3.3d Oxygen and CO2 in blood/Measurements/Effects

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(Continues Chapter 3. About oxygen in blood and effects. Also CO2 and effect on brain)

Update 01 Aug. 2021

Oxygen Saturation of Arterial Blood (SaO₂)

 Recall that hemoglobin (Hgb) is the red pigment inside the red blood cells that, in the lungs, combines with oxygen O₂ and, as Hgb-O₂, goes in the blood inside each freshly oxygenated RBC to the tissues where it releases O₂ to cells in exchange for CO₂, carbon dioxide. The combining of O₂ with Hgb in the lungs is efficient. Nearly every molecule of Hgb is filled at its bond site with O₂ (97%-100% saturation by oxygen, or SaO₂). Testing SaO₂ by Pulse Oximeter is a simple, painless finger pinch that checks for low O₂ in breathed air at high altitude or with poor lung air-exchange; and it also checks the efficiency of air-exchange in your lungs as affected by lung disease or by heart failure or by shunting of low-O₂ venous blood from right to left side of heart. The SaO₂ test is not affected by anemia, i.e. you can be severely anemic with low RBC yet have SaO₂ 99%. The oximeter also measures the heart's pulse rate, and the latest one may measure the hemoglobin concentration in capillary blood.

The SaO₂ while at rest breathing sea level air should stay above 95%. If one gets in a jet and goes up to 35,000 feet, the SaO₂ may fall to 90% with no problem. But if SaO₂ goes much below 90%, it means critical lack of oxygen (hypoxia) is developing, and, if below 75%, damage from lack of oxygen to brain may be imminent. Low SaO₂ requires 100% oxygen inhalation.

Oxygen Physically Dissolved in your Blood Plasma: the SaO₂ measures how much O₂ is in chemical combination with Hgb in RBC in artery blood but it does not measure the physically dissolved O₂ in blood plasma. Normally, this is insignificant – 0.3% of the total O₂ in blood circulation while breathing air at sea level. But in acute myocardial infarction (AMI, a coronary artery blockage heart attack), brain stroke or in an underwater diver ascending too quickly and having nitrogen bubbles in blood, the damage may be prevented by prompt breathing 100% oxygen or even better by hyperbaric (at pressure of several atmospheres) oxygen in sealed chamber. In AMI and stroke, the good effect is because pure O₂ under high pressure breathing will increase the oxygen dissolved physically in the tissues as molecular gas, even when the SaO₂ is 100%, and this difference gives better heart and brain tissue oxygenation and may make the difference between life and death for heart or brain cell already affected by the low oxygen of poor circulation.

   Carbon monoxide (CO) comes mainly from the incomplete burning of carbon fuel (the complete burning gives the greenhouse gas, CO2, carbon monoxide). In the blood it forms an irreversible bonding with the RBC hemoglobin (carboxyhemoglobin) that in high enough concentration prevents adequate oxygenation of tissues (a frequent means of suicide is breathing in CO from illuminating gas or in a closed car with engine on). One-to-two-pack per day smokers typically have 3 to 8% of their hemoglobin as carboxyhemoglobin with non smokers having <2%. Five to 10% blood level is associated with impaired cognition and above 30% leads to anoxic death with a cherry-red color to lips. Life-saving treatment if you come upon a possible CO asphyxiation is immediate removal to fresh air and high-pressure oxygen administration. At physiological level,  CO is produced in the body and serves as an atypical neurotransmitter used in the hippocampus to enhance memory process (LTP, long term potentiation).

Color Appearance of Whole Blood depends on ratio of the O₂ to the CO₂ connected to hemoglobin in the red blood cells. When the heart’s right side chamber pumps high-CO₂ venous blood into lungs, the blood exchanges its CO₂ acquired in tissue for newly breathed-in O₂ and becomes bright red. Then the bright red blood is returned to left heart, pumped into aorta, and distributed via arteries to tissues where it gives up the O₂ for CO₂ to the cells of your body tissues, and becomes dark, as seen in blood returning in veins to right side of heart. When such vein blood, freshly drawn by needle syringe, is put into a glass tube, it should appear dark red. But occasionally the vein blood appears almost black due to severe disease affecting heart or lungs causing low O₂ in veins. Because the tissue extraction of the already low O₂ depletes oxygenation of vein blood more than usual, it leads to blood more black than red and a dusky skin appearance in fair-skin person. Blood color changes relating to oxygenation also are the cause of the blue-black tinged skin of methemoglobin poisoning (seen with overdose of APC tablets against fever) and the cherry red color about a patient’s lips, of carbon monoxide poisoning. The late President JFK's wife Jackie Kennedy's father was nicknamed Black Jack Bouvier because he had the methemoglobinemia condition as genetic trait.

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3.3e White Blood Cells, Types & Count & Percent i...