Exam 4 Book Notes PET3380CChapter 24• high altitude = 10,000 ft to 18,000 ft above sea level• prolonged exposure of an unacclimatized person to high altitudes can cause death from the ambient air's subnormal oxygen pressure (hypoxia), even if the person remains inactive• hypoxia - the pathological condition in which the body is deprived of adequate oxygen supplyThe Stress of Altitude• altitude's physiologic challenge comes directly from decreased ambient Po2, not from reduced total barometric pressure or any change in the relative concentrations of gases in inspired air• oxygen transport cascade - the progressive change in the environment's oxygen pressure and in various body areas• as you increase in altitude, the density of air decreases• at sea level, the air contains about 21% oxygen, .03% CO2, 79% N• density of gas = barometric pressure x partial pressure of gas• acclimatization - adaptations produced by changes in the natural environment, whether through a change in season or place of residence• acclimation - concerns adaptations produced I a controlled laboratory environment that simulate high altitude or microgravity, hypoxic environments, and extremes of thermal stressOxygen Loading at Altitude • only a small change occurs in hemoglobin's percentage saturation with oxygen until an altitude of about 3,000 meters◦ does not really affect little to moderate exercise◦ does however reduces performance during vigorous aerobic activities• altitude does not impair the short term anaerobic energy system ◦ ie. glycogen storage, glycolysis pathways, and enzyme activity◦ in fact, due to a reduction in air resistance (lower air density), sprinters actually perform better at higher altitudes• in addition to impairment in oxygen transport capacity, high altitude exposure impairs the homeostatic regulation of immune balance (ie. can increase risk of infection)Acclimatization• altitude acclimatization - adaptive responses in physiology and metabolism that improve tolerance to altitude hypoxia• typically, it takes longer to lose your acclimatization to an area than it takes to achieve itImmediate Responses to Altitude Exposure• there are immediate physiologic adjustments to compensate for thinner air and accompanying reduction in alveolar Po2• two more important responses:◦ increase in respiratory drive to produce hyperventilation (ie. body breathes more)◦ increase in blood flow during rest and submaximal exercise (ie. less O2 = need more blood)Hyperventilation• it is the most clear-cut immediate response to reduced arterial Po2 at a higher altitude• hypoxic drive increases for 1st few weeks and can remain elevated for a year or longer after...• the aortic arch and carotid arteries in the neck contain peripheral chemoreceptors that are sensitive to reduced oxygen pressure◦ they increase alveolar ventilation causing alveolar Po2 to rise toward the level in ambient air◦ small increase in Po2 facilitate oxygen loading in the lungs and provide the first line ofExam 4 Book Notes PET3380Cdefense against reduced ambient Po2Increased Cardiovascular Response• your blood pressure increases in the early stages of altitude adaptation ◦ this largely compensates for arterial desaturation◦ ie. the less saturated your blood, the more it has to pump• also, submaximal heart rate and cardiac output can rise to 50% above sea level values, while the heart's stroke volume remains the sameCatecholamine Response• increased blood pressure and heart rate at altitude coincide with the steady rise in plasma levels and excretion rates of epinephrine• norepinephrine levels peak after 6 days of altitude exposure• epinephrine levels increase only slightly during exposure and peak also• during submaximal exercise, increased cardiac output can entirely compensate for the blood's oxygen content◦ with this increase in cardiac output, the sea level values and altitude values were the same• in comparison, during maximal exercise, the body fails to compensate for the depressed arterial oxygen content• at higher and higher altitudes, pulmonary ventilation (breathing) progressively and disproportionately increases, but the amount of oxygen consumed becomes less and lessFluid Loss• the air in mountainous regions is typically cool and dry, allowing body water to evaporate as inspired air becomes warmed and moistened in the respiratory passagesSensory Functions• 5% decrease in sensitivity to light at 1524m• 25% decrease in light sensitivity/30% decrease in visual acuity at 3048 m• 25% deterioration occurs in coding task performance and simple reaction time at 6096 mMyocardial Function• except for possibly people with coronary artery disease, people with stable chronic heart problems do not seem to be adversely affect by altitudeLonger Term Adjustments to Altitude• short term counter: hyperventilation and increased submaximal exercise cardiac output• three longer term adjustments that improve tolerance to relative hypoxia of medium/high altitudes:◦ regulation of acid-base balance of body fluids altered by hyperventilation◦ synthesis of hemoglobin and red blood cells and accompanying changes in local circulation and aerobic cellular function◦ elevated sympathetic neurohumoral activity reflected by increased norepinephrine that peaks within 1 weekAcid-Base Readjustment• the effect of hyperventilation at altitude to increase alveolar Po2 produces opposite effects on the body's carbon dioxide level• ambient air contains almost no carbon dioxide, so increased breathing dilutes the CO2 concentrations• this creates a larger gradient for diffusion of CO2 from the blood to the lungs◦ this causes a decrease in arterial Pco2Exam 4 Book Notes PET3380C• the weak acid H2CO3 readily dissociates in H+ and HCO3- and moves to the lungs• the H+ and HCO3- recombine in the pulmonary capillaries to form carbon dioxide and water• carbon dioxide then diffuses from the blood into the alveoli and leaves the body• ie. a decrease in CO2 level with hyperventilation increases the pH from loss of carbonic acid, making bodily fluids more alkaline (basic)Reduced Buffering Capacity and the “Lactate Paradox”• the more your body tries to acclimatize to an altitude, the less able your body gets at buffering acid ( increased sensitivity to it)• there is a greater reliance on anaerobic glycolysis that increases lactate accumulation•
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