Gas Exchange and Transport

Composition of nitrogen in air

78.6%

Composition of oxygen in air

• 20.9%

Composition of carbondioxide in air

• 0.04%

Composition of water vapor in air

• ranges from 0-4%

  • typical 0.5% is used

what is the average atmospheric pressure at sea level? 

760 mm Hg 

Daltons Law

• total atmospheric pressure is the sum of the contributions of these gases 

Partial Pressure 

• contribution of each SEPARATE gas in a mixture 

How to calculate partial pressure

• multiply average sea level atmospheric pressure (760 mm Hg) x % of gas in atmosphere 

• ex: calculating PO2 

  • oxygen: 20.9% 

  • 760 mmHg x 0.209 = 159 mm Hg 

Average Partial Pressures of atmospheric air

PN2: 597 mm Hg

PO2: 159 mm Hg 

PH2O: 3.7 mm Hg 

PCO2:  0.3 mm Hg 

How does atmospheric air differ compared to alveolar air?

• it has a higher N2 and O2 concentration 

How does alveolar air differ from atmospheric air?

• it has a higher H2O and CO2 consentration

why do alveolar air and atmospheric air differ in their composition amounts? 

• air is humidified by contact of mucus membranes 

  • causes H2O to be higher

• inspired air mixes with residual air left in lungs

  • O2 is diluted amd enriched with CO2

• alveolar air exchanges O2 and CO2 with blood

  • PO2 is is lower and CO2 is much higher 

What is the only way that oxygen can enter the bloodstream 

• dissolving in water and passing through the respiratory membrane that separates air from the bloodstream 

What is the only way the CO2 can leave the blood

• it must pass out of bloodstream and diffuse out of water film into alveolar air 

Alveolar Gas Exchange

• back-and forth traffic of O2 and CO2 across the respiratory membrane 

Why does gas exchange depend on the ability for gas to dissolve in water ? 

• O2 can only enter bloodstream by dissolving in water and passing through respiratory membrane 

• CO2 can only exit bloodstream by diffusing out of water film into alveolar air 

Why do CO2 and O2 diffuse in different directions?

• each gas has its own partial pressure gradient 

How does gradient diffusion work with O2 and CO2?

• when in contact with air and water, each gas fissue down their gradients till the partial pressure of each gas in air equals partail pressure in water 

• simple:

  • pp air = pp wwater

what would happen if a gas had a greater partial pressure in the water than air?

• it would diffuse into the air  

what would happen if the partial pressure of gas is greater in the air compared to water? 

• it will diffuse into water 

Henrys Law

• at the air water interface for a given temperature,the amount of gas that dissolves in water is determined by its solubility in water and its partial pressure in the air 

what does it mean if PO2 is in great amounts in alveolar air?

• the blood is going to pick up more O2

What does it mean since blood arriving at an alveolus has a higher PCO2?

• it is going to be released into alveolar air 

What happens at the alveolus

• CO2 is unloaded

• O2 is loaded

what does the efficiency of CO2 unloading and O2 loading dependon on in the alveolus

• how long an  RBC spends in an alveolar capillary compared to how long it takes for eachgas to be fly loaded or unloaded

• how long it takes for the to reach equilibrium concentrations in blood capilary 

what happens sice PO2 of alveolar air is initially higher than Po2 of blood arriving at alevolus? 

• oxygen diffuses into blood until equilibrium is reached

What happens since PCO2 of arriving blood is higher than PCO2 of the alveolar air? 

• CO2 diffuses into alveolus until they are equal 

what factors can impact alveolar gas exchange?

• pressure gradients of gases

• solubility of gasses

• membrane thickness

• membrane area

How does pressure gradient differ when at high elevations

• partial pressure of O2 is much lower 

• O2 gradient from air to blood to much less 

  • less O2 diffuses into blood 

why does the PO2 of blood drop to 95 mm Hg before it leaves the lungs? 

• oxygen dilution occurs due to pulmonary vein anastomosis 

  • oxygen rich/poor blood is mixed 

Blood Entering Lungs

PO2: 40 mm Hg

PCO2: 46 mm Hg 

Blood Leaving Lungs

PO2: 95 mm Hg

PCO2: 40 mm Hg 

How does pressure gradient differ when using a hyperbaric oxygen chamber

• oxygen in air is much higher

• steep gradient of PO2 from alveolus to blood means diffusion is accelerated 

How does solubility of gases impact alveolar gas exchange

• CO2 is 20x more soluble than O2 

• O2 is twice as soluble as N2

• pressure gradient of O2 is much greater than CO2 but since CO2 is more soluble gases are exchanged equally 

How does membrane area impact alveolar gas exchange

• decrease in alveolar surface area= low blood PO2 

How does membrane thickness impact alveolar gas exchange

• thicker membrane = gases have to travel further between blood and air = blood leaving lungs have low PO2 and high PCO2 

• O2 cant get to RBC quick enough to fully load hemoglobin 

Ventilation Perfusion Coupling

• physiological responses that match airflow to blood flow 

• changes in blood flow to region of lung stimulate contraction/dilation, adjusting ventilation so air is directed to better perfused parts of lung

what happens if an area of a lung were poorly ventilated due to destruction or an airway obstruction? 

• local vasoconstriction occurs

• blood is rerouted to better ventilated areas of lung so it can pick up more oxygen 

what happens if there was an area in the lungs that had increased ventilation? 

• local vasodilation occurs 

• blood flow to specific region increases to take advantage of oxygen availability 

What happens during gas transport

• O2 carried from alveoli to systemic tissues

• CO2 carried from systemic tissues to alveolis 

what happens dyring systemic gas exchange?

• O2 is unloaded
  CO2 is loaded

Response to Increased Ventilation

• increased airflow → elevated PO2 in blood vessels → vasodilation of pulmonary vessels → increased blood flow → blood matches airflow

Response to Reduced Ventilation

• Decreased airflow → reduced PO2 in blood vessels → constriction of pulmonary vessels → decreased blood flow → blood matches airflow

Response to increased perfusion

• increased blood flow → elevated PCO2 in alveoli → bronchioles dilate → increased airflow → airflow matches blood flow

Response to decreased perfusion 

• decreased blood flow → reduced PCO2 in alveoli → constriction of bronchioles →  decreased airflow → airflow matches bloodflow

What is the purpose of ventilation adjusting to changes in perfusion? 

• hopes to result in airflow matching blood flow

What is the purpose of perfusion adjusting to changes in ventilation? 

• hopes to match blood flow with airflow 

Alveolar Air levels

PO2: 104 mm Hg

pCO2: 40 mm Hg

Tissue fluid air levels

PO2 : 40 mm Hg

PCO2: 46 mm Hg 

Oxygenated Blood air levels

• PO2: 95 mm Hg

• PCO2: 40 mm Hg 

deoxygenated Blood air levels

PO2: 40 mm Hg

PCO2: 46 mm Hg 

Expired Air levels

• PO2: 116 mm Hg 

• PCO2: 32 mm Hg 

Gas Transport

• process of carrying gases from alveoli to systemic tissues and vise verse 

what percentage of oxygen is transported via hemoglobin? 

98.5% 

what percent of oxygen is transported via being disolved in the blood plasma? 

1.5% 

oxyhemoglobin

• if one or more molecules of O2 are bound to hemoglobin 

Oxyhemoglobin dissociation curve

• relationship between hemoglobin saturation and ambient PO

• compares partial pressure of PO2 and percentage of O2 saturation of hemoglobin 

what happens on the Oxyhemoglobin dissociation curve?

• when PO2 is low, the curve rises slowly 

• then there is a rapid increase in oxygen loading as PO2 increases 

  • this reflects hemoglobin binding to oxygen

  • at high Po2 levels, saturation reaches 100% saturation and cant load anymore oxygen

What are the three modes of CO2 transport in the blood?

• carbonic acid 

• carbamino compounds

• dissolved gases 

Carbonic acid for CO2 transport

• transports about 90% of CO2

• dissociates into bicarbonate and hydrogen ions 

Carbamino compounds for CO2 transport

• about 5% binds to the amino groups of plasma proteins and hemoglobin 

where does O2 bind to on hemoglobin?

• heme moiety 

Where does CO2 bind to on hemoglobin?

• polypeptide chains 

Carbaminohemoglobin

• what hemoglobin is called when bound to CO2 

• HbCO2 

• Hb + CO2 → HbCO2 

Dissolved gas for CO2 transport

• carries 5% of CO2 

About 70% of exchanged CO2 comes from where?

• carbonic acid

About 23% of exchanged CO2 comes from where?

• carbamino compounds

About 7% of exchanged CO2 comes from where?

• dissolved gas 

T or F: blood gives up the dissolved CO2 gas and CO2 from carbamino compounds more easily than it gives up the CO2 in bicarbonate

True 

Systemic Gas exchange

• loading and unloading of O2 and CO2 in systemic capilaries 

Gradient of CO2 in tissue fluid

• 46 → 40 mm Hg from tissue fluid to blood

  • this is due to tissue fluid having high PCO2 

Carbonic andydrase

• enzyme that speeds up reaction that converts CO2 and H2O to HCO3-  and H+

Chloride Shift

• occurs when chloride bicarbonate exchanger pumps most of HCO3- out of RBC in exchange for Cl- from true blood plasma

• allows more CO2 to diffuse from the tissues into the red blood cells, ensuring efficient and continuous loading of carbon dioxide

What happens when H+ binds to oxyhemoglobin

• it reduces its affinity for O2 

• makes hem release it 

pressure gradient of O2 during systemic gas exchange

• O2 consumption by respiratory tissue keeps Po2 of tissue fluid low 

• pressure gradient is 95 → 40 of oxygen from arterial blood into tissue fluid

O2 concentration as blood enters systemic capilaries

• Hemoglobin: 97% saturated 

• O2 concentration: 20 ml/dl

O2 concentration as blood exits systemic capilaries and enters tissues

• hemoglobin: 75% saturated

• O2 concentration: 15.6 ml/dl 

  • 22% of oxygen load has been given up

  • marked by utilization coefficient  

what factors effect the rate of oxygen uloading

• ambient PO2

• temperature

• ambient pH

• BPG 

how does ambient PO2 effect rate of oxygen unloading in tissues

• at a low PO2, HbO2 releases more oxygen 

• tissue fluid PO2 is typically low 

how does temperature effect rate of oxygen unloading in tissues

• rising temperature = more oxygen unloading 

how does ambient pH effect rate of oxygen unloading in tissues

• increased H+ = lower pH

• increased H+ = weakend bond between hemoglobin and oxygen = promotes oxygen unloading 

how does BPG effect rate of oxygen unloading in tissues

• metabolic intermediate that binds to hemoglobin/ promotes oxygen unloading 

what hormones can promote oxygen unloading?

• thyroxine

• growth hormone

• testosterone 

• epinephrine 

what can impact the utilizing coefficient 

• some tissues having a higher need for O2

  • ex: skeletal muscles might have a utilizing coefficient of 80% when exercising

How does alveolar gas exchange differ compared to systemic gas exchange

• hemoglobin loads O2

• causes loss of affinity for H+

• H+ dissociates from heme and binds to HCO3- transported into RBC

• Cl- transported out of cell vie chloride shift

• reverse hydration reaction generates CO2 that diffuses into alveolus to be exhaled

why does the haldane effect occur? 

• HbO2 doesnt bind CO2 as well as deoxyhemoglobin 

• HHb binds more H+ ions than HbO2 does and removing H+ from solution, Hbb shifts carbonic acid reaction to the right 

what allows for more transfer of CO2? 

• low levels of oxyhemoglobin 

  • haldane effect 

what would cause a left shift in the oxyhemoglobin dissociation curve 

• decreased temp 

• decreased H+

• decreased 2-3 DPG

what would cause a right shift in the oxyhemoglobin dissociation curve 

• increased temp 

• increased H+

• increased 2-3 DPG

• (reduced affinity) 

what 3 factors stimulate central and peripheral chemoreceptor?

• pH

• CO2

• O2

what is the effect of pH and stimulating receptors

• has the strongest influence on breathing

• helps body adjust to changes alkalosis and acidosis

What is the correct response to alkalosis

• hypoventilation

• allows CO2 to accumulate in body fluids faster than we exhale it

• raises H+ concentration and lowers pH

what is the correct response to acidosis

• hyperventilation

• blowing off CO2 faster than body produces it 

• H+ declines 

• pH rises 

what is the effect of CO2 and stimulating receptors

• more of an indirect influence

• mediated through effects on pH

• at beginning of exercise, rising blood CO2 may stimulate peripheral chemoreceptors and trigger an increase in ventilation quicker than central chemoreceptors

what is the effect of O2 and stimulating receptors

• little effect on respiration 

how does exercise increase respiration?

• Brain sends motor commands to muscles and sends that information to respiratory centers

• Respiratory centers increase pulmonary ventilation in anticipation of the needs of exercising muscles 

• Exercise stimulates proprioceptors of muscles and joints and they transmit signals to brainstem of respiratory centers

• Respiratory centers increase breathing because they were informed the muscles have been told to move 

• This keeps gas values normal in spite of elevated O2 consumption and CO2 generation by muscles