Oxygen Content II

This session focuses on the delivery of oxygen to tissues, specifically pertaining to Chapter 23 of Berne & Levy Physiology, 6th Edition.

• Primary learning objectives include:

  • Calculating arterial and venous oxygen content.

  • Calculating oxygen delivery rates to tissues.

  • Understanding alterations in oxygen content during physiological or pathological states such as polycythemia (a condition with too many red blood cells) and anemia (a condition with too few red blood cells).

  • Identifying factors affecting the extraction (how much oxygen is taken from the blood) and overall delivery of oxygen to systemic tissues (the tissues throughout the whole body).

Calculation of Arterial and Venous Oxygen Content

• Total oxygen content in blood is the sum of oxygen bound to hemoglobin (the protein in red blood cells that carries oxygen) and oxygen dissolved in plasma (the liquid part of blood). The formula for total oxygen content ($C$) is:

  • $C = ([\text{Hemoglobin}] \times \text{Capacity} \times \text{Saturation}) + (P_{O_2} \times \text{Solubility Coefficient})$

• Arterial Oxygen Content ($C_aO_2$):

  • Hemoglobin concentration is measured at $15\,g/dL$. This indicates how much hemoglobin is present in the blood.

  • The binding capacity of hemoglobin is $1.34\,mL\,O_2/g\,Hb$, which means each gram of hemoglobin can carry this much oxygen.

  • Arterial saturation ($S_aO_2$) is determined at $98\%$ (or $0.98$), indicating that a high percentage of the hemoglobin is saturated with oxygen.

  • To calculate the amount of oxygen bound to hemoglobin: $15 \times 1.34 \times 0.98 = 19.7\,mL/dL$.

  • The arterial partial pressure of oxygen ($P_aO_2$) is $100\,mmHg$, which reflects the pressure of oxygen in the arterial blood.

  • The solubility coefficient is $0.0031\,mL\,O_2/dL/mmHg$, showing how much oxygen dissolves in plasma.

  • Dissolved oxygen is calculated as: $0.0031 \times 100 = 0.3\,mL/dL$.

  • Total $C_aO_2$ is found by adding the bound oxygen and dissolved oxygen: $19.7 + 0.3 = 20\,mL$ of oxygen per deciliter ($dL$) of blood.

• Venous Oxygen Content ($C_vO_2$):

  • To calculate venous content, we look at the relationship between the venous partial pressure ($P_vO_2$) and hemoglobin saturation through the oxyhemoglobin dissociation curve, which shows how hemoglobin releases oxygen at different pressures.

  • Normal $P_vO_2$ is $40\,mmHg$.

  • According to the oxyhemoglobin curve, a $P_vO_2$ of $40\,mmHg$ corresponds to a saturation ($S_vO_2$) of approximately $75\%$.

  • The amount of oxygen bound to hemoglobin is calculated as: $15 \times 1.34 \times 0.75 = 15.075\,mL/dL$ (approximately $15.08\,mL/dL$).

  • The dissolved oxygen is calculated as: $0.0031 \times 40 = 0.124\,mL/dL$.

  • Total $C_vO_2$ is therefore: $15.075 + 0.124 = 15.199\,mL/dL$, which is typically rounded to $15.2\,mL/dL$ or simply $15\,mL/dL$ for easier comparison.

Tissue Oxygen Extraction and the Oxygen Gradient

• Extraction Calculation:

  • The amount of oxygen that tissues extract from the blood is the difference between arterial content and venous content.

  • We can calculate this extraction as follows: $C_aO_2 - C_vO_2 = 20\,mL/dL - 15\,mL/dL = 5\,mL$ of oxygen per deciliter ($100\,mL$) of blood. This means that 5 mL of oxygen is used by tissues from every deciliter of blood.

• Oxygen Partial Pressure Gradient:

  • As oxygen moves from the atmosphere to the tissues, there is a stepwise decrease in the partial pressure of oxygen:

  • Atmosphere: $160\,mmHg$

  • Inspired Air (humidified): $150\,mmHg$, as air gets moist when we breathe in.

  • Alveolar Air ($P_{AO_2}$): $100\,mmHg$, which is the pressure of oxygen in the air sacs of the lungs.

  • Arterial Blood ($P_aO_2$): $100\,mmHg$, indicating that oxygen has moved into the bloodstream.

  • Venous Blood ($P_vO_2$): $40\,mmHg$, which shows a significant drop in oxygen pressure after tissues have consumed the oxygen.

  • This drop from $100\,mmHg$ in arterial blood to $40\,mmHg$ in venous blood represents the oxygen consumption by metabolically active tissues that need oxygen for energy.

Quantification of Oxygen Delivery Rates

• General Delivery Equation:

  • Oxygen delivery is expressed mathematically as the product of cardiac output ($Q$) and the arterial oxygen content. This can be understood as how much oxygen is carried to the tissues per minute.

  • The equation is: Delivery ($D_{O_2}$) = $Q \times C_aO_2$

• Calculation of Global Oxygen Consumption:

  • By examining how much oxygen is extracted and the amount of blood being pumped out by the heart (cardiac output), one can determine how much total oxygen the body uses per minute.

  • For instance, if extraction is $5\,mL\,O_2$ per deciliter (which is also expressed as $0.05\,mL\,O_2/mL$ of blood) and cardiac output is $5000\,mL/min$:

  • Total consumption = $5000\,mL/min \times 0.05\,mL\,O_2/mL = 250\,mL$ of oxygen per minute.

  • This value ($250\,mL/min$) reflects the average resting oxygen demand of the human body, indicating how much oxygen is needed in a state of rest.

Physiological and Artificial Modulation of Oxygen Delivery

• Variables for Increasing Delivery:

  • Oxygen delivery can be changed by various factors including cardiac output (which is determined by heart rate times stroke volume, the amount of blood pumped with each beat) and oxygen content (which depends on the hemoglobin concentration and saturation levels).

  • Cardiac Output: In scenarios such as exercising, the heart rate and stroke volume both increase, making oxygen delivery more efficient.

  • Hemoglobin Concentration: An increase in hemoglobin ($Hb$) concentration significantly affects oxygen delivery. For example, increasing $Hb$ from $15\,g/dL$ to $20\,g/dL$, as seen in the condition known as polycythemia, enhances the ability of blood to carry oxygen.

  • Saturation: Usually, arterial blood is saturated at $98\%$, and increasing this to $100\%$ only has a small impact on total oxygen delivery.

  • Partial Pressure/Dissolved Oxygen: Under normal conditions, enhancing the arterial pressure of oxygen ($P_aO_2$) does not significantly affect the total content. However, in specialized high-pressure environments (like hyperbaric therapy), the dissolved oxygen amount can be substantially increased.

• High Altitude Adaptation:

  • Training at high altitudes, like in Colorado Springs (the location of the US Olympic Training Center), can lead to a state known as hypoxemia (reduced oxygen levels in the blood).

  • This triggers the body to naturally increase its hemoglobin concentration over time, enabling better oxygen delivery to tissues in those challenging conditions.

• Athletic Doping:

  • Some elite athletes, such as professional cyclists like Lance Armstrong, often reach maximum cardiac performance, meaning their hearts are working as hard as they can.

  • To enhance oxygen delivery to their muscles for activities like climbing or intensive performances, they might artificially increase hemoglobin levels.

  • Methods of Doping:

  • Erythropoietin (EPO) administration: A synthetic hormone that boosts the production of red blood cells, which increases the blood's ability to carry oxygen.

  • Blood transfusion: Athletes may donate their own blood and then re-infuse it before competitions to raise their red blood cell count and improve performance.

Clinical Scenarios: Carbon Monoxide Poisoning and Anemia

• Carbon Monoxide (CO) Poisoning and Hyperbaric Therapy:

  • Carbon monoxide interferes with hemoglobin saturation, potentially reducing it to $50\%$ instead of the normal $98\%$, which causes the overall arterial content to drop from $20\,mL/dL$ to $10\,mL/dL$. This can lead to severe oxygen deprivation.

  • Hyperbaric therapy (being in a high-pressure environment, like $3\,atm$) can increase the amount of dissolved oxygen, potentially raising total content to about $16.82\,mL/dL$.

  • Is this enough? Yes, because if the content reaches $16.82\,mL/dL$ and the cardiac output remains at $5000\,mL/min$, the delivery becomes $841\,mL/min$. Given that the body only demands $250\,mL/min$, this delivery rate is ample "breathing room" until the harmful carbon monoxide is eliminated from the system.

• Anemia:

  • Anemia is characterized by low hemoglobin concentration, which results in markedly reduced oxygen content (for example, dropping to $13.44\,mL/dL$).

  • Compensatory Mechanism: Patients with anemia often present with elevated cardiac output, meaning that their hearts work harder to supply enough oxygen.

  • If the oxygen content is low, the body compensates by increasing heart rate and/or stroke volume (the amount of blood ejected per heartbeat) to maintain adequate total delivery: $D_{O_2} = Q \times C_aO_2$.

  • Due to the low content, normal oxygen therapy at typical pressures does not improve conditions for these patients; the root issue is the lack of hemoglobin to carry oxygen effectively rather than the partial pressure of oxygen itself.

Relationship Summary

• The arterial $P_aO_2$ of $100\,mmHg$ plays a vital role in determining the saturation ($98\%$) and content ($20\,mL/dL$) of arterial blood, as explained by the sigmoidal oxyhemoglobin dissociation curve. This curve illustrates how hemoglobin binds and releases oxygen based on pressure levels.

• Reductions in hemoglobin (as seen in Anemia) or a decrease in oxygen-carrying capacity (as seen in CO Poisoning) lead to lower overall oxygen content despite normal partial pressures, presenting a significant challenge for the body.