CO₂ in arterial blood

СО2 release is indissolubly related to the oxygen consumption and СО2 generation in organism. СО2 is generated in organism as a result of biochemical transformations of glucose, amino acids, fats in the liver and blood under the influence of enzymes. Since glucose is one of the main oxygen source for a cell, its level is connected with СО2 generation and release in organism. Under the influence of glucose oxydase, glucose is oxidized by air oxygen to gluconic acid, and hydrogen peroxide in equimolecular quantities is formed. In this relation, СО2 generation rate must be lower than СО2 release rate, and total venous blood CO2 must be higher than total arterial blood CO2.

Too much CO2 in the blood can be a sign of many conditions, including:

• Lung diseases
• Cushing's syndrome
• Kidney failure
• Metabolic alkalosis, a condition in which your blood is not acidic enough. You may lose acid from conditions such as vomiting, dehydration, and anorexia.

Too little CO2 in the blood may be a sign of:

• Addison disease, a complication of type 1 and type 2 diabetes
• Diabetic ketoacidosis
• Shock
• Metabolic acidosis, a condition in which your blood is too acidic. It may be caused by many things, including kidney or liver disease, or long-lasting diarrhea.
• Respiratory alkalosis, a condition in which your blood is not acidic enough because of lung or breathing disorders, including hyperventilation (rapid, deep breathing).

Transport of carbon dioxide from tissue cells to lung

The carbon dioxide journey (FIGURE 1a) begins in the mitochondrion of tissue cells where it is produced. Due to the prevailing concentration gradient carbon dioxide diffuses from mitochondria (where pCO2 is highest) across the cytoplasm, out of the cell and into the capillary network.

FIGURE 1a: CO2 in tissues

A little of the CO2 arriving in blood from tissue cells remains physically dissolved in blood plasma (see above) and an even smaller proportion binds to NH2 (amino) terminal groups of plasma proteins, forming so called carbamino compounds. Most however, diffuses down a concentration gradient into red cells.

A little of this remains dissolved in the cytoplasm of the red cell and some is loosely bound to amino terminal groups of reduced hemoglobin forming carbamino-Hb. However most of the carbon dioxide arriving in red cells is rapidly hydrated to carbonic acid by the red cell isoform of the enzyme carbonic anhydrase. At physiological pH almost all (≈ 96 %) of this carbonic acid dissociates to bicarbonate and hydrogen ions:

Equation 1:

The hydrogen ions are buffered by reduced hemoglobin and most of the bicarbonate ions pass from the red cell to plasma in exchange for chloride ions (this so called ‘chloride shift’ maintains electrochemical neutrality). It is clear from the above that most carbon dioxide is transported as bicarbonate (predominantly in blood plasma), but there are in total four modes of CO2 transport [1,2]:

90 % is transported as bicarbonate in plasma (65 %) and red cells (25 %)

5 % is transported physically dissolved in plasma and red cell cytoplasm

5 % is transported loosely bound to hemoglobin in red cells and < 1 % to proteins in plasma - so called carbamino compounds

< 0.1 % is transported as carbonic acid

Total carbon dioxide blood content is the sum of these four components.

Within the microvasculature of the lungs (FIGURE 1b) the partial pressure gradient across the alveolar membrane determines that dissolved CO2 passes from blood to alveoli. This loss of carbon dioxide from blood favors reversal of the red cell reactions described above.

Thus bicarbonate passes from plasma to red cell, buffering hydrogen ions released from hemoglobin, as it is oxygenated. Reversal of the carbonic anhydrase reaction, results in production of CO2 that diffuses from red cells to plasma and onwards to alveoli.

Mixed venous blood arriving at the lungs has a total CO2 content of approximately 23.5 mmol/L (or 52 mL/dL) whereas arterial blood leaving the lungs has a total CO2 content of 21.5 mmol/L (48 mL/dL).

This arterio-venous difference (2 mmol/L or 4 mL/dL) represents the amount of CO2 added to blood from tissue cells and lost from blood as it passes through the pulmonary microvasculature, to be eventually excreted from the body in expired air.

FIGURE 1b: CO2 in lungs

https://medlineplus.gov/lab-tests/carbon-dioxide-co2-in-blood/

https://www.nps.org.au/assets/AP/pdf/The-interpretation-of-arterial-blood-gases.pdf

https://acutecaretesting.org/en/articles/parameters-that-reflect-the-carbon-dioxide-content-of-blood