2 Gas Exchange (Blood, Lungs, Tissues) As blood enters the lungs via pulmonary circuit, oxygen levels are low and carbon dioxide levels are highIn the alveoli oxygen is taken up and carbon dioxide is unloadedOxygen is delivered to body tissues and cellsThree factors influence the movement of these gases(1). Partial pressure and gas solubilities(2). Alveolar ventilation and pulmonary blood perfusion(3). Structural aspects of respiratory membrane
3 Gas Laws and Properties of Gases Gas exchange in the lungs and tissues takes place via simple diffusionWe measure gases in partial pressuresDalton’s Law of Partial Pressure states that the total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas (it’s partial pressure) in the mixture.N and O (21%) account for 99% of the gas in the atmosphere; 0.5% carbon dioxideTotal partial pressure of gases in atmosphere = 760 mmHgPO2 = 159 mmHg (decreases at higher altitude = 110 mmHg)Henry’s Law states that when a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion to it’s partial pressureThe amount of gas that dissolves in a liquid is dependent upon the solubility of the gas and the temperature of the liquid.
4 Gas Solubility Solubility of gases depends on: 1. Solubility of a particular gas in a liquidCarbon dioxide most soluble in air than O22. Temperature of the liquid/PressureAs Temperature rises, gas solubility decreasesAs Pressure increases, gas solubility increasesThe composition of gases in the alveoli differs from that of the air (table 22.4)Atm (mostly O2 and N2)Alveoli – more CO2 and water vapor and less % of oxygenOxygen moves into the blood (lower in alveoli); mixture of new gases in the conducting tubesP02 = 104; PCO2 = 40 mm HgThe differences in PP of the gases allows for gas exchange via diffusionAlveoli and gas pressure
5 Factors Influencing Movement of Gases 1. Partial Pressures and Gas Solubility'sPressure difference across membranes allow movement2. Ventilation-Perfusion CouplingVentilation (gas reaching the alveoli) must be almost at an equal rate to perfusion (blood flow in the capillaries)3. Membrane Thickness0.5 – 1.0 micron is optimal and efficient
6 Partial Pressure Gradients Top: gradients promoting gas exchange in the lungsBottom: gas exchange at the tissue levelGases measured in partial pressuresPO2Movement is ‘down’ the concentration pressure gradientGas ExchangeGas Exchange II
7 Ventilation-Perfusion Coupling in the Lungs DefinitionsVentilation = gas that reaches the alveoliPerfusion = blood flow in the pulmonary capillariesAutoregulatory controls (affecting the alveoli)1. If ventilation is poor (low PP of oxygen); terminal arteries contract shunting blood to areas where PP oxygen is high2. If ventilation is high (high PP of oxygen); arterioles dilate which increases blood flow in the pulmonary capillariesPP of carbon dioxide affect bronchiole diameter1. Areas where PP CO2 is high, bronchioles dilate so that it can be eliminatedResults1. This balances ventilation with perfusionPoorly ventilated alveoli (low oxygen and high carbon dioxide), bronchioles dilate, and more air is brought in (more oxygen).
9 Oxygen Transport Oxygen-hemoglobin dissociation curve (22.20) Most oxygen is transported via (1) hemoglobin and by (2) dissolved in plasma (1.5%)- low solubility in waterHemoglobin is very important in that it transports around 98% of the oxygen to the tissuesHbO2 (oxyhemoglobin) HHb + O2 (deoxyhemoglobin)Oxygen gas is ‘loosely’ attached to hemoglobinHHb + O2 ↔ HbO2 + H+ (oxyhemoglobin)Loading and unloading of hemoglobinRight: lungs; Left: tissuesRate at which Hb binds or releases oxygen depends on temperature, blood pH, PCO2, BPG (organic chemical)Influence of PO2 on hemoglobin saturationOxygen-hemoglobin dissociation curve (22.20)
10 Oxygen-hemoglobin Saturation Curve Influence of PO2 on hemoglobin loading and unloading
11 Influence of PO2 on Hemoglobin Saturation During resting conditions, PO2 = 100 mm Hg (98% saturation of Hb in arterial blood)100 ml of systemic blood contains 20 ml of oxygen (20 vol %)Blood flows into the systemic circuit (capillaries) - Hb saturation is 75%5ml of oxygen is released / 100 ml of bloodOxygen content is now 15% volumeImportant points1. At PO2 mm Hg, Hb almost fully saturated with oxygen. Increase in pressure causes very little increase in saturation.Higher altitudes there is enough oxygen (sufficient Hb loading) when PP oxygen is lower
12 Influence of PO2 on Hemoglobin Saturation Only around 25% of oxygen unloading occurs during one systemic circuitHigh reserve of oxygen is left in the venous blood (venous reserve)Applications for exercise (more oxygen will dissociate from hemoglobin during vigorous exercise and move into the tissues with out increasing respiratory rate or CO
14 Other Factors Influencing Hemoglobin Saturation Temperature, blood pH, PCO2 and BPG(2,3-bisphosphate) affect saturation at a given PO2 in bloodCells metabolize glucose using oxygen and produce CO2 and H+ (lowers pH) levels in the capillary bloodOccurs during cell respiration. More oxygen is needed by the cell to continue respirationAll factors modify hemoglobin structure-affects it’s affinity for oxygenIncrease in all of the factors listed above will decrease Hb affinity for oxygen (curve to shift right)Declining pH and increasing PCO2 weakens the Hb bond which is called the Bohr Effect.Allows for oxygen to be unloaded when really neededDecrease in factors causes higher affinity and curve shifts leftIncreasing pH causes higher Hb affinity for oxygen
15 Carbon Monoxide and Hb Affinity Any inadequate delivery of oxygen to the tissues is called hypoxia.Hb saturation falls below 70%1. Anemic2. Ischemic – congestive heart failure blocks blood flow3. Histotoxic – cells cannot use oxygen due to poisons (CN)4. Hypoxemic – reduced air flow to lungs; CO poisoningCarbon monoxide poisoning200 times Hb affinity for CO compared to oxygenTakes oxygen’s placeCyanosis, headache and respiratory distress
16 Making Sense of CO2 Transport Transported by following means(1). Blood plasma (7%)(2). Chemically bonded to hemoglobin (carbaminohemoblobin) (23%)CO2 + Hb ↔ HbCO2(3). HCO3- (70%)Bicarbonate ion in plasmaCO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-Carbonic anhydrase in RBCsCarbonic acid converted to H+ and binds to Hb increasing the RELEASE OF OXYGEN!Hb loading increases O2 unloading!HCO- is released into the plasma and onto the lungsCl- moves into the cell to account for the loss of the negative ion (chloride shift)
18 Haldane EffectLower PO2 and lower Hb saturation - the more CO2 can be transportedAs CO2 enters systemic bloodstream, causes more oxygen to dissociate from Hb (Bohr effect) 22.22AAllows for more CO2 to combine with HbHbO O2 + HbOpposite in pulmonary circulationHb saturated with oxygen, the H+ released combines with HCO unloads carbon dioxideO2 + HHb HbO2 + H+H+ + HCO H2CO CO2 + H2O unloaded in lungs 22.22b
19 Buffering pH Changes Carbonic Acid – Bicarbonate Buffer System CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-PointsResists shifts in pHIf concentration of H+ rises (pH falls) will be tied up by bicarbonate to form carbonic acid (weak acid- changes pH very little)Maintains pH of around 7.3
20 Neural Pathways Medullary groups (control respiration) 1. DRG (dorsally near root of CN IX)Integrate peripheral sensory input; modify rhythms of VGRTakes input from peripheral stretch and chemoreceptors and sends to the VGRVRGRhythm generators ‘drive’ respirationVRG neurons fire which activate respiratory motor neurons which cause inspiratory muscles to contract (inspiration)Expiratory neurons fire inhibiting the VRG and expiration occursNeural Pathways
21 Neural Pathways Pons PGR (pontine respiratory group) Sends impulses to the VRG of the medullaHelps to modify the rhythm of the VRG (during sleep, voice, exercise)
22 Neural and Chemical Influences on Medullary Respiratory Centers Irritant ReceptorsMucus, dust constrict bronchioles (-)Inflation Reflex (stretch)Baroreceptors: lungs over-inflate sends signals to medulla which causes decrease in inspiration (-)Chemical FactorsPCO2 ; PO2; pH
23 Chemical FactorsChemoreceptors monitor gas levels and H+ levels in the blood1. Effects of PCO2 (central chemoreceptors)As carbon dioxide rises, pH falls in the cerebrospinal fluid --- increase in respiration (hypercapnia)2. Effects of PO2 (peripheral chemoreceptors)Levels must drop below 60 mm Hg before it causes increased ventilationWill stimulate ventilation even if carbon dioxide levels are normal3. Effects of pHFalling pH levels is monitored by the peripheral chemoreceptorsCauses increased breathingCentral chemoreceptors are not involved due to the lack of H+ diffusing from the blood to the cerebrospinal fluid
25 Adjustments Exercise High Altitude Breathing becomes hyperpnia but not hyperventilationGas levels maintained (actually oxygen may increase slightly)Higher Exercise induced ventilation caused byPsychic stimuli, proprioceptors relay stimuli to respiratory centersHigh AltitudeAs oxygen levels drop above 8000 ft. carbon dioxide sensors become more sensitiveLow oxygen levels means increased ventilationAfter 2 or 3 days you acclimatize or respiratory volume stabilizes at a level 2-3 L/min higher than at sea levelHb saturation is only 67% at 19,000 feet; but Hb unloads only 25% of oxygen so tissue oxygen needs are metIf all out effort takes place then fatigue sets in. Kidneys will produce EPO and more erythrocytes
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