Gas Exchange and Transport - Hemoglobin and Diffusion

Flashcards covering the mechanisms of gas exchange, diffusion barriers, V/Q relationships, oxygen transport via hemoglobin, and clinical abnormalities in gas exchange.

Respiration

The process of moving oxygen to tissues for aerobic metabolism and the removal of carbon dioxide from the tissues to the atmosphere.

Emphysema

A clinical condition characterized by alveolar-capillary destruction that limits gas diffusion.

Atelectasis

A clinical condition involving alveolar collapse which decreases the rate of gas diffusion.

Alveolar Fibrosis

A condition involving the thickening of the alveolar wall, leading to diffusion-limited problems.

Pneumonia

Alveolar consolidation that decreases ventilation to the affected lung lobe.

Pulmonary Edema

A clinical condition involving frothy secretions or interstitial edema that impairs gas diffusion.

Alveolar Carbon Dioxide ($PACO_2$)

Gas tension that varies directly with the body's $CO_2$ production and inversely with alveolar ventilation ($V_A$), normally maintained at $35$ to $45\,mmHg$.

Dead Space

Gas that does not participate in gas exchange, which can lead to an increased $PACO_2$.

Alveolar Oxygen Tension ($PAO_2$)

Partial pressure of oxygen in the alveoli, which falls when oxygen diffuses out of the alveoli faster than $CO_2$ diffuses into it.

Water Vapor Pressure ($PH_2O$)

The pressure exertion of gas fully saturated with water vapor at BTPS, which is $47\,mmHg$.

Alveolar Nitrogen ($PAN_2$)

Determined by Dalton's law as $PAN_2 = P_B - (PAO_2 + PACO_2 + PH_2O)$, where nitrogen is inert but occupies space and exerts pressure.

Fick's Law

A principle stating that the greater the surface area, diffusion constant, and pressure gradient, the more diffusion will occur.

Pulmonary Blood Exposure Time

The time blood is normally exposed to alveolar gas, which is approximately $0.75\,\text{seconds}$ at rest and may fall to $0.25\,\text{seconds}$ during exercise.

Anatomical Shunts

Right-to-left shunts such as bronchial venous drainage and Thebesian venous drainage that cause $PaO_2$ to be lower than $PAO_2$.

A-a Gradient ($P(A-a)O_2$)

The difference between alveolar and arterial $PO_2$ used to measure gas exchange efficiency and estimate hypoxemia and shunting.

Thebesian Vessels

Vessels of the left ventricular myocardium that drain directly into the left ventricle, contributing to the normal alveolar-arterial oxygen difference.

Ventilation/Perfusion ($V/Q$) Ratio

The ratio of ventilation to blood flow; an ideal ratio is $1$, while ratios greater than $1$ indicate ventilation exceeds perfusion.

Alveolar Dead Space

Areas with ventilation but no blood flow, which increases $PO_2$ and lowers alveolar $PCO_2$.

Anatomic Deadspace

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

Hypoxic Vasoconstriction

A reaction in the pulmonary capillary where blood flow is decreased to lung lobes with decreased ventilation, such as in pneumonia.

Henry's Law

Law allowing calculation of dissolved oxygen in plasma: $\text{Dissolved } O_2 \text{ (ml/dl)} = PO_2 \times 0.003$.

Hemoglobin (Hb)

A conjugated protein with four linked polypeptide chains (globin) and heme, where $1\,\text{gram}$ can bind $1.34\,ml$ of oxygen.

SaO2

The percentage of hemoglobin carrying oxygen compared to total hemoglobin, calculated as $[HbO_2 / \text{total Hb}] \times 100$.

Arterial Oxygen Content ($CaO_2$)

The sum of oxygen bound to Hb and dissolved in plasma, calculated as $(Hb \times 1.34 \times SaO_2) + (PaO_2 \times 0.003)$, with a normal value of $17\text{-}20\%$..

Mixed Venous Oxygen Content ($CvO_2$)

Total oxygen carried in mixed venous blood, normally $14\,\text{vol \%}$, which decreases when cardiac output ($Q_t$) decreases.

Swan-Ganz Catheter

A balloon-tip, flow-directed catheter inserted into the pulmonary artery to draw mixed venous blood.

Affinity

The characteristic of hemoglobin that determines whether it holds onto or releases oxygen.

Bohr Effect

The impact of pH on hemoglobin affinity; low pH shifts the $HbO_2$ curve to the right, enhancing oxygen unloading at the tissues.

2,3-Diphosphoglycerate (DPG)

A substance in Red Blood Cells that stabilizes deoxygenated Hb; increased levels shift the curve to the right to promote oxygen unloading.

Sickle Cell Hemoglobin (HbS)

A fragile hemoglobin variant that leads to hemolysis (RBC destruction) and the formation of thrombi.

Methemoglobin (metHb)

Hemoglobin with abnormal iron ($Fe^{3+释放}$) that cannot bind with oxygen, causing a left shift in affinity; often caused by nitric oxide or lidocaine.

Carboxyhemoglobin (HbCO)

Hemoglobin bound to carbon monoxide with an affinity $200$ times greater than oxygen, preventing oxygen unloading.

Oxygen Delivery ($DO_2$)

The total amount of oxygen delivered to tissues, calculated as $DO_2 = CaO_2 \times \text{Cardiac Output}$.

Hypoxemia

An abnormally low $PaO_2$, most commonly caused by $V/Q$ mismatch.

Shock

Circulatory failure where tissue oxygen deprivation is widespread, representing a reduction in blood flow.

Ischemia

Localized reductions in blood flow that can cause localized hypoxia, anaerobic metabolism, and metabolic acidosis.