood Larger pressure differentials with vigorous exercise Small pressure gradient, but adequate CO2 transfer occurs rapidly because of high solubility Vigorous exercise Large pressure PCO2 90 mmHg differential establishes diffusion PO2 3 mmHg gradient Oxygen and Carbon Dioxide Transport  Oxygen Transport in Blood In physical solution: Dissolved in the fluid portion of the blood; establishes the PO2 of the blood and tissue fluids Combined with hemoglobin: In loose combination with the iron-protein Hb molecule in the RBC; ↑ blood’s O2-carrying capacity 65-70 times above that normally dissolved in plasma Oxyhemoglobin- requires no enzymes, PO2 solely determines oxygentation of Hb 2,3-diphosphoglycerate (2,3-DPG) Compound produced as anaerobic metabolite in RBCs during glycolysis Facilitates O2 dissociation by combining with subunits of Hb to reduce its affinity for oxygen Elevated levels in individuals with cardiopulmonary disease and high-altitude inhabitants Compensatory adjustment to facilitate oxygen release from cells Adaptations in 2,3-DPG occurs relatively slowly compared with immediate Bohr effect Myoglobin and Muscle Oxygen Storage Similar to hemoglobin, found in skeletal and cardiac muscle Combines reversibly with oxygen, adds additional oxygen to the muscle Facilitates oxygen transfer to mitochondria, especially at start of exercise and during intense exercise when cellular PO2 decreases considerably No Bohr effect Carbon Dioxide Transport in Blood CO2 formed in cells, only means to “escape” is diffusion and transport to lungs in venous blood 10% transported as physical solution in plasma This small quantity establishes PCO2 20% transported in loose combination with Hb Carbamino compounds (carbaminohemoglobin) Formation reverses in lungs as PCO2 decreases Hb’s oxygenation reduces capacity to bind CO2 o “Haldane effect” – interaction between oxygen loading and carbon dioxide release facilitates carbon dioxide removal from lungs 70% combined with water as bicarbonate  Carbon Dioxide as Bicarbonate Most of CO2 combines with water to form carbonic acid Slow reaction, accelerated 5000x by zinc-containing enzyme within RBCs (carbonic anhydrase) Tissue level CO2 + H20 → H2CO3 → H+ + HCO3- Protein portion of Hb buffers H+ to maintain pH HCO3- ‘s high solubility causes it to diffuse from RBC to plasma in exchange for chloride ion which moves into cell to maintain ionic equilibrium; “chloride shift”- higher content of Cl- in venous RBCs than arterial blood Pulmonary level PCO2 decreases at lungs, disturbing equilibrium between carbonic acid and formation of bicarb ions H+ + HCO3- → H2CO3 → CO2 + H20 Carbon dioxide can exit at lungs Plasma bicarb concentration decreases at pulmonary capillaries, permitting Cl- to move back to plasma Regulation of Pulmonary Ventilation Neural Factors Normal respiratory cycle comes from inherent, automatic activity of inspiratory neurons whose cell bodies reside in the medial medulla of the brain Lungs inflate as neurons activate diaphragm/intercostals As expiration proceeds, Inspiratory neurons cease inspiratory center released from because self-limiting and inhibition and progressively inhibitory influence from become more active medulla’s expiratory neurons Expiration begins as Inflation stimulates stretch passive recoil. Activation of receptors in bronchioles. expiratory neurons further Inhibit inspiration/stimulate facilitate. expiration Humoral Factors Chemical state of blood largely regulates pulmonary ventilation at rest Variations in arterial PO2, PCO2, acidity, and temperature activate sensitive neural units in the medulla and arterial system Hypoxic threshold: point at which decreased PO2 stimulates ventilation (~60-70mmHg) Chemoreceptors (and other specialized receptors): Structures that stimulate ventilation in response to increased carbon dioxide, temperature, and acidity, a decrease in oxygen and blood pressure, and perhaps an increase in circulating potassium PCO2 in arterial plasma provides the most important respiratory stimulus at rest Hyperventilation and Breath-Holding Breath-holding following normal exhalation lasts ~ 40 sec Urge to breathe mainly from stimulating effects of increased arterial PCO2 and [H+], not decreased arterial PO2 Break point for breath-holding generally corresponds to increase in arterial PCO2 to about 50 mmHg Conscious increase in alveolar ventilation increases CO2 leaving blood, decreasing arterial PCO2 below normal levels Application in swimmers and sport divers Ventilatory Control During Exercise Neurogenic factors Cortical Influence- “central command”- primary drive Neural outflow from regions of the motor cortex during exercise and cortical activation in anticipation of exercise stimulate respiratory neurons in the medulla Peripheral Influence- help to fine tune ventilation Sensory input from joints, tendons, and muscles adjust ventilation during exercise (most likely mechanoreceptors) Ventilatory Control During Exercise Humoral Factors- also help to finetune ventilation Po2 Arterial PO2 in exercise does not decrease to the point that stimulates ventilation by chemoreceptor activation Pco2 [H+] Chemical stimuli cannot fully explain the hyperpnea during physical activity Pulmonary Ventilation During Exercise Ventilation in Steady-State Exercise Light to moderate exercise VE increases linearly with oxygen uptake Mainly through increases in TV VE/VO2 (Ventilatory equivalent for oxygen) Indicates breathing economy Maintained around 25 in healthy, young adults during submaximal exercise up to about 55% VO2max Individual differences but complete aeration takes place because 1. Alveolar PO2 and PCO2 remain at near-resting values 2. Transit time for blood flowing through pulmonary capillaries proceeds slowly enough to permit complete gas exchange Ventilation in Non-Steady-State Exercise Ventilation in Non-Steady-State Exercise Ventilatory threshold- point at which pulmonary ventilation increases disproportionately with oxygen uptake during graded exercise At and above this threshold, pulmonary ventilation no longer links tightly to oxygen demand at the cellular level “Excess” ventilation relates directly to CO2’s increased output from the buffering of lactate that begins to accumulate from anaerobic metabolism Lactate + NaHCO3 → Na lactate + H2CO3 → H2O + CO2 Non-metabolic CO2, stimulates pulmonary ventilation, disproportionately raising VE/VO2