Lecture 16 - Gas exchange and gas transport

what are the functions of the respiratory system?

oxygen-exchange at the alveolar-capillary interface, oxygen transport, oxygen exchange at cells where drops of O2 for metabolism then picks up carbon dioxide in the venus side and transports it back to the heart, CO2 exchange at the cells, CO2 transport and CO2 exchange at alveolar-capillary interface

what makes up the total atmospheric pressure?

the sum of the partial pressures so PO2, PCO2 an PN2

what makes up partial pressure?

fractional concentration multiplied by the total pressure

fractional concentration

only applicable to gases

concentration

only applies to gases in liquids like blood

water vapour

added as soon as air inspired, contributes to overall pressure, because it has a partial pressure, PO2 and PCO2 go down when the air is humidified in the respiratory tract

what is gas in solution determined by?

partial pressure of the gas, solubility of the gas and temperature of the soluton

gas in solution

initially there is no O2 in solution, then the oxygen dissolves due to diffusion, at equilibrium the PO2 in air and water is equal but low solubility concentrations even though the pressure is the same

how does gas exchange occur?

through passive diffusion, carbon dioxide diffuses from the capillary back into the alveolus

fick’s law of diffusion

this is the law that gas transfer is equal to constant x partial pressure gradient x area/wall thickness, constant includes solubility of gas in the alveolar membrane so CO2 is 20X more soluble than O2 so water holds more CO2

partial pressure gradient

this is the difference in pressure between one compartment and another, we want to maximize this to promote more gas diffusion, same with area

wall thickness

we want to minimize this for better movement of gas

what does low arterial PO2 cause?

it causes low blood O2 concentration, smaller gradient between the blood and tissues which hinders ability to push oxygen into the tissues, low O2 delivery to tissues and poor tissue function and disease

what diseases affect gas exchange?

emphysema, fibrotic lung disease, pulmonary edema and asthma

emphysema

very large and misshapen alveoli causing PO2 to be lower

fibrotic lung disease

connective tissue build up around the alveolus causing low PO2

pulmonary edema

this occurs when an increased amount of liquid is in the alveolus and the capillary itself causes increased diffusion resulting in low PO2 in the blood, the exchange surface is normal but there is increased diffusion distance

asthma

the alveoli is normal but the bronchioles are constricted causing low PO2

PO2 and PCO2 levels in the arterial blood and alveolar air

PO2 is 100 and PCO2 is 40, the alveolus does have some air due to metabolism so PCO2 goes up, this allows efficient transfer of gases between the alveolus end and the arterial blood

PCO2 and PO2 levels in the tissues and venous blood

PO2 is 40 and PCO2 is 46, there is offloading of O2 from arteries into those tissues driving O2 into them, all the O2 at the tissues is dropped off into the venous blood and picks up CO2

PCO2 and PO2 in the atmospheric air

the PO2 is 160 and the PCO2 is 0.3

how is oxygen transported in the blood?

first the O2 is dissolved in plasma which is less than 2% (around 3ml of O2/L of blood), then there is a lot of O2 is carried in the red blood cells (more than 98% so around 197 ml of O2/L of blood) and this gets transported to the cells so they metabolize it, only a small fraction is dissolved into the plasma cuz there is very little solubility for oxygen, all of it is dissolved into the tissues and used in cellular respiration

hemoglobin structure

it is made up of 4 globin chains and each one is centered around a heme group, each heme group has a porphyrin ring with an iron atom at the centre

alveolar O2-hemoglobin saturation

98% of Hb is saturated with O2 so almost completely

tissue O2-hemoglobin saturation

73% of Hb is saturated with O2

how much O2 is released at the tissues at rest?

25% is released (98% - 73%)

how does O2-hemoglobin saturation change during exercise?

more O2 is offloaded at the tissues (55%) because the PO2 is 20, there is therefore a stronger gradient for diffusion and high offload cause PO2 goes down since you’re using more oxygen

what factors decrease affinity of hemoglobin for oxygen?

lower pH so a more acidic environment, increased temperature, increased PCO2 so the Bohr effect and 2-3 DPG which in pregnancy helps offload O2 into the fetus

Beatrice case study

in her third trimester of pregnancy and informs her doctor that she has been feeling short of breath

what explains beatrice’s symptoms

the fetus has high oxygen demand so 2,3 DPG in maternal blood reduces HB affinity for oxygen allowing offloading of oxygen from the mother’s blood into the fetus, this increases oxygen transfer to the fetus

what increases the unloading of oxygen at the tissues?

increased H_

what increases the unloading of CO2 at the lungs

higher PO2

how does hemoglobin change during development?

most adult Hb is HbA so there is 2 alpha and 2 beta chains, the fetal Hb is HbF so there are 2 alpha and 2 gamma chains, the fetal Hb has greater affinity for oxygen than maternal Hb, the fetal Hb improves fetal survival by increasing O2 uptake from the mother

carbonic anhydrase

an enzyme that catalyzes formation of carbonic acid

chloride shift

for bicarbonate to leave the cell, chloride has to enter (exchanges it)

how is carbon dioxide transported in the blood?

70% of it gets transported by chloride shift as bicarbonate so CO2 will diffuse into red blood cells and react with water catalyzed by carbonic anhydrase to form carbonic acid, then this will dissociate quickly into bicarbonate and hydrogen ions, the bicarbonate then exits the RBC into plasma while the chloride enters via chloride shift, 23% of it is transported by binding to hemoglobin (HbCO2) occurring at different sites than oxygen and influenced by the haldane effect so oxygenation of blood reduces CO2 binding, lastly 7-10% is dissolved in the plasma so a small amount of CO2 remains dissolved in plasma as free gas and this directly contributes to PCO2 in the blood

how is carbon dioxide released at the lungs?

the dissolved CO2 will diffuse directly into the alveolus due to partial pressure gradient (7-10%), some will do so by dissociation from hemoglobin so it is released from HbCO2 and diffuses into the alveoli (23%) and a majority (70%) gets converted back to CO2 from bicarbonate via reversed chloride shift

Haldane effect

as PO2 increases, the amount of CO2 carried decreases, as a consequence the oxygenated blood carries less CO2 and allows Co2 to be offloaded at the lungs, in the deoxygenated state the venous blood carries more CO2 and less O2 allowing CO2 to be pushed out at the lungs, in the venous 40 mm Hg of PO2 is carried while in the arterial blood 100 mm Hg of PO2 is carried