Respiratory 3 Flashcards
What does gas exchange refer to in the respiratory system?
-diffusion of O2 & CO2 in the lungs & peripheral tissues
Describe O2 gas exchange.
-O2 transferred from alveolar gas -> pulmonary capillary blood -> tissue from systemic capillary blood -> cells
Describe CO2 gas exchange.
-delivered from tissues to venous blood -> pulmonary capillary blood -> alveolar gas
Describe the steps of external respiration.
Describe partial pressure.
‘Fractional composition’
-composition of gas mixture
>21% O2
*at high altitudes = low oxygen partial pressure (but same 21%)
—lower barometric pressure
—O2 less densely packed
What is partial pressure of O2 in a dry gas mixture determined by?
-barometric pressure
-fraction of oxygen
*in atmosphere (dry sea level) = 160 mmHg
*PO2 decreases in high altitudes (due to barometric pressure decrease)
*PO2 = 149 in mmHg humid air
What is alveolar gas composition determined by?
-alveolar ventilation & exchange of O2 & CO2
-source of CO2 in lungs is from tissue in blood
-PACO2 = determined by rate of carbon dioxide production in relation to the amount of alveolar ventilation
*of the rate of CO2 increases = amount of alveolar ventilation increases (maintain PACO2 = 40mmHg)
Why is alveolar partial pressure of oxygen (PAO2) lower than inspired air?
-bc O2 & CO2 exchange occurs continuously
-during breathing PAO2 fluctuates around 100mmhg
>increasing during inhalation & decreasing during exhalation
-PAO2 = determined by partial pressure of inspired O2 & exchange of O2 for CO2
*whenever PACO2 increases, PAO2 decreases (VICE VERSA)
What are the causes of alveolar hypoventilation?
- CNS depression by drugs/injury
- Injury to phrenic nerve
- Damage to thorax & respiratory muscles
- Airway obstruction
- Lung disease (decreases compliance)
What are the causes of alveolar hyperventilation?
- Hypoxia
- Acidosis (blood pH > 7.4)
- Increase in body temp
-heat stress = respiratory alkalosis (decreasing CO2 in blood)
What is the rate of gas movement between the alveolus & blood (VO2) determined by?
- Physical properties of gas
-depends on molecular weight & solubility - SA for diffusion
-surface of perfused capillaries s - Thickness of air-blood barrier
-less than 1micrometer - Driving pressure gradient of gas between alveolus & capillary blood
-driving pressure gradient = rapid diffusion of oxygen from alveolus into capillary
What covers the surface of the alveolus to facilitate O2 & CO2 exchange?
-capillary network
-diameter of pulmonary capillaries = makes RBCs squeeze through them
-RBC membrane touches capillary wall = so O2 & CO2 dont pass though a lot of plasma as they diffuse between alveolus & RBC
>increases rapidity of diffusion
What does the respiratory membrane include?
- Layer of liquid & surfactant lining the alveolar surface
- Epithelial layer formed by type 1 epithelial cell
- Epithelial BM
- Variable thickness interstitum (gets bigger w edema)
- Capillary BM
-fuses w epithelial BM - Layer of capillary endothelium
Describe mixed venous blood.
-deoxygenated
-blood entering alveolar capillary from small pulmonary arteries
-returned to the R side of heart in veins from systemic circulation
-driving pressure for gas diffusion = difference between alveolus & capillary blood
What is the partial pressure & alveolar partial pressure of O2 in mixed venous blood?
-partial pressure = 40mmHg
-alveolar partial pressure = 100mmhg
-driving pressure for diffusion of O2 = 60 mmHg
>oxygen diffuses from alveoli into blood
-in resting = equilibrium between alveolar & capillary oxygen tension happens in 0.25s - 1/3 time blood is in pulmonary capillaries
What is the partial pressure & alveolar partial pressure of CO2 of venous blood returning to the lungs?
-partial pressure = 46 mmHg
-alveolar partial pressure = 40 mmHg
-driving pressure for diffusion = 6mmhg
-the amount of CO2 that diffuses per min from capillaries into alveoli = similar to amount of O2
*the greater solubility of CO2 compared to O2 compensated for the small driving pressure gradient
How much air & blood should the alveoli receive?
-amounts that are optimal for gas exchange
-alveolar ventilation (VA) & perfusion (Q) = matched
-in disease VA/Q ratio = mismatching is more extreme = leads to hypoxemia
What happens when the alveolus is served by an obstructed bronchiole?
-no ventilation
VA/Q = zero
no gas exchange
What happens normally during ventilation/perfusion ratios in O2 & CO2 tensions?
-receives VA/Q ratio = 1
-mixed venous blood arrives at each alveolus with a PO2 of 40 & PCO2 of 46 mmHg
-after equilibrium with alveolar gas tensions = blood leaves the normal alveolus with PO2 of 100 mmHg & PCO2 of 40 mmHg
What happens when alveoli receives no blood flow but continues to ventilate?
VA/Q = infinity
-composition of air is similar to atmosphere
-alveoli dont contribute to gas exchange = dead space
What is the composition of the systemic arterial blood determined by?
-by the composition of the capillary blood that drains each alveolus
-blood returns from the lungs to the LV for distribution to the tissues comes from capillaries associated w alveoli
What happens to the diffusion of oxygen in a diseases lung?
-impeded due to inflammation & edema
-thickens respiratory membrane
-reduce surface for gas exchange
>administer O2 = increase PAO2 = provide driving pressure
What does the exchange of gases between the tissues & blood occur by?
-diffusion
-PAO2 of blood entering tissue capillaries from the systemic arteries is 100 mmHg (oxygenated)
-PACO2 = 40 mmHg
-as blood passes through capillaries = exposed to tissues that are consuming O2 & producing CO2
>partial pressure of O2 & CO2 in tissues is 40 & 46mmhg respectively
-partial pressure differences between the tissues & capillaries = O2 diffuses into tissue & CO2 into blood
>until partial pressure of blood & tissue are equal
Describe tissues with a high oxygen demand.
-more capillaries per gram of tissue
-larger surface for diffusion
-max distance between the tissue & nearest capillary is less than in poorly vascularized tissue
How are the partial pressures of O2 & CO2 used to evaluate gas exchange?
-systemic arterial blood sample = pulmonary gas exchange
>acid base balance in blood
-hypoventilation = PACO2 increase & PAO2 decrease
-hyperventilate = PACO2 decrease & PAO2 increase
