Lecture 21: Gas Diffusion And Gas Transport Flashcards
Again what is Dalton’s law of partial pressure?
The total pressure of a mixture of gases is a sum of the partial pressures exerted by each gas.
Partial pressure is the pressure exerted by an individual gas in a mixture
What is the composition of air?
Like what’s in it and what %?
-78% nitrogen
-21% oxygen
-0.033% carbon dioxide
Water vapour dilutes gases in air.
Each gas in the mixture contributes to the total pressure. It’s contribution yo the total pressure is called ‘partial pressure’
Partial pressure of gases?
What is it?
What does it depend on?
How do you calculate it?
The partial pressure is a gas (mmHg) is the proportion of total air pressure contributed by that gas. It depends on:
1. Total pressure exerted by air
2. % of that gas within the air
P(gas) = percent composition (gas) x total pressure (mixture)
Eg nitrogen
Composition = 78 %
P(N) = 0.78x 760mmHg
Gases in liquids. Gases also exert pp when dissolved in liquid.
What does it depend on?
-The amount of gas dissolved in the liquid depends on both the partial pressure gradient and the gas solubility.
-when Gas is in contact with a liquid, net movement of gas will occur between Compartements
-this will occur until equilibrium PO2 in air = PO2 in liquid is reached. This is equilibrium.
BUT equilibrium doesn’t imply that no. Of gas molecules in liquid = no. In air.
-the final concentration of gas in a liquid at equilibrium will depend in its solubility.
-CO3 is 20x more soluble than O2 a the same partial pressure gradient.
Slide 10
Respiratory gas exchange
Gas diffusion
O2 and CO2 diffuse between alveolar air and blood across the respiratory membrane
- diffusion occurs because there is a partial pressure gradient for both CO2 and O2.
- each gas diffuse from high partial pressure to low partial pressure
- so O2 diffuses from alveoli into the blood, and CO2 diffuses from blood into alveoli
Tell me about the partial pressure gradients in the lungs?
- O2 diffuses from alveoli to blood down a PP gradient (100-40mm Hg)
- CO2 diffuses from blood to alveoli down PP gradient (46➡40mmHg)
- gas exchange complete when equilibrium is complete between blood and alveoli
Give me three reasons as to why P(O2) is lower and P(CO2) is higher in the alveoli than in the atmosphere.
- Gas exchanges continuously between alveoli and blood
- atmospheric air mixes with dead space gas (⬆CO2 and ⬇O2)
- Air in alveoli is saturated with water
Slide 15
What are the Factors that affect the rate of gas diffusion
- Partial pressure
- Surface area of membrane
- Permeability of membrane
- Diffusion distance
What is hyperventilation and what is its consequences?
- alveolar ventilation exceeds demands of tissues
- excess CO2 is removed and excess O2 is inspired for bodies requirement.
- P(CO2) 100mmHg in alveoli and arterial blood
- Hypocapnia (low PCO2) causes vasoconstriction in the brain leading to reduced O2 delivery to brain and dizziness
What is hypoventilation and what are its consequences?
- Alveolar ventilation is insufficient to meet tissues demands
- could be in disease were normal ventilation is difficult eg asthma, overdose of sleeping tablets
- cells continue to produce Co2 and consume O2 yet ventilation unable to keep up with demand
- P(CO2) increases > 40mmHg in alveoli and arterial blood
- PO2 decreases < 100mmHg in alveoli And arterial blood
What are some diseases etc that could cause diffusion problems?
- ⬇Surface area eg emphysema -breakdowns of the alveoli reduces surface area for diffusion/gas exchange.
- ⬇permeability of the respiratory membrane eg fibrosis-gases diffuse more slowly through fibrous scar tissue
- ⬆diffusion distance eg pulmonary oedema➡ fluid build up in lungs due to increased pressure in pulmonary capillaries (congestive heart failure, lung infections etc. this excess interstitial fluid ➡ greater diffusion distance ➡decreases exchange
Gas transport in blood
Oxygen transport in blood. Tell me how da shit it works
-Oxygen is poorly soluble in plasma
-most oxygen is transported bound to haemoglobin
-dissolved oxygen is important for tissue supply
-O2 dissolved in the plasma determines the PO2 I’d the blood
Slide 23
Tell me about haemoglobin?
Hb is a protein found in RBC
Has 4 subunits each consisting of:
-a protein (globin) chain
-a heme unit (one Fe2+ and one protoporphyrin molecule)
-O2 binds reversibly to the Fe2+ of the heme
Hb + O2 ⬅➡Hb x O2
-each Hb molecule can bind up to 4 molecules of O2
-binding of O2 is reversible
-the amount of O2 bound to Hb depends on the PO2
-carbon monoxide competes with O2
Describe and explain the Hb: O2 dissociation curve
Slide 25
- P(O2) of surround fluid is primarily a determinant of Hb O2 saturation.
- Oxygen saturation= % of Hb binding sites occupied by O2
- relationship between PO2 and Hb saturation described by the hemoglobin-oxygen dissociation curve
- ability of Hb to bind to O2 depends on how much O2 is already bound
- O2 uptake in lungs, O2 release in tissues
- in alveolar capillaries, high PO2 in the plasma promotes O2 binding to Hb
- in tissues capillaries, low PO2 in the plasma promotes O2 unbinding from Hb.
At rest Hb carries much more oxygen than tissues repaire
- Hb in blood returning from tissues in systemic veins still 75% saturated with O2
- O2 reserves that can be drawn in if needed eg during exercise
What are the factors affecting Hb: O2 binding affinity
- The PO2 is the most important regulator of binding by Hb
- 3 other factors can change Hb: O2 binding affinity
- they shift Hb: O2 dissociation curve to left or right
Temp, PCO2/H and 2,3 diphosphoglycerate all affect affinity
