Gas Exchange in Lungs

• Chapter 12 discusses gas exchange and transport in the body.
• Learning objectives include describing oxygen & carbon dioxide movement, determinants of alveolar oxygen, calculating partial pressure, and effects of ventilation & perfusion on gas exchange.
• Additional learning objectives cover computation of total oxygen content, factors affecting arteriovenous oxygen difference, oxygen loading, and carbon dioxide transport.
• Final learning objectives involve oxygen/carbon dioxide transport interrelation, factors that impair oxygen delivery, and factors that impair carbon dioxide removal.

Respiration Recall

• Respiration is the process of moving oxygen to tissues for aerobic metabolism and removing carbon dioxide.
• Respiration involves gas exchange at lungs and tissues.
• O2 moves from atmosphere to tissues for aerobic metabolism.
• CO2 moves from tissues to atmosphere.

Clinical Diffusion Problems

• Some clinical conditions decrease gas diffusion
• Diffusion-limited problems that occur are:
• Alveolar collapse
• Emphysema
• Interstitial edema
• Alveolar capillary membrane is normal
• Alveolar fibrosis
• Pneumonia
• Pulmonary edema

Diffusion Continuation

• O2 shows a downward "cascade" of partial pressures from atmospheric to the cellular level.
• CO2 is moving from the opposite direction where it is higher in the cells and cascades downward.

Determinants of Alveolar Gas Tensions

• PACO2 varies directly with the body's CO2 production and inversely with alveolar ventilation (VA).
• Under normal conditions, PACO2 is maintained at 35 to 45 mm Hg.
• PACO2 will increase above normal if carbon dioxide production increases while alveolar ventilation remains constant.
• An increase in dead space can lead to an increased PACO2
• PACO2 decreases if CO2 production decreases or alveolar ventilation increases.
• If CO2 production increases (exercise or fever), ventilation automatically increases in order to maintain the PACO2 within a normal range.

Alveolar Air Equation

• Alveolar oxygen tension is (PAO2 *)
• In the lungs, atmospheric oxygen is diluted by water vapor and CO2.
• Gas fully saturated with water vapor at body temperature pressure saturated (BTPS) is 47 mmHg.
• Moving into the alveoli, PO2 is less because it contains water vapor and CO2.
• Healthy PCO2 is 40 mmHg (range is 35-45).
• Since the sum of all gases must equal PB (Duh Dalton), the PO2 falls by 40 mmHg when it enters the alveoli.
• O2 diffuses out of the alveoli faster than CO2 diffuses into it.

Practice

• Example Practice questions
• PB 760mmHg, PH2O 47 mmHg, FIO2 is 70%, PaCO2 50 mmHg
• PB 750 mmHg, PH2O 47 mmHg, FIO2 is 45%, PaCO2 47 mmHg
• Рв 750 mmHg, PH2O 47 mmHg, FIO2 is 30%, PaCO2 55 mmHg

Alveolar Gas Tensions cont

• Changes in alveolar gas partial tensions
• Gases O2, CO2, H2O, and N2 normally compose alveolar gas
• N2 is inert, occupies space, and exerts pressure.
• Partial pressure of alveolar nitrogen (PAN2) is determined by Dalton's law: PAN2 = PB – (PAO2 + PACO2 + PH2O).
• Only changes seen will be in O2 and CO2.
• At constant FiO2, PAO2 varies inversely with PACO2.
• Prime determinant of PACO2 is VA.

Mechanisms of diffusion

• Diffusion occurs along pressure gradients
• There are several barriers to diffusion
• A/C membrane has three main barriers: Alveolar epithelium, Interstitial space and its structures, and Capillary endothelium
• Also, the RBC membrane itself
• Fick's law: The greater the surface area, diffusion, constant, and pressure gradient, the more diffusion will occur.

Mechanism of gas diffusion

• Given an relatively constant alveolar-capillary membrane in humans, gas diffusion depends on gas pressure gradients.

Mechanisms of Diffusion

• Pulmonary diffusion gradients include:
• Diffusion occurring along pressure gradients
• Time limits to diffusion:
• Pulmonary blood is normally exposed to alveolar gas for 0.75 second, during exercise may fall 0.25 second
• Normally equilibration occurs in 0.25 second
• With diffusion limitation or blood exposure time of less than 0.25 seconds, there may be inadequate time for equilibration.

Diffusion & Exercise

• O2 is at 100 mm Hg while transferring from the alveoli to the capillary, while CO2 is at 40 mm Hg while transferring from alveoli to capillary
• During normal resting conditions, blood moves through the alveolar capillary membrane for 0.75 seconds.
• During exercise or stress, the transit time is shorter than normal.
• When the diffusion rate is decreased from alveolar wall thickening, the oxygen equilibrium event may not occur

Shunting

• Ventilation and perfusion are not necessarily perfect in the normal lungs
• PaO2 is usually 5–10 mmHg less than PAO2 due to shunts.
• Anatomical shunts are right to left
• Bronchial venous drainage
• Thebesian venous drainage

PAO2-PaO2 Gradients

• Measures the difference between Alveolar and arterial PO2.
• Indicates efficiency of gas exchange.
• Estimates the degree of hypoxemia and shunting.
• Normal values:
• 5-10 mmHg on 21%
• 25-65 mmHg on 100% FIO2.
• 66-300 mmHg = V/Q mismatch
• Greater than 300 mmHg = shunt
• Check normal values to ensure parameters are correct

PaO2 Values

• If there are imbalances in the body, there may be instances in which abnormal O2 exchange.
• A smaller difference between alveolar and arterial of O2 is related to a small number of veins that bypass the lungs.
• Veins emptying blood into arterial cirulatory system result in blood deoxygenating
• Thebesian vessels of the left ventricular myocardium drain directly into the left ventricle.
• Some bronchial and mediastinal veins drain into pulmonary veins and decreases arterial PaO2

Normal Variations From Ideal Gas Exchange cont

• Ventilation/Perfusion ratio Ideal ratio is 1
• Areas with ventilation and no blood flow is called deadspace
• Alveolar dead space comes from: PE, partial obstruction of the pulmonary vasculature, destroyed pulmonary vasculature (like in COPD), and reduced cardiac output.
• Anatomic dead space is the portion of VT that never reaches the alveoli for gas exchange

Ideal Gas Exchange Cont.

• If ventilation and blood flow are mismatched, there can be impairment of both O2 and CO2 transfer.
• If ventilation exceeds perfusion the V/Q is greater than 1.
• If perfusion exceeds ventilation the V/Q is less than 1.
• In the instance of pneumonia there is a decrease in ventilation, if perfusion is unchanged, there can be hypoxia vasoconstriction.

Ideal Gas Exchange cont

• Ventilation with zero blood flow = alveolar dead space (increases PO2 and lowers alveolar PCO2).
• Lower alveolar PO2 increases PaCO2; perfusion but no ventilation occurs.
• In an upright person the V/Q at the top of the lung is increased, meaning increased ventilation relative to little blood flow in pulmonary circulation because of gravity.

Oxygen Transport

• Oxygen is transported two ways, either dissolved or bound
• Physically dissolved in plasma
• Gaseous oxygen enters blood and dissolves.
• Henry's law allows calculation of amount dissolved: Dissolved O2 (ml/dl) = PO2 * 0.003.
• Chemically bound to hemoglobin (Hb) is the other majority of oxygen
• Each gram of Hb can bind 1.34 ml of oxygen. ([Hb g) 1.34 ml O2 provides capacity.
• 70 times more O2 transported bound than dissolved.

Oxygen Transport cont.

• Hemoglobin saturation is the % of Hb that is bonded to oxygen
• SaO2 = [HbO2/total Hb] 100
• Normal range is 95% to 100%
• HbO2 dissociation curve relationship between PaO2 and oxygen saturation
• Flat portion above 90% saturation that facilitates oxygen loading at lungs even with low PaO2
• Steep portion below 90% saturation that facilitates unloading at tissues in the capillaries.

Molecular Hemoglobin cont.

• Hb is a conjugated protein w/ four polypeptide chains (globin), and it combines to a heme.
• O2 binds to Hb via the Fe+. With complete bonding the Hb is converted to HbO2, is called oxyhemoglobin.
• Deoxygenated Hb shows characteristics of being a weak acid.

Measuring Oxygen Saturation

• When O2 binds with Hb, there are shape changes to the molecule that reflects or absorbs light differently.
• Bright red is usually arterial blood
• Deeper purple is usually the venous blood
• Using spectrophotometry can measure amount of Hb is saturated
• If all molecules carry four oxygen molecules, the patient is said to have a 100% saturation
• Light technology is not perfect, as it does not fully examine the effect of Hb
• One may ask, what is the normal value of Hb? -If the a patient's hemoglobin is 7 and they fully fill, are they accurately reading 100%
• There are calculations that can be used to assess overall blood content

Hemoglobin Saturation

• Measured as a proportion of oxygen
• SaO2 = (HbO2 / total Hb) x 100
• Expressed as a percent

Oxygen Transport explained

• The CaO2 equation is (Hb x 1.34 x SaO2) + (PaO2 x 0.003)
• The first half represents the measure of bound oxygen
• Seconds half represents the portion which is dissolved
• The better measure of oxygen delivered to the tissues, or the better index of measurement

Oxygen Transport Continued

• Round to the nearest tenths
• The CaO2 is essentially a measurement of what is being sent by the body
• 40/50/60/70/80/90 rule.

Oxygen Transport cont.

• CvO2 (mixed venous oxygen content)
• Equation: (Hb x 1.34 x SvO2) + (PvO2 x 0.003)
• Normal value: 14 vol % (12 – 16% vol)
• CvO2 will decrease when Qt decreases
• This represents : Total amount of oxygen carried in the mixed venous blood.
• Blood is typically drawn from the arteries (Swan-Ganz catheter)

Associations with affinity

• Hemoglobin binds with oxygen, depending on how the body reacts

Associations with Oxygen transport

• Increase hemoglobin = oxygen
• A decrease in hemoglobin's affinity for oxygen, will result in releasing to other tissues.

Oxygen Transport Continued

• Important to note is the fact that the factors affect loading and unloading
• Both these characteristics come down to the shape of hbo2
• Describer as effect of pH in relation to hbo2
• As temperature is increased, the more delivery in other tissues
• As temperature is decreased, the more metabolic demand

This quiz covers the key concepts of gas exchange in the lungs, including the role of hemoglobin, the process of gas exchange in the alveoli, and the effects of altitude. It also addresses how diseases affecting alveolar surface area can impact respiration. Test your knowledge of respiratory physiology.