Respiration and Gas Exchange

• Respiration involves moving oxygen to tissues for aerobic metabolism and removing carbon dioxide, including gas exchange in the lungs and tissues.

PACO2 and PaCO2

• These are maintained at approximately 35 to 45 mm Hg under normal conditions.
• These values increase above normal if carbon dioxide production increases while alveolar ventilation remains constant.
• These values decrease if CO2 production decreases or alveolar ventilation increases.
• An increase in dead space, where gas does not participate in gas exchange, can also lead to an increase in these values.
• If CO2 production increases (e.g., during exercise or fever), ventilation automatically increases to maintain these values within a normal range.

Air in Lungs

• Diluted by water vapor and CO2.

Healthy PCO2

• 40 mmHg, with a range of 35 to 45 mmHg.

Changes in Lungs

• Only changes seen will be in O2 and CO2.

Constant FiO2

• PAO2 varies inversely with PACO2.

Diffusion

• Occurs along pressure gradients, from high to low pressure.

Barriers to Diffusion

• The alveolar-capillary membrane has three main barriers: alveolar epithelium, interstitial space, and capillary endothelium.

Fick's Law

• States that diffusion increases with greater surface area, diffusion constant, and pressure gradient.

Diffusion in Lungs

• In healthy individuals, diffusion in the normal lung mainly depends on gas pressure gradients, given relatively constant area and distance across the alveolar-capillary membrane.

Pulmonary Diffusion Gradients

• Diffusion that occurs along pressure gradients.

Pulmonary Blood Exposure

• Normally exposed to alveolar gas for 0.75 seconds, but during exercise, this may fall to 0.25 seconds.

Equilibration

• Normally occurs in 0.25 seconds.

Diffusion Limitation

• If blood exposure time is less than 0.25 seconds, there may be inadequate time for equilibration.

Shunting

• Perfusion without ventilation.

Normal V/Q Ratio

• 0.8.

Ventilation & Perfusion

• Not perfect in the normal lungs.

A-a Gradient

• Measures the difference between alveolar and arterial PO2.
• A normal gradient is 5-10 mmHg on 21% oxygen or 25-65 mmHg on 100% FIO2.
• A V/Q mismatch corresponds to 66-300 mmHg.
• A shunt corresponds to >300 mmHg.
• Increased levels indicate abnormal O2 exchange.

Ventilation/Perfusion Ratio

• An ideal ratio is 1, where V/Q is in balance.

Deadspace

• Areas with ventilation and no blood flow.

Alveolar Deadspace

• Caused by pulmonary embolus.

Anatomic Deadspace

• The portion of tidal volume (VT) that never reaches the alveoli for gas exchange.

Mismatched Ventilation and Blood Flow

• Impairs both O2 and CO2 transfer.
• If ventilation exceeds perfusion, the V/Q is greater than 1.

Pneumonia

• Ventilation is decreased to the affected lobe.
• If perfusion is unchanged, then perfusion is in excess of ventilation, and the V/Q is less than 1.
• Hypoxic vasoconstriction occurs in the pulmonary capillaries supplying the lobe due to decreased ventilation, which decreases perfusion.

Alveolar Dead Space

• Ventilation with zero blood flow increases PO2 and lowers alveolar PCO2.

Perfusion Without Ventilation

• Lowers alveolar PO2 and increases PaCO2.

Oxygen Transport

• Occurs in two forms: dissolved in plasma and bound to hemoglobin.

Hemoglobin Normal Value

• 12-16 g/100mL

Oxygen Transport: Plasma

• Oxygen is physically dissolved in plasma.

Oxygen Transport: Hemoglobin

• Oxygen is chemically bound to hemoglobin, where the majority is carried.

HBO2 Dissociation Curve

• Describes the likelihood for oxygen to offload at the tissues.

Hemoglobin and Oxygen

• Iron present in the hemoglobin allows oxygen to bind to it.

Blood Color

• When O2 binds with Hb, it changes its shape, so arterial blood appears bright red while venous blood appears deep purple.

Spectrophotometry (Oximetry)

• Measures the amount of saturated hemoglobin.
• Does not account for the quantity of Hb present.

Hemoglobin Saturation Equation

• SaO2 = (HbO2 / total Hb) x 100, expressed as a percentage.

CaO2 Equation (Arterial Oxygen Content)

• CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003)
• The first part of the equation reflects oxygen bound to Hb.
• The second part reflects how much oxygen is dissolved in plasma.

CaO2 Normal Value

• 17-20 vol%.

CvO2 Equation (Mixed Venous Oxygen Content)

• CvO2 = (Hb x 1.34 x SvO2) + (PvO2 x 0.003)

CvO2 Normal Value

• 14 vol % (12 - 16% vol).

CvO2 Measurement

• Measures the total amount of oxygen carried in the mixed venous blood.

Blood Draw Location

• Blood is drawn from the pulmonary artery via the balloon-tip, flow-directed (Swan-Ganz catheter).

Affinity

• Describes whether Hb holds onto or releases oxygen, either associating or dissociating.

Increased Hemoglobin Affinity

• Oxygen strongly binds to hemoglobin and is less available to the tissues (holds onto oxygen).

Decreased Hemoglobin Affinity

• Allows hemoglobin to easily off-load oxygen to the peripheral tissues (lets oxygen go).

O2Hb Curve: Left Shift

• Indicates increased affinity (holds onto O2).
• Caused by increased pH (Alkalosis), decreased PaCO2, decreased body temperature, and decreased 2-3 DPG.

O2Hb Curve: Right Shift

• Indicates decreased affinity (release O2 to the tissues).
• Caused by decreased pH (Acidosis), increased PaCO2, increased body temperature, and increased 2-3 DPG.

Diphosphoglycerate (DPG)

• Stabilizes the hemoglobin (Hb) molecule.

Stored Blood

• Loses 2-3 DPG, which can significantly impair tissue oxygenation after large transfusions.

HbS (Sickle Cell)

• Fragile, leading to hemolysis (RBC destruction) and thrombi; sometimes binds to oxygen due to its shape.

Methemoglobin (metHb)

• Abnormal iron (Fe3+) that cannot bind with oxygen and alters HbO2 affinity (left shift).
• Does not bind to oxygen; patient's blood appears dark brown, and the patient physically looks like a corpse.

Carboxyhemoglobin (HbCO)

• Due to carbon monoxide poisoning, Hb binds CO with 200 times greater affinity than O2.
• Does not bind to oxygen; treat with hyperbaric therapy or 100% oxygen.

DO2 Equation (Impaired Oxygen Delivery)

• DO2 = CaO2 x CO (also abbreviated as Qt, it means cardiac output).

Hypoxia

• Defined as decreased O2 in the tissues.

Hypoxemia

• Defined as abnormally low PaO2.

Most Common Cause of Hypoxemia

• V/Q (ventilation/perfusion) mismatch.

Other Causes of Hypoxemia

• Hypoventilation, diffusion defect, shunting, and low PiO2 (altitude).

Ischemia/Shock

• Reduction in blood flow.

Circulatory Failure (Shock)

• Tissue O2 deprivation is widespread; the body tries to compensate by directing blood flow to vital organs.

Local Reductions in Perfusion (Ischemia)

• Can cause localized hypoxia, resulting in anaerobic metabolism, metabolic acidosis, and eventual death of the affected tissue.

Dysoxia

• DO2 is normal, but cells undergo hypoxia because they are unable to adequately utilize oxygen.
• Occurs in cyanide poisoning.
• In critically sick patients (sepsis, ARDS), oxygen debt may occur at normal levels of DO2.

Impaired CO2 Removal

• Disorders that decrease VA (alveolar ventilation) relative to metabolic need impair CO2 removal.

Inadequate Ve (Minute Ventilation)

• Usually a result of low VT (tidal volume) and low f (respiratory rate), such as in drug overdose.

Increased Deadspace Ventilation (VD/VT)

• Caused by increased physiologic dead space, as in pulmonary embolus.

Normal PaCO2 Value

• 35-45 mmHg

Normal Hb Value

• 12-16 g/100 mL of blood

Normal A-a Gradient (21% FIO2)

• 5-10 mmHg

Normal A-a Gradient (100% FIO2)

• 25-65 mmHg

A-a Gradient with V/Q Mismatch (100% FIO2)

• 66-300 mmHg

A-a Gradient with Shunt (100% FIO2)

• 300 mmHg

CaO2 Equation (Measures Oxygen Delivered to Tissues)

• CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003)

Normal CaO2 Value (Oxygen Delivered to Tissues)

• 17%-20%

CvO2 Equation (Measures Mixed Venous Oxygen Content)

• CvO2 = (Hb x 1.34 x SvO2) + (PvO2 x 0.003)

Normal CvO2 Value (Mixed Venous Oxygen)

• 14 vol % (12 - 16% vol)

DO2 Equation (Impaired Oxygen Delivery)

• DO2 = CaO2 x CO (also abbreviated as Qt, it means cardiac output)

The lesson covers respiration, emphasizing oxygen transport to tissues for aerobic metabolism. It also deals with carbon dioxide removal. The content highlights the importance of maintaining PACO2 and PaCO2 within the normal range of 35 to 45 mm Hg and factors affecting these values.