Cardiovascular Oxygen Delivery and Metabolism

Dissolved Oxygen and Henry's Law

A much smaller fraction of oxygen is dissolved in the plasma. According to Henry's Law, the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas. Therefore, the dissolved portion is calculated using the partial pressure of oxygen ($PaO_2$) multiplied by a solubility coefficient of 0.003.

Combining these, the formula for arterial oxygen content is:

$$CaO_2 = (Hgb \times SaO_2 \times 1.34) + (PaO_2 \times 0.003)$$

Similarly, the Venous Oxygen Content ($CvO_2$) can be calculated using venous saturation ($SvO_2$) and venous partial pressure ($PvO_2$):

$$CvO_2 = (Hgb \times SvO_2 \times 1.34) + (PvO_2 \times 0.003)$$

The "Engine" of Delivery: Cardiac Output ($CO$)

Even if the blood is highly saturated with oxygen, delivery will fail if the blood is not moving. Cardiac Output ($CO$) is the volume of blood pumped by the heart per minute. It is the "engine" that drives the transport of oxygenated blood to the systemic tissues.

Cardiac output is determined by the interaction of three fundamental hemodynamic factors:

1. Preload: The initial stretching of the cardiac myocytes prior to contraction.
2. Contractility: The innate force of myocardial contraction during systole.
3. Afterload: The resistance the heart must pump against to eject blood into the circulation.

Exam Tip: Remember that Cardiac Output is the product of Heart Rate and Stroke Volume ($CO = HR \times SV$). Changes in preload, contractility, or afterload will all influence the stroke volume and, consequently, the total cardiac output.

Respiration: External and Internal

The delivery of oxygen is a continuous cycle of external and internal respiration.

• External Respiration involves the process of ventilation, distribution, diffusion, and perfusion that brings oxygen from the atmosphere into the arterial blood.
• Internal Respiration is the final step where oxygen is transferred from the blood into the systemic tissues to support cellular function.

Oxygen Extraction and Consumption ($VO_2$)

Once oxygenated blood reaches the tissues, the cells extract what they need for metabolism. This process is quantified by the Oxygen Extraction Ratio ($EO_2$), which represents the fraction of delivered oxygen that is actually utilized.

$$EO_2 = \frac{CaO_2 - CvO_2}{CaO_2}$$

In a healthy, whole-body state, the typical extraction ratio is approximately 25%. The actual amount of oxygen consumed by the tissues per minute is known as Oxygen Consumption ($VO_2$). It can be calculated by multiplying the cardiac output by the difference between arterial and venous oxygen content:

$$VO_2 = CO \times (CaO_2 - CvO_2) \times 10$$

Clinical Correlation: If oxygen delivery ($DO_2$) drops (e.g., due to anemia or low cardiac output), the body may attempt to compensate by increasing the oxygen extraction ratio ($EO_2$) to maintain a constant oxygen consumption ($VO_2$). However, there is a limit to how much oxygen can be extracted before tissue hypoxia occurs.