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Human Phys
Gas exchange
| Question | Answer |
|---|---|
| Main types of gas exchange | 1) alveolar: picks up O2, expels CO2 2) gas transport: oxygenated blood travels to systemic tissues 3) systemic gas exchange: blood unloads O2, picks up CO2 - relies on pressure gradient (different partial pressure of gases) |
| Atmospheric gases | - mixture of 78.6% N, 20.9% O and 0.04% CO2, and other minor gases - dalton's law: total atmospheric pressure is the sum of all gases |
| Gas moves by two mechanisms | Bulk flow – a) BREATHING b) TRANSPORT Diffusion – a) Between ALVEOLI and PLASMA b) Between PLASMA and CELLS [high] to [low] partial pressure |
| Atmosphere PO2 | Atmosphere PO2 is different to alveolar PO2 (due to inhaled and exhaled air mixing) Alveolar PO2 is only about 65% that of inhaled air Alveolar PCO2 130 times higher that inhaled air |
| Alveolar gas exchange | Oxygen/ carbon dioxide must travel across: 1. The air must dissolve in the water 2. Pass through the Alveoli cells 3. Interstitial space/ basement membrane 4. Endothelial cells of the capillaries - gases pass this way due to partial pressure CG |
| Alveolar tissue/gas exchange | - fresh air O2 diluted, CO2 enriched - O2 into blood down PP gradient - at tissues, PPO2 low PO2 is low - O2 down gradient to tissues - CO2 byproduct, moves down PP gradient into blood - lungs: PCO2 low, moves into alveoli |
| What factors affect alveolar gas exchange? | Pressure gradients of the gases Solubility of the gases Membrane thickness Membrane area Ventilation-perfusion coupling |
| Pressure gradient of gases | PO2 in the alveolar is 100mmHg PO2 in the blood is 40mmHg Oxygen therefore diffuses into the blood and reaches about 100mm Hg PCO2 in the blood arriving at the alveolus is 46mmHg - at high altidude, PP lower, O2 gradient lower, longer to diffuse |
| Gas solubility | - movement of gases dependent on water solubility - CO2 20 times more soluble then oxygen - equal amounts exchange at respiratory membrane |
| Respiratory membrane thickness | - thinner membrane, faster diffusion - RM 5 micrometres thick - some diseases cause pressue to build and thicken the membrane - gases have to travel further as O2 diffuses slowly - under these conditions, blood leaving lungs has low PO2 and high PCO2 |
| Membrane area | The amount of gas exchange is directly proportion to the contact surface between the blood and the alveoli air the lungs have an area 70M2 of respiratory membrane available for gas exchange |
| Gas exchange diseases | Diseases that reduce gas exchange produce low blood oxygen levels - diseases reduce the surface area of gas exchange Emphysema/lung cancer |
| Ventilation-perfusion coupling: good ventiation | Gas exchange requires: Good ventilation Good perfusion of the capillaries Coupling refers to physiological responses that match the airflow to the blood flow |
| Poor ventilation | Poorly ventilation leads to low P02 in that region of the lungs Stimulates vasoconstriction Reroutes blood to better ventilated region Increased ventilation, raises local O2 Stimulated vasodilation, increases blood flow |
| Oxygen transport | RBCs have 250 mil Hb molecules, binds up to 4 O2 41% rbc, 4% wbc, platelets, 5% plasma increases blood oxygen concentration sevety-fold 98.5% bound to RBC low solubility |
| Haemoglobin | HHb - deoxyghaemoglobin HbO2 - oxyghaemoglobin At low PO2, O2 saturation of Haemoglobin is slow, then rapid increase in O2 loading as PO2 rises - shape changes to accomodate other O2 molecules |
| T and R states | Tense - low O2 affinity Relaxed - high O2 affinity - state triggered by binding of O2 - as one oxygen binds to a heme group it causes conformational changes to the other heme groups making them have more affinity for oxygen |
| Oxy-haemoglobin disscociation curve | The oxygen dissociation curve shows how the amount of oxygen bound to haemoglobin changes depending on the partial pressure of oxygen in the blood In the lungs PO2 is high= 100mmHg In tissues PO2 is lower= 40mmHg During exercise PO2 = 25mmHg |
| At specific conditions | - at alveoli: highly saturated (100mmHg) - in tissues: lower PO2 pressure (40mmHg) - exercising: lowest O2 levels (25mmHg) - more O2 needed, more released |
| Shape of curve | - sigmoidal: cooperative binding of Hg - steep part: range of PP of O2 when haemoglobin changes from oxy to deoxy (pressure drop releases more O2) - flat part: rate of PP when oxy saturation is high (large pressure only has small increase of O2 binding) |
| Curve can shift left or right | - left: Hg high affinity for O2, releases less O2 - right: Hg low affinity for O2, releases more O2 |
| What causes a right shift? | - increased CO2 ie. exercise - decreased pH: increased carbonic acid formed - temp increase: active tissues promote more O2 unloading |
| What causes a left shift? | - higher pH - lower CO2 - lower temp - lower BPG (metabolic product) - foetal Hg: high O2 affinity |
| CO2 transport | 1) bicarbonate ions (75%) 2) binds to Hb (20%) 3) dissolved in plasma (2%) - preferred carried as HCO3- in plasma |
| The chloride shift | - HCO3 exchange for Cl- - mitigates pH change - cell membranes generally impermeable to charged ions - increases CO2 carrying capacity in the blood, and increases O2 unloading |
| Transport and exchange into alveoli | 1) CO2 dissolved in plasma moves to alveoli 2) CO2 from Hb moves to plasma then alveoli 3) HCO3- exchanged for Cl- and move back into the cell, carbonic anhydrase produces CO2, diffuses into plasma, then into alveoli |
| Blood gases and respiratory rhythm | The brainstem respiratory centres receive input from the central and peripheral chemoreceptors that monitor the composition of blood and CSF - potent chemical stimulus for breathing: 1) pH (most significant) 2) CO2 3) O2 - hypoxia: too little O2 |
| Brain and CO2 control | Automatic breathing influenced by: Peripheral chemoreceptors (detect chemical changes) Central chemoreceptors - exercise the rising CO2 levels stimulate the peripheral chemoreceptors and trigger increased ventilation more quickly than chemoreceptors |
| Blood gases: H+ alkalosis | Alkalosis: corrective homeostatic response is to decrease ventilation. CO2 builds up, reaction shifts to the right, H+ increase, pH decrease |
| H+ acidosis | Acidosis: corrective homeostatic response is to increase ventilation. CO2 is eliminated, reaction shifts to the left, H+ reduces, pH rises |
| A low partial pressure of carbon dioxide in the blood causes alkalosis (hyperventilation) dizziness Pulmonary ventilation is adjusted to maintain pH of the brain | |
| Once in brain CO2 reacts with H2O to produce carbonic acid which dissociates to HCO3 and H + CSF contains little protein so cant buffer H+ , remains free and stimulates the chemoreceptors | |
| Peripheral chemoreceptors mediate 25% of respiratory response to pH changes | |
| Respiration and exercise | FEED-FORWARD MECHANISM Via higher brain Brain stem sends signals to muscles Also send information to the respiratory centres Increase pulmonary ventilation in anticipation of the needs of the muscles |
| EXERCISE ALSO STIMULATES PROPRIOCEPTORS Exercise also stimulates proprioceptors Signals to the brain stem respiratory centre - pulmonary ventilation helps keep blood gases stable |
Created by:
reub8n
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