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Oxygen transport

German: Sauerstofftransport

1 Definition

The transport of oxygen is the connection between alveolar gas exchange and internal respiration performed by the transport of oxygen dissolved in blood plasma and bound to hemoglobin. The oxygen concentration in arterial blood is about 20ml/dl at 158mmHg oxygen partial pressure (pO2) like it is at sea level or 104mmHg pO2 in alveolar air. under these circumstances hemoglobin is nearly 100% saturated and the amount of dissolved oxygen in blood account for 1.5% and the amount of Oxyhemoglobin for 98.5% of total oxygen transported in blood.

2 Oxygen dissolved in Plasma

The amount of oxygen dissolved in plasma is directly proportional to the Temperature, the water solubility and partial pressure. In human body the temperature (37°C) and the solubility of oxygen in plasma is constant. Therefore according to Henry's law the oxygen concentration just depends on the pO2.

The amount of oxygen dissolved in blood plasma is 0.3ml/dl in arterial and 0.15ml/dl in venous blood. In the case of 100% oxygen respiration the concentration increases to 2-3ml/dl.

Physiological relevance of these small amount of dissolved oxygen is given by the fact that the dissloved oxygen in blood is the first portion that is absorbed at level of the alveoli and also the frist portion that is released to the tissue.

3 Oxyhemoglobin

In red blood cells hemoglobin ist the binding unit for oxygen by heme which is the prosthetic group of hemoglobin. Each hemoglobin can bind four oxygen molecuels and the amount of hemoglobin in blood is 15g/dl. The saturation of hemoglobin with oxygen depends on the pO2 and is depicted by the oxyhemoglobin dissociation curve with hemoglobin saturation (%) on Y-axis and pO2 (mmHg) on X-Axis.

The amount of oxygen that can be bound by 1 gram of hemoglobin is defined as oxiphoric power or oxygen capacity.

3.1 Oxyphoric power

Under normal conditions at sea level and in healthy persone hemoglobin would be 100% saturated with oxygen. A 100% saturation leads to an oxyphoric power of 1.39ml oxygen in 1 gram hemoglobin.

  • oxygen concentration under ideal conditions: 15g hemoglobin/dl blood x 1.39ml oxygen/g hemoglobin = 20.85ml oxygen/dl blood

These optimum value is reduced by physiological shunt between systemic circulation which proviedes nutrition for the lung tissue and the pulmonary circulation plus small amount deoxygenated blood delivered by Thebesian veins. The physiological shunt decrease the saturation of arterial oxygen after mixing within the left ventricle and atrium of the heart to 97%. Moreover there is a small amount of non-functional hemoglobin which also influence the ideal value by decreasing the oxyphoric power to 1.34ml of oxygen/g hemoglobin (Hüfner's constant).

  • Oxygen concentration under physiological conditions: 15g hemoglobin/dl blood x 1.34ml oxygen/g hemoglobin x 97% = 19,5ml oxygen/dl blood

In the end we have to add 0.3 ml oxygen dissolved in the plasma leads to a physiological amount of 19.8 ml oxygen/dl blood.

After the blood passed the tissue and hemoglobin released oxygen, a process that is triggered by Bohr effect and done by Brownian motion, the pO2 in venous blood is about 46 mmHg. The 46 mmHg pO2 is related to an saturation of 75%.

4 Reference

Textbook of Medical Physiology/Guyton and Hall - ISBN-10: 1416045740

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